Where are JTM45 Ri bias points
- Thread starter gjhawkins
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Oh no one answered this yet?
The JTM 45's have no adjustments from outside the chassis. There is a trimpot on the circuit board that you have to turn with a very small flat head screwdriver. There are no test points either, you must get a bias probe. You can do the 1ohm resistor method but I highly recommend the bias probe. It will make your life much easier. They can be found for very cheap at Eurotubes
There are more expensive ones out there that are easier than that one and more convenient but for a man with a budget that's what I got. They just plug into a standard multimeter. $25 USD
The JTM 45's have no adjustments from outside the chassis. There is a trimpot on the circuit board that you have to turn with a very small flat head screwdriver. There are no test points either, you must get a bias probe. You can do the 1ohm resistor method but I highly recommend the bias probe. It will make your life much easier. They can be found for very cheap at Eurotubes
There are more expensive ones out there that are easier than that one and more convenient but for a man with a budget that's what I got. They just plug into a standard multimeter. $25 USD
MajorNut1967
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Absolutely not! The RI's are majorly differrent then the originals, mostly for international safety code reasons and also for pratical reasons. One of the main differences is the old ones used either 5881 or KT66 power tube, I believe some of the RI's even had EL34's in them definately not a JTM45 sound? The other main reason is the OT, the RI's do not use the RS Deluxe Output Transformer which is key to the old JTM 45 sound.
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MajorNut1967
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Just needs a little rebias, that's all. It is possible that a bias resistor might need to be changed but that is not certain, you won't know for sure until you try to get the new tubes within the right range. As far as the bias current of 40mA, that isn't necessarily true. Before setting the bias you need to know the plate dissipation of the tube and plate voltage of the amplifier and work those into an equation. The plate voltage needs to be read every time a bias adjustment is performed. The plate voltage will change whenever the tubes are changed, resistors, etc.
(plate dissipation of tube / plate voltage of amp) x 0.7 = typical bias current in milliamps.
This result is only a generalized number, typically where crossover distortion ends and that is what is generally accepted as the sweet spot. Adjust up or down a few milliamps to taste.
Ok, now I've changed the tubes from 5881 to KT-66 and rebiased the JTM 45RI.
But something came up:
- I messured platevoltage to 360v before and 351v after adjusting. This seems low? Is this normal or would it help to change the rectifing tube as well?
Another tubeamp guy I talked to said the voltage is maybe 100v off. In that case, how does that effect bias (and the wear of the tubes)? According to my calculations, I adjusted to 45mA, but it should be lower if my friend is right.
Any thoughts on this? Please?
But something came up:
- I messured platevoltage to 360v before and 351v after adjusting. This seems low? Is this normal or would it help to change the rectifing tube as well?
Another tubeamp guy I talked to said the voltage is maybe 100v off. In that case, how does that effect bias (and the wear of the tubes)? According to my calculations, I adjusted to 45mA, but it should be lower if my friend is right.
Any thoughts on this? Please?
