Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on October 20, 2022, 10:45:01 am

Title: 6A7 as a guitar preamp
Post by: scstill on October 20, 2022, 10:45:01 am

In the Prewar42 project (design schematic and pics here https://stillampd.com/prewar-42-guitar-amplifier)
The radio chassis has a extra 6A7 tube that wasn't planned for. https://frank.pocnet.net/sheets/127/6/6A7.pdf
As an experiment, was wondering if I might use the 6A7 as a second channel preamp configured as a pentode?
Since it is a Pentagrid tube there are lots of grids and wasn't sure what grids to use and how :-)
Thinking to apply screen voltage to pin 3, Tie cap and pin 4 to cathode (as a pentode would do internally)
And then use pin 5 as the signal grid, or should the cap be signal grid.
Would this be workable or better way?

Title: Re: 6A7 as a guitar preamp
Post by: PRR on October 20, 2022, 05:05:05 pm

I think you are the first to ask that in 85 years. It is only 4 pins. Time for Original Research.

Title: Re: 6A7 as a guitar preamp
Post by: scstill on October 20, 2022, 05:37:37 pm

This tube is 7pin (with 2 fat pins) and a cap
I am just not sure if all the grids need to be connected or if I could leave unused grids unconnected.

Title: Re: 6A7 as a guitar preamp
Post by: PRR on October 20, 2022, 09:34:10 pm

It's a "Pentagrid" but pinned-out as 4 grid pins. You could tack-solder many variations faster than type about it.

I'm not being dismissive. But I don't think you will find a ready-baked answer.

Title: Re: 6A7 as a guitar preamp
Post by: scstill on October 20, 2022, 09:39:43 pm

I have been finding some old stuff in this forum when referencing Heptode.
Convinced the scavenger hunt will show it's possible.