Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TheKT88KilledJFK on November 06, 2017, 09:58:01 am
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I have a test power amp setup that uses the dynaco mk3 power & output transformers, but using the typical fixed bias el34 power amp section of a JCM800 2204. It's class AB push-pull 2xEL34 at the moment.
B+ sits at 520vdc.
The primary of the OT is 5k a-a.
When calculating the class B current (z=5k/4) ,520VDC/1250R I come out with 416mA.
When calculating the class A current (z=5k/2) , 520VDC/2500R I come out with 208mA.
When looking at EL34 plate characteristic chart, tracing the class B line puts nearly all of it over the 25w line. Isn't this a problem? There are plenty of old JCM800s running 500+VDC Va, so I'm left to believe I've made an error once again!
Does biasing shift BOTH the class A & class B lines down? It doesn't look like the amp ever reaches class A operation with such a high plate voltage.
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Yes, the B load line puts the tube over power, but when it switches off (on the other part of the cycle) it is under power again, so the power condition averages out.
Biasing the tubes to run deep in Class B can help with tube longevity.
What fries EL34s (and OTs) in these amps is high screen voltages. If you run the screens at 380-400V regulated, the EL34s will last longer.
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Yes, the B load line puts the tube over power, but when it switches off (on the other part of the cycle) it is under power again, so the power condition averages out.
Biasing the tubes to run deep in Class B can help with tube longevity.
What fries EL34s (and OTs) in these amps is high screen voltages. If you run the screens at 380-400V regulated, the EL34s will last longer.
Unfortunately, The straight class A load line runs tangent to the crest of the curved Power limit line so I don't know if it's operating in a safe way for the valves. I'll check the screen voltage, but I think it's going to be about 14mA*1kR so Va-Vrs(voltage across screen resistor) =Vs (screen voltage) which is 520VDC-14VDC= 506VDC.
This is a condition present in most older marshalls, is it not?
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It is, but those Marshalls also eat tubes!
EL34 datasheets show full-signal screen current around 25ma per tube for voltages like this.
You actually lose more voltage across the screen resistor if you're in class AB or B, since 25ma average is (potentially) 50ma for half the cycle and 0ma for the other half. You lose 50v for 470v effective screen voltage, average current of 25ma... that's 11.75w of screen dissipation! EL34s are rated for 8w.
If you're closer to class A, drawing current more evenly throughout the cycle for the same average, you lose even less voltage from the screen resistor (more like 25v for typical voltage of 495) and dissipation is even higher.
Regarding plate dissipation, the issue is not if the loadline ever passes the 25w mark, but if it's above 25w average throughout the cycle. In class A, you're probably fine if it just hits that maximum at one point, because it's usually less.
In class B, it could be as much as 50w since its 0w for the other half of the cycle. In practice, you'd probably want it to be less than that, but anyways, the 25w line has very little relevance in those situations.
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It is, but those Marshalls also eat tubes!
EL34 datasheets show full-signal screen current around 25ma per tube for voltages like this.
You actually lose more voltage across the screen resistor if you're in class AB or B, since 25ma average is (potentially) 50ma for half the cycle and 0ma for the other half. You lose 50v for 470v effective screen voltage, average current of 25ma... that's 11.75w of screen dissipation! EL34s are rated for 8w.
If you're closer to class A, drawing current more evenly throughout the cycle for the same average, you lose even less voltage from the screen resistor (more like 25v for typical voltage of 495) and dissipation is even higher.
Regarding plate dissipation, the issue is not if the loadline ever passes the 25w mark, but if it's above 25w average throughout the cycle. In class A, you're probably fine if it just hits that maximum at one point, because it's usually less.
In class B, it could be as much as 50w since its 0w for the other half of the cycle. In practice, you'd probably want it to be less than that, but anyways, the 25w line has very little relevance in those situations.
Thanks man!