[Question] Master or post PI bright cap

[Bombacaototal]

Dumble ODS #102 uses a 68pF bright cap across the master knob
Dumble SSS #002 uses a 68pF bright cap right after the Phase Inverter loads

What are the differences between them as far as end result?

Would polystyrene be the best option for 68pF (since it is smoother sounding) or ceramic or even silver mica would be better? Which one is your favourite?

[metalguy]

The 68pF cap in #002 is a shunt cap. It's there to roll off some highs and prevent oscillations..
Concerning the 68pF bright cap in #102 in my recent build for some reason (maybe because of too much staring at schematics) I used there a 15pF cap.
After playing the amp for some time and checking many times with the schematic if everything is correct I noticed that the schematic calls for a 68pF Master cap.
I'm not an expert in Dumble tones but with 68pF the amp is too bright for my taste (especially at low volumes). I did some A/B tests between 15pF and 68pF and I prefer more the 15pF cap.

[Colossal]

I used 25pF across the master in a D-Lite type build and it was plenty

[aflynt]

The 68pF on the master of 102 is quite sadistic if you're not using a d-lator. It's like punishment for not running the amp at ear bleed levels. The hypothesis is that it's there to make up for loss of highs to cable capacitance when using the loop. In that regard the stock 102 schematic is viewed as part of a "system" that includes the Dumbleator and specific loop cables.

I think RF no longer uses the Loop but looking at the amp I seem to recall that he still had a cable plugged into the loop jacks. Perhaps it's just there to smooth it out. That or he just plays loud enough for it not to have an effect.

-Aaron

[Bombacaototal]

Thanks a lot everyone for the explanations specially regarding the shunt cap

I found an older post with a few discussions about the master bright cap

https://tubeamparchive.com/viewtopic.ph ... b&start=30

Thanks for the feedback on the 68pF. I will probably stay away from them and try values between 47pf and 15pf

About using a .02 coupling cap after V1B on #124, #102 type builts, there are comments on the mentioned thread above that it wil make the bass knob less effective but with a bit of a tighter bass. Has anyone tried it?