I recently picked this up on eBay. It's a 2 channel Heathkit preamp from 1958. Each channel has 3 tubes, EF86, 12au7, 12ax7. There isn't a whole lot out there about this model, outside of one pretty decent video on YouTube and a few tidbits here and there. If you know differently, please let me know. Anyway, I'd like to outline a few issues I'm having as I slowly restore this unit a bit at a time. As it was from the getgo, it amazingly worked with all it's original parts, and sounds very good minus some slight hum and sizzling at very high volumes. The only reason I turned it on was because the seller had already done so extensively. So here goes:
1) First thing I did was replace the selenium diodes with modern diodes. That brought the DC voltages back up closer to the specs outlined in the schematics. I did not add a resistor in series, as the info I see online is contradictory on whether or not this is necessary.
2) I worked on the power section first, replacing capacitors and resistors. Here is the first major rub. The resistors called for in the plans and schematics for the section that creates stepped down voltages to send to the tubes do do match what was in there. Also, the video I previously referenced, that guy found the exact same "wrong" resistors. The voltages were close to where they are supposed to be (270,180,110,90). When I switched out to the "proper" specs resistors, the voltages went up on the first two legs, both at around 305v. From what I can tell these are the voltages that go to the 12ax7 plates. My instinct tells me there is some addendum lost to time that instructed kit builders to use different resistors. What should I do here? It still works and sounds great (maybe even better), I just don't want to burn up tubes.
3) The kit instructions reference tube shields. There are no shields on these tubes. Should I get some? Where?
4) I'll be working on the channels next. The caps are going to be replaced, that's a given, but should I also go after every resistor in there as well? I read that these composite resitors from the 50's are awful, but it looks like a daunting task to replace all of them. Should I concentrate on certain areas of the circuit?
1) First thing I did was replace the selenium diodes with modern diodes. That brought the DC voltages back up closer to the specs outlined in the schematics. I did not add a resistor in series, as the info I see online is contradictory on whether or not this is necessary.
2) I worked on the power section first, replacing capacitors and resistors. Here is the first major rub. The resistors called for in the plans and schematics for the section that creates stepped down voltages to send to the tubes do do match what was in there. Also, the video I previously referenced, that guy found the exact same "wrong" resistors. The voltages were close to where they are supposed to be (270,180,110,90). When I switched out to the "proper" specs resistors, the voltages went up on the first two legs, both at around 305v. From what I can tell these are the voltages that go to the 12ax7 plates. My instinct tells me there is some addendum lost to time that instructed kit builders to use different resistors. What should I do here? It still works and sounds great (maybe even better), I just don't want to burn up tubes.
3) The kit instructions reference tube shields. There are no shields on these tubes. Should I get some? Where?
4) I'll be working on the channels next. The caps are going to be replaced, that's a given, but should I also go after every resistor in there as well? I read that these composite resitors from the 50's are awful, but it looks like a daunting task to replace all of them. Should I concentrate on certain areas of the circuit?
Yes it has mic, phono, and 3 aux inputs. I attached a schematic that shows the power section and one channel. It's a modular design and you added the second channel if you wanted it.
Leaky caps will affect voltages - don't worry too much about them until caps are changed. Resistors can be measured in place, for the4 most part - if they measure high, they're bad, if low, check for a parallel path that may be responsible for low reading. Plate and screen resistors may be high, and if so, they'll be noisy. Shields are probably the "wrap" style, unless the sockets have a bayonet or snap-on ring. Plenty of shields around from vintage equipment - I have a cubic foot or two of them...