I have a torn down Laney VC50 I'm converting to a 1987 plexi with 6L6GC's. I have quite a few laying around so I don't bother with EL34 for the time being as money is a little tight at the moment. The B+ is 440V unloaded, so maybe 420-430 at idle. The OT is 3k3 with 4, 8 and 16 ohm taps, a bit low for 6L6GC. If I double the loads, I get 6k6, which looks fine loadline-wise. A 6k6 OT also makes the loadline hit well below the knee, so a nice amount of screen compression is possible. I'm considering using a chunky 1k screen node resistor in place of a choke to further accentuate screen compression.
Is a 6k6 load with ~430v B+ risky in regards of arcing/flyback in the OT? I've not seen many builds 6L6GC builds going much higher than 4k2-4k5.
worst scenario: you are risking in destroying the output tubes with playing long time on high output power. because your anodes will swing low and your g2 will pull all the electrons in it , which would be more attracted to g2 high voltage then anode voltage of 50volts in that moment.. so current through g2 will be high and your 1k resistor will fail with time.. g2 is not designed to accept all anode current so.maybe tube will fail.or you will find your 1k resistor below the socket unsoldered from thw
circuit with the heat
6k6 load with those voltages is not risky in any way. As load impedance gets higher plate current goes down and screen current goes up. With 1k screen node resistor and series resistors with screen grids there's no way the screen dissipation will ever get too high.
but how i allready mentined , all the dissipation will be taken by g2 resistor, heated up too much and fail over time . or will be desoldered by heat like i've seen many time before
At full power output the screen grid current for two 6L6GC tubes at these voltages is going to be about 25mA probably even less. So let's say the current through the 1k power supply resistor is 30mA. Oh noes that's 0.9W dissipation in that resistor. How in the world will a resistor survive such a heinous dissipation without desoldering itself and emitting enough green house gases to destroy the whole planet?
bepone wrote: ↑Mon Oct 11, 2021 11:59 am
worst scenario: you are risking in destroying the output tubes with playing long time on high output power. because your anodes will swing low and your g2 will pull all the electrons in it , which would be more attracted to g2 high voltage then anode voltage of 50volts in that moment.. so current through g2 will be high and your 1k resistor will fail with time.. g2 is not designed to accept all anode current so.maybe tube will fail.or you will find your 1k resistor below the socket unsoldered from thw
circuit with the heat
bepone wrote: ↑Tue Oct 12, 2021 9:44 am
but how i allready mentined , all the dissipation will be taken by g2 resistor, heated up too much and fail over time . or will be desoldered by heat like i've seen many time before
As I mentioned, that 1k screen node resistor is supposed to go in place of the choke, and as usual there's gonna be the usual 470R/1k screen grid stoppers. The screen node "choke" resistor will be a 25W, like the one in the TW Express, so it should hold up just fine. When screen current tries to increase, the voltage across on the screen node will fall due to increased current through the 1k/25W. Lower screen voltage squishes the G1 0V knee down to (or below) the loadline, which lowers screen current. So I think screen current should be held well in check this way. With a standard choke supply that has minimal voltage drop, I could see 6k6 becoming a problem for the screens though.
Lauri wrote: ↑Mon Oct 11, 2021 2:40 pm
6k6 load with those voltages is not risky in any way. As load impedance gets higher plate current goes down and screen current goes up. With 1k screen node resistor and series resistors with screen grids there's no way the screen dissipation will ever get too high.
Thanks Lauri, that's what I was suspecting as well. Excessive screen current is easy enough to prevent, I'm more concerned about what happens on the other end of the loadline, so to speak. I've read a lot of vague or contradicting info about spiking and even arcing in the OT when using higher impedances than the standard Marshall 3k4 or Fender 4k2. At the same time, not much is said about what conditions are needed to get into trouble, so I felt the need to ask directly about my setup. Thanks anyway.
for the other side, depending how good is insulation in the transformer..usually OT are tested on 4kV breakdown voltage. peaks in guitar amp can be in kV in OT specially if you have bad connnection on the cable, jacks, speaker connection. then breakfown in OT depending on humidity, insulation quality and thickness (0.3mm standard between P and S is not enough, and i'm putting 0.4-0.6mm in my transformers for this reason)
