65Amps PI alteration: Seen this?

[rooster]

I recently opened a Marquee amp from 65Amps that had a few repair issues and noticed a 22pf silver mica cap in parallel with the PI cathode tail. (On the Rocket, this would be the 47K resistor that attaches to ground off the PI - with me?)

Well, let me add that this is a discontinued amp from the company and the only quad EL84 type amp of theirs that I have looked at internally. AND, BTW, this is prolly one of the brightest (!!) amps I have ever heard - and this would be 'globally' bright, not just one channel.

So, it occurs to me that this cap was added as a means to tame some of the super bright characteristics of the amp, or perhaps some high frequency parasitics. Have any of you here run into this somewhere else? It's so small a value it would seem for the benefit of a parasitic but I have to wonder if it's something any of you have tried on your Rockets of Liverpools?

[fperron_kt88]

Hello rooster,

Interesting find!

However, my understanding is that if you bypass a cathode in a gain stage (be it differential or single stage), you will get higher gain for the decoupled frequencies. In other words, my intuition tells me this bypass cap will boost higer frequencies more than anything...

Am I missing something here?

[rooster]

Interesting thought, fperron_KT88. If such is the case, then they were attempting to increase the highs? Well, I have to doubt this, as would you if you heard this amp. Too, considering the frequency base implications of a 22pf cap, this would be like, 20-100KHz? No need for that, eh? Which still makes me think it is some attempt at a shunt capacitor to remove either a parasitic or high end in general. (Fender does something like this with their BJr, BTW, but prior to the MV and PI, and here the value is realtively significant, 1500pf, compared to whatever the 22pf cap is doing on the PI tail.)

So I guess it remains to add such a cap to my Rocket and see what exactly it does. ..Er, unless someone else has run into or already tried something like this? Like I said, not seen this type of thing before and am assuming it is a bandaid device aimed at some high frequency information. Oh, and when I say this amp is bright, compared to an AC30TB or Rocket, this amp is like a shot to the backside with two barrels of coarse glass buckshot vs. two of barkdust.

Any other thoughts?

[XgamerGt03]

Is the tail resistor 47k? Does it have any negative feedback?

[rooster]

Yes. No.

[surfsup]

Wouldnt it be better to remove the cap from the amp that has it to hear the effect?

[rooster]

Well, sure, that would have been simple, but I didn't and still don't have that much background with that amp. But actually, at 22pf, I am pretty sure I could have pulled it and not heard anything out of the ordinary.
So I thought I would ask here if anyone has seen this type of thing before - and what exactly - beyond something that I could hear - this cap might be doing.

Trying it on my Rocket, it's possible, since I'm familiar with it, that I might 'hear' a change. And, too, I could up the value if need be to see if I could hear that. Er, so it's trial and error then - if - nobody here knows anything about what strapping a cap on the PI tail would do *scientifically*. I've never seen it before. And here then, yeah, sure, better to try it on my Rocket than somebody's $2000 deal.

I suspect it's some parasitic control but, again, any more thoughts?

[angelodp]

Ok, Rooster, let us know what you find once its on.

A

[dave g]

There is no degenerative feedback through the tail resistor since the grid reference is between the cathode and tail resistors. The tail resistor is there simply to raise the PI's local "ground" to a DC voltage.

I doubt you will hear a difference with or without the cap there.

[passfan]

Perhaps take a look at it with an o scope and remove it and compare ?