5E3 Overheating/Bias Issue?

[Chuck H]

Problems like an unwanted current drain will almost never be in the preamp signal chain. Which is what you are talking about rewiring. Probably moot since the voltages you report don't seem to correspond to a fault in the circuit drawing current. HV will usually be dragged down a lot more in when that happens.

Please comply with post 41 above, and one more thing...

If you pull all the tubes from the amp and take voltage readings at the rectifier output with the amp in standby. Then with the standby switch in play mode at each B+ node.

[Chodrick1]

Alright Chuck, I took those measurement. I have 0V on pin 5 for both tubes.

With tubes removed:
In standby 410 VDC out of the 5Y3

In play mode:
547.3 VDC at B1 and rectifier output
544.5 VDC at B2
542 VDC at B3

[Chuck H]

Well that all looks normal. I just have to chalk this up to modern tubes drawing more current than vintage tubes.?. I know this to be true of el84's but my experience with 6v6's is limited. With el84's, for example, it was common to use a 130R cathode resistor at any voltage for class AB. Vox used a 47R for four tubes in class A. IME modern tubes are way too hot with values like this at relative plate voltages and I usually end up at values around 160R for a pair in AB (very hot AB which is actually biased to around 90+%!) and to keep a Voxish design from meltdown I use 68R (still over 100% dissipation at idle). Again, at relative plate voltages. So if this trend is also true of modern 6V6's it seems reasonable that you may simply need to cool the bias with a larger value cathode resistor. There's no slight to any vintage ideology in doing this. It's more vintage correct to bias the tubes to similar current as the vintage models than it is to leave an arbitrary part value in the amp that biases the tubes much too hot. IMHO. Try the 330R and even a 360R. Ignore the 70% rule as this simply doesn't apply to cathode biased amps. Cathode biased amps are self regulating and will end up too cold at full output due to the circuits functional bias shift with current. Part of the 5e3 sound is a hot bias. I'd shoot for about 90% at idle. And trust me, this will self regulate to a more sane bias at full output and this bias shift is part of the sound. That is, a 5e3 wouldn't sound right if it didn't idle hot. But you're clearly too hot right now.

[Chodrick1]

Awesome! I will have to try the 330Ohm resistor, if I feel it effects too much of the tone in a negative way, I could save up a bit for the Mojotone 120v to 330v transformer. That or try the Zener diodes going from the center tap to ground, I have heard of people doing this with great results. Im aiming for about 370VDC out of the rectifier, but with the 5Y3, I am always above 390v and with the solid state bridge rectifier, I end up with around 410v due to having no voltage sag.