HH Scott 272 Restoration
It's been a long time since I really had any interest in vintage tube audio as my life has been quite busy with a newer career and settling into our first house in the past year. I have enjoyed some nice solid state seperates in the mean time (MAC MC-250, Full Mitsubishi System DA-A10DC W/Meters) but when I saw this pristine 272 in barter town not long ago I knew it was time to give it another go. I was able to work out a deal and she's here on my actual bench in my new place. No more attic listening space/workshop!
It is in excellent shape and has the optional wooden case to go with it. It has original highly sought after Mullard EL-34 outputs and Telefunken small signal tubes throughout. Sweeet. The previous owner brought it up slowly on a variac and it passed signal and had some scratchy controls but that's the extent of testing.
My original plan was to wait until I'm off for 2 months or so for the winter (I do construction) to start this project. I couldn't wait In the past I tend to get impatient and do what it takes to get a project safely up and running. I figured starting it now would give me the time and patience to attack this in a methodical manner and give it the care and love it deserves. Hand picking each and every part. Also I have on hand better gear and speakers then I've ever had since getting into the hobby so hurrying up to get something better isn't in my mind. Who knows there is a chance this won't be the best sonic gem I have in the stable when done? BUT I think it has the chance to be the coolest piece of 60's audio engineering I own, and be a hell of a tube integrated with the right love.
Today I decided to do some light cleaning and fire her up without the dual rectifiers in the see what the voltages measure in the bias system. If they are close I was going to carefully and slowly fire her up and hear what we have in bone stock form myself. That's not going to happen.
The sam's photofact shows the voltage coming off the selenium rectifier going into the first 33ohm 2W resistor of the supply is supposed to be -55V, I'm only reading -45V so the selenium rectifier must be tired. Coincidentally I'm only reading around -25V on the grid pins of the EL-34's when they should be -35V according to the schematic so that makes sense. This is the first thing to replace.
As far as the power supply goes, I've already contacted Tom at Hayseed Hamfest for 6 custom replacement cans as the originals has some funky values and there is ZERO real-estate under the chassis. He got back to me and said they can do everything I need.
This is the most intimating tube amp I've opened the bottom of. A Fisher receiver honestly looks juvenile in comparison. What strikes me the most is the massive amount of tiny wiring EVERYWHERE. Components stacked on top of components. I'm going to think about it some more but I think honestly the best plan for this amp is going to be to do it section by section and then TEST. By this I mean If part of the amp is too cramped to neatly recap without toasting everything around it, I'm going to completely replace all the resistors in the way as I go. In the past I've always just ran with it and recapped everything, then plug it in
I'm going to order a nice pair of new production EL-34's to use for all the testing/turning on and off stuff. Since this model has full bias control, the only addition I need to make is individual bias test points on top which will be a piece of cake.
I am looking for suggestions on capacitor selection. If you've recapped a Scott amp before you know what size the stock Ceracaps are. I'd like to put some fancy schmancy caps in this but do not want to exceed the size of the originals. There's many spots that there's really not room. Just going with those little Illinois yellow axial caps crossed my mind since many have said that would be as good or better then stock sound but I'm not interested in keeping something completely original for the sake of keeping it original. I was thinking of bumping the stock coupling caps from 0.1uf to 0.22uf. Any thoughts?
These are some open thoughts on my plan of attack and you are welcome to encourage or constructively criticize!