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Attached are the manuals...

Ok, I think I see it now. For example, suppose the output transistors on the positive rail are in (or near) saturation. Little voltage across them, but that puts almost the total rail voltages across the output transistors on the negative rail, which are cutoff. Output signal is near positive...

I should have added 70 to 76 vac, which would give rail voltages between 99 and 107 under ideal conditions.

Sorry, I'd just like to pull on this thread a bit. Which transistors are being subject to the full + and - rail voltages? Worst case (I see) is either the + rail voltage or the - rail voltage. Not both added together. Secondly, dirty power - isn't that dirty power just coming through the...

Nope - can't find it. Secondary goes from 55v to 60v to 110v - nothing between 60 and 110. How about rails of 170 volts? Eliminate the transformer altogether and go direct to the 120v (actually this puts the transformer outside your house on the power pole). Is that just too hot? Could the...

Yep, I get it now - you're right. When I looked at toroidal transformers a couple of years ago, I seem to remember some that greater voltage output on the secondary. I'll do another search.

This explains it better: Mathematics of tiling

Hmm. I wonder if the transformer needs to be 75 volts to be able to put out say 400 watts. 400 watts at 8 ohms is about 57 vac (or 800 watts at 4 ohms). Rectified (with no load) a 60 volt transformer will produce a DC voltage of 85 volts. I would think this will still be adequate. As the...

Einstein

How about using two of these - one for the left channel & one for the right. Avel Lindberg Y236904 Mount them left and right to balance (in weight) the unit. Still would be a heavy mother.

Just follow Joe's advice and use film capacitors in the signal path. Most of the polarized electrolytic caps in your equipment are there to bypass the power supply, but I understand that this requires looking at the schematic and being able to discern which are which. Using a bipolar...

Ideally you'd like your source signal to be as free of harmonics & noise as possible. That's what the HP8903B does, i.e., it's output signal has very low noise and THD so that when measuring a DUT the readings are attributed to the DUT. That just makes everything easier. But you could measure...

So the schematic that was posted shows 515vdc at point A. If this is correct, then the transformer would be 515*0.707 = 364 vac (assuming no load). I think the 350/0/350 would be just fine as tubes will work over a wide spread. However that first cap is shown with a working voltage of just...

George, does (or can) the QA400 account for the harmonics of the generated sine wave such that it can take it out for the UUT (unit under test). I know nothing about the QA400, so this is kind of a general question. Back in my old testing days, we'd first account for the harmonics of the...

Always start with a visual inspection with everything unplugged. I use my nose too, to check for anything toasted. Check fuses. If that looks and smells ok, then I'd bring it up on a variac and dim bulb tester while monitoring the power supply voltages. Everything begins and ends with the...

Two thumbs up from me. Love it.

PS - also clean all the tube sockets (each and every pin) and the tube pins (each and every).

DC on the grid is normal, but it depends what polarity and how much. That's how tubes control how much plate to cathode current flows. Generally, grid bias is a couple of volts negative - but it depends on the tube and the application. This amp does not have a rectifier tube. I'd start by...

Maybe this: Toroidal shield

The nice thing about toroidal transformers are their lower "hum field". You might also consider mumetal shielding around it. Cured a problem I was having although steel will work too. It's a function of the strength of the field.