Bad Sozo caps???

[dehughes]

Several months back I had a set of Sozo Vintage Mustard capacitors installed in my Germino Classic 45 (JTM-45 clone). They sound GREAT.

However, I noticed that ever since they were installed, the amp periodically thins way out....major loss of low end and some overall volume....and this is only remedied by turning the amp to standby and then back on again. I have tried the following: different channels on the amp, different eq and volume settings, installed known good tubes, tried different guitars, different cables, different electric outlets, and even a different building entirely. Nothing has eliminated the problem. Granted, it is relatively infrequent (maybe happens once every few times I play the amp), but it never happened before the caps were installed. I sent the whole amplifier into Greg Germino and he tested the caps and went through the entire amp, and found nothing amiss. The caps tested out fine and the whole amp was gone over, and he play-tested the amp extensively with no issue. He said he'd experienced such a thing before when he first started using Sozo caps, and found that when he installed the supposedly problematic caps in another amp, it ran fine with no issues...and he was baffled as to what the cause might have been.

Is this part of the natural break-in of these type of handmade caps? I've used the standard Sozo caps and never experienced any problems, so I'm unsure. As well, the caps are the only constant in this equation, so I'm fairly certain the issue lies with them. Before I go order new caps and begin a lengthy process of elimination, I was wondering if you all had any ideas as to what the problem might be?

[Rich M]

Bad solder joint?

[dehughes]

Bad solder joint?

The whole amp was checked by Greg Germino himself, so I'd say probably not. I figure between him and myself we probably would have caught a bad joint.....but technically, it is possible....just dubious.

[Structo]

That's a strange one!

I'm sure you have ruled out all the usual suspects.

Only time I ever experienced anything close to that it was a bad preamp tube.

Perhaps something in the tone stack is dodgy?

[dehughes]

That's a strange one!

I'm sure you have ruled out all the usual suspects.

Only time I ever experienced anything close to that it was a bad preamp tube.

Perhaps something in the tone stack is dodgy?

It's possible. It does the same thing regardless of what channel I'm using, so that rules out the first pair of coupling caps. That leaves the caps in the tone stack and everything downstream from there. The thing that makes me suspect the caps is that this never cropped up before the caps were installed.

[bluefireamps]

Was Germino able to recreate the problem when he had it? It sounds like a bad Bass or Mid pot. Do you have a cathode bypass cap on the triode driving the cathode follower/tone stack? That being wonky could cause that kind of problem too.
Dave

[dehughes]

Was Germino able to recreate the problem when he had it? It sounds like a bad Bass or Mid pot. Do you have a cathode bypass cap on the triode driving the cathode follower/tone stack? That being wonky could cause that kind of problem too.
Dave

He was unable to re-create the problem. Figures...just like when your car is acting up and the mechanic can't find a thing wrong.

The amp is essentially a JTM-45 clone, so yes, it has the cathode caps, etc., just like the original.

I tried moving all the pots when it acted up last time, and nothing changed. It's possible it's a pot issue, but I'm leaning towards a cap problem being as the pots are sealed, mil-spec PEC caps.

[paulster]

Do you have silver mica caps in there?

Before you pull out the Sozo's I'd be looking at these as they are notorious for leaking DC.

[dehughes]

Do you have silver mica caps in there?

Before you pull out the Sozo's I'd be looking at these as they are notorious for leaking DC.

One SM cap, in the tone stack.

I've used silver mica caps for years and never experienced this type of phenomenon, though. Not saying it's not the problem, but it doesn't correspond with the timeline of the problem surfacing, as the cap has always been in there, and it wasn't touched during the soldering process (different solder lug).

I'll test it for DC leaks, though.

One question: Would a leaky cap manifest itself in such a way as I'm experiencing? I figured a leaky cap would introduce noise/hum into the circuit, not cut out partially. There's no additional noise when my amp does its thing, just a drastic reduction of low end and volume.

[paulster]

It would depend on exactly where in the circuit it sits, and I haven't got time to pull out a schematic right now to check it out.

What you can end up with is shifting bias points in the next stage because it becomes grid-biased as well as cathode-biased, for instance, but it all depends on where in circuit it sits.

If I remember rightly the cap will be on the input to the treble pot, which I think is DC-coupled to ground. If that's the case then when you get the problem again try turning the treble pot and see if you get the telltale crackle that'll tell you you've got DC on the pot.

You might well have a bad cap elsewhere though, or possibly a dry solder joint, but it does sound like a leaky cap that taking the amp into standby mode is allowing the DC charge on it to diminish.

[Cliff Schecht]

One of my recent amp builds would dip out for a second when I turned the bass pot. Turned out to be a bad coupling cap that was allowing the next stage to shift bias for a second or so. Relatively common problem in older amps and when using less-stable old cap technologies (even with NOS).

Every NOS (heck, even used) US-made paper-in-oil cap I have used has never caused me an issue. Specifically I'm talking Sangamo, Vit Q's and West-Cap. The first time I tried Japanese PIO caps almost every one of them was bad. The Russian PIO caps have the same leaking problem.

I haven't tried Sozo's yet but I'm not convinced that these are your problem. Any part manufacturer worth their salt will test every part to assure it's operating within spec. Is there anything special in your JTM45 clone that isn't in a normal JTM? A thermistor on the heaters or a varistor that could be acting up as the amp heats up?

[dehughes]

...but it does sound like a leaky cap that taking the amp into standby mode is allowing the DC charge on it to diminish.

Now we're talking. So putting the amp on standby will allow DC on a leaky coupling cap to "drop off"? That's describing the problem pretty well....it's like the amp starts to go half-way into standby when the issue manifests itself. Kinda like a voltage drop, but not as "brown", but with the same type of bass-reduction and volume drop. Flipping the standby switch off and then back on remedies the issue.

[dehughes]

Attached is a picture of the chassis, post caps. There are two SM caps...one under the voltage divider resistors, and one in the tone stack.

[Cliff Schecht]

There are a LOT of carbon comp resistors in that amp, and not necessarily in places that make sense to use them in a modern amp (i.e. anywhere in the first stage). I would start by pulling the tubes and measuring all of the resistors that are floating on one end to see if one or more of them is the culprit. It's a top notch build quality-wise so hopefully it's something simple like this. CC's drift over time and can absorb moisture and go bad over time. The silver-mica caps are also a variable but being that there is only two those you could replace with known good caps to eliminate this as an issue. If there is a fizz-cap across the PI caps and it's leaking, this could cause the issue you describe.

[dehughes]

I'll post an updated picture of the amp, but since that picture was taken it had been shipped back to Greg and he'd replaced a lot of the CC resistors with CF ones...leaving only a select few in the circuit, and only after they'd tested within spec.

I'll be using the amp tomorrow, so sometime next week I'll open it up and start testing voltages to get a baseline for "normal", so that if it acts up again I can test again and see what's up. I suppose this is good timing, as I'm considering selling the 2x12 combo cabinet and going with a head/cabinet setup anyway. May as well take it all apart now!