1982 JCM 800 2204 problems

[frankeg]

Voltage Questions: I installed the 1 ohm resistors on the pwr tube cathodes and they work fine. The amps overall voltage seems low: I have 594 vac rms (840 pk) going into a full wave rectifier and have 390 vdc out or roughly 30 vdc lower than the math (1/2 the pk 2ndary voltage for full wave). The diodes check out good with a fluke tester and the amp works fine. The PT is a Drake 1202-324/3. Does this bode well with those more experienced here? The amp was loaded (tubes in) when the measurements were taken. If all is well it means biasing for 60-70 % at 390 vdc plate would take 40-50 ma. I have 6550's (new) rated at 35 watts max. Again does this seem correct or am I missing something?

[Chuck H]

That seems about right for that amp. Marshall distributors never set up the 6550's to run at 35 watts each. Off the floor those amps were usually biased at about 50 to 60%. That's one reason for the poor acceptance of this change to 6550's and a big part of their bad rep for guitar (though IMHO EL34's do sound better anyway). Biased cold 6550's will sound very hard and unyielding. Still, the 6550's can sound good when biased up hotter like you have them. If it sounds good then use the tubes as they are biased now. It should be fine but I doubt you'll ever get 35 watts per tube out of that amp. I think you can expect wattage in the low 60's though.

Chuck

[frankeg]

Thanks Chuck for the quick reply and info! I really appreciate what you and the other "wise" ones here offer to those of us learning the ropes! frank

[Ronsonic]

Voltage Questions: I ... have 390 vdc .... If all is well it means biasing for 60-70 % at 390 vdc plate would take 40-50 ma. I have 6550's (new) rated at 35 watts max. Again does this seem correct or am I missing something?

Had to clip there.

390 - 405 is normal for that series. I suppose you're thinking "that's less than a Deluxe Reverb?!" It is.

40 mils or so is also normal for that series.

I like doing old Marshalls on the 'scope, old schools style @ 400Hz for a clearer picture than 1K. Sorta traditional.

[frankeg]

Thanks Ron! Yea Fenders and especially the Deluxe can be pretty hot. I was thinking that with the 6550's, Marshall would have run them pretty hot too but as Chuck said they didn't. I haven't developed the knack yet to distinguish clearly the cross-over notch so I rely on getting close with the current and tweeking with my ears. This is the 1st time the OT shunt method failed me but I do like the 1 ohm resistor trick! BTW this amp may sound thin at low vol but she really smokes when you turn it up. I'm practicing the "old school" guitar vol trick which is about all you can do to clean her up - it seems... A one trick pony I guess but a rocker for sure!

[Robert M. Martinelli]

...I was thinking that with the 6550's, Marshall would have run them pretty hot too but as Chuck said they didn't.

Chuck's right...

AFAICR they (Marshall) were then having reliability problems with EL34s, so decided to use four 6550s because they were (and are) capable of matching the performance (power-wise) of four EL34s without sweating.

As you have already found, to switch "back" to EL34s you need at least to reduce the bias voltage (EL34s need a voltage in the -40V range) and this can be accomplished easily enough by modifying the bias voltage divider.

Hope this helps

Best regards

Bob

[frankeg]

Thanks Bob. I'm going to stick with the 6550's for now since I bought a new set. It really is a fun amp to play with a HB equiped guitar.

[frankeg]

FACEPLATE: I got a new faceplate from mojo and it's about 1/16" off. The original was metal which I removed. I needed to cover a previous modders numerious holes. The new one is a flexable plastic/rubber type... Lined up to the original holes it is offset to the left - translating to all the holes fit except the input jacks. Although it's flexable, I couldn't stretch it enough. My best guess is to enlarge the input jack holes in the chassis. I don't think I could heat and stretch the faceplate or drill larger holes there without damaging it or marring the finished side. Any suggestions?