Phase inverter capacitor?

[Enzo]

The problem is that the math for "bad part" is hard to come by. If the cap shorts or gets really leaky it is going to squash your sound. But a good cap in that spot will make the amp sound like all the rest of them. The question isn't whether a cap there can cause trouble, the question is whether YOUR cap is causing touble.

[Enrico]

Hi, and thank you for the input. I tried and unsoldered the cap, but there's basically no change to either the sound of the amp nor the volume situation. I checked the capacitor and is actually fine--but I am taking it out anyway since it's not supposed to be there.
I suspect that the culprit is the reverb mod (they added an extra tube to feed the reverb), so I am going to revert the amp back to original and see if that works...

[tubeswell]

Hi, this thread might be kind of old, but it relates to what I am going through. So here I quote a post of mine that I uploaded today to a different section of this forum:

Hello,

I recently acquired a Mesa Boogie SOB, 60W/reverb version, which is my third one (plus one that I used to have way back when and regretfully I sold). I really like these amplifiers, as you might imagine.

So, this "new" one has an issue, which is a quite obvious lack of volume compared to my other ones. It sounds good, everything works--tone stack, reverb, volume controls etc, but the volume is basically half of what it should be.

I went through the entire layout and compared it with the other two amps. The only difference I can spot is a 230pF capacitor across pins 1 and 6 on the PI tube, which has been retrofitted into the "quiet" amp. ... only thing is that capacitor on the PI. Can it really account for such a drop in volume? Where else should I look? I checked all of the capacitors and they measure the same as on the good amps.

If all the parts are working properly the effect of the smoothing cap between the PI plates on a LTP inverter is to make the amp a bit more hi-fi sounding.

Now if the LTP smoothing cap is leaking DC, it could cause a loss of gain in both sides (because the LTP plates are out of phase, so all other things being equal, if they were directly connected, the opposing signals may tend to cancel each other out).

However, who's to say its the LTP smoothing cap causing the problem? (apart from Enzo ;-) ). It could be another cap leaking DC (e.g.; like a coupling cap earlier in the signal chain, that could be affecting the bias of another stage. (Have you check all the cpas for DC leakage?) It could be something else like a bad tube (did you swap the tubes out?) or failing power supply voltage (Have you measured the DC voltages throughout?). The situation calls for systematic diagnosis - not hunches and guesses.

To be more precise, the amp has been modded so that the reverb circuit has its own tube, therefore instead of V3 feeding reverb and PI, there are now two dedicated tubes. I could revert everything back to original quite easily,

Modded huh? So is this a custom-built amp (where the build problems were not sorted out)? Or is it a modded factory-built amp (where the mods were not sorted out)? Have you got gut shot pictures you can upload? Have you got a schematic of the modded amp you can upload?

[Enrico]

Hello, and thank you for the really accurate reply. The amplifier is a Mesa Boogie S.O.B., therefore factory-built around 1982. It is quite an amazing amplifier, basically an early reincarnation of the original Mark 1. Incredibly responsive and dynamic (and yes, loud!), it is essentially a hot-rodded black face with cascading preamp stages. Nothing to do with more modern Mesa Boogies in fact...so I am on the lookout for these amps and when I find one at a decent price I buy it. It only has one sound, but it is a good sound! Versatility is not its thing, but that one tone is fantastic.
The problem with these is, schematics are almost non-existent. Mesa offered different options on these amps: 100W (4 6L6) vs 60W (two 6L6), spring reverb vs. no verb, and the last pot could be either presence or limiter (sort of an attenuator, but not that great sounding). Furthermore, the version with reverb had no mid control, whereas the no-verb version had mids. On the net you can only find schematics for one of these versions (60W, no-verb, presence) and it's hard to read too.

The modification to the reverb circuit looks so old and integral to the amp that, supported by the fact that these amps were practically custom-made at customer's request, I thought it was done at the factory. I sent pictures to Mesa Boogie to find out, and this is what they (very kindly, right away) replied: "It appears some one has moved the rev drive circuit (which was 1/2 of v3) to use its own complete tube. It was not done by us."

I will try and post pictures ASAP--in fact I should upload photos of both my reverb-SOB's, this one and the other one which is all original. By the way, I compared the two--all caps measure the (within tolerance) same, I swapped all tubes, checked voltages...there's no difference other than the modification.

[overtone]

Quick question:
you said that all the caps measured the same within tolerance, are we to understand that you tested them for DC leakage?
Maybe this is all belongs in a new thread now?