Marshall jcm 2000 dsl blowing ht fuse.

[Jerm]

Hello techs! I am working on a marshall jcm 2000 that had volume issues. I noticed that R9 was cracked and burned. I replaced it, put the power board back together and turned the amp on with a bias rite to set the bias. after a min i turned standby off and noticed that one side was pulling almost double MA than the other. One side was reading 35ma and the other 65ma. I leveled it out to 34ma both sides. after about 5 mins i notice the plate voltage dropping from 483 to 0v. that is when i notice the HT fuse blew. I cant get the amp to run without blowing that fuse. Whats next? please help! Thank you in advance!

[Enzo]

DSL what? DSL201? 401? 50? 100?

Take the old power tubes out, you most likely have a bad one. RUn the amp with no power tubes and see if the voltages around the empty sockets look reasonable. The bias voltage doesn;t fade away for example. And see if the amp keeps a fuse that way.

Most blown tube amp fuses are due to failed power tubes.

[Amp Kat]

^^^ This and check the 1 ohm resistor from Cathode to ground. There are two on the board 1 for each side and if they are open it will redplate. Check the socket that is running away and see if it has continuity from Cathode to ground. If they are ok and the tubes are ok put a meter on pin 5 and see if the bias voltage fluctuates when it goes into runnaway and if it does it could have a bad coupling cap.

[Jerm]

Ok amp is up and running! 1ohm resisters are reading 1ohm. voltages are stable. now while i still have you guys here, would one of you know of a credible " how to " website that has alot of information on tube amps? Biasing, testing, modding ect?

Also what would be a good bias point for this marshall? its a dsl 100 to answer the earlier question.

I really admire what you guys do. You are always so helpful and informative. Thanks again.

talk to you soon.

[Jerm]

hmmm. the ma readings on power tubes are looking weird to me. I hate the dual adjustment pots. outer tubes are reading 35ma inner are reading 15ma.
is this normal? any suggestions

[Enzo]

Outer/inner?

Why it is this way I will never know, but looking from the rear with the two little controls and three pins staring you in the face, the control on the right adjusts the two tubes on the left, and the control on the left adjusts the two tubes on the right. Two on the let, two on the right, not inner/outer.

Have you determined that all four tubes work, and that all four power tube sockets are working?

[Jerm]

The outer two (end tubes) are reading 34ma and the two in the middle ( middle tubes) are reading 15ma. should I worry about the variance of ma?
as far as I can tell, the amp is running stable. Power tubes work and power sockets are working ( I say that based on playing on it at high volume ).

[Amp Kat]

What happens if you swap the tubes around so does the low current follow the tubes are the sockets ?

[Jerm]

I decided to just use my dmm instead of my bias rite because the readings were all over the place and never consistant. with dmm i was able to get steady mV readings of 90 on each side. safe range for a dsl 100? I have a scope and a noise osciliater but never learned how to use them. sad....

[g1]

The DMM readings were taken off the 3 pins? Sounds like you have unmatched tubes. Like Enzo said, the test points show the sum of the 2 tubes on the left and the sum of the 2 tubes on the right. Why your DMM reading don't match the bias probe readings is odd. (34 + 15 = 49mA for left pair and same for right pair).
Maybe try rearranging them so the inner pair is on left or right side and outer pair is moved to the other side, then see if they are more evenly matched.

[Jerm]

The 3 pins in the middle of the two bias pots. thats where i decided to take my readings. Thanks. i'll try that and see if it levels out.

[Jerm]

Hey guys sorry for the delay. Switched tubes around and the variances followed with the tubes. As I moved them to different sockets the numbers followed the tubes. I went ahead and put a new set of matched tubes and was able to see even readings. Thanks for all the input!