JCM 800 Bias Question

[mikeboone]

I am looking at a JCM800 for a friend and have a question about bias measurement.

First! I lost my bias probe when I moved or I would just do this the old fashioned way. So I removed the HT fuse and placed my Fluke 185 amp meter across the contacts on the back of the fuse holder and I am seeing about 60 mA there. I looked at the schematic of the 4210 model and it shows the HT fuse just before the power tubes so I think I am fairly close to an accurate reading. Tubes are EL34. The plates are 475.

[Jazz P Bass]

Sounds like a nice number.
Not too hot.
Not too cold.

[Chuck H]

That seems a little cold to me. About 14W per power tube plate MINUS screens and whatever the preamp is drawing. You could go old school with a 1ohm resistor between the cathodes and ground to isolate the power tubes. But it's really not significant. Put it on a scope and check for crossover distortion at full clean power and as the amp starts clipping. If you have objectionable amounts of crossover distortion bias hotter (within safe limits).

[mikeboone]

I was thinking of running it up to around 75 to be around 37mA per tube. I started it off running cold to see if there were going to be issues. He brought it to me because the 6550's that were in it were putting on pretty blue light show and making loud noises so I thought it best to put it on my limiter and start a little cold.

It sounds pretty good. I am going to let it run with a test tone for a few hours then start cranking on it a bit.

My question was certainly directed at the fact that I don't have the draw for the pre's factored in. I assumed I was close but like to seek the advice those who know more. I do, however, write all this stuff down so I don't have to ask the same questions over and over.

[Gtr_tech]

My calculator tells me 7.1w/tube. Cold....

[Mark Black]

My calculator tells me 7.1w/tube. Cold....

Only if it's a 100W head...

[mikeboone]

It's 50... Guess I could have tossed that in there.

[Gtr_tech]

That makes a difference then...

[big_teee]

Is it a 4 input or 2, or 4 holer or 2 holer?
I have the 2 holer, and I put the 1 Watt Resistors on mine when I built it.
T

[mikeboone]

Single input jack. Split channel reverb model with MV. I think this is a later model 800.

I ran it up to 37 per and it sounds pretty darn good.

[big_teee]

Single input jack. Split channel reverb model with MV. I think this is a later model 800.

I ran it up to 37 per and it sounds pretty darn good.

At your voltage level, I would think 35-40 ma.
Mine Runs 492v Loaded and I run it 35-37ma.
T

[mikeboone]

Thanks T...