Sherwood S-1060 Amplifier Restoration

Thanks, Gadget. Appreciate your comments. Will do further follow-up as you suggest. Also attaching schematics: Sherwood for entire set, Sams output tubes portion and Sams power supply portion--for any comments you may wish to offer.
 

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Here is a slightly better version of Sherwood schematic.
 

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Sounds reasonable. Without making it too complex, I'm thinking breaking the connection between R54, R62 and ground, and putting a 5-10K trimmer between ground and those two resistors would make for a workable bias adjustment if needed.

Of course if its working properly now, leaving it alone will also work just fine.
 
Gadget. Thanks again for your advice. I'll do a bit of experimenting on your suggestion. I had a couple of 7189s red plate. They were old tubes & I suspect had been red plating in a previous life . Replaced with new ones which are not red plating.
 
Gadget, Thinking the best thing to do is to custom bias each of the 3 pairs of output tubes since I don't have matched pairs. Presume the center tap of the output transformer is the point between C36 and C37. Is that correct?
 

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Individual bias with a 10 ohm cathode resistor per-tube would be the ultimate. It would need a little tinkering with since the stock balance pot would have to go. A matched set of 6 tubes would still be a good idea though.

Trafo CT goes through the time delay relay. Looks like it has a resistor to ground for soft-start and the relay bypasses the relay for full power. Not sure what that has to do with bias adjustments though.
 
The bias method I was going to follow starts with getting voltage and resistance measurements between the center tap and output tube plate pin and eventually gets to a plate dissipation measurement--then biased to 70% of PD.
 
The M15 gizmo is listed on the Sams as a "thermal switch". Never seen one of these on an amp before. I surmise its purpose is to shut the amp off above a certain temperature. This makes sense considering the enclosed design of this amp which builds up a lot of heat. Just a guess.
 
Looks more like it gets warm and then engages to bypass the 47 ohm resistor between the transformer CT and ground.

I've really never been a fan measuring stuff through the transformer resistance. It changes with temperature, and it involves measuring high voltages and more complex math. A 10 ohm to ground will give you readings well under 1 volt and the math is trivial. 0.35 volts is 35ma. it will also let you read it per-tube, which reading through the trafo will not do. No point having 6 bias adjustments when you can only measure the sum of 3 tubes at a go. If you're doing it that way, it would make more sense to add a single bias adjustment and use that plus the stock balance adjust to get the sides even. Or honestly you can just get the voltage drop across that first resistor correct with the bias control, and use the balance control to minimize hum in the stock way. Its all about as effective.
 
Gadget, Again, very much appreciate your willingness to provide expertise. Under stand the "dangers" of high voltages associated with OPTs. I'll be going on vacation until July 9 and will get back on this project after return. Will keep you informed. Thanks again.
 
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