One of the issues with this amp was trying to get the correct schematic. Some stuff can be fixed without one. But when you are tracing through the circuit looking for a noise source or an anomaly, the schematic, the correct one is really needed. I could find the old one, but there were things on this amp, in the area where the problem was, that were way different from the old schematic. I looked an looked but could only find the older version, which from what I read did not have this issue.
Once I fixed it I was really curious as to why that would work, so I wanted even more to get a schematic. It occurred to me that Marshall also makes this same amp in a 100w version. In fact the extra power tubes have places on this amp's circuit board where they would be added. I assume a bigger power transformer and output transformer are also needed, but perhaps they use the same ones on both amps? So I did a serch for a schematic for the DSL100HR, which is the 100W Head version of this amp. And to my delight it was out there. Someone on Music Electronics was looking for the same on I was, and someone first sent the old one. But then later in the post someone provided the latest schematic. It actually had the parts that Marshall told me to change. I wish I had figured that out a bit sooner, it would have saved me some troubleshooting times and grief.
Here is the section of the schematic that has the parts I replaced that seemed to fix the issue. R141, R148, and C139. They are in the section with a pot called "Resonance". It is interesting. They are taking the negative feedback, which is also adjusted by the "precense" control, in the Phase Inverter section, Tube V4 in the upper left. That type of Precense control is pretty standard. But the Rosnance control is something new. It appears to use the negative feedback signal from the speaker out, to actually load down the B+ supply. That's the HT1 signal. So I guess this causes the B+ to droop a little, which causes the output to droop, which causes the Resonance signal to let the B+ go back up. So it is a way to add global reverb sort of? Perhaps this makes it behave more like a tube amp that uses a tube rectifier? But you can adjust it to be more like a solid state rectifier, or a tube rectifier, or somewhere between the two.
The effect must have had a low frequency waver in the output caused by some weird heating effect in the resistors R141 and R148, and cap C139. I don't like to sell myself short, although I do it all the time, but I doubt I would have been able to figure this one out? Maybe, but it would have taken a lot longer.
Thank you Marshall for refusing to help, but then helping me anyway! And I repay that by divulging their new secret circuit? Just the kind of guy I am, I can't keep a secret. I just thought it was interesting as heck, and sometimes new tube amps have some new innovative stuff going on. Now that I know where the issue is, I need to make sure it doesn't come back with other settings of that pot.
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Dennis Kelley, on Flickr