V-Champ power tube cap

[g1]

Hardly any LTPI in a PP amp using global NFB that doesn't have a lag cap between the PI plates.

I will counter that AB763 is a lot of amps. In my experience with Fender, the cap between PI plates is more the exception than the rule. But that is just Fender.

As far as the 'penciled in' addition to the schematic, I have no problem believing Fender at that time might just draw in production changes and have a bunch of copies run off. Perhaps there would have been a service bulletin as well, especially if warranty work was involved.

[Chuck H]

I don't doubt that it may have been a last minute, where can we put this circuit change to the design. But I think Helmholtz is really on point with mentioning that an OT difference in prototyping could have been responsible. The little, crappy OT's used in these amps didn't stabilize before the AA764 schematic. Before that the Champs used an off the shelf Triad OT. It was also a small, even marginal OT. But it was a different OT. So the first run of AA764 Champs did something less than great and Fender rebounded with a correction for the new OT. Which, while probably managed with an existing system in place, involved the added capacitor on the power tube socket and it worked. Job done. (<period). Leo Fender once said: "We don't make them like we used to. And we never did." Kind of brilliant, right? So this next era of Champ's with the proprietary OEM OT needed a "correction". Perhaps. There is another aspect... It's told that after the sale of Fender to CBS that the engineers under Fenders employ took the liberty of implementing circuit corrections that the old guard was opposed to. And they were opposed to these changes because the amps sounded good and people were buying them. These are good reasons. But once the sale happened these changes were introduced anyway. There is a lot of speculation that this was a detriment to the tonality of Fender amps. Hard to say really. The wiring in the silver face amps was a lot more cavalier than the black face era. And some circuits were addressing clean reproducion more than acceptible distortion. It all makes a difference and "acceptible distortion" ended up being favored over HiFi clenliness. Which should be obvious by where we've landed with guitar amp circuit designs So the CBS engineers were wrong on this point. But no slouches. As a result each circuit in these period specific amps needs to be evaluated very specifically (IMHO).for how their function holds up to modern ideologies.

[Helmholtz]

I will counter that AB763 is a lot of amps. In my experience with Fender, the cap between PI plates is more the exception than the rule. But that is just Fender.

You're right, I should have been more careful with my statement.

In fact it seems that Leo always used the PI cap with a 12AX7/7025 LTPI but not with a 12AT7 LTPI (so most AB763 models).
Likely reason is the lower gain of the 12AT7 which tends to increase stability.
But interestingly it was those 12AT7 designs which soon after the CBS takeover received the large value (2nF) power tube grid capacitors.
Lifting those grid caps on my 1970 Vibrolux Rev. did not show instability with a resistive load but the amp liked to oscillate on speakers.

https://music-electronics-forum.com/...ibrolux-reverb


BTW, the 330p cap from 6V6 grid to cathode is also shown on the original Bronco AB764 schematic and it doesn't look "hand drawn" there.

[Tom Phillips]

...BTW, the 330p cap from 6V6 grid to cathode is also shown on the original Bronco AB764 schematic and it doesn't look "hand drawn" there.

Interesting. Thanks for pointing that out. I have both the Bronco AA764 and AB764 in my files and I never noticed the 330pF cap addition.

[Chuck H]

From post 14:

Also, you can look on Schematic Heaven at the Bronco AA764 and AB764 schematics and layouts. The AA shows no cap in either but the AB does show the cap on both. Unlike the AB VC layout where the cap looks added in the AB Bronco layout the cap looks to be drawn with the rest of the amp.

That the cap is included in the AB764 Bronco layout and looks like it may be original is interesting because it certainly looks to have been added to the AB764 Champ layout.