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posted earlier, and removed tubes checked bias still sending too much into the tubes, no cracks on soldering around pots. what else could this be?
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Can we be more specific than "sending too much into the tubes?" I don't know what sky high and through the roof refer to.
Your bias voltage is way high, as in the -50v at each grid is more like -75v? or do you mean the reading at the test point is too high, indicating too much CURRENT through the tube? That is usually the result of LOW bias voltage.
Please provide whatever voltage readings you are using and where in the amp you took them.
I am not sure what version of the amp you have. What EXACTLY does it say on the serial number tag for a model? And for that matter, printed on the amp does it have extra letters, like DSL100 XX or something. And what is the board number for hte circuit board in your amp? Something that looks like 62-02-02.Education is what you're left with after you have forgotten what you have learned. -
There has been an issue in the past with resistors used in the bias circuit having negative temp coefficient so these R's did drift a lot when the amp warmed up. To ensure that this is not the case I would just plainly replace the bias resistors (R68=33k, R77=10k, R67=220k, R69=220k) with 1W metal film resistors.
While having the amp open also check if it still has the 220k grid stoppers R7, R10, R66, R70 installed. Replace them with a piece of wire and solder 5k6 resistors directly to the socket (pin 5), this will remove a huge blanket from the sound...
cu
bluesfreakI can fix everything, where is the duct tape?Comment
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I have a question regarding the 220k grid stopper resistors.
I have an issue 1, board JCM2-60-00-00 tube board first generation DSL 50 from 1997. A lot of the schematics floating around on the net are for the later amp, more specifically, issue 7, JCM2-60-00-02.
The later schematics specify 5.6k grid stoppers while the schematic for the earlier amps specify 220k. It seems that both values are correct, depending on the version that you have.
Even though the schematic calls for 220k, would there be any benefit from swapping them out for 5.6k? Some people say that it can make a big, positive difference in tone but it there something we're missing here? Marshall must have chosen 220k for a reason.Comment
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hey guys thanks for the input on the amp, i got the new motherboard and its running as should. alohaComment
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The 220k Grid Stopper had been introduced due to the poor quality of the tubes available back then. As now the tube quality has improved a lot again you can change them to 5.6 without any issue. I would recommend to solder them directly to the socket pin and replace the 220k Resistor on the PCB with a wire jumper...
cu
bluesfreakI can fix everything, where is the duct tape?Comment
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Can you tell us what version board you originally had in there? i.e. Issue 3, JCM2-00-02?Originally posted by kalaheo View PostComment
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