Reviving a supro thunderbolt type build

[mikepukmel]

Hi All,
Working on reviving a supro thunderbolt build, using Weatherford schematics fount on this site. Getting back to the project, and fixing some things that could be better in the original:

There is one honking shared 200R cathode bias resistor for both output tubes.





If I use one resistor for each tube as some other amps do, is the following correct:

Vx = I* 200R

If I give each tube its own resistor, then to keep the voltage drop the same, if current through the resistor is then halved, would I start with a 400R resistor with about half the wattage rating?

Thanks!!
Mike

[eschertron]

The shared cathode resistor helps reduce cathode degeneration from the lack of a cathode bypass cap. If you split the resistors, it'll have an impact on the gain of the power tubes if you don't then add bypass caps. Just a thought. Yes, the value of each is double. Keep them separate, or parallel them as per Ohm's law.

You didn't ask, but I thought I'd share - I like the design of this amp. Looks bone-simple, with an economy of parts. Can't wait to find out how you like the sound

[mikepukmel]

Awesome, thanks Eschertron. I got really interested in the project last year, ran out of steam due to an overload of house repairs, then heard a comparison by that cool guy Johan Segeborn did a few comparisons on Youtube, and that really revived my interest. (The tone!!!). The first idea I had was to pickup a marshall style head box (so dang much room in those things and the tubes face up!!!) . Carey at Juicy Amplification will cut a chassis head and leave out some holes. Then started looking at layouts, found a few here and there, and then found Morts thread on his scratch build, and wow that layout looks just fantastic, and easy since on an eyelet and not point to point.

Thanks for the info on cathode resistor, will stick with 'stock'.