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[J.J. Blair]

I have a pair of GE "uni-level amplifiers."  Each channel has a pair of 6V6s, another tube I can't remember and a 6386.  Are all limiters with a 6386 considered vari-mu?  This is a subject I am completely ignorant on.

[Jakob Erland]

The 6386 is a remote-cutoff dual-triode, a type that you probably wouldn't want to use for any other purposes than varying gain - be it RF or audio.

So yes, there's reason to believe that you have a variable-mu based limiter.

Jakob E.

[John Klett]

yes... and pretty much every tube limiter made is variable-mu - there are a few obvious exceptions (optical limiters like the LA-2 and it's variants are not variable-mu - I'm not sure what a Drawmer 1960 is - I think it's a VCA of some sort but I lost the schematice years ago)

Most (non-optical) tube limiters, but not all, used variable mu triodes or remote cutoff pentodes that have a grid that is wound with the wires spaced unevenly - closer together at the ends and more spread apart in the middle.  

When the grid-wire spacing and diameter and the spacing between the grid and the cathode are uniform along the entire length of the grid, the field strength over the entire cathode surface covered by the grid does not vary much from point to point.  This means that the negative grid voltage required to prevent electrons from going from the cathode to the plate is going to be the same for all points along the grid.  A grid with a uniform structure like this is called a sharp-cutoff grid.

When the spacing of the wires varies along the grid the field at the cathode varies at the cathode surface.  A greater negative grid voltage is required to prevent electrons from moving to the plate at points where the grid-wire spacing is wider than it is at points where the grid-wire spacing is narrower.  This means that cut-off takes place a different grid voltages at different parts of the grid.  As grid voltage varies, some portion of the grid can be at cut-off (generally the wire spacing is narrower at the ends and wider in the middle) while another portion is not.  A grid with a structure like this is called a remote cut-off, variable-mu or gradual cut-off grid.

You don't have to use a tube with a remote cut-off grid to control gain in a tube.  One can also modulate plate voltage, grid and cathode bias to change the gain or mu of a tube stage.  I have heard of this being done - maybe seen a schematic - but all the limiters I can think of right now (outside of optical units and after only one cup of espresso - 1 of 3 typically required to achieve prober brain bias) use remote cut-off or variable-mu tube(s) either directly in the signal path to control gain in that stage or in feedback to control the amount of gain through some or all of the unit by varying the amount of negative feedback applied.  

[Larrchild]

6386= vari-mu.

in the 60's Gates Urei Fairchild, GE, CBS, and a host of European limiters used the 6386. It was an industrial rf agc tube, i think.

The minimalist audio path was usually an input transformer, a push-pull set of tubes and an output transformer. Not bad.

How to make the control voltage to vari the mu,  usually consisted of some low level amplification connected to the limiter's output and driving a tube diode to make a varying dc voltage which went back to the 6386's to do the gain control. These models are denoted by being able to physically lift easily.

Fairchild and perhaps a few others took a more bombastic approach with large audio power amplifiers (10-20w) in the sidechain to make the control voltage. These are heavy and full of tubes.

But both do the same thing, slightly differently

[Brian Roth]

I see that Manley has gone to a pair of 6BA6 bottles, each wired as a triode:

http://www.manleylabs.com/techpage/TBAR.html

Bri

[Larrchild]

This is off my Tek 577 curve tracer.  They are close!

[Brian Roth]

Larry, quite interesting, considering that the 6BA6 and 12BA6 were built "by the bazillions" for TV sets, radios, and hence seem plentiful as NOS parts.  I bet I have more than a few in my "garage accumulation" of receiving tubes.

Now I need to look through my olde RCA manuals for other possible candidates <g>.

Bri

[Larrchild]

I should be more fair and use more than 1v ramp steps so i can show more of its higher bias range.

I'll use 2v/step and start it at say -4v, so's as to not eat up traces on the low bias sweeps.

But they still stay very similar in the 0 to minus 20 db range of gr.

And the tube to tube variations in both 6386's and 6AU6's are staggering!

It's all about the hand-matching.

[Brian Roth]

Larry..I have no doubt that matching remote cut-offs is a biiiitch, I guess like matching 6L6's, etc...

However, the writer at Manley also made the comment that sections within a 6386 aren't necessarily matched, and no way in hell can any of us fix THAT!  So, it sorta makes sense to me to split the "twins" out into "two singles" so they can be matched....like output pairs on an audio amp.

Bri

[maxdimario]

this is why the fairchild has two pairs of tubes wired in opposites..if you have one tube that draws more current or less you can find the combination between tubes to even it out by changing sockets.

[Larrchild]

Bri: "I guess like matching 6L6's, etc..."

it's like trying to match a pair of 6L6's at 100 different bias points, lol!

In a stereo limiter, not only are you matching the push-pull sides, but also the gain/voltage plots of both channels for image tracking.

lots of small piles of tubes on your bench with numbers on them.

[Brian Roth]

I wonder how well a small pile of 6BA6's from the same manufacturer and a similar manufacturing date match up......

Bri

[Larrchild]

they are not bad. but within a batch, ya still have a lot of work matching.

heres some 2/v per step sweeps of both. I used a -4v fixed bias and so yer looking at 10 sweeps starting at -4 and sweep line 10 (right one) is the -24 volt sweep. that would be the 10-15 db gain reduction area of the 6386 and they still match right-well.
6BA6 is on the first. 6386 is next.

[Brian Roth]

Larry, looks pretty damn close!

The writer at Manley made mention that sections within a given 6386 don't always match, which I assume would mean the entire bottle would be a "reject".  How often have you encountered that?

Bri

[maxdimario]

all variable-mu tube compressors apart budget ones such as the altec use a balancing circuit to get both sides of the tube to work in a similar way.

part of the Fairchild sound was the non-linearity which came as a result of 4 sets of triodes in push-pull, each one with a slightly different curve..You can't match perfectly.. it's impossible.

[Larrchild]

brian: "The writer at Manley made mention that sections within a given 6386 don't always match, which I assume would mean the entire bottle would be a "reject". How often have you encountered that?"

Back in the mid 80's when I built a dozen clones using 6386's Richardson was still making them. They truly sucked ass. 2 in 10 made it.  Now NOS GE's rocked. But that was then.

[Brian Roth]

Larry, that sounds like 6(12)BA6 bottles would be an even better deal!  They appear to be dirt cheap since there were bazillions made for radios, TVs, etc.  The trick would be working up a test jig to make matching "easy" without a bunch of effort via a Tek curve tracer (unobtanium!).

Bri

[Larrchild]

Well...
sadly the curves are the most revealing of tests.

The good news is that you can use a modern transistor curve tracer like mine that has 400+ anode volts and a few watts of current dissipation and do small signal tubes like this by merely adding a filament supply, and building a plug-in tube socket jig.

So rather than trying to find the 3 tektronix 570's left in the world..I chose a modern storage unit with an A-B switch. a 577D1

ebay has em'


By the way PRR, thats a 12BH7 on the screen. the 'AU7's bigger brother would also make a good vca tube eh?

[PRR]

> Are all limiters with a 6386...

It is possible to use the 6386 for other things. Could be an OK mike-amp or line-out tube. BUT there are a bazillion other tubes to do such chores, so in general if you see a 6386 it is doing variable-gain work.

> considered vari-mu?

Pet Peeve: The Mu may vary but that is NOT the key feature of an audio variable-gain tube.

Mu of 6386 drops about 1:3, maybe 1:4, but you can force the gain down over 1:100 and still get useful output.

I do not know of any audio gain-control scheme where shift of Mu (Amplification factor) is the primary effect.

As John says: "You don't have to use a tube with a remote cut-off grid", nor one with a variable Mu. In fact ANY tube "can" be used for electronic audio gain control (a limiter is such a gain-control plus level detector). The basic idea in all of these isn't vari-Mu, but Variable Transconductance (vari-Gm). I don't know why we call them "vari-Mu"; maybe "moo" or "meu" sounds more mystical than "gmm".

Any useful amplifying tube has Transconductance (Gm): change of grid voltage causes change of plate-cathode current. And Gm always reduces when tube current is reduced. In a conventional voltage-amplifier stage, gain is total load impedance times Gm. If Gm is reduced, gain is reduced.

But tube current normally falls faster than Gm. For a limiter, where maximum input wants minimum gain, sooner or later the current will be too small to handle the output level, and distortion gets gross. Using "ordinary" tubes like 12AU7, with a post-amplifier, you can get 15dB-20dB of gain reduction before it all falls apart.

That's where the tapered grid winding that John mentions comes in. This trick allows Gm to fall almost as fast as current. We still eventually run into gross clipping, but in audio use (push-pull plus a post-amp) we can easily top 30dB of gain reduction with 6386 or 6BA6 or a host of others (6ES8 is another good type, though it was one of the last TV-set tubes and can be hard to find).

In any case, the output of a vari-Gm stage in deep gain reduction is low. (The original intent was to drive 100K+ tuned circuits, not wide-band audio loads.) Most vari-Gm limiters follow the vari-Gm stage with an amplifier. Fairchild went a bit nuts: massively parallel 6386 with beefy iron can just-barely make recording studio line levels, and avoid the flavor of a post-amp. I think much of the sound of the Fairchild 660 is the super-simple yet highly-tuned audio path.

And in any case: a lot of the "working sound" of a limiter is the sidechain and the time constants. Fairchild has a massive sidechain and fine time control. Things like the 175 have "good enough" sidechains that will chip transients.

There are other ways to control gain electronically. LDRs are light sensitive resistors. Simple, cheap, no trims, but not precise. You can sharpen them up but then it makes as much sense to go vari-Gm. PWM has been used, but it isn't easy. Instead of using a tube as a gain-stage, you can use it as a cathode follower: reduce current enough, output falls. Since cathode impedance is just 1/Gm, this can be seen as another kind of vari-Gm, but the rate of Gm drop with grid voltage is very different. There is another form using plate resistance, either as passive load or as was done in the GE BA-6.....

> GE "uni-level amplifiers."

The BIG broadcast GE limiters use a 6386 front end, plus a post-amp (twin 6V6 to drive WATTS into long transmitter lines), plus a whacky feedback scheme that only happens at VERY high levels where the 6386 loses control. I think it can be pumped 35dB above nominal level and still not splatt your signal. There is no good reason a broadcaster would drop +35dB into a limiter, but stuff happens. Network feed accidentally gets patched through a mike-amp, etc. A lot of the old broadcast stuff was run with nominal level 30dB or 40dB below clipping to get low THD, like 0.1%. So it is possible to mis-patch too hot, wind up barely clipping at 3% THD but 30dB over nominal level, and then the BA-6 will whomp that down to 0dB and deliver a lightly-flavored loud an dlegal signal instead of splatting clip-garbage all over the radio band.

> I guess like matching 6L6's

6L6 has a bent-enough Gm curve that you could build an effective vari-Gm stage around a pair. You may not be able to get 30dB clean reduction without heroics; anyway two 6L6 is pretty heroic already (idle ~200V ~180mA). But the tubes are readily available on friday night.

[Larrchild]

Dude, my very first post on Gearslutz, as a newbie to gear forums,, I used the term "Variable transconductance" and it seemd like about 5 Ravens swooped down and started pecking at my head. Since then, I just use vari-mu so people understand me.

I give up on fighting marketing.

Funny you mentioned the plate resistance effect. One day I accidently had a few k of resistor from the reg'd dc voltage to the 6386's. What should have been a stiff supply was slightly choking and starving the tube on peaks, and giving it a totally different limiting sound. Later,I realized this wasn't coming from grid control and made a note.

I'm curious how the BA-6a differs. I have one and always thought it was remote cutoff based.

I think a fundimental point omitted in all that stuff there, PRR, is that the reason Fairchild used a big power amp, was to get risetime on the control voltage that a 6AL5 couldnt muster.. trying to charge a cap and bias a batch of grids fast enough.  So UREI 175's and Altec 436's  etc. have a transient distortion on attack from the momentary period where the overmodulation soft clips in the tube while it waits to be gain controlled.

But we digress.  Yes JJ if you see 6386's ..it's variable gain or variable transconductance or *shudders* vari-mu. That was really the only part you needed to read.

[JGreenslade]

Last December I attended a historical lecture relating to "fashion in audio", beginning around the dawn of the last century.

I discovered that the first "variable Gm" (aka "Delta

[John Klett]

Larrchild wrote on Tue, 09 August 2005 00:56

Dude, my very first post on Gearslutz, as a newbie to gear forums,, I used the term "Variable transconductance" and it seemd like about 5 Ravens swooped down and started pecking at my head. Since then, I just use vari-mu so people understand me. I give up on fighting marketing.

Actually you are fighting history

The technical term "variable mu" has been in use since the late 1920's and has been applied in technical literature and documentation to a number of techniques for changing gain on a dynamic basis in a tube stage.  This technical term was picked up and used as a marketing tool for one product.  Variable Mu has since been registered as a Trade Mark for a product manufactured by one company which has existed for only a fraction of the last 75+ years since the technical term entered common use in engineering circles...  sort of like trademarking "State Variable" or "Parametric Equalizer".  Now anyone manufacturing a dynamics control device using that gain control technique has to refrain from using a technical term that had been in common use for many many years.  The term can't appear on the device itself or even in the technical specification or literature that describe the device....  pretty cool trick and a testament to the stupidity of the people PTO and/or the skill of the attorney who did the filing...  and great marketing.

[JGreenslade]

In patent law, if prior art can be proved, the patent becomes null and void - would the same rule apply to trademarks? Did the PTO examiner do enough research?

Trademark law also forbids "descriptive" trademarks, i.e. it has to be abstract; you could probably trademark "tastyburger", but "tasty burger" would fall foul of the requirements. Surely "variable mu" is descriptive? Or is it legal because it's abbreviated to "vari-mu", which would mean "variable mu" is fine?

I'm thinking this is a "marketing trademark" - one that would not stand up in court if push came to shove...

Justin

[Larrchild]

Quote:

In patent law, if prior art can be proved, the patent becomes null and void - would the same rule apply to trademarks? Did the PTO examiner do enough research?

There is a "First used in Commerce" precident that applies to it's Trademarkability.

Trademark of vari-mu was abandoned by said company.


vari-mu vari-mu vari-mu vari-mu vari-mu vari-mu vari-mu vari-mu vari-mu.

Quote:

In fact ANY tube "can" be used for electronic audio gain control

A 12AX7 gets biased off after a few volts. So tubes that bias off farther out are better candidates. That 12BH7 in my curver tracer shot looks good, actually.

[JGreenslade]

Quote:

Typed Drawing ------------------------------------------------------------ -------------------- Word Mark VARI MU Goods and Services (ABANDONED) IC 009. US 021 023 026 036 038. G & S: audio signal processor, namely a compressor/limiter Mark Drawing Code (1) TYPED DRAWING Serial Number 78275083 Filing Date July 16, 2003 Current Filing Basis 1B Original Filing Basis 1B Owner (APPLICANT) Manley Laboratories Incorporated CORPORATION CALIFORNIA 13880 Magnolia Ave. Chino CALIFORNIA 91710 Attorney of Record Bruce K Jamison Prior Registrations 2654443 Type of Mark TRADEMARK Register PRINCIPAL Live/Dead Indicator DEAD Abandonment Date June 14, 2004

[Larrchild]

When I'm building stuff, the cat is pretty fussy about when I have it sounding right. If anything is not quite right, she vanishes. When the circuit is close, she comes back into the lab and meows.

This effect I call "Variable Mew".

[PRR]

> I discovered that the first "variable Gm" (aka "Delta

[Larrchild]

ED: PRR has forgotten more than we will ever know about tubes and
is a god on a major propellerhead forum where he has some inexpensive and functional variable transconductance circuits out there that others have built with delight.

[Larrchild]

maxdimario wrote on Sat, 23 July 2005 09:06

the Fairchild has two sockets wired opposite the other two, so that by inverting the positions of various 6386 tubes you balance the mismatch (all 4 tubes are in parallel). all variable-mu tube compressors apart budget ones such as the altec use a balancing circuit to get both sides of the tube to work in a similar way.

Absolutely. the sockets in a fairchild were wired so they alternated internal sections of the dual tube so you can get them to balance better by juggling them. But thats just idling current.

How they track across their control and gain range can be way off amongst similar current tubes. I discovered this when minimum thd null didnt agree with the cathode tube balance null.
use a THD meter to balance, if ya can.

Gotta plot the load lines on a screen to know, and then match pairs.

I just had a flash. If you kept adding gain reduction with a tone and checking a-b cathode balance throughout the gain reduction span until you found tubes that matched, that would mimic a curve tracer somewhat.

[JGreenslade]

PRR wrote on Wed, 10 August 2005 02:32

> I discovered that the first "variable Gm" (aka "Delta Audio is a vocational affliction "there is no "homeopathic" effect in bits and bytes." - HansP JGreenslade Hero Member Offline Posts: 824 Re: are all 6386 limiters vari-mu? Tek 577D1. What do you think a fair price for a storage (and non-storage 577) should be? I see the D1 going for $2,000 USD+ , but I'm guessing a shrewd engineer can find them for less? What's the D2 worth? Thanks, Justin edit: Another one here:  http://cgi.ebay.com/Tektronix-577-177-Curve-Tracer_W0QQitemZ  7537304581QQcategoryZ1504QQssPageNameZWDVWQQrdZ1QQcmdZViewIt em Audio is a vocational affliction "there is no "homeopathic" effect in bits and bytes." - HansP Larrchild Hero Member Offline Posts: 3972 Re: are all 6386 limiters vari-mu? I was bidding on several 577d1's in the $500+ US range on Ebay. Then I found one for $375US that had a busted a/b switch handle I fixed in 10 mins. They go for more like 500-1000 US from my experience. D2's are more. But high voltage option is needed in either one. edit: the top one ain't a D1  it's a 577 non storage. too much. it will say "D1 Storage" in the upper right corner over the intensity knob.  this one does not. Larry Janus http://2ubes.net JGreenslade Hero Member Offline Posts: 824 Re: are all 6386 limiters vari-mu? Many thanks for the advice Larry. I was asking because a local dealer has a 577 in stock, and they claim it's fully working / serviced... I've bought from the dealer before, and they're usually ok for haggling, so I thought it best to get an opinion on the price... With the 570 not being an option, are there other curve-tracers worth having that are not Tek? Did B+K make a good unit for valves? I'm guessing there are probably units around from the likes of AVO / GEC / Marconi etc, although the costs of restoration may be prohibitive I guess... I should clarify that I'm only looking to match / test small-signal valves for front-end apps, not power-tubes. Cheers, Justin Audio is a vocational affliction "there is no "homeopathic" effect in bits and bytes." - HansP vanimal Jr. Member Offline Posts: 61 Re: are all 6386 limiters vari-mu? Well guys, with regards to the trademark issue related above, as far as we know, we were the first company to make a dynamic limiting rack mount product which was called the "Variable Mu" Limiter Compressor and so we trademarked that product name to be ours in order to protect our mark the same as I trademarked "VOXBOX" "ELOP" "LANGEVIN" "MANLEY" "STARBIRD" "SLAM!" et al. Those are our product and brand names. Trademark law protection dictates that I can't start manufacturing an audio product called "The Manley Distressor" no more than some other company can start making one called the "[insert company name here] Variable Mu". Yes we can all still talk about variable mu dual triodes as they were called in tube manuals, and yes, "variable mu" is still a swell engineering term as discussed above. Yes of course there were many other limiters throughout history which used similar methods as ours to achieve limiting. Obviously we built upon this history, literally. We never claimed to have invented the method or the concept of this type of limiter. We were just the first who applied those two words "Variable Mu" to the faceplate and called our product that. The trademark means no other company can call their audio signal processor product a "Variable Mu". That's our name that we applied to our box in 1993 and later trademarked. That's it folks. Besides this aside, this is a great thread.... -- Cheers, EveAnna Manley http://www.manleylabs.com Larrchild Hero Member Offline Posts: 3972 Re: are all 6386 limiters vari-mu? waves from the east! Larry Janus http://2ubes.net Larrchild Hero Member Offline Posts: 3972 Re: are all 6386 limiters vari-mu? Quote: I should clarify that I'm only looking to match / test small-signal valves for front-end apps, not power-tubes. Triodes only.. Tektronix is the only one worth messing with. And the 577 has 1600v @ 20 watts, so it's possible to launch a tube right out of it's socket with too much throttle. =) For pentodes, you can fashion a screen supply too. Use pomona plugs and protoboard to make a tube socket jig. And it has a chimp-proof hv interlock you need to push on that the original jig has a pin for. I have dual jigs for all my fave tubes. It was easier than jumpers. Hint: 4 tube sockets per jig allows you to compare different tubes on each side and each sides' sockets are just wired in parallel so the unused socket becomes inert. So my main jig can compare 6386's to each other or 6BA6's to each other or 6BA6's to 6386's etc. Larry Janus http://2ubes.net PRR Newbie Offline Posts: 6 Re: are all 6386 limiters vari-mu? > The variable-mu valve is probably from the late 1920s or early 1930s. I know that they were commercially available by 1933. The full development was prompted by the superhet receiver but I have a feeling that manual bias changes were applied to earlier TFR receivers. The core problem (40dB-60dB range of radio signals) is the same TRF or superhet. But TRF was a fussy thing anyway, so manual gain control was tolerated. Superhet allowed one-knob tuning (good TRF had three tuning knobs) so it began the trend to "dummy" radios that any fool could operate. The oldest way to change gain was to reduce filament current. This not only killed Gm, it made your batteries last longer. But it had the same flaw as grid-bias control: for large reduction of gain, the plate current times the load impedance gets so small that the output overloads. Antenna potentiometers were sometimes used. But Cunningham's data for the '01 (their number CX-301-A) says "Volume control ... variation of either the grid bias or the plate voltage...", no mention of filament control. (This is stated to be a "storage battery" tube, but at 5V heat they are assuming a 4-cell 6.3V battery and a rheostat that you adjust to keep an even 5V glow as the battery runs down from 6.3V to ~5V.) The type 35/51 is a fairly old "Super-Control" tube (another phrase that someone should trademark) and the 1937 data (the tube is older) shows grid voltage needed for 37dB gain reduction (unspecified current, but obviously way too small for useful output with anything other than a tuned circuit). That sheet references a page 15; this seems to be the page with a hasty note about super-control tubes. BTW: this page says "variation in amplification factor", which could also be spelled "Variable Mu". Prior art is no bar to trademarks: the word "coke" was widely used for cooked-coal and cocaine products long before a soft-drink company registered it. In fact they were reluctant to call their brew "coke" because of the Drug Act (if it contained cocaine, it was illegal; if it didn't, it was misleading) but "had" to register it so that no other drink company could use it and be confused with Coca-Cola. "Variable Mu" was apparently up for grabs when EveAnna applied for a Trademark. A specific implementation of "Variable Mu" might be valid as a Design Patent. Clearly there should not be an Invention Patent on "Variable Mu" at this point in time (though the current US Patent Office is liable to approve anything and let you fight it out in court). In all these old notes, know that "Volume Control" does not mean what it did later. Early radios had tame audio stages that were run wide-open, no audio gain control. With the rise of the dummy superhet/AVC radio, and with better audio stages, the idea became to let AVC action keep the detector output up as close to 10V as possible (by AVC-reducing RF IF gain) and then use an audio potentiometer to reduce audio gain to the desired listening level. In local reception, it keeps all stations in/around town at similar level, so you can (one-knob) scan the band without deafening yourself. (In short-wave use, a single station will fade in and out as the ionosphere wobbles; AVC does act to keep a constant volume with fading.) > I trademarked ... "STARBIRD" ... et al. Those are our product and brand names. In the audio racket only. If you started making custom cars (you'd be good at it) and called them "Starbird", you would run right into Darryl Starbird. I don't know if he trademarked his name, or if that would bar your use of it in his field; I leave that to lawyers. Even in the audio racket only, you are only a few notches away from being razzed by the Songbird hearing-aid company. Hey; hearing aids are audio. And modern hearing-aids include limiters and equalizers. But you didn't step ON their name, and hearing-aid customers are unlikely to confuse a hefty Manley for something they can stick in their ear. Larrchild Hero Member Offline Posts: 3972 Re: are all 6386 limiters vari-mu? Thanks for the clarification on variable mu as a generic technical term vs a unique product name, EveAnna. I get it, and will type with less fear now =) .. even though PRR says it's a flawed term. But I suspect over time, one could save a few engraver bits by not having to keep engraving the word 'transconductance" over and over. Wasn't that Kraftwerk's 2nd album "Transconductance Express" ? Hey PRR, what about pentagrid converter tubes? Ones with a remote cutoff grid, and a normal one? Audio goes into the straight grid and control into the remote cutoff grid. Anyone done that? and would they mix like a dual gate mosfet and create artifacts? Eh? Quote: If you started making custom cars (you'd be good at it) Edit: If Manley made a car, I'd buy it in a New York Second. Imagine a purplish billet of streamlined aluminum with 4 black tires and some windshields. Larry Janus http://2ubes.net JGreenslade Hero Member Offline Posts: 824 Re: are all 6386 limiters vari-mu? Larrchild wrote on Fri, 12 August 2005 20:31 Quote: I should clarify that I'm only looking to match / test small-signal valves for front-end apps, not power-tubes. Triodes only.. Tektronix is the only one worth messing with. And the 577 has 1600v @ 20 watts, so it's possible to launch a tube right out of it's socket with too much throttle. =) For pentodes, you can fashion a screen supply too. Use pomona plugs and protoboard to make a tube socket jig. And it has a chimp-proof hv interlock you need to push on that the original jig has a pin for. I have dual jigs for all my fave tubes. It was easier than jumpers. Hint: 4 tube sockets per jig allows you to compare different tubes on each side and each sides' sockets are just wired in parallel so the unused socket becomes inert. So my main jig can compare 6386's to each other or 6BA6's to each other or 6BA6's to 6386's etc. Thanks again Larry. J Audio is a vocational affliction "there is no "homeopathic" effect in bits and bytes." - HansP ssltech Hero Member Offline Posts: 4780 Re: are all 6386 limiters vari-mu? vanimal wrote: Besides this aside... ...I'm still trying to figure out how you do that! Keith MDM (maxdimario) wrote on Fri, 16 November 2007 21:36 I have the feeling that I have more experience in my little finger than you do in your whole body about audio electronics.. PRR Newbie Offline Posts: 6 Re: are all 6386 limiters vari-mu? > what about pentagrid converter tubes? Ones with a remote cutoff grid, and a normal one? Audio goes into the straight grid and control into the remote cutoff grid. I've seen it proposed. I've even seen a plan, though not detailed. One monster advantage is you don't need a floating transformer winding to get both audio and control on the same grids. Couple chips would do great. > would they mix like a dual gate mosfet and create artifacts? Eh? There's always artifacts. The minimum-artifact approach for speech/music is usually fast attack and slow release. That still clobbers the first over-level wave; but that is inevitable unless you can see into the future. (There is a class of limiters with a delay-line so the control can preview the audio and act a little ahead of time.) Larrchild Hero Member Offline Posts: 3972 Re: are all 6386 limiters vari-mu? Quote: There's always artifacts. Right-on, but I was thinkin if you combine 2 signals into a common gate its not the same as if you combine 2 signals into 2 gates of a dual gate mosfet. It acts more like an rf mixer due to additional non linearities. f1+f2  f1-f2 f1+f2-f1+f2 ,lol  all that. Easy enough to breadboard. I'll get back to ya.=) Quote: that is inevitable unless you can see into the future Well, I just need to copy my vocal track in PT and slide it a few ms early and drive my control amp from that. Ain't no big thang! Larry Janus http://2ubes.net JGreenslade Hero Member Offline Posts: 824 Re: are all 6386 limiters vari-mu? Larrchild wrote on Fri, 12 August 2005 16:45 I was bidding on several 577d1's in the $500+ US range on Ebay. Then I found one for $375US that had a busted a/b switch handle I fixed in 10 mins. They go for more like 500-1000 US from my experience. D2's are more. But high voltage option is needed in either one. edit: the top one ain't a D1  it's a 577 non storage. too much. it will say "D1 Storage" in the upper right corner over the intensity knob.  this one does not. Larry, How do you confirm one has the high-voltage option? What if you bought one without - does it have to be a factory option? What about the other options - I assume the 177 fixture isn't  essential?   Thanks again! Justin Audio is a vocational affliction "there is no "homeopathic" effect in bits and bytes." - HansP maxdimario Hero Member Offline Posts: 3811 Re: are all 6386 limiters vari-mu? Part of the fairchild sound was derived from the fact that it's almost impossible to get truly linear sound out of it. the good thing about the fairchild is that it is a one-stage design. It does amplification, gain reduction and line driving all in one stage of 4 parallel 6386 tubes. this makes it direct sounding, distortion aside. If we focus on the distortion artifacts, it distorts differently depending on the amount of gain reduction, and it is a mix of all the 4 tubes' non linearities, as well as being class-a push-pull which means that some of the low-order distortion cancels out, but not completely because the two sides are never truly matched. grainy agressive sound. with two tubes, you can match the characteristics better, and end up with a cleaner sound, but then you've got to put a line driver after the GR circuit and that adds it's own kind of distortion as well. the fairchild 660 was a unique box. Larrchild Hero Member Offline Posts: 3972 Re: are all 6386 limiters vari-mu? Quote: How do you confirm one has the high-voltage option? What if you bought one without - does it have to be a factory option? Well, I should say its handier to have the high voltage option. Look and see if the voltage knob goes to 1600v.   If not, find a tracer that has at least has 400+ The 177 is the general test fixture and probably the one you want. Also the most common. avoid test fixtureless units. Quote: the good thing about the fairchild is that it is a one-stage design. It's also a no-feedback amp relying heavily on push pull cancellation of distortion products. Larry Janus http://2ubes.net maxdimario Hero Member Offline Posts: 3811 Re: are all 6386 limiters vari-mu? Yep. but all vari-mu GR circuits are no feedback. only the post GR line drivers such as in the u176 or Ba6a etc used feedback. others, like some collins, altec etc. used no feedback at all throughout. But yeah... a big part of it is the sheer simplicity of the audio path. Print Pages: 1 2 3 4 [All]   Go Up « previous next » R/E/P Community > R/E/P > R/E/P Archives > Bruno Putzeys (Designer) - Dave Hecht (Master Tech) > are all 6386 limiters vari-mu?   SMF 2.0.19 | SMF © 2021, Simple Machines | Terms and Policies XHTML RSS WAP2 Site Hosted By Ashdown Technologies, Inc. Page created in 0.035 seconds with 18 queries.

[JGreenslade]

Tek 577D1.

What do you think a fair price for a storage (and non-storage 577) should be? I see the D1 going for $2,000 USD+ , but I'm guessing a shrewd engineer can find them for less? What's the D2 worth?

Thanks,

Justin

edit: Another one here:  http://cgi.ebay.com/Tektronix-577-177-Curve-Tracer_W0QQitemZ  7537304581QQcategoryZ1504QQssPageNameZWDVWQQrdZ1QQcmdZViewIt em

[Larrchild]

I was bidding on several 577d1's in the $500+ US range on Ebay. Then I found one for $375US that had a busted a/b switch handle I fixed in 10 mins.

They go for more like 500-1000 US from my experience.

D2's are more. But high voltage option is needed in either one.

edit: the top one ain't a D1  it's a 577 non storage. too much.

it will say "D1 Storage" in the upper right corner over the intensity knob.  this one does not.

[JGreenslade]

Many thanks for the advice Larry.

I was asking because a local dealer has a 577 in stock, and they claim it's fully working / serviced... I've bought from the dealer before, and they're usually ok for haggling, so I thought it best to get an opinion on the price...

With the 570 not being an option, are there other curve-tracers worth having that are not Tek? Did B+K make a good unit for valves? I'm guessing there are probably units around from the likes of AVO / GEC / Marconi etc, although the costs of restoration may be prohibitive I guess...

I should clarify that I'm only looking to match / test small-signal valves for front-end apps, not power-tubes.

Cheers,
Justin

[vanimal]

Well guys, with regards to the trademark issue related above, as far as we know, we were the first company to make a dynamic limiting rack mount product which was called the "Variable Mu" Limiter Compressor and so we trademarked that product name to be ours in order to protect our mark the same as I trademarked "VOXBOX" "ELOP" "LANGEVIN" "MANLEY" "STARBIRD" "SLAM!" et al. Those are our product and brand names. Trademark law protection dictates that I can't start manufacturing an audio product called "The Manley Distressor" no more than some other company can start making one called the "[insert company name here] Variable Mu".

Yes we can all still talk about variable mu dual triodes as they were called in tube manuals, and yes, "variable mu" is still a swell engineering term as discussed above. Yes of course there were many other limiters throughout history which used similar methods as ours to achieve limiting. Obviously we built upon this history, literally. We never claimed to have invented the method or the concept of this type of limiter. We were just the first who applied those two words "Variable Mu" to the faceplate and called our product that. The trademark means no other company can call their audio signal processor product a "Variable Mu". That's our name that we applied to our box in 1993 and later trademarked. That's it folks.

Besides this aside, this is a great thread....

[Larrchild]

waves from the east!

[Larrchild]

Quote:

I should clarify that I'm only looking to match / test small-signal valves for front-end apps, not power-tubes.

Triodes only..

Tektronix is the only one worth messing with.
And the 577 has 1600v @ 20 watts, so it's possible to launch a tube right out of it's socket with too much throttle. =)

For pentodes, you can fashion a screen supply too.

Use pomona plugs and protoboard to make a tube socket jig.
And it has a chimp-proof hv interlock you need to push on that the original jig has a pin for.

I have dual jigs for all my fave tubes. It was easier than jumpers.

Hint: 4 tube sockets per jig allows you to compare different tubes on each side and each sides' sockets are just wired in parallel so the unused socket becomes inert. So my main jig can compare 6386's to each other or 6BA6's to each other or 6BA6's to 6386's etc.

[PRR]

> The variable-mu valve is probably from the late 1920s or early 1930s. I know that they were commercially available by 1933. The full development was prompted by the superhet receiver but I have a feeling that manual bias changes were applied to earlier TFR receivers.

The core problem (40dB-60dB range of radio signals) is the same TRF or superhet. But TRF was a fussy thing anyway, so manual gain control was tolerated. Superhet allowed one-knob tuning (good TRF had three tuning knobs) so it began the trend to "dummy" radios that any fool could operate.

The oldest way to change gain was to reduce filament current. This not only killed Gm, it made your batteries last longer. But it had the same flaw as grid-bias control: for large reduction of gain, the plate current times the load impedance gets so small that the output overloads. Antenna potentiometers were sometimes used.

But Cunningham's data for the '01 (their number CX-301-A) says "Volume control ... variation of either the grid bias or the plate voltage...", no mention of filament control. (This is stated to be a "storage battery" tube, but at 5V heat they are assuming a 4-cell 6.3V battery and a rheostat that you adjust to keep an even 5V glow as the battery runs down from 6.3V to ~5V.)

The type 35/51 is a fairly old "Super-Control" tube (another phrase that someone should trademark) and the 1937 data (the tube is older) shows grid voltage needed for 37dB gain reduction (unspecified current, but obviously way too small for useful output with anything other than a tuned circuit).

That sheet references a page 15; this seems to be the page with a hasty note about super-control tubes.

BTW: this page says "variation in amplification factor", which could also be spelled "Variable Mu". Prior art is no bar to trademarks: the word "coke" was widely used for cooked-coal and cocaine products long before a soft-drink company registered it. In fact they were reluctant to call their brew "coke" because of the Drug Act (if it contained cocaine, it was illegal; if it didn't, it was misleading) but "had" to register it so that no other drink company could use it and be confused with Coca-Cola. "Variable Mu" was apparently up for grabs when EveAnna applied for a Trademark. A specific implementation of "Variable Mu" might be valid as a Design Patent. Clearly there should not be an Invention Patent on "Variable Mu" at this point in time (though the current US Patent Office is liable to approve anything and let you fight it out in court).

In all these old notes, know that "Volume Control" does not mean what it did later. Early radios had tame audio stages that were run wide-open, no audio gain control. With the rise of the dummy superhet/AVC radio, and with better audio stages, the idea became to let AVC action keep the detector output up as close to 10V as possible (by AVC-reducing RF IF gain) and then use an audio potentiometer to reduce audio gain to the desired listening level. In local reception, it keeps all stations in/around town at similar level, so you can (one-knob) scan the band without deafening yourself. (In short-wave use, a single station will fade in and out as the ionosphere wobbles; AVC does act to keep a constant volume with fading.)

> I trademarked ... "STARBIRD" ... et al. Those are our product and brand names.

In the audio racket only. If you started making custom cars (you'd be good at it) and called them "Starbird", you would run right into Darryl Starbird. I don't know if he trademarked his name, or if that would bar your use of it in his field; I leave that to lawyers.

Even in the audio racket only, you are only a few notches away from being razzed by the Songbird hearing-aid company. Hey; hearing aids are audio. And modern hearing-aids include limiters and equalizers. But you didn't step ON their name, and hearing-aid customers are unlikely to confuse a hefty Manley for something they can stick in their ear.

[Larrchild]

Thanks for the clarification on variable mu as a generic technical term vs a unique product name, EveAnna. I get it, and will type with less fear now =) .. even though PRR says it's a flawed term. But I suspect over time, one could save a few engraver bits by not having to keep engraving the word 'transconductance" over and over.

Wasn't that Kraftwerk's 2nd album "Transconductance Express" ?


Hey PRR, what about pentagrid converter tubes? Ones with a remote cutoff grid, and a normal one? Audio goes into the straight grid and control into the remote cutoff grid.

Anyone done that? and would they mix like a dual gate mosfet and create artifacts? Eh?

Quote:

If you started making custom cars (you'd be good at it)

Edit: If Manley made a car, I'd buy it in a New York Second.
Imagine a purplish billet of streamlined aluminum with 4 black tires and some windshields.

[JGreenslade]

Larrchild wrote on Fri, 12 August 2005 20:31

Quote: I should clarify that I'm only looking to match / test small-signal valves for front-end apps, not power-tubes. Triodes only.. Tektronix is the only one worth messing with. And the 577 has 1600v @ 20 watts, so it's possible to launch a tube right out of it's socket with too much throttle. =) For pentodes, you can fashion a screen supply too. Use pomona plugs and protoboard to make a tube socket jig. And it has a chimp-proof hv interlock you need to push on that the original jig has a pin for. I have dual jigs for all my fave tubes. It was easier than jumpers. Hint: 4 tube sockets per jig allows you to compare different tubes on each side and each sides' sockets are just wired in parallel so the unused socket becomes inert. So my main jig can compare 6386's to each other or 6BA6's to each other or 6BA6's to 6386's etc.

Thanks again Larry.

J

[ssltech]

vanimal wrote:

Besides this aside...

...I'm still trying to figure out how you do that!

Keith

[PRR]

> what about pentagrid converter tubes? Ones with a remote cutoff grid, and a normal one? Audio goes into the straight grid and control into the remote cutoff grid.

I've seen it proposed. I've even seen a plan, though not detailed.

One monster advantage is you don't need a floating transformer winding to get both audio and control on the same grids. Couple chips would do great.

> would they mix like a dual gate mosfet and create artifacts? Eh?

There's always artifacts. The minimum-artifact approach for speech/music is usually fast attack and slow release. That still clobbers the first over-level wave; but that is inevitable unless you can see into the future. (There is a class of limiters with a delay-line so the control can preview the audio and act a little ahead of time.)

[Larrchild]

Quote:

There's always artifacts.

Right-on, but I was thinkin if you combine 2 signals into a common gate its not the same as if you combine 2 signals into 2 gates of a dual gate mosfet. It acts more like an rf mixer due to additional non linearities. f1+f2  f1-f2 f1+f2-f1+f2 ,lol  all that.
Easy enough to breadboard. I'll get back to ya.=)

Quote:

that is inevitable unless you can see into the future

Well, I just need to copy my vocal track in PT and slide it a few ms early and drive my control amp from that.
Ain't no big thang!

[JGreenslade]

Larrchild wrote on Fri, 12 August 2005 16:45

I was bidding on several 577d1's in the $500+ US range on Ebay. Then I found one for $375US that had a busted a/b switch handle I fixed in 10 mins. They go for more like 500-1000 US from my experience. D2's are more. But high voltage option is needed in either one. edit: the top one ain't a D1  it's a 577 non storage. too much. it will say "D1 Storage" in the upper right corner over the intensity knob.  this one does not.

Larry,

How do you confirm one has the high-voltage option? What if you bought one without - does it have to be a factory option?

What about the other options - I assume the 177 fixture isn't  essential?  

Thanks again!

Justin

[maxdimario]

Part of the fairchild sound was derived from the fact that it's almost impossible to get truly linear sound out of it.

the good thing about the fairchild is that it is a one-stage design.

It does amplification, gain reduction and line driving all in one stage of 4 parallel 6386 tubes.

this makes it direct sounding, distortion aside.

If we focus on the distortion artifacts, it distorts differently depending on the amount of gain reduction, and it is a mix of all the 4 tubes' non linearities, as well as being class-a push-pull which means that some of the low-order distortion cancels out, but not completely because the two sides are never truly matched.

grainy agressive sound.

with two tubes, you can match the characteristics better, and end up with a cleaner sound, but then you've got to put a line driver after the GR circuit and that adds it's own kind of distortion as well.

the fairchild 660 was a unique box.

[Larrchild]

Quote:

How do you confirm one has the high-voltage option? What if you bought one without - does it have to be a factory option?

Well, I should say its handier to have the high voltage option. Look and see if the voltage knob goes to 1600v.  

If not, find a tracer that has at least has 400+

The 177 is the general test fixture and probably the one you want. Also the most common. avoid test fixtureless units.

Quote:

the good thing about the fairchild is that it is a one-stage design.

It's also a no-feedback amp relying heavily on push pull cancellation of distortion products.

[maxdimario]

Yep.

but all vari-mu GR circuits are no feedback.

only the post GR line drivers such as in the u176 or Ba6a etc used feedback.

others, like some collins, altec etc. used no feedback at all throughout.

But yeah... a big part of it is the sheer simplicity of the audio path.