Help I'm a Newbee, DSL 100 Problems

[blues666]

My amp started to loose power and basicilly sound like crap, hum appeared also,I changed the power tubes and and rebiased the amp, sounds great now and power is there, hum still there although its not loud enough to hear when playing, my other marshall is plugged in to the same power surge protecter - no hum.
also changed preamp tubes- no difference.
I also should mention that the dumb stereo/mono switch on my cab went out and basicilly went from 16 ohms to 4 while I was playing it, amp problems started around the same time or shortly after.
Any help or suggestions would be greatly appreciated since I live in a area where there is no tech nearby and would have to ship it off to be repaired.
-Dave

[Chuck H]

Dave,

Assuming there is hum with no guitar plugged in...Power up the amp and pull the first preamp tube. if the hum is still there, pull the second preamp tube, etc. If pulling a preamp tube stops the hum then the problem can be isolated to that part of the circuit. If pulling all the preamp tubes does not stop the hum then the problem is in the power amp circuit. The amp losing power is very suspect. I wouldn't be surprised if it happens again with the new tubes you installed. If the hum indeed wasn't there before, then there is a componant failure that caused the first problem and that componant is still bad and could likely cause another failure. If the hum is in the power amp circuit (my guess is that it is) you should check the bias supply, bias supply feed resistors, power tube coupling caps and B+ filter cap for the plates. Probably some other stuff I'm forgetting now. Check for ground faults as well as componant integrity.

Also check the impedance selector on the amp. Make sure the switch isn't toasted and there are no ground faults. After that...Well, I suppose there could be a problem with the OT. But since the amp is working and making sound, I doubt it. If you don't find anything obvious, take voltage measurements and post again.

HTH

Chuck

[blues666]

thanks

thanks Chuck, It'll be a few days before I can have time to look at it again, I'll repost what I find, thanks
-Dave

[Amp Kat]

These amps have a 1 ohm current sense resistor from Cathode to ground. Make sure it is not open . When the cabinet switch went it is possible that a resistor fried as these amps are very sensitive to tube and output failures. Check all of the resistors around the output tubes and also make sure the bias on both sides is correct. Pots PR1 and 2 set each sides respectively to around 70 to 80 mv's depending on your plate voltage.