trouble with burning screen resistor

[Luthierwnc]

put a 1k resistor in for the choke and nearly fried it. It did stay on long enough to hear the whine of the signal generator become intermittent. I'm guessing a bad OT. If so, I'll probably do a complete overhaul since the layout wasn't as good as I do it now. Thanks, sh

[SoulFetish]

If you do end up overhauling the amp, I strongly suggest considering changes to the design. From what I gathered, the amp is trying to do what it should be doing, given the component values and operating voltages here. Using the lower 35mA idle current for some of the calculations, (just to err on the conservative side),
here is what the amp is working with, and correct me if I'm wrong.
•360V on the plates (350-358V on the screens?)
• 250Ω cathode resistors per tube.
250Ω*35mA providis a grid bias of -8.75V. EL84s rated at 12W max dissipation, this would put each EL84 over 100% at idle,
• 100Ω screen resistors
• 5k5 p-p primary impedance
Take a look at this data:

Screen Shot 2020-03-19.png

The red line is the load line, and it extends way beyond the safe operating range.

Here are the approximate calculations for these operating conditions.

Screen Shot 2020-03-19_2.png

[Luthierwnc]

I dropped a 1k 10w resistor in place of the choke. It was an old-school hollow ceramic unit. The enamel coating started to bubble but it held its integrity long enough to check a few things. I'm going to desolder a bunch of connections and check the filter caps, resolder some of the preamp connections and root around the hard way. IMH experience, at times like this, it is probably something that looks fine and isn't. Thanks again and if any other ideas come to mind, please keep them coming. Cheers, Skip

[Luthierwnc]

Had some spare time after lunch. The big resistor on the second node gets the screen voltage down by about 13 volts. The waveform at the speaker jack looks fine at very low volume. At about three on the dial, for either channel, the whine of the signal generator starts cutting in and out when the rail resistors get hot. I would think it was a cold joint but I've checked those at least three times -- and this is a really simple design. The cathode resistors don't get hot. Draw seems to be mis-matched tube dependent rather than which side of the trannie.

Still stumped. Cheers, sh