Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bieworm on May 05, 2021, 06:47:54 am

Title: JTM1 build
Post by: Bieworm on May 05, 2021, 06:47:54 am

Hey there

Was looking into the marshall JTM1. I would prefer to build that one, but the PT on the schematic calls for 12.6VAC filament voltage. I can only score 6.3V types ..preferably the Hammond 369EX.
What also puzzles me on that schematic is the center tap of the heaters at the 12AU7. It's connected to the cathode of the tube, but there are no resitors between heaters and cathode..is that normal? Any idea how to approach this with a 6.3V secondary?

Title: Re: JTM1 build
Post by: JB on May 05, 2021, 06:58:32 am

It's just DC elevating the heaters by whatever amount the cathodes are sitting at, for hum reduction.  You'd do the same for 6.3V, or don't bother, might not need it.

Title: Re: JTM1 build
Post by: Bieworm on May 05, 2021, 07:26:32 am

Would it make a difference running the filaments at 6.3V instead of 12.6V ?

Title: Re: JTM1 build
Post by: dbishopbliss on May 05, 2021, 08:09:44 am

You can wire the heaters using 6.3v secondary. That is what most Fender amps do. Check out this link to see how to wire it up.


http://www.valvewizard.co.uk/heater.html

Title: Re: JTM1 build
Post by: Bieworm on May 05, 2021, 09:57:25 am

Stoopid me...I didn't look at the heater connections too well. I never used a triode at the power stage before...so I  made the wrong assumptions.