Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fossilshark on August 04, 2023, 08:21:06 pm

Title: power supply bias design
Post by: fossilshark on August 04, 2023, 08:21:06 pm

Hi all, went to order parts to design the power supply PCB and realized the transformer I plan on using does not have a 50v CT (Hammond 273BX). The 700v center tapped HT line is rated for 201ma, could I throw a resistor on the 700v line (or divider between the 350v line and GND) to get 50V? How would I calculate the value(s) needed?

This is for a 100w amp meant to be able to use 6l6 or el34.

I greatly appreciate any advice!

Title: Re: power supply bias design
Post by: Dave on August 04, 2023, 08:41:54 pm

You can use one leg of the secondaries. Look at just about any Marshall schematic. That's how they did it.


Dave

Title: Re: power supply bias design
Post by: fossilshark on August 04, 2023, 09:01:01 pm

Looking at 100w jcm800 schematics it seems that marshall uses 350vac instead of 700vac center tapped, would I just double the values of the resistors and pot to get around where id need to be?

How much current could I expect the bias of an EL34 and 6L6 to draw at 100w? I would be able to use that to calculate the voltage divider needed more accurately.

Title: Re: power supply bias design
Post by: sluckey on August 04, 2023, 10:02:39 pm

Quote from: fossilshark on August 04, 2023, 09:01:01 pm

Looking at 100w jcm800 schematics it seems that marshall uses 350vac instead of 700vac center tapped

Bad example! Do it like this...

     http://sluckeyamps.com/phoenix/phoenix.pdf

Title: Re: power supply bias design
Post by: fossilshark on August 04, 2023, 11:03:01 pm

perfect thank you!