It would behoove you to get a copy of Aspen Pittman's "The Tube Amp Book". In it is a great deal of tube amplifier knowledge; although much of it is geared toward guitar amps, the concepts, methods, and troubleshooting tips are nearly universal. Another good read is the Radio Engineer's Handbook (Terman 1943), and it has a TON of useful electronics information; it also shows how very little has changed in 60 years, on a fundamental level at least.
Looking at the Sta-Level schematic, the 6V6 output tubes are arranged in a cathode-bias setup, similar to some Ampeg amplifiers. It's not all that common in the amplifiers on which I've worked, as most are grid biased. Notice that 10-watt resistor feeding the balance pot? Yup, that sucker will likely get hot. Anyhow, if it's used for 'balance', I'd
assume all you're looking to do is get both tubes working proportionally in this push-pull setup. Therefore, let's go for the approach of biasing a tube amp, so you can see whether they're balanced using your O-scope (again, a bit of assumption here, so bear with me, and others, please jump in!).
If it's similar to a guitar amplifier (which it appears to be, albeit with differences), what you're
probably going to be doing is dummy-loading the output, apply a ~2000 Hz (pure-) sine signal to the input, crank the device's output to about 70-80% of its nominal operating level (on a guitar amp, that'd be full blast), but before clipping or distortion occurs (if that's even possible with this device, I don't know). Probe the output with your oscilloscope, and change the 'scope settings until you get the waveform on the screen so that you can see about two or three full cycles of the signal. Now, ordinarily with a guitar amp, you'd adjust the bias until the cross-over distortion (a little 'bump' or notch at the zero-volt axis) just barely disappears. But here, and this is an educated
assumption, you're going to be turning that potentiometer until the top and bottom halves of the signal are symmetrical, i.e. they're of equal amplitude; there should be no cross-over distortion, either.
I've never seen anything configured like this, but that doesn't mean much. Heck, I've done guitar & bass amps, and all the effects are usually analog solid-state parts. However, a few things rub me weird. First, this design uses not one but TWO rectifier tubes. I can't honestly think of a reason anyone would actually WANT a rectifier tube in his/her new design, especially in this day & age of cheap and reliable solid-state FWB rectifiers. They don't affect the sound, and only serve to keep the rest of the amplifier fed with clean DC -- beware that on vintage equipment, things can go POP! when only the rectifier tube is changed out for a silicon FWB rectifier. The higher, more-consistent voltage might be too much for some old and crusty components to handle. I'm sure there's a reason that they specify two tube rectifiers, but it's beyond my comprehension (that doesn't go too far, anyway
). It looks like maybe this design was lifted from some existing one, from the 1950s...? Strange that the designer wouldn't at least update it, or at least provide some freshened schematics. The .pdf layout is making my stomach turn....
Second, I'm not certain why this piece of equipment needs that very-expensive 6386 tube. Yes, it has certain characteristics that DO make it desirable (remote cutoff for AGC, low noise, low harmonic distortion, etc.), but wow they're pricey. Almost everything I've worked with uses 12AX7A or equivalents, which, for all intents and purposes, appears very similar: dual triode, low noise, but high-µ instead of the 6386's medium-µ. Again, there has to be a reason...and the implementation of AGC is the only thing I can think of or see. Actually, that might make a lot of sense on a mic amp, now that I think of it. lol
If you want to look at tube data sheets, check out
http://tubedata.tubes.se. They've even got the General Electric sheets for each of the tubes in that design!
Please take everything here with a grain of salt and a swig of whiskey. This is the first time I've seen such a beast, but I'm fairly confident in my procedure outlined. There's no harm that can come from doing it that way either, so worst-case, I wasted your time and I look like an idiot. If it helps, I'm rockin' the DS1052E as well, and I am absolutely loving it. Best Christmas gift I've ever received.
Keep your eyes on the voltages of some branches of the schematic. The highest you'll have to worry about looks to be +332V, with others in the +137 to +282 V range, but remember, these are ballpark numbers, and it's DC, so if it hits you, IT. WILL. HURT. Unlike AC, which (aside from its pleasant tingling sensation) typically will allow you to let go or detach from the circuit because of the zero crossing, DC won't kick you away nicely. IME, it makes your hands grab HARD, your jaw clench SHUT, etc., and can make your heart STOP. Don't be afraid. Just be safe and aware, keep one hand in your pocket, don't prod about with anything but an insulated device (chop stick, high-voltage-rated screwdriver if you must, etc.), wear good insulated shoes, and for the love of Pop-Tarts, don't rest your free hand on anything at earth potential (ground) while probing.
If you don't already have a good DVOM/multimeter, now's the time. Just make certain it's up to handling the higher voltages; this Sta-Level is running at the relative low end of voltages seen in audio vacuum tube equipment , so if you're planning on building any other tube projects in the future, consider looking for a meter & leads rated Category III, for safety.
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