V-Champ power tube cap

[trem]

What is this little cap doing ... is it 350 pF ... from 6V6 grid to cat resistor.?
Thank You

https://el34world.com/charts/Schemat..._schematic.pdf

[Justin Thomas]

Pretty sure it's to snub any oscillations that may or may not happen. Since the wiring got so messy over the years Fenders got lots of these in various locations. You can probably remove it, or at least try it without it...

Jusrin

[trem]

Pretty sure it's to snub any oscillations that may or may not happen. Since the wiring got so messy over the years Fenders got lots of these in various locations. You can probably remove it, or at least try it without it...

Jusrin

Hhhmmmm
This is a Pre CBS schem.
If there was oscillation at the grid, would the cap got to the bias resistor instead of ground.?

Are they trying to fine-tune away some of the highs from the "fully" bypassed cat resistor.?

[Chuck H]

Local NFB vs blunt snubber. I'm sure whomever did whatever at the time to make it work and that became the schematic.

[Jon Snell]

That is a hand drawn modification.
It is a 330pF probably silver mica or polystyrene type. Voltage doesn't matter.
A 22k 1/2W CC resistor between the 0.02u and the grid pin would do the same job.
It helps to stop HF instability but at the expense of a full range sound.

[Chuck H]

Looking at the schematic you can see that cap wasn't drawn in with the same care as the rest of the schematic. Making it seem like an after thought. If you look at the following VC AB764 schematic it's drawn in with more care. On the layout drawing for the VC that cap is right on the tube socket and also drawn in with less care than the rest of the circuits. This would also make it seem to be a late design add on. To me it seems like the only reason that cap goes to the cathode resistor rather than ground could be nothing more than convenient mounting. So it may just be a coincidence that it goes to the cathode rather than ground.

[Helmholtz]

Together with the source impedance of the preceding stage (around 40k) the cap constitutes a low pass filter with a corner frequency of 12kHz.
It doesn't matter if the cap is connected to the cathode or ground.
As the global NFB tends to equalize the frequency response there shouldn't be a noticeable treble loss (even if the speaker could reproduce it).

Rather the cap will be needed to generate a phase lag in the forward path to stabilize against oscillation.
The effect is similar to the small cap between the PI plates in PP amps.
Sometimes a different OT necessitates such additional measure.

[trem]

Was Fender having oscillations troubles with this amp, or was it just good design from their perspective.?

I owned a 1969 VC and i built a few clones.
I do not remember seeing this cap before, but i might have missed it or forgotten

[Chuck H]

Together with the source impedance of the preceding stage (around 40k) the cap constitutes a low pass filter with a corner frequency of 12kHz.
It doesn't matter if the cap is connected to the cathode or ground.
As the global NFB tends to equalize the frequency response there shouldn't a noticeable treble loss (even if the speaker could reproduce it).

Rather the cap will be needed to generate a phase lag in the forward path to stabilize against oscillation.
The effect is similar to the small cap between the PI plates in PP amps.
Sometimes a different OT necessitates such additional measure.

Yes. This is what the cap IS DOING. But you make it's presence sound more mathy and calculated than it probably was. I get the distinct impression from the drawing on the schematics and layouts that the amps were misbehaving and someone just did something to make the problem stop. Of course the cap value was considered for the circuit. But I imagine the thought "Well I guess we could just stick a cap there on the tube socket." entered someones mind.

[Justin Thomas]

But I imagine the thought "Well I guess we could just stick a cap there on the tube socket." entered someones mind.

I bet Fender considered this seriously on the bigger amps like the TR & Bassman, but I bet they just said "well, maybe it'll work here." The Champ & VC were the bottom-line amps.

Also, it IS a CBS schematic. They just didn't see fit to draw up a new one til several years later when we got the 420V B+ one. Bet this one dates to late 67/early 68; that's when they started adding caps to the output sections of the big amps too.

Jusrin

[Helmholtz]

Using real numbers (and a bit of theory) helps to avoid guessing wrong regarding possible effects.
So I can exclude that the cap was added to modify the treble response.

An experienced amp designer knows how to test for stability.
Instability does not necessarily show with a sine test signal or a resistive load.
I bet many a diy amp builder is not aware that his amp is oscillating under certain circumstances.
Might be just short HF bursts at certain signal levels resulting in a "harsh" sound.

With a power amp using global NFB the main contributor to phase rotation at HF is the OT.
So a different (e.g. from the prototype) OT might have required adding the cap.
Even using a different speaker can have an effect on stability when an amp is on the verge of oscillation.

[Tom Phillips]

Looking at the schematic you can see that cap wasn't drawn in with the same care as the rest of the schematic. Making it seem like an after thought...

I think that it was drawn by someone other than Fender and then released to the wild on the internet .

[Justin Thomas]

Here's the actual CBS Champ Schematic, and that cap is definitely part of it. So I'll bet Tom called it. Somebody just either couldn't find or couldn't get the VC schematic & doctored the old BF one.

Jusrin

[Chuck H]

Well I just looked at the AA764 schematic in Aspen's "The Tube Amp Book" aaaaand... No cap. So I think Tom has it right.

EDIT: 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.

[Helmholtz]

To me the question by whom and when the cap was added to the drawing is subordinate.
Question should be if it's beneficial to the actual amp.
A thorough stability analysis with and without the cap could tell.

In any case judging from the cap value I'd say it won't have a detrimental effect even if it's not needed here.
Phase lag compensation with global NFB is a widely used and accepted practice.
Hardly any LTPI in a PP amp using global NFB that doesn't have a lag cap between the PI plates.