WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I just did an upgrade to the amp. I changed out the oem .47uF 400V caps with Sonicap's .47uF 600V. I bypassed the larger electrolytics with some Jantzen .1uF 400V caps. Replaced the oem power tubes with some winged C EL-34's. Replaced the pre-amp tubes with some Russian 6N3P-EB's. Replaced the oem binding posts with some gold plated ones. Replace the oem rca jacks with some gold plated/shielded type. I just purchased a 100k pot that will be replacing the crappy oem pot. After I install the new pot, I am going to let the amp warm up for at least an hour, and then I will measure the bias once again to see if it needs adjusting.
The first stage is a long-tailed-pair. You might want to check the amplitude balance of the two outputs of the phase splitter.
try adding some 0.1 - 1uF plastic film or paper in oil type capacitors to the big electrolytics so their ESR's are more predictable and controlled.
The 5k output transformers are also a limitation. They are not the greatest. If you want to do some surgery, you can replace them with some Dyna A430's or maybe the 4.2k 60watt push-pull Edcors. Which are fairly inexpensive. Either will be an upgrade. I have some early 60's Thordarson "Super-Iron" that I will be putting in mine soon. The steel chassis has funny mounting centers. You may need to fabricate something to adapt the transformers or drill some holes.
The power transformer is actually over-specified for the B+ and filament currents. If you replace the output transformers, you ought to be able to comfortably run 6550/KT88 type tubes in the output stage. The 70V tap on the power transformer should give you sufficient grid bias to use those tubes instead of the EL34's.
Thanks Walker112 for the info! I have a question regarding the volume pot. When I turn it up and down, it produces noise that comes out of the speakers, and also noticeable are the blue gases inside the power tubes that will glow more and then get dim as the volume knob is turned up and down. I had asked someone about this problem and was told that it appeared to be leaking some dc voltage somewhere. I will also check the flux between the solder joints on the pre-amp sockets as some look like they may cause some problems. One thing that was noticed on the preamp tube sockets is that it doesn't appear to be a factory solder, as if someone did it by hand. Can you think of what may be causing this problem?
Sound like there is some residual gas inside your power tubes. The gas can ionize during operation and cause a blue glow.
Keep an eye on it. Unless you think they are starting to arc, I wouldn't worry about it.
Regarding the noise, are you using a true continuous wire-wound or carbon POT? Or is this some type of step attenuator? A clean continuous POT should not cause "noise".
If it's a break before make type step attenuator, you should make certain that you have a ground path for the grid of the input triode at all times like a 500k-1meg ohm resistor to ground. I think when I traced the schematic out I didn't see a resistor at the input, just the POT after the input selector.
Make before break type switches for the attenuator are better in this regard.