Topic: Pultec EQP-1a EQ cap replacement (Read 14594 times)

ssltech · « **Reply #1 on:** September 22, 2007, 04:14:48 PM »

...You sure you're not talking about the inductor???

And are you certain that you're sufficiently familiar with working on gear not to do more harm than good...?

Be SURE before you start...

Keith

Klaus Hiney · « **Reply #2 on:** September 22, 2007, 09:43:45 PM »

ssltech · « **Reply #3 on:** September 22, 2007, 10:10:17 PM »

Well, if one tap of the inductor was broken, the frequency(ies) associated wouldn't boost. The caps should be discrete, the INDUCTOR is more likely to be wax-potted.

IIRC, they 'share' inductors on adjacent frequencies... if that's true as I remember it, then one open-circuit inductor tap would cause two dead -but adjacent- frequencies...

Keith

Klaus Hiney · « **Reply #4 on:** September 22, 2007, 10:38:17 PM »

Klaus Hiney · « **Reply #5 on:** September 25, 2007, 09:28:08 AM »

David Kulka · « **Reply #6 on:** September 25, 2007, 11:43:13 PM »

I was a little afraid to try to advise since this is such an expensive piece, with so many dollars potentially at risk. If one is not a very experienced tech then one should tread very lightly with a unit like this.

Even though the circuit is in essence pretty simple, there are ways to go wrong. The problem could be caused by the switch, wiring, inductor network, a capacitor, a previous repair, etc. For an experienced tech, patient, methodical fault finding could probably identify the problem in an hour or so but it seems difficult to give easy to follow, safe advice for fault finding. It might be helpful to verify the accuracy of the frequency settings that do work, along with cut/boost amounts.

The problem might or might not be a capacitor. Don't do anything that might damage the potted network, because it's basically irreplaceable. It might be difficult (or impossible) to remove and repair it without doing even more damage. Sometimes, if the exact fault is known, a part can be added inside the main chassis to repair a fault in the network.

If you can, check the other frequencies and cut / boost amounts carefully. I'll look at the schematic and see if I can offer any other suggestions.

(I edited this reply a couple of times because I wasn't comfortable with some of my original comments.)

drpat · « **Reply #7 on:** September 26, 2007, 06:41:52 PM »

The earlier 1A’s used a 4 tap inductor, while the latter used a 5 tap. Most of the schematics that I’ve seen on the internet are for a 6 tap inductor.

The inductor for the 1A is wax potted in the same filter can that the HF BOOST and HF ATTEN caps are in.

All of the 1A’s (that I’ve seen) use the same cap for 8k and 10k, and most have a common cap for 3k & 4k (I own one that doesn't).

All of the Pultecs that I’ve seen share an inductor tap for 8k and 16K, and also share a tap for 10k and 12k.

You can get all of this info from your HF BOOST switch. Just count the number of colors coming from the filter network to the switch. ...for the small gauge wires and the bigger gauge wires.

BEFORE YOU REMOVE THE BEEZWAX!!!

Your switches could be dirty. To test, you can manually short the inductor tap to the cap tap on the switch. Be sure to do this while the switch is in the same position as the frequency that you're shorting. If it works while doing this, clean your switches.

If this doesn't work, it's time to rule out the caps and inductor...

You could start by isolating the HF BOOST caps in the filter network. The HF caps can be tested by removing the wire that’s connected to the wiper of the HF BOOST pot. Now you can use a cap meter between the switch connections (the bigger wire gauges) and this wire. To completely isolate the caps, be sure that the switch is NOT set to the position that you’re testing. If you find an open cap, lucky you, you can mount a new one to the switch. If it’s shorted, I hope you like the smell of honey!

Testing the HF BOOST inductor involves removing the small wire that’s coming from the filter can that’s connected to the bandwidth pot. You can now do an inductance reading from the switch connections (the smaller wire guages) to this wire.

Hope this helps...

Klaus Hiney · « **Reply #8 on:** September 26, 2007, 06:57:44 PM »

ssltech · « **Reply #9 on:** September 26, 2007, 07:04:58 PM »

Open caps in that circuit... -Anyone ever seen one?

Keith

drpat · « **Reply #10 on:** September 26, 2007, 07:32:30 PM »

That's why I said "Lucky You"... Very Happy

Klaus Hiney · « **Reply #11 on:** September 27, 2007, 09:34:33 PM »

David Kulka · « **Reply #12 on:** September 28, 2007, 04:00:28 AM »

I do and obviously Patrick does -- or at least he knows them very well.

ssltech · « **Reply #13 on:** September 28, 2007, 10:35:01 AM »

I know Pat's familiarity with these and other tube pieces, and David... well you'd be VERY hard-put to find anyone else who will stand more fully behind such excellent work on such a wide array of gear.

Whereabouts are you...?

Keith

drpat · « **Reply #14 on:** September 28, 2007, 07:15:30 PM »

I've been inside a few of these filter cans, if you need somebody to look at this unit, I'd be more than happy to do so. Where are you located?

Klaus Hiney · « **Reply #15 on:** September 29, 2007, 03:21:31 PM »

ssltech · « **Reply #16 on:** September 29, 2007, 06:32:45 PM »

Independance?

Just curious...

Keith

Klaus Hiney · « **Reply #17 on:** October 03, 2007, 10:43:09 AM »

David Kulka · « **Reply #18 on:** October 03, 2007, 11:30:23 PM »

I often use the yellow axial caps from Illinois Capacitor (image below), or the tried and true Orange Drops.

index.php/fa/6369/0/

drpat · « **Reply #19 on:** October 05, 2007, 03:35:28 AM »

Those caps are pretty common. I've got a bunch. Panther parts has them in stock. It's a metallized, paper-plastic type.

Klaus Hiney · « **Reply #20 on:** October 05, 2007, 03:49:25 PM »

Klaus Hiney · « **Reply #21 on:** October 05, 2007, 03:55:35 PM »

Andy Peters · « **Reply #22 on:** October 05, 2007, 06:27:57 PM »

Jim Von Nordheim wrote on Fri, 05 October 2007 12:49

I see the cap in the Pultec says .25 MFD 400vdc and it is an Aerovox Aerolite. Wouldn't an orange drop change the sound drastically? Wouldn't a paper cap be the way to go if I can't find an Aerolite exact replacement? it's hard to find the pre-war .25 value. Everything is .22 now it looks like. Shocked

Methinks a modern 0.22uF cap would be well within the tolerance of that ancient paper thing.

-a

drpat · « **Reply #23 on:** October 05, 2007, 09:36:13 PM »

Are you in the filter can, or is the cap in the chassis? This is one of the 2 Aerovox .25 caps in the chassis...

http://i74.photobucket.com/albums/i276/velvettone/SIMG0040.jpg

Again, PANTHER PARTS HAS THESE IN STOCK. No need for a substitute, unless you're trying to save a few bucks...

David Kulka · « **Reply #24 on:** October 05, 2007, 10:18:28 PM »

I wish folks could get away from this idea that virtually equivalent caps will change the sound "drastically". I'm not sure who started this rumor but if his car has a flat tire tomorrow morning, it will make me quite happy.

If anything, a modern film cap will probably sound better than those nasty old paper jobs since the leakage is likely to be far lower and the tolerance will probably be better. But in the real world I'd defy anyone to hear or measure any sonic difference.

There are lots of excellent quality film cap, choose the kind that you like. Appearance is a valid consideration in a case like this. I'd really avoid NOS "paper" type caps though, because of the leakage issue.

Klaus Hiney · « **Reply #25 on:** October 06, 2007, 04:38:24 PM »

drpat · « **Reply #26 on:** October 06, 2007, 08:34:22 PM »

Have we changed model numbers since this thread started???

dcollins · « **Reply #27 on:** October 06, 2007, 09:51:19 PM »

David Kulka wrote on Fri, 05 October 2007 19:18

I wish folks could get away from this idea that virtually equivalent caps will change the sound "drastically".

Is that before or after the film cap is properly "broken in?"

DC

compasspnt · « **Reply #28 on:** October 07, 2007, 12:39:32 AM »

Just like batteries, they can last longer if you refrigerate them first.

RMoore · « **Reply #29 on:** October 07, 2007, 07:08:54 AM »

Apparently if you use music to break in the caps as opposed to just electricity, pink noise or sine waves, this helps form the caps in a more musical way. I'm not sure if there is a difference between running say classical through them or euro disco but I'm certain experiments are now being performed by top golden ears which will determine the differences thereof..

I read it on the net so it must be true

Klaus Hiney · « **Reply #30 on:** October 07, 2007, 08:13:49 AM »

thermal idiot · « **Reply #31 on:** October 07, 2007, 11:15:12 AM »

Jim Von Nordheim wrote on Sun, 07 October 2007 05:13

This morning I tried 2 NOS Aerovox caps and also Sprague orange drops in the EQP-1A. Orange drop caps sounded very sterile compared to the Aerovox.

So is it that your program material sounds sterile in the 3k region or does it all gets sterile when the 3k is selected? I'm having trouble imagining the capacitor in the LC circuit for 3khz is causing the entire program to become "sterile". What kind of orange drop cap is it? Polypropelyne, Polyester? All orange drops are not the same. What kind of test are you performing to discover the sound alteration? Program material? What kind? Live, recorded, etc.
I'd be curious to know.
Charlie Bolois

drpat · « **Reply #32 on:** October 07, 2007, 12:38:48 PM »

That's not a filter cap he's talking about. That cap resides in the amplifier section. This could definitely have an effect on the overall sound, since all audio passes through the amp, even in bypass.

I just had this conversation about Pultec caps with somebody who owns about 15 Pultecs, and we both agreed that there are a few caps in the thing that need to be kept original if you want to maintain that Pultec sound. He had similar (bad) results with some orange drops, I've never tried them in that position, so I can't comment on how they sound.

It seems to me that the sound of these units are the sum of all parts, with three caps in particular that absolutely do change the sound too much if not kept original. I would think that if you kept replacing caps with substitutes, eventually it would have a cumulative effect on the sound. Personally, I've always played it safe, and used original NOS parts, so I don't know this for sure, just a hunch.

Jim...

With regard to David's comment about leakage, this has nothing to do with the cap's tolerence.

Klaus Hiney · « **Reply #33 on:** October 07, 2007, 12:47:17 PM »

Klaus Hiney · « **Reply #34 on:** October 07, 2007, 04:09:17 PM »

Jim Williams · « **Reply #35 on:** October 12, 2007, 11:17:41 AM »

Just drop in a Mundorf silver foil paper in oil cap and call it a day.

Your ears will thank you.

Klaus Hiney · « **Reply #36 on:** October 12, 2007, 01:33:16 PM »

maxdimario · « **Reply #37 on:** October 23, 2007, 05:41:41 AM »

there is a difference in sound between paper and plastic film caps.

especially if the paper cap is old.

if you want a treat replace the cap from the tone control of a gibson or fender with it's high.voltage paper equivalent.. big difference.

if you want the vintage (as in OLD and decayed caps) you need a NOS equivalent cap, which seemingly is available according to the above posts.

as far as .22 to .25 it is nearly impossible for a .25 cap to measure .25 especially a 50's cap.... and an aged 50's paper cap will have drifted out of value..

you are trying to keep it original so LOOK FOR ORIGINAL NOS CAPS.

if it were just for sound's sake Jim W.'s advice is probably spot on.

Jim Williams · « **Reply #38 on:** October 23, 2007, 11:15:08 AM »

One can just add a smaller cap across the .22uf versions to obtain .25 uf. A .033 uf film cap will get you there. It will also allow faster small signal transients to pass if it's a quality cap.

I've never heard of anyone replacing their silver foil caps with any vintage or old NOS caps unless they wanted a darker, more muffled sound. Maybe in a guitar amp, not in recording gear.

The difference is whether you want it to sound great or if you want it to sound old.

Klaus Hiney · « **Reply #39 on:** October 24, 2007, 10:26:03 AM »

Klaus Hiney · « **Reply #40 on:** November 04, 2007, 04:21:51 PM »

MagnetoSound · « **Reply #41 on:** November 25, 2007, 03:27:48 PM »

Jim Von Nordheim wrote on Sun, 04 November 2007 21:21

I know that the 12au7 will burn out prematurely because the cap I need to change is leaking DC.

I presume from this that the DC is leaking into the tube, then?

If so, the tube will be badly biased, be out of it's correct operating area and will sound bad.

Why would you not replace the cap?

R/E/P Community

Author Topic: Pultec EQP-1a EQ cap replacement (Read 14594 times)