Plexi 50 out of a Bassman 100

[rfgordon]

Merry Christmas, all! Santa was good to me this year and brought a Strymon Mobius! Woohoo!

In other news, I finished my Bassman 100 head to Plexi 50 conversion yesterday. I am extremely happy with it. It rocks and has virtually no 60 Hz hum. I'm gonna swap peanuts till I find some with less hiss and hash, but I love the way it sounds.

As y'all will see from the pictures, I made lots of changes. For one, I completely redid the grounding scheme, so that made it quiet. I added a pot upstream from the stock bias balance pot. This allows me to adjust the bias voltage, then use the balance pot. I can dial in the tubes to less than half a milliamp difference. It can take a range of tubes, up to KT88s, thanks to the 100 watt tranny set.

I also up bias test points in the small plates that cover the two unused power tube holes. I put new octal sockets in. The filament work was redone, along with a new hum balance pot.

I put pot after the tone stack to allow that part of the circuit to be overdriven or not, which yields more control over the amp's texture.

The original Deep switch cuts in a bigger cathode bypass cap on V1, to fatten things up. That's kind of like jumpering the channels and dialing in a little of the dark side of a Plexi.

[Stevem]

When you get the chance, change out all 6 rectifiers with fast recovery type UF5408.
The leads on these are too large to get two leads in one ring hole like stock so you will need to rap the center rectos leads around the ends of the two other rectos tha boarder it.
This will make for near zero switch nose hash, which is something you do not realize untill its gone.
That the one good thing about tube recto`s, no noise hash!

[rfgordon]

Would a 1N4937 do the job? I can get them from CEDist, but not the ones you mentioned.

[Bieling3]

Did you have a 125P34A power transformer to start out with? I'd love to convert a Fender PA 100 to an el34 based amp.

[rfgordon]

It had whatever the stock Bassman 100 PT is. I can't check on it now, because the amp is at a studio as a loaner until I make the same amp out of a Bantam Bass 50 that the studio has.

I was hoping to sell my amp when I took it to them. I had no idea he had two Bantam Bass chassis on a shelf. One of them has a replacement PT with loads of iffy changes. I'll use the good one for his Plexi build.