Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ALBATROS1234 on October 28, 2018, 01:12:16 pm

Title: questions about a capacitor on pp power tube kathodes
Post by: ALBATROS1234 on October 28, 2018, 01:12:16 pm

latest project is a harp amp for a friend. i am esentially doing only the microphone input/channel of an older gibson ga-20. i am soldering it up and connected to the cathodes of the power tube there appears to be a cathode bypass cap. it is 20uf so that makes sense but the placement seems odd as it appears to have both sides connected to ground/negative. it does not indicate if it is an electrolytic (polarized) or film(nonpolarized) just kinda confused about this. below is the schematic.

Title: Re: questions about a capacitor on pp power tube kathodes
Post by: VMS on October 28, 2018, 01:23:28 pm

I think it's electrolytic and heaters are elevated with power tubes cathode voltage. This schematic shows it more clearly:


https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga20_12ay7_preamp.pdf (https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga20_12ay7_preamp.pdf)

Title: Re: questions about a capacitor on pp power tube kathodes
Post by: PRR on October 28, 2018, 01:28:46 pm

It's badly drawn, faded, brutally scanned.

Fix the missing cross-overs. Dot the connections. Crayon the main buses.

The 20u is across the 220r and this point is about +15V or +20V. This is also extended (blue line) to the heater CT, to "raise the heaters".

Or what VMS said while I was playing with crayons.

Title: Re: questions about a capacitor on pp power tube kathodes
Post by: ALBATROS1234 on October 28, 2018, 01:46:16 pm

aaah gents thanks much, this is kinda what i was thinking but as you saw the schematic was pixelated and funky. that makes sense more how the later version is drawn