JCM 900 problems

[Teleman55]

I have a JCM 900 Model 4500 50wt. to fix.The customer bought the amp with no tubes and installed EL34,s.It worked for a while,became noisey and crapped out.I found R30 was burned and can't read the value.I believe it should be 22k 1wt ?.The schematics I have found are confusing-Help.R28 is 56k which makes me think this had 5881's.R30 is on the mode switch,what could have caused this to burn up?

[52 Bill]

I found R30 was burned and can't read the value.I believe it should be 22k 1wt ?.

There are schematics for 2 versions at the Dr.Tube site, one shows that resistor as 22K 1 watt Metal Oxide, the other shows it as 10K 1 watt Metal Oxide.

R28 is 56k which makes me think this had 5881's.

Both versions of the schematic show R28 as 47K

R30 is on the mode switch,what could have caused this to burn up?

R30 feeds the power supply to the driver and pre-amp tubes. Maybe a bad tube or a shorted cap?

Check the amp out and see what version it is supposed to be and look up the correct schematic. The Dr. Tube site has very readable ones there.

Hope this helps.

[Teleman55]

Bill-Those are the schematics I finally referenced.I found the bias pot was completely turned to one side,checked the bias caps and they are fine.I installed 1ohm resistors to set the bias and a 22k at R30,powered up and found that the bias maxed at 24mv with 432v at the plates.I replaced the R28 with 47k and set the bias at 39mv.All seems well.The owner has no manual,so who knows,maybe the mode switch was between positions?The bias was not optimal,maybe wrong speaker impedance or no speaker when powered or a combination of these things.I will give him a web address to get a manual.