Help needed with my Marshall

bluesbend

bluesbend

Lovely BIG Starologist
I'd like you amp guru's to help me out if you can.
I was given a JCM2000 DSL401 and was told that the output transformer is shot.
When plugged in the power tubes glow but the pre-amp tubes don't and there is no sound from the speaker.
I'd like to know which of the transformers in the pic I googled is the output.



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Re: Help needed with my Marshall

It's the smaller one on the far right. Yours won't look like that, though, since the amp in the picture has been modded with Mercury Magnetics transformers.
 
Re: Help needed with my Marshall

Thanks Rich, that's what I needed to know.
I've contacted Heyboer and they need the bolt spacing so they can wind one for me.
 
Re: Help needed with my Marshall

That model (I know I have one) runs very hot. Like can't touch the front panel hot. Mine bit the dust after maybe two years. I'm pretty sure it was the output transformer. If you get it working keep a fan on it.
 
Re: Help needed with my Marshall

bluesbend said:
I'd like you amp guru's to help me out if you can.
I was given a JCM2000 DSL401 and was told that the output transformer is shot.
When plugged in the power tubes glow but the pre-amp tubes don't and there is no sound from the speaker.
Click to expand...

The glow of the tubes is a product of the heaters. This has nothing to do with the output transformer. If the heaters are working for the power tubes but not for pre-amp tubes that is a problem not with the output transformer. Heater circuit supply for preamp and power tubes should be the same source. The power transformer.

If the heater supply from the PT was faulty then they should be both out. This makes me think it is something only for the pre-amp tubes such as a faulty ground or something. Do the tubes mount directly to the printed circuit board? It could be the pcb.
 
Re: Help needed with my Marshall

Yeah. If the preamp tubes don't glow the output transformer is not your (only) problem.
 
Re: Help needed with my Marshall

The tubes look like they are mounted to the pcb.
I've not tried plugging into the effects return yet.
 
Re: Help needed with my Marshall

I plugged into the FX Return and there's no sound at all.

What next?
 
Re: Help needed with my Marshall

Well, I tried switching the pre-amp tubes around and now they all glow, still no sound from the amp though.
 
Re: Help needed with my Marshall

bluesbend said:
Well, I tried switching the pre-amp tubes around and now they all glow, still no sound from the amp though.
Click to expand...

^^Bad connections or bad traces on their PCB. You should get a PM. You can substitute a known good 12AX7, one at time, for each 12AX7 position starting with the phase inverter and working toward V1. If its just a bad preamp tube and if the good tube suddenly makes the amp work, then it was that tube. Usually with a bad power tube and the other one good, it will still make sound.
 
Re: Help needed with my Marshall

The chances of this being an OT issue are really, really low. Unfortunately, since your preamp tubes won't light up, there isn't any backyard method of determining if it is problematic. I'd suggest you take it to a qualified tech and have them determine which 10 cent part is causing this rather than blindly throwing parts at it...you'll be money ahead if you do.
 
Re: Help needed with my Marshall

Are you absolutely 100% sure its the OT?
One thing ive learned on this forum is that whatever glassman recommends is probably what you need to do.
 
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Re: Help needed with my Marshall

You guys do know that some brands of 12ax7's dont glow right? That the filaments are encased so they dont light up? I still think he needs to take it to a tech but the non-lit preamp tube thing might just be the brand
 
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