Marshall JCM2000 DSL100 low power weird distortion

[samgraysound]

Hi,

Trying to fix a Marshall JCM2000 DSL100 head. The amp was on for about 5 hours in standby then in the middle of a take lost most of its volume and became all fizzy.

The amp currently has a max output of 2v rms into a 16ohm load. The output on the oscilliscope has some extreme crossover distortion.

Tracing the signal it looks fine until after the power tubes.

Two of the power tubes are all burnt and a little melty around their base. I put in four good tubes and the problem persists. Two of the new tubes now have the color burnt off their logos.

Plate voltages and grid voltages look fine, about 475v, and -33v on all sockets (with tubes in). Heaters are at 6.6v.

Did the bias adjust according to the documentation from marshall and had no problem getting to the specified voltage.

I've tested all the resistors around the power tubes and they are fine. The grid stoppers have been changed from 220k to 6.8k.

What am I missing? Schematics attached.

 

[samgraysound]

Could it be a parasitic oscillation out of the audible spectrum?

 

[Khron]

Is the transformer still ok?

 

[Junction]

Is the transformer still ok?

Does sound like a loading problem, either the transformer is cooked or could even be a problem with the speaker, have you matched the speaker with the transformer output impedance ...  measure the resistance of the speaker with your multimeter and make sure the impedance switch is set the same.

 

[samgraysound]

Does sound like a loading problem, either the transformer is cooked or could even be a problem with the speaker, have you matched the speaker with the transformer output impedance ...  measure the resistance of the speaker with your multimeter and make sure the impedance switch is set the same.

I've been using a dummy load with the 16 ohm output which is unswitched. I did basic continuity tests on the transformer but it could be something weirder. There is a spare OT transformer available so I will try swapping in that.

Sam

 

[CJ]

probably shorted turns in the output, is it a Dagnall with the masking tape around the secondary?

China makes some good stuff, unfortunately, magnet wire is not one of them.

Fender twin reverb OPT will last forever,  slight Z mismatch but better than breaking down in the middle of song, put another dagnall in there and you will get the same hing down the line, 

look for a Drake on evilbay, have somebody rewind your bad one with Essex wire,