John Mayer Preamp Build

[the_jessmo7]

Hey guys! After much deliberation and examining my needs from an amp, I’ve decided to build a full John Mayer Signature preamp. My initial questions are what power transformer I should use and how to determine the power filtering needed. I know many have said the reverb circuit on the JM Sig is delicious and a crucial part of its sound, so I want to make sure I have it included in my build. I will build the full preamp up to the Master Volume/Effects Send. Are there any initial suggestions or tips from those who tried something similar? I will run this into power amp and cab sim 90% of the time and into the effects loop of my Hot Rod Deluxe on the rare occasion I get to have stage volume.

[the_jessmo7]

Here are some more specifics if anyone can help me.

Choosing power transformer
- I will use a diode bridge rectifier
- how much VAC do I need?
- how much HT amperage do I need?

Bridge rectifier
- how do I calculate B+ leaving the rectifier?

Filter caps, dropping resistors, and choke
- how much filtering is needed?
- what formula is needed to calculate how filter caps and dropping resistors lower the B+
- is a choke needed?

Cathode follower
- to make the output of the preamp low impedance, I’ve read a cathode follower is the way to go. Should I use a tube circuit or a MOSFET? Are there any schematics I can drop in to the end of this preamp?

For the JM Sig preamp section, I need to have three filtered B+ nodes at:
403v for the V3 reverb driver
256v for V2 and V4
248v for V1

I really need some help figuring this out. Any advice, tips, resources, etc would be greatly appreciated.

Thanks guys!

[erwin_ve]

Some recources:
http://www.valvewizard.co.uk/
https://robrobinette.com/How_Amps_Work.htm

[timrobertson100]

It’s cool that you’re keen to try and build something bespoke rather than following a known build and tweaking. I am sure you’ll learn a ton.

Bearing in mind I’ve never built anything like this, I think I’d start looking at something like the power transformer and filtering from the stand alone Fender reverb unit and see if it can supply the current for the valves in the JM preamp. The voltages may not get up there, but I guess this is always going to be an approximation of the real amp. For higher voltages you’ll likely be on something like the Princeton Reverb PT or similar (for which you might as well build a low power 6v6 output stage)

Best of luck, and please keep us updated. Curious.

[the_jessmo7]

For higher voltages you’ll likely be on something like the Princeton Reverb PT or similar (for which you might as well build a low power 6v6 output stage)

Best of luck, and please keep us updated. Curious.

Thanks for your reply, Tim! I’ve considered doing a 22 watt version with a dual 6V6 power section. I’m not sure what components would need to be changed other than using Deluxe Reverb PT and OT. I’m sure the filter caps would need to be different values?

[timrobertson100]

I’m about to build that exact thing. I’m planning on using 40uF on the first filter as a start, and GZ34 rectifier similar to a typical deluxe reverb. You might want to keep an eye on max plate voltage for your 6v6s if you use solid state rectifier. The bias circuit will probably need adjusted too, to get the right range for 6v6.

[martin manning]

For a 2x 6V6 version DR power supply would be fine. If just the preamp is to be built, a Champ PT would be more than adequate.