JCM 800 2205 power tube issues...

Rob Bergman

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I did a really stupid move the other day. Was playing my JCM 800 2205 and then put it on standby after i was finished. But forgot to turn the power switch off, and left it idle on standby for 4 days.. Now its cooked. I tested the power tubes, one gets a bias reading of 44mv, the other is zero. Originally the tubes were biased at 35mv matched. The amp only has a very quiet signal now. The tubes are ok as i switched them around and get the same readings. Even tried new power tubes and there's no change. Preamp tubes were also replaced to rule out issues.

Capacitors: C3 and C7 in the signal chain were microphonic and were replaced. Still no change. Seems like the power tube reading zero is not getting bias power. Does anyone know what might cause this issue?
 

scozz

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Rob Bergman

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Interesting posts. Hopefully my beloved amp will be fixable.... I feel like such an idiot
 

marshallmellowed

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I did a really stupid move the other day. Was playing my JCM 800 2205 and then put it on standby after i was finished. But forgot to turn the power switch off, and left it idle on standby for 4 days.. Now its cooked. I tested the power tubes, one gets a bias reading of 44mv, the other is zero. Originally the tubes were biased at 35mv matched. The amp only has a very quiet signal now. The tubes are ok as i switched them around and get the same readings. Even tried new power tubes and there's no change. Preamp tubes were also replaced to rule out issues.

Capacitors: C3 and C7 in the signal chain were microphonic and were replaced. Still no change. Seems like the power tube reading zero is not getting bias power. Does anyone know what might cause this issue?
Do a visual inspection of all the components around the non-functioning power tube. You may have a fried resistor that's gone open. If you can't see anything, you'll have to start measuring the values. Also check all internal fuses.
 

Rob Bergman

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Ok thanks. Fuses are both ok. I’ll take a closer look around the power tube
 

Gunner64

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I'd see what I had for a measurement at pin 5 of the offending tube socket. If wonky I'd look at the resistor there at pin 5. R60, or R61, depending on the tube. 5.6k. May be open as mentioned above.
 

Rob Bergman

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I'd see what I had for a measurement at pin 5 of the offending tube socket. If wonky I'd look at the resistor there at pin 5. R60, or R61, depending on the tube. 5.6k. May be open as mentioned above.
Thanks so much. Amp will be on the bench tomorrow and i will let you know what it reads. Hopefully its just a cooked resistor and not a transformer.... I was told to test for parasitic oscillation but the meter read zero when in AC so its not that i don't think.
 

Spanngitter

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If you left it just powered on with Standby active then there is only heaters running and you could get cathode poisioning on the valves.
I doubt that there had been any damage to grid resistors or even output or power transformer as there could be no activity at all in the circuitry due to missing B+
So said, pls measure voltages on the power tube sockets and if there is Bias Voltage measureable at Pin5 you might have plainly a poisoned/dead power tube...
 

Gunner64

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He said he switched the valves around and the problem stayed at the same socket, he said he even tried new valves with the same results.

I wonder how he is measuring bias. A probe or 1 ohm resistors at the sockets?
 
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Pete Farrington

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Yes, I was thinking that the ‘no current’ valve might have some sort of spurious failure to its screen grid feed.
Whatever, that shouldn’t near mute the signal, so as the other valves are ok, something additional is amiss, and so some tech time is called for.
I suppose it might just be that a layer of oxide built up on the shorting contact of the fx loop return socket.
 
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Rob Bergman

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So in this pic the power on the right is the problem one getting no reading. Those big rectangle white resistors might be the issue? The one on the left reads 1.0k like it says on the label, the right one does not read. Would that do it?
 

Rob Bergman

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He said he switched the valves around and the problem stayed at the same socket, he said he even tried new valves with the same results.

I wonder how he is measuring bias. A probe or 1 ohm resistors at the sockets?
1ohm at the sockets. And they test exactly correct
 

Pete Farrington

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The one on the left reads 1.0k like it says on the label, the right one does not read. Would that do it?
The valve in the right socket won't conduct any HT current, may as well not be there.
There's no reason being left in standby would cause it to fail, as standby switch the HT current off anyway.

That bad resistor wouldn't cause the very quiet signal issue.
 

Spanngitter

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Those big rectangle white resistors might be the issue? The one on the left reads 1.0k like it says on the label, the right one does not read. Would that do it?

This means that this valve will not work properly as it misses the screen supply.
However, I doubt that this resistor failed due to the amp being 4 days on with heaters on only.
When replacing those I would recommend to avoid the white bricks and get a proper 1k 5W Welwyn, Arcol or similar round / enameled type instead on both, R62 and 63
 

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Rob Bergman

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Ok thanks so much. Would this cause the amp to have next to no sound? It only plays at talking level volume….. wouldn’t it just play at normal volume running on the one power tube?
 

Rob Bergman

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This means that this valve will not work properly as it misses the screen supply.
However, I doubt that this resistor failed due to the amp being 4 days on with heaters on only.
When replacing those I would recommend to avoid the white bricks and get a proper 1k 5W Welwyn, Arcol or similar round / enameled type instead on both, R62 and 63
Ordered some arcol 1% should be here in a few days. Now just have to figure out what is making the amp quiet. When turned on at first it is loud for about 5-10 seconds then goes quiet
 


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