Biasing JCM800 help!!!

[grtamp]

I'm adjusting the bias to a JCM800 of a friend but the voltages and current consumption are strange! however, the amplifier function well!

The bias voltage from -66.5 to -62.2V ( for EL34 must be less?)
Plate voltage 476V

current measured( 1ohm res. from pin.1/8 to ground) 00.7mV this is strange to me!!!

My check mode is wrong?

Thanks

[Phil_S]

You should have ~75mA per tube at idle. Bias voltage should be around -36. It is possible you have the tubes biased so cold they aren't generating much of anything.

[Jana]

75 ma per tube is way too hot! It should be more in the range of 30 to 35 ma per tube.

[Phil_S]

Sorry, I was looking at the KT66 data sheet earlier. I got mixed up. Jana is right about the mA.

[grtamp]

I'm sure , the bias voltage is -62Vdc (the bias trim is at min.)and the current of power tubes is 0.7mA with 476 Plate and screen voltage, with no signal input.

Bias Voltage would be about 42/43V in marshall amps with EL34?

There are crossover distorsion also , I tryed to connect the scopemeter to my dummy load.

[marcoloco961]

I'm sure , the bias voltage is -62Vdc (the bias trim is at min.)and the current of power tubes is 0.7mA with 476 Plate and screen voltage, with no signal input.

Bias Voltage would be about 42/43V in marshall amps with EL34?

There are crossover distorsion also , I tryed to connect the scopemeter to my dummy load.

Maybe a bad component in the bias circuit? Have you checked the values? Something might have drifted. Are you measuring the -62V on pin 5, or elsewhere in the bias circuit?

Also, I am not understanding the .7mA. .7 would = 700mA, .0007 would = .7mA.
At 700mA you would fry tubes in seconds. At .7mA I doubt the amp would make any noise. How are you measuring?

[grtamp]

Problem solved....was a bad component in bias circuit , the 56K res. measured with DMM give me 115K value.
moreover this res. should be 47K and not 56K!! This is strange , the res. is an original parts!

Now the crossover distortion go away and the current for power tube is 34mA with 476V plate voltage and -47V bias voltage!
Thanks all

[marcoloco961]

Problem solved....was a bad component in bias circuit , the 56K res. measured with DMM give me 115K value.
moreover this res. should be 47K and not 56K!! This is strange , the res. is an original parts!

Now the crossover distortion go away and the current for power tube is 34mA with 476V plate voltage and -47V bias voltage!
Thanks all

That sounds much closer to what you would expect. Rock on.

Does the amp seem louder than before?

Sometime people will change the value of the 56K bias resistor so they can get the tubes a little more headroom if the range is limited using a 47K. It all depends on where you are targeting your bias for the power tubes. Not everyone likes it the same.

[grtamp]

now the amp sounds more louder than before , it's much appreciable now!
Thanks all!