I'm repurposing a '50s amp chassis from a phono console into a guitar amp (see attached). The tube complement was 6au6a, 12ax7, (2) el84 and ez81. I'm replacing the 6au6a with another 12ax7. The original power transformer is 210-0-210. The circuit (attached) I'm using calls for a 290-0-290 supply. How do I design the power rail to get the most out of the transformer I have? I'm using two 32uf caps and two 22uf caps for filtering. What I can't figure out is the correct resistors for the power section that supply the el84s. I've searched the web for an easy explanation for calculating the dropping resistors, but nothing has made sense in this application--or with my math skills. Even the online calculators have asked for a spec I don't have (or don't understand). Can anyone help? Thanks!

 

The transformer you now have is going to do its best.
Why do you want to modify the design?
I would opt to go with what you already have.

 

Thanks for the answer. Just to clarify, would you suggest keeping the Marshall design or go with the original specs on the Voice of Music circuit? My thought about modification comes from wondering whether a change in the dropping resistors would allow more of the B+ from the transformer to make it to the EL 84s. Perhaps I have my theory wrong.

 

Presumably the designers of the amplifier designed the circuit based on what they've got and did their best. Squeezing a few more volts out of the HV supply is not going to make a lot of difference.

You should focus on how much additional gain you will need at the preamp stage and how much distortion you can tolerate, unless you play grunge.

 

Thank you, sir. I appreciate it. Stay safe.