Ampeg V4 - preamp-type question, white whale

[Dreams]

This was my first big-boy amp. It's never worked right. I'm looking for another set of eyes on this problem, because the two techs I've taken it to over the years have both failed to fix it. I have also failed to fix it.

It occasionally does this thing where the dry signal cuts out, and only the reverb signal gets through. When I say occasionally I mean only at those times where it is most inconvenient; I can practice with it for hours with no problems, and it is rock solid whenver it's on the bench. It's been re-capped, etc... by said techs, and is generally in good shape.

Here is the clearest schematic I could find:

www.recordingjunkie.com/Documents/TechSpecs/BassAmps/Ampeg/VSeries/vt22-v4-74.gif

My problem concerns V201, the 6k11. The reverb returns after this stage, and everything before it seems to be working perfectly. Another hint is that whoever serviced this in the seventies made notes on the schematic, and most of them involve replacing this particular tube.

So.

Measured voltages by pin (plate and cathode for each section):
2:  201v
3:  22.6v

5:  143v
6:  1.27v

10:  322v
4:  151v

All voltages are pretty damn close, except that the plate voltage is 40v too low on pin 5, and the cathode voltage is 10v too high on pin 4.

This makes the grid voltage 10v negative WRT the cathode in the last section, where the schematic shows it should be ~40v above it.  I have a NOS tube in there, and I swapped it with the old one I have, voltages stay the same.

Anyone have any ideas? I can't even THINK about it anymore. It seems like that section (2nd triode, pins 5, 6, 7) is drawing too much current? Plate resistor (R207) has been changed. Are these voltages a problem?

Sorry, I'm kinda throwing up my hands here. Any ideas/suggestions are welcome.

 

[emrr]

Cuts out suggests an intermittent, I'd look at VR104 treble pot first, then all the solder joints around the 6K11 and associated parts.  Could be the circuit board itself. 

The voltages are off, but they should be consistently off and produce a consistent result. 

 

[Dreams]

The signal splits right after the treble pot, one for the reverb send, and one for the mid-eq control. Wet and dry. They combine just after the 6k11, and the reverb never cuts out.

So I believe everything up to just after the treble pot is cool.

Um, I think I was a little unclear (whatelseisnew)

I don't think that my off-voltages are necessarily the cause of my problem, but they're the only things that look strange. I wish that the cutting-out problem would happen while the amp is on my bench, but it never does. So the voltages are the only clue I got.

Solder joints are all good. Checked and rechecked and redone and....

Circuit board, eh? What exactly do you mean by that? Broken traces? This amp has been in my possession and broken for over 10 years. I'm not above running wires over the traces. I've thrown enough good money after bad at this point.

Does it seem odd that the grid voltage is lower than the cathode when the schematic indicates it should be higher? Should I not worry about it? Maybe I'm just grasping for ANYTHING to get this damn thing to work right. I love the sound, I just can't rely on it EVER.

 

[emrr]

Right on the treble pot, I saw it wrong in haste. 

Yeah, broken traces that flex with heat.  last thing I'd check unless obvious.

Anything from c201 to r16/c8 may be intermittent.  Could be the 6K11 socket. 

Consistent off voltages are a red herring, that will always act the same. 

 

[Dreams]

Word. Well at this point, just about everything between c201 and r16/c8 has been replaced, one by one, with no success.

I hate doing that; I feel like I'm cheating.

So I'm gonna make the traces redundant with some wire, and order a new tube socket. I did clean and tighten the socket pins (socket sockets?), but I suppose yeah, something funny could still be going on there. Cool.

I'll do that and put it back together and wait for something important to use the amp for to see if that helped. Thanks for the second opinion.

If anyone has anything to add, please do.

 

[PRR]

> grid voltage 10v negative WRT the cathode in the last section, where the schematic shows it should be ~40v above it.

Grid is NOT more positive than cathode.

Schematic is wrong. Either mis-noted, or observed with heavy meter loading (poke the grid, *both* grid and cathode fall).

It's not the problem.

You have a loose contact. Solder joint isn't really wet down to bare metal. Insert jack. Pot wiper. Tube socket contact. Maybe a resistor internally cracked. Or a capacitor.

On the bench: Work it hard, poke with a chopstick (try not to die).

Reading further: you've done all the logical things. At this point you must be looking for something that "can't" be wrong (but is). Yes, PCB traces do crack. I personally doubt ALL solder joints until I take them apart, sand the leads, tin the leads separately, then re-joint under bright light (and now, magnifiers).

Would it be better if it worked without that wonky MID control, reliably? Easy to bypass. Or if it does fail when bypassed, at least a whole section has been eliminated as a primary problem.

 

[Dreams]

I would LOVE it if I could just bypass the mid control. Hate that thing. Thought about doing it, but I like to have things in more-or-less perfect working order before I start hacking at it. Would it be as easy as lifting one side of c201 from the plate and lifting one side of r16 from where it connects to c205 and joining the two lifted parts together? (I haven't looked very hard at this yet)

Well, that's a thing for next time I open it up. I connected a bunch of parts with wires, I'm gonna see what happens.

I've been working on this, incrementally, for years now. Piece by piece. Every component is new except for the tube socket. I probably should have replaced that earlier, yeah, but like I said, I've thrown a lot of good money after bad on this guy. Most of the other parts I already had, so...

Anyway, I've ruled out the solder joints. Sanded them all before they went in. Well, scotchbrite-d 'em.

Thanks!

 

[emrr]

Yeah, don't die.  This is a serious high voltage/current circuit.  Touching the wrong thing even barely with one finger will numb you to your shoulder and make you see black.  It hurts for a few days. 

 

[Dreams]

HAHA! Funny story...

The first tech I brought it to all those years ago did the recapping. A very popular and respected tech who shall remain nameless. Anyway, the first caps after the rectifier are connected in series/parallel, so the cap common of one can is connected to the positive side of another can. So the outside of the one can (which IS usually grounded, I suppose) has about 275 volts on it. It's the one right next to the power tubes, yeah that one. So I was reaching around the back for...I don't know, something...and there you have it.

Stopped going to that guy.

Long story short, this amp is why I repair amps now. I'm usually pretty ok at it; I hope I don't sound TOO green. It's just that this amp has been hanging over me like a black cloud since I got it, and I needed to talk it out. See if there was something I was missing.

 

[PRR]

> Would it be as easy as

Looking at it more...

It's not that simple to bypass it right. The bass/treb network is high impedance and the dry/reverb mix is a lower impedance. Gain and knob-action will be different.

However it would work well enough to try and prove the problem is between these two points. Depending how thoroughly you can test it without getting into a play-or-be-fired situation.

 

[Dreams]

A little update:

I tried replacing a some of the traces, but the problem was still there. I did a few more, and the amp sounds way more...solid...or something. Doesn't breakup at the same spot it used to...hmmm. Maybe I've got something.