B+ voltage drop via Zener/MOSFET question

[J M Fahey]

Congratulations.
I was going to chime in to suggest a flatback, finned heatsink on the outside of the chassis, sandwiched against it with some thermal grease and the MosFet on the inside.
Even if heat goes through 1 mm steel (iron chassis thickness) it's not that bad, and the fins rest in free air, not trapped inside the chassis.
Something like this:

[casey73]

Congratulations.
I was going to chime in to suggest a flatback, finned heatsink on the outside of the chassis, sandwiched against it with some thermal grease and the MosFet on the inside.
Even if heat goes through 1 mm steel (iron chassis thickness) it's not that bad, and the fins rest in free air, not trapped inside the chassis.
Something like this:
[ATTACH=CONFIG]28391[/ATTACH]

I appreciate that. The problem is, I had a very narrow space between the back of the amp and the cabinet. This is an old Kay 720 Bass Amp. The chassis is on the bottom and the controls are on the top There was about a half inch between the lip of the back of the cabinet (he never had the back panel - which is good because it needs open air). So I had to fashion something in a "field expedient" fashion. Fortunately, the amp, being an economy model, had a very thin steel chassis. I greased up every point of contact, bolted it together and it is working. It looks a little strange, but it does the job. If I had my druthers, I'd have used something along the lines of what you suggested. There is a local surplus electronics place that has many such heat sinks they sell by the pound.

This project turned out to be a challenged that I brought upon myself. The amp was a rats nest, had been FUBAR and sounded thin and lifeless. So he gave me permission to make what I could out of it. I left the existing power supply alone. The original FP cap was shot and had been replaced by several snap in caps with 220K voltage dividers to up the voltage handling. It was adequate. I set it up like a tweed Fender Pro with a Princeton preamp with an added mid pot. I ended up with an extra triode so wired them parallel ala Matchless Spitfire for a tad extra gain and touch response. The existing shielded cable was RG-6, the typed used for cable which I replaced it with RG-174. Much to my surprise, the thing is quiet as a church mouse. Talked to him this morning and he said he gigged with it this weekend and was quite happy. I dubbed it the Proton.