Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on July 30, 2011, 06:06:19 pm

Title: Elevated Heaters
Post by: TIMBO on July 30, 2011, 06:06:19 pm

Hi Guy's,I pinched an idea off one of Tubeswell's Amps (EF 86 PP gigging amp) and thought it very good. The amp i am working on has two pairs of power tubes that can be switched on and off, which means that i am unable the use the power tubes cathode to elevate the heater CT.This idea of the heaters having own injection of DC via this circuit i hope will do the trick. Any thoughts, Thanks

Title: Re: Elevated Heaters
Post by: labb on July 30, 2011, 06:50:52 pm

It will work. voltage divider with cap. Shoot for about 70 vdc elevation.

Title: Re: Elevated Heaters
Post by: jojokeo on July 30, 2011, 07:17:36 pm

I actually keep mine between 35v - 40v. I don't think going higher gives any extra benefit and is what was suggested as the range to shoot for from Weber. He gave his reasons in one of his books. It works is all I know. Use a 220k/1W & either a 27k or 22k 1/2watt depending on B+ voltage.

Title: Re: Elevated Heaters
Post by: tubeswell on July 30, 2011, 08:15:43 pm

Looks good Timbo. With the 2 x 390k in parallel you have 195k above the knee. With 22k beow the knee you will have:

V out = 22k/(195k+22k) x V in, which boils down to:

V out = .10 x V in

Make sure the decoupling cap (~10-50uF) is rated for about at least 20% more than the V out and you'll be fine.

Title: Re: Elevated Heaters
Post by: Fresh_Start on July 30, 2011, 08:43:00 pm

The only time you need to shoot for a DC voltage reference (AC ground) greater than 50 volts is if you have a cathode follower and are worried about the maximum cathode-to-heater voltage.  Check tube data sheet for CF triode and voltage on that cathode.  Jojokeo's got good suggestions.

Cheers,

Chip

Title: Re: Elevated Heaters
Post by: TIMBO on July 30, 2011, 11:47:52 pm

Thanks guys, i not sure if i'll need it yet as i have DC heaters at the front end.