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Vintage threads from the first ten years
JCM900 retube
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 6/2/2000 8:17 AM |
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Gibson |
JCM900 retube I am retubing a JCM 900 100W head. There is some confusion about the power tubes that go in it. I was told by more than one source that it uses 5881's, but the guy I bought the amp new from said you can tell the tube type by visual inspection. As far as I can tell, it had EL34's in it. Also, I am not sure if I need to rebias the power tubes,(I bought a matched quad of Svetlana EL34's), since I am replacing with the same kind of tubes. The amp has a bias pot. I know a fair amount about electronics but am somewhat unfamiliar with tubes, I am assuming they work more or less like FET transistors, and the more negative the bias, the less cathode to plate current. Can the bias voltage be set without the tubes in place? Unfortunately I don't know the tube supply voltage. I don't have an oscilloscope or a dummy load. I do have a multimeter, and I could put together a sine wave generator. I got Sovtek 12AX7WB preamp tubes so I may have some distortion before the power stage. Any advice would be appreciated. The nearest authorized tech is a long trip away, and I would be lucky to escape without lossing my wallet. |
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| 6/2/2000 12:16 PM |
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| Ray Ivers |
Gibson, The schematic I have indicates the output tubes are EL34's (a relatively tall, thin tube - the 5881's are shorter and thicker), but that doesn't mean that there aren't any 5881-equipped JCM900 amps out there. The place to check is inside the chassis, on the rear board over the output tubes. If you find 470 ohm, 5 watt resistors connected to pin 4 of the output tube sockets, it's a 5881 amp; if you find 2.2K ohm resistors, it's an EL34 amp. The schematic indicates -38 to -40 volts bias (you are correct about the FET-like biasing behavior of tubes), but please see Randall Aiken's web site (www.aikenamps.com, technical section) for more info on amp biasing. Ray Ivers R.A.G.E. Electronics |
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| 6/2/2000 4:35 PM |
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| Tonefactor |
They came with both types, and can use either with a minor mod to the bias supply.
Yep.. just like JFET's.
Most people just bias them by adjusting the idle current with no signal at the input. You'll want each duet to draw around 70mA of current at idle. You can measure this by putting a current meter in series between the cathode and ground, or by installing a 1 ohm resistor between cathode and ground and measuring the voltage drop across it which will be equal to the current draw, due to ohms law.
These are the right type of preamp tubes for the amp. | ||||
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| 6/3/2000 5:10 AM |
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| Gibson |
Does anyone know the cathode to plate voltage in this amp? I measured the resistance of the bias pot and it was about 18K out of a possible 20K. So the amp was biased almost as negative as possible. I am reluctant to fool with it. I read the Aiken articles, and it sounds involved and I don't want to monkey with the circuitry. Will the tubes last longer under-biased, (more negative)? Also I tend to keep it to conservative volumes for my hearings sake, so wouldn't I be able to get away with under-biasing if I keep to lower power levels,(5dist.,7clean)? One of the old tubes is visibly darkened, it was probably the worst of the quad. The amp has transformer fuses to prevent its damage during tube failure. I put my multimeter across the tube power supply caps and they still had considerable voltage in them,(~200V), but they quickly discharged through the multimeter. This suprised me since the amp hasn't been plugged in for a week or more. Why don't they put discharge resistors across the caps to discharge them within a reasonable time,(~10meg)? |
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