1981 jcm 800 2204

cap217

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I have an 81 2204 that I am making sure is good to go. I am going to do the jel mod to it but wanted to question a few things vs 6550 amps.

Bias split resistors - 150k, not 220k like el34. I read that the lower value with el34 is fine and some people like it. But then I read some posts that say it’s reduces hogs a little at 150 and another saying that it reduces lows at 150k. What is the impact of each value here?


Bias voltage resistor - 15k. What effect does this have?

Bias resistor - 47k. I know on my other 6550-el34 I would piggy back this to get the amp into bias range for el34. But this amp is biasing fine. I had to actually lower the bias to the lowest setting to get it to bias where my other amps I wouldn’t be able to get it high enough. So this is why I am posting and confused.

Amp is 385 plate voltage that I have biased at 43ma which is 65%. Is this the proper voltage for a 2204? Or should I expect higher?
 

Pete Farrington

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When reporting voltages inside an amp, it’s a good idea to check and include the mains voltage, measured at the wall outlet. If that’s off, all voltages inside the amp will be too.
And measure the heater voltage, across the heater supply, eg 6.3VAC.
If you’re only getting 3.xVAC, move the meter probe from chassis 0V common to the other side of the heater supply.
Rather than measuring output valve anode voltage, I suggest to measure the HT voltage, eg at the HT fuseholder.

I recall that some mid to late 70s 50 watters did have a lowish HT voltage, under 400V.
 

cap217

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When reporting voltages inside an amp, it’s a good idea to check and include the mains voltage, measured at the wall outlet. If that’s off, all voltages inside the amp will be too.
And measure the heater voltage, across the heater supply, eg 6.3VAC.
If you’re only getting 3.xVAC, move the meter probe from chassis 0V common to the other side of the heater supply.
Rather than measuring output valve anode voltage, I suggest to measure the HT voltage, eg at the HT fuseholder.

I recall that some mid to late 70s 50 watters did have a lowish HT voltage, under 400V.

Wall voltage is 117v through a brown box.
 


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