JCM2000 DSL Silver Edition problems.

[g1]

Back in post #5 you had -58VDC of bias coming out of D9 & D10. Do you still have that? Was that with or without the power tubes installed?
I would think that should be sufficient to run the tubes fairly cool. If you don't have that anymore I would try replacing C43 and C44.
It's better to look for a fault right now rather than modify a possibly defective circuit.

[gtrplayr1976]

Back in post #5 you had -58VDC of bias coming out of D9 & D10. Do you still have that? Was that with or without the power tubes installed?
I would think that should be sufficient to run the tubes fairly cool. If you don't have that anymore I would try replacing C43 and C44.
It's better to look for a fault right now rather than modify a possibly defective circuit.

Replacing R68 to 33k got it I believe. I now have 62mA per side to start off. I want to let it warm up before I adjust it. I will get it to about 80mA per side ,and let it go at that if it will stay steady. The schematic I have shows it as a 33k , but there was a 22k in it. The plate volts seems a little high at 512 ,but that should come down when I bring the bias up.

[gtrplayr1976]

I have it at about 80mA per side. I know Marshall says 90 ,but I had seen a video from Eurotubes that shows setting it at 80 may be better ?

[g1]

I would agree, but it's important what the plate voltage gets to. It must have dropped from the 512V ?

[gtrplayr1976]

I would agree, but it's important what the plate voltage gets to. It must have dropped from the 512V ?

It's down to 500. Should I go up some on the bias ?

[Enzo]

80, 90, doesn't much matter, it isn't that critical.

Just be aware that current and plate voltage are interactive. As you reduce the current by increasing bias voltage, your B+ will rise. And vice versa. So you don't just take one plate voltage reading and start adjusting.

[gtrplayr1976]

80, 90, doesn't much matter, it isn't that critical.

Just be aware that current and plate voltage are interactive. As you reduce the current by increasing bias voltage, your B+ will rise. And vice versa. So you don't just take one plate voltage reading and start adjusting.

Plate seems to be consistent on all. Maybe I'll bring it up to 90 to reduce the voltage some. Just seems hot for el34s?

[Enzo]

I thought you said it dropped from 512 to 500?

What kiils tubes is dissipation, not voltage. EL34s are rated to something like 800v on the plate anyway, though the screens need to be lower than that.

[gtrplayr1976]

I thought you said it dropped from 512 to 500?

What kiils tubes is dissipation, not voltage. EL34s are rated to something like 800v on the plate anyway, though the screens need to be lower than that.

Yes the plate voltage is at 500, maybe a shade under. That is with the bias set at about 80mA per side. Screen voltage is 478. Maybe these are high due to our wall voltage ? It runs at 128vac.

[Leo_Gnardo]

Maybe these are high due to our wall voltage ? It runs at 128vac.

High supply voltage sure would explain it. FWIW I set 2000 series 100W at 70 mA per side, just a smidge past the point the crossover notch disappears.

[gtrplayr1976]

High supply voltage sure would explain it. FWIW I set 2000 series 100W at 70 mA per side, just a smidge past the point the crossover notch disappears.

I'm considering setting it through my regulated supply at 117 ,and tell my friend to get one if his wall voltage is as high as mine.

[Enzo]

Again, why worry about it? This isn't NASA. You said you had it down to 62ma a side, right, 31ma per tube? That is less than 16 watts dissipation per tube. If you feel lacking in confidence in it, set the thing as though the mains are at 125, and if they are ever lower, well, it will run a watt or two cooler.

The EL34 is a 25 watt tube. if you go by the internet canon of 70%, then 17.5 watts is your target. Thinking about that, 90ma is 45ma per tube, and at 500v, that is 22.5 watts a tube, way too hot. Even 40ma is 20 watts. Assuming B+ holds at 500v.

REgulating the mains is a solution for a problem that doesn;t exist.

That is just my opinion, however.

[gtrplayr1976]

Again, why worry about it? This isn't NASA. You said you had it down to 62ma a side, right, 31ma per tube? That is less than 16 watts dissipation per tube. If you feel lacking in confidence in it, set the thing as though the mains are at 125, and if they are ever lower, well, it will run a watt or two cooler.

The EL34 is a 25 watt tube. if you go by the internet canon of 70%, then 17.5 watts is your target. Thinking about that, 90ma is 45ma per tube, and at 500v, that is 22.5 watts a tube, way too hot. Even 40ma is 20 watts. Assuming B+ holds at 500v.

REgulating the mains is a solution for a problem that doesn;t exist.

That is just my opinion, however.

If I run it at 62mA (31 per tube) then the B+ goes up above 500. It is at 500 ,or just below with 80mA per side. As you said these will handle 800 volts on the plates , but the screens are at 478. Isn't that too high ?

[Jazz P Bass]

If I run it at 62mA (31 per tube) then the B+ goes up above 500. It is at 500 ,or just below with 80mA per side. As you said these will handle 800 volts on the plates , but the screens are at 478. Isn't that too high ?

Too high for what?

'Most' amps have the screens riding darn close to the plate voltage.

The screen tap (and the rest of the preamp) runs through a 270 ohm 7 watt resistor.

I would say it was wise of Marshall to knock down the screens a bit.

[Enzo]

It is good to 800v with appropriate screens, but you are 300v lower. I mentioned 800v only to point out that the voltages you have will not threaten the tube. Don't focus on 500v as magic, focus on the dissipation instead, as that is what will melt your tube. Your screen resistors 1K or more?

There are milions, or at least many thousands of MArshall amps with your 500v on EL34 plates and screens not terribly lower..