1st, snip out the .047uF/600v 'death cap' on the ground switch before you do anything else. With the 3 wire cord you don't really need the ground switch any more. Just snip it, you don't need to heat up the iron.
Read about failure modes that can cause smoke in tube amp and continue electronics 101 refresher.
When something draws too much current it over heats, and will burn up, causing smoke. That's why we use fuses, to limit the current in case of failure. That's why putting in a 10A fuse where it's supposed to only be 3A's is a really bad idea.
That amp looks to be in original condition mostly. Power tubes look stock, electrolytic caps (e-cap) look stock and must be changed.
A tube can have it's screen grid short out internally and take out the screen grid R that's mounted on the tube socket. Pins 4 to 6. Measure the resistance of all 4 and post the #'s you get. Should be 470 ohms. Those R's are carbon comp and will start to smoke when heated up and can/will catch fire.
And you need to change out
every e-cap. The electrolytic paste inside dries out over the years. They can short out to ground and draw massive current. After 20/30 years, their done, taking a chance leaving them in. F&T's are very good caps at good price, many guys use those, I do. (See at the very end.)
And the B+ dcv power supply dog house board has a few dcv dropping R's on it, 1 of those could have smoked. When you change the B+ filter caps change those R's, their CC (carbon comp) to flame proof metal film R's. Same for the 6L6GC screen grid R's. And the 1K5 grid stopper R on the tube socket, pins 1 to 5. A lot of heat there and if the tube shorts could cause a fire with CC.
will have a tube tester
Buying a tube tester can be/is a discussion on it's own. I don't know much about them, never owned 1. Many of the used 1's need to be calibrated.
Will check vdc and vac from rectifier and bias to relevant tube sockets using schematics and layout y'all kindly provided.
I'd rebuild that rectifier board too. Those solid state (SS) diodes are pretty old. Use new 1N4007's. Ultra fast are nice, UF1N4007, they switch on for a shorter time and so are a little quieter because of that. For pretty much same price now a days, I use them, not a must though. Don't forget to change the e-cap in the bias balance circuit.
An amp needs a full load, all tubes in, to give the correct acv/dcv measurements. Unloaded all readings will be high, sometimes very high.
(Will look for something near -52vac going to balance thingy.)
That amp has a -bias balance pot, not a -bias adjust pot. It it to try and get both sides of the 6L6's that are working in push/pull to draw the same current on each side. There's 2 6L6GC tubes on each side. Class AB PP, when 1 side is on, push, the other side is off resting.
I would add a 25KL (linear) pot after the 15K R that is hooked up to the -bias balance pot's wiper. Then you can set the -bias balance and set the overall -bias. Read up on negative bias.
And when you check/measure for -bias, you always measure for it at the tube socket pin. Always!
DO NOT put power tubes in until you know for certain you have enough of a -dcv at the tube socket.
Only question - looks like bias acv from TR1 is above 20% "tolerance." Assume that transformer windings are stable over time and it likely has been like that but do you gurus see it differently?
We don't use the term 'gurus' here.
And TR1 we call PT, power transformer.
Tube amp transformers are very stable the years. Your getting a high reading because the PT is not loaded, it's not doing any work, not under any strain. Think of a car going up a steep hill, engine is under strain, it will slow the engine down.
That 20% tolerance note, in most/pretty much all cases, is way larger then what you really see.
Our host Doug, has the F&T caps in his on line store along with many/most of the parts you'll need. And his shipping is very fast.
https://hoffmanamps.com/MyStore/catalog/parts4.htm
Topic: Fender Silverface Twin Reverb Repair - 1973 (Read 7628 times)