Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on April 11, 2022, 03:03:03 pm

Title: Where to ground power-amp grid-leak resistors
Post by: PharmRock on April 11, 2022, 03:03:03 pm

Should the grid leak resistors on the power tubes (in this case a cathode-biased 2x EL84) be grounded with the power amp cathodes, or with the PI tail resistor?  I'm thinking it would make sense to go with the power tube cathodes.  I've read this http://www.valvewizard.co.uk/Grounding.html (http://www.valvewizard.co.uk/Grounding.html) and it seems that is the case based on some of the figures I checked out.

There is no NFB in this amp, and the PI plates are tied to a can cap that shares the ground with the pre-amp tubes filter cap.

Thanks

Title: Re: Where to ground power-amp grid-leak resistors
Post by: Latole on April 11, 2022, 03:19:17 pm

This Fender amp layout and schematic may help you

Title: Re: Where to ground power-amp grid-leak resistors
Post by: Latole on April 11, 2022, 03:19:46 pm

Schematic

Title: Re: Where to ground power-amp grid-leak resistors
Post by: sluckey on April 11, 2022, 04:11:58 pm

Quote

I'm thinking it would make sense to go with the power tube cathodes.

Yes

Title: Re: Where to ground power-amp grid-leak resistors
Post by: tubeswell on April 11, 2022, 04:59:02 pm

Yet another schematic/layout. (Adapt to suit for different pre-amp type-ologies)

Title: Re: Where to ground power-amp grid-leak resistors
Post by: PharmRock on April 11, 2022, 05:42:12 pm

Thanks everyone.  I've changed the schem/layout so that the EL84 grid leak resistors connect to the ground for the shared cathode resistor on the EL84's. 

Title: Re: Where to ground power-amp grid-leak resistors
Post by: PRR on April 11, 2022, 07:19:27 pm

It should not matter.

Maybe less to go wrong if nailed to the cathode return. (Since we keep hearing of amplifiers with two ground busses/points not firmly connected.)

Title: Re: Where to ground power-amp grid-leak resistors
Post by: PharmRock on April 11, 2022, 07:28:07 pm

Quote from: PRR on April 11, 2022, 07:19:27 pm

Maybe less to go wrong

Good enough reason as any as far as I'm concerned.