2203/2204 470pf V2 Bypass Caps

[jgab]

Hi,
Have any of you messed with these two 470pf bypass caps (circled in black in the image below)?

If so, I was wondering:
- What sort of effect did you notice and was a particular value something that you stuck with other than 470pf or 500pf?
- If tested, did you hear a difference between a 470pf and a 500pf?
- Is there a difference in the characteristic of the amp from one 470pf vs the other 470pf?

 

[Jon Snell]

30pF difference will change the slope very slightly.
If you fit a 220pF instead, there will be a slight gap in the upper frequencies, a bit like a dip in d=signal.
One 470pF versus another 470pF ... only differ if they are different tollerances.
Many Fender tone controls replace the 220pF with a 270 or 330pF to give a wider range on the higher frequencies.

 

[Pete Farrington]

The left hand 470pF is the tonestack treble cap, to see the effect of different values have a play with C1 here https://www.guitarscience.net/tsc/info.htm

The right hand 470pF raises the level of higher frequencies about 6dB above lower frequencies, ie a shelf filter.
It can be modelled using this calculator https://www.ampbooks.com/mobile/amplifier-calculators/bright-boost/calculator/
Set Rv to 940, volume setting to 50, and Cbp to 470 (or whatever value you wish to model).
With a stock 10k unbypassed cathode on the preceding stage, the output impedance setting should probably be increased, to 100, but it doesn't make much difference.
I recall @neikeel recommending a value of 2n2F there (2200pF in the calculator)?
That seems to bring the filter's -3dB upper corner frequency down from about 1kHz to about 250Hz, so increases the level of upper mids.
I intend to try that next time I've got the chassis out

 

[jgab]

Thanks guys. I went with a couple vintage RMC ceramics around 485pf and they sound great. Old ones were not stock (changed by some tech at some time. Measuring ~440pf). I wish I had the original red dog bones ceramics.
With these RMC ceramic caps, I noticed a bit more clarity and some harmonic squishiness to the amp. It is slight but I can noticed a change for sure. I like it.

EDIT: The amp I worked on is my 1976 2203 MV. Excellent amp and full of Mustards.

 

[Pete Farrington]

I recall @neikeel recommending a value of 2n2F there (2200pF in the calculator)?
That seems to bring the filter's -3dB upper corner frequency down from about 1kHz to about 250Hz, so increases the level of upper mids.
I intend to try that next time I've got the chassis out

So I give @neikeel 's suggestion my thumbs up, with the 470pF treble peaker increased to 2n2F, its tonal balance seems improved, it's become a bit thicker, less edgy / bright.

 

[syscokid]

So I give @neikeel 's idea my thumbs up, with the 470pF treble peaker increased to 2n2F, its tonal balance seems improved, it's become a bit thicker, less edgy / bright.

And now, I need to try this!

 

[jgab]

That's interesting.

My amp seems pretty thick overall so the 470pf works great. I play a lot with my band and I find the bright and edgy works well in the mix when recording or rehearsing. I think if I were a bedroom player that I would want my amp to sound thicker since I wouldn't be playing with a bass player or drums.