Marshall DSL100H weird bass response

BoomTexan

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I just got an amp that I'll be repairing for a friend. It's a Marshall DSL100H, and the issue is that the bass response is off. The owner put some new tubes in, and V4 is reading a terrifying 1.2V with bias probes. Not mV, V. I don't know how that's possible, maybe his multimeter was just malfunctioning. V3 is reading 47mV. The other two are fine, with readings of 50mV.

Upon looking inside, the first thing I noticed was that several solder joints on V2 and V4 were burnt. Also, resistor R67 (100 ohm 2W) blew under testing. The owner said that he bought it new 2 years ago, and that the only mod ever performed was a capacitor clip on the ultra channel, which doesn't affect the preamp signal. When he runs the preamp signal out to a power amp, the sound isn't there, so it's definitely the power section. Apparently the only thing wrong with it is the obvious tube bias, and when cranked, the bass response is weird.

I'd be willing to bet that the bass response is caused by the obviously dying or dead V4 providing a significant wattage mismatch at higher volumes, causing AB power to be mismatched and creating a weird distortion effect. I'm planning on replacing at least the bias cap and doing some serious adjustments and rolling tubes to make sure that V4 isn't just caused by a bad tube. Past that, I'd put money on failing filter caps, but if anyone else has experience with the DSL line of Marshalls or knows what could be causing this, please let me know your thoughts.
 

dan40

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Some of the older DSL series amps had issues with the bias circuit but this amp is only two years old so that shouldn't be an issue. Do you see any redplating occurring in the v4 power tube? With the bias reading that you reported, that tube should be redplating. I would measure your bias voltage at pin 5 of each power tube to see if every socket is showing the correct negative DC voltage. If the bias voltage is good and the tube is not redplating, there may be an issue with the 1 ohm current sensing resistor in the amp that is causing a false reading.
 

BoomTexan

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Yeah, I'm actually pretty sure the issue is with the 1 ohm resistor. The old one for the V4 was completely blown, measuring infinity on my multimeter. Just put in a new 2W metal film resistor and it should be just fine. I'm gonna check bias and reconnect the mod he did to the capacitor and see how it sounds.
 




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