Phase inverter capacitor?

[EFK]

Why would there be a little blue ceramic cap across the cathode pins on my PI tube? Pins 3 and 8, right on the tube socket. I think it's a 100 pF but I'll have to go back and check. Anyone?

[Daver]

Good question. What type/model/brand of amp? What style of phase inverter?
Dave

[EFK]

Gibson GA42RVT, 2 X 6L6 type amp. Kind of a darker Fender/Marshall cross. Standard long tail PI with 'different' values (see other thread). I've never seen a cap put there like that before but then I have not seen a whole lot. These amps apparently are all 'one-offs' as it seems they kept monkeying with the layout and they're all different. Very odd.

[tubeswell]

On some LTP inverters, they are like local feedback caps for high frequencies. See the 5F6A for example.

[daz]

Ok, i'm probably missing something and will make a fool of myself with this question but what the heck...ain't like i haven't made a fool of myself here plenty in the past. Why would a cap from the PI cathodes do anything when the cathodes on a LTP are tied together anyways?

[Merlinb]

Why would a cap from the PI cathodes do anything when the cathodes on a LTP are tied together anyways?

I was wondering the same thing...

Sure it's across the cathodes??

[EFK]

I'll pull the chassis back out today and double check the two pins it is placed across. Thanks so far guys!

[EFK]

Oops! Well I feel stupid now - closer look actually taking my time and it is across pins 1 and 6. 1KV ceramic code 101 (100 pF, correct?) Any ideas?

[daz]

Yeah, thats there to attenuate the high end a bit and/or eliminate oscillations. It cuts the very very high frequencies. I've tried them before using common and not so common values and it never helped really. More harm than good to my ear.

[tubeswell]

Of course I knew all the time he was talking about the plate pins (ESP ). (But you others are technically procedurally correct for pulling him up about it ).

[EFK]

How high frequency are we talking here? Just about out of the range of normal hearing high freq.? I guess I ought to just clip it to find out. How much of a difference could 100 pF make?

Is it possible that the oscillation issue could arise being as this PI is a little 'different' than the common 470/10 long tail Fender-type? This PI is setup in a more balanced manner (See the other thread). Could the oscillation possibility be tied to the 2-channel issue?

[Steve Conner]

The oscillation mostly depends on the characteristics of the output transformer. If you have a good enough OPT you don't need this capacitor. You also don't need it if you're using less than "X" amount of NFB. Where X = goodness knows what. But if you're not using any NFB, you certainly don't need it.

If the amp is only marginally stable without it, you'll see little bursts of oscillation at a few hundred kHz near the peaks of the waveform, when it's cranked up into a dummy load and the output observed on a scope. You should check for this on every amp you work on that has a NFB loop. If there's a presence control it should be turned to 0.

[Enrico]

On some LTP inverters, they are like local feedback caps for high frequencies. See the 5F6A for example.

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. 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, 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.

Any help is greatly appreciated!

[Enzo]

Hi Enrico.

SO if that is the only difference that you can see, and you wonder if it is the problem, unsolder one end and lift it away from the circuit and find out, then you tell us.

[Enrico]

Hi Enrico.

SO if that is the only difference that you can see, and you wonder if it is the problem, unsolder one end and lift it away from the circuit and find out, then you tell us.

Hi Enzo,

thank you for the reply and yes, that would be #1 thing to do. I was just wondering if there's any "math" to the issue--can in fact a capacitor there suck half of the power? It seems weird to me--but of course it won't take long to unsolder and see if there's any difference.

As for the "double" thread, it is just because I see a lot of the usual competent people here and I figured they would receive a notification of my post. I was afraid the new thread that I opened wouldn't get much attention. Anyway sorry for the double trouble!