Marshall DSL40CR farting

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CapnDenny1

CapnDenny1

Member
Joined 2016
#1
I have one of these amps to fix for a customer. The complaint was it sounded like water gurgling thru the peaker. It is basically a low freq rumble, like a wavy lp. I determined it is past the preamp section. Even with all knobs on zero.

I tried different tubes and the same thing. I checked the bias currents, and for the high power setting it is ight at 33 mA. There is a low power setting, part of the Standby switch, they swith the B+ to 150v. The C version they had a Pentode/Triode switch that changed the screen grd voltage only to 150. But the CR version they appear switch the voltage to the transformer center tap, and the screen resistors to 150v. They also change the bias voltage and increase the bias current to about 85mA. But with the plate voltage at 1/3 the normal value, you can have 3x the bias current and the tube is at the same power dissipation.

I guess that is a clever way to make sure you get tube breakup even at the low power setting? Clever guys there at Marshall.

I ound some talk on the Marshall forum tht these amps had this issue when first released. They said there it was linked to internal cable routing too near the reverb effects board inside. I will try contacting Marshall, but they usually just say take it to an authorized service center. They don’t mind fessing to them, but the general public doesn’t have a need to know.

I was just wonderig if anybody has seen or heard this kind of issue, and what the cuse was. The only time I heard something like it, was a power tube issue. I tried different tubes already with no help.
 
#3
Not yet. I don’t have a schematic so it’s slow going.

I took the board out and gonna check for poor solder.

I verified removing thephase splitter tube makes it stop.

I will lift the coupling caps from the PI tube to see what happens.
 
#4
I sent an email about this to Marshall. They responded, which surprised me! They told me to replace the 1M resistors in the PI circuit.

44567231410_bdf7d67ba6_h.jpg
Untitled by Dennis Kelley, on Flickr

44567231410_bdf7d67ba6_h.jpg
Untitled by Dennis Kelley, on Flickr[/IMG]

And the the cap on the right of the one 1M resistor. We shall see.
 
#5
No they did not. They told me R141, R148, C139. I did that and it fixed the issue. It was in the “Resonance” circuit. It takes the negativefeedback signal and uses it to apply a load to the B+ power supply. I think that’s what it does anyway.

I found the schematic by searching for its sister amp, the DSL100HR. Same circuit and pcb, just has 4 power tubes.
 
#9
CapnDenny1 said:
I have one of these amps to fix for a customer. The complaint was it sounded like water gurgling thru the peaker. It is basically a low freq rumble, like a wavy lp. I determined it is past the preamp section. Even with all knobs on zero.

I tried different tubes and the same thing. I checked the bias currents, and for the high power setting it is ight at 33 mA. There is a low power setting, part of the Standby switch, they swith the B+ to 150v. The C version they had a Pentode/Triode switch that changed the screen grd voltage only to 150. But the CR version they appear switch the voltage to the transformer center tap, and the screen resistors to 150v. They also change the bias voltage and increase the bias current to about 85mA. But with the plate voltage at 1/3 the normal value, you can have 3x the bias current and the tube is at the same power dissipation.

I guess that is a clever way to make sure you get tube breakup even at the low power setting? Clever guys there at Marshall.

I ound some talk on the Marshall forum tht these amps had this issue when first released. They said there it was linked to internal cable routing too near the reverb effects board inside. I will try contacting Marshall, but they usually just say take it to an authorized service center. They don’t mind fessing to them, but the general public doesn’t have a need to know.

I was just wonderig if anybody has seen or heard this kind of issue, and what the cuse was. The only time I heard something like it, was a power tube issue. I tried different tubes already with no help.
Click to expand...
I bought the same amp 2nd hand and it was fine for a few months. It's been back to Marshall twice for this issue and they say it's because of a gyrator in the circuitry to improve the bass response. Apparently, it can't be fixed. I've changed all the preamp valves and it's still there so I'm going to try the output valves next. It's not acceptable though and it's definitely getting worse. I'm not an electrician and I don't repair amps so I'm not qualified to judge whether this is acceptable behaviour so should I just be expected to live with it? Marshall kindly haven't charged me anything, but I'm going to send it to an engineer I know who used to work for Marshall and is building his own tube amps now. Thanks, Rob.
 
#11
It is a common issues with some of those amps.

I see two solutions, one that I would not apply (substitute the gyrator with a wah inductor) and one that IME is an improvement (use the old resonance circuit with the pot in series with the feedback and a cap across it).
 
#12
I've had the amp looked at by another tech who also builds valve amps. He looked it over, changed the output valves and it was fine for a couple of weeks, but the problem is back again. I was going to get some new preamp valves, but Ives noticed on the spec for this amp that it require 4 ECC83 and V4 has to be a balanced valve. What is balanced, why does this matter, and will it cause a problem if I just fit 4 standard valves instead of 3 standard ECC83 valves, without the one at V4 being balanced?

Thanks

Rob
 
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