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Author Topic: u67 running warm  (Read 4521 times)

underblu

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u67 running warm
« on: January 12, 2015, 04:28:45 pm »

I've just replaced a mullard ef86 that went bad in my u67 with a NOS telefunken EF86.  I actually bought 3 and with all three of them the mic feels a bit warm to the touch.  Not hot, but noticeable whereas before I didn't notice any warmth.

I'm wondering if that is normal.  I plan on taking it in to Tom Onofrio up in Connecticut when the weather gets a little better for a full checkup so I just want to know if slight perceptible warmth is within the normal operating realm.

Thamks in advance
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klaus

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Re: u67 running warm
« Reply #1 on: January 12, 2015, 06:57:00 pm »

Before you continue running the mic: check the heater voltage! It should not exceed 6.3VDC, measured inside the mic, or 6.4VDC measured in the power supply. (Careful! Very high voltages present!)

 Heater voltages will vary with current draw which varies from tube to tube, even when using tubes of the same brand.
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Klaus Heyne
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underblu

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Re: u67 running warm
« Reply #2 on: January 12, 2015, 07:43:49 pm »

Before you continue running the mic: check the heater voltage! It should not exceed 6.3VDC, measured inside the mic, or 6.4VDC measured in the power supply. (Careful! Very high voltages present!)

 Heater voltages will vary with current draw which varies from tube to tube, even when using tubes of the same brand.

Thanks Klaus,  I'll see if i can find a schematic online as I'm not sure what points I should be measuring.  If that proves too daunting, I'll take i up to Tom Onofrio sooner rather then latter.  I know you recommended him on GS so that's good enough for me.
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klaus

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Re: u67 running warm
« Reply #3 on: January 12, 2015, 08:21:07 pm »

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Klaus Heyne
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soapfoot

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Re: u67 running warm
« Reply #4 on: January 12, 2015, 10:28:42 pm »

...and based on Klaus's recommendation, we sent one of our 47FETs to him, and he did a great job.
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Piedpiper

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Re: u67 running warm
« Reply #5 on: January 15, 2015, 12:52:54 pm »

Before you continue running the mic: check the heater voltage! It should not exceed 6.3VDC, measured inside the mic, or 6.4VDC measured in the power supply. (Careful! Very high voltages present!)

 Heater voltages will vary with current draw which varies from tube to tube, even when using tubes of the same brand.

I was running into trouble years ago with a single ended triode stereo amp that was blowing very expensive output tubes periodically. At the time I didn't understand much about tube circuits, or any other circuits for that matter. After much trouble shooting, education and help from experts, I found that my voltage out of the wall was hovering around a very high 125 volts. Since the power supply was unregulated, the filaments were running too high. I ended up rebuilding the amp with upgraded parts and an improved circuit, as well as tailoring the heater supplies appropriately. I'm starting to have some elusive issues with that amp again after many years of seamless service and I checked the voltage and it's now in a bit more normal range around 121. I'm not sure where the issues are with this amp but I'm looking into it all again. What is your experience as to a workable range of heater voltage?
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klaus

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Re: u67 running warm
« Reply #6 on: January 15, 2015, 01:35:05 pm »

What is your experience as to a workable range of heater voltage?

It depends very much on the tube and its original design and application purposes, etc.
Some tubes you can underheat dramatically (VF14) or overheat a good 10% or more (some 6072/12AY67 types, and other preamp miniatures), other tubes, like the AC701 are very finicky: Overheat that tube by 10% and it's gone quite quickly.

There are other considerations for choosing the 'right' tube heater voltage: noise and sound.
The AC701, for example, can tolerate voltages down to 3.6VDC from its nominal 4.0VDC without incurring an appreciable noise increase. Whereas an EF86 cannot maintain its noise floor in a U67 mic with supply voltages lower than 6.0VDC (nominal: 6.3VDC).

P.S.: My observations described here only pertain to heater voltages with tubes used in microphones. Some circuit designers use extreme under-heating in tube pre-amps or phono pre-amps without any seeming adverse effects.
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Klaus Heyne
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boz6906

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Re: u67 running warm
« Reply #7 on: January 15, 2015, 04:37:31 pm »

Many tube mics use regulated DC for the filaments, you can dial in the exact voltage and you remove a source of hum.

That should work for any filament supply.

Regulated B+ tube power amps are a different matter, some folks like the power compression.

I have a friend who builds a neat little variable voltage reg board to retro fit in tube gtr amps, he uses a mosfet:

http://www.hallamplification.com/main.html?src=%2F#2,2

It would be interesting to try on a tube mic B+ supply...

http://www.hallamplification.com/main.html?src=%2F#2,2
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underblu

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Re: u67 running warm
« Reply #8 on: January 15, 2015, 07:40:57 pm »

I'm trying to measure the voltage of my u67, 

I think I read that it's pin 4 but how would I access it with the mic plugged in.  Also should  I put the red lead on Pin 4 and the black lead on the metal enclosure. 

I apologize asking for such rudimentary instructions on the board  but I haven't been able to find it anywhere else online.   Again a clear  explanation of how to do this would be most appreciated.

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klaus

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Re: u67 running warm
« Reply #9 on: January 16, 2015, 01:59:30 am »

I am not fond of explaining high voltage electrical experimentation to lay people. Please find a qualified tech who can measure and adjust your heater and B+ voltages according to Neumann specs.
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Klaus Heyne
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Piedpiper

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Re: u67 running warm
« Reply #10 on: January 16, 2015, 02:23:40 am »

Thanks for your reply, Klaus. So in the case of unregulated supplies, what can be done when AC mains voltage varies from place to place and to a lesser extent (hopefully), day to day? Must one measure it and adjust the filament resistors, as I have done for my amp, to accommodate the average voltage for a given location if one is to keep the heater supply optimized for the tubes that are touchier about their acceptable range?
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klaus

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Re: u67 running warm
« Reply #11 on: January 16, 2015, 04:02:48 am »

Exactly, and well put.
I always ask clients who work in a specific location, or in countries, the following question:

What is the usual voltage measured at the usual AC outlet you usually encounter at the usual times when you record?
That will be the voltage I will dial in here with a variostat, to adjust the DC voltage(s) in order to approximate the condition found at the client's location.
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Klaus Heyne
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Piedpiper

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Re: u67 running warm
« Reply #12 on: January 16, 2015, 12:10:49 pm »

Interesting... makes sense, but I don't think I've ever heard of that being even mentioned by a manufacturer. Ever hear of using a variac in front of a given component to keep the voltage adjusted correctly? Just thought of that...
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boz6906

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Re: u67 running warm
« Reply #13 on: January 16, 2015, 02:23:18 pm »

I've used one of these for 17 years, I'm in a rural location, at the very end of the line so I see daily variations from 103Vac to 127Vac plus all the line noise.

http://www.tripplite.com/line-conditioner-1800w-avr-system-automatic-voltage-regulator-power-conditioner-ac-surge-protector~LC1800/

The Tripplite uses a tapped xfmr with a relay, it's fast and clean, maintains 120Vac very well at a great price.
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Piedpiper

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Re: u67 running warm
« Reply #14 on: January 16, 2015, 05:17:25 pm »

Yikes! That's a lot of variation! Mine's pretty stable, probably + or - a volt, and a bit high, but not as high as it was when I first discovered the issue with the amp. Just completed recalibrating that amp with .39 Ohm resistors in place of the .42 that was in there from the first time I redid them. The output tubes are very rare modern 20 watt versions of a 300B and are very pricey to replace at about $1000 a pair so I'm a bit skittish about getting this right.

I've been concerned about the current restriction of the Triplite and the like. Any observations or valid theories as to the sonic effect of these, aside from the advantages of a reliable supply? Sorry if this is too much of a tangent off the OP, but it does seem relevant.
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