Marshall 6100 problems.

[StratTone]

I'm working on a 6100 for a buddy. He said it would would in pentode but not triode so I tested the ceramic resistors first and sure enough R252 and R253 were open so I took the board up and damn all 4 of the 220u 400V caps solder joints were broke. What I need your help on is that this amp has some different components than what the schematic calls for??? What is up with that. The amp has in it 5W 1k resistors and the schematic says 470ohm. The caps are 220u 400V but the schematic says they should be 47u 450V. I must have the wrong 6100 schematic but I can't find another one anywhere. Any ideas?

[HTH]

I'm working on a 6100 for a buddy. He said it would would in pentode but not triode so I tested the ceramic resistors first and sure enough R252 and R253 were open so I took the board up and damn all 4 of the 220u 400V caps solder joints were broke. What I need your help on is that this amp has some different components than what the schematic calls for??? What is up with that. The amp has in it 5W 1k resistors and the schematic says 470ohm. The caps are 220u 400V but the schematic says they should be 47u 450V. I must have the wrong 6100 schematic but I can't find another one anywhere. Any ideas?

I'm assuming the 470ohm resistors you're referring to are the screen grid resistors. I seem to recall that Marshall were still using 5881s in many amps round the time of the 6100 Anniversary head, so the 470ohm screen grid resistors would make sense. However, you will harm nothing by keeping the 1k, 5w screen grid resistors in there - thats what you'd want with EL34s anyway, and with 5881s it'll just protect the screens a little more.

The 47uF, 450v caps I'm assuming will be smoothing caps on the B+ line. Maybe the 220uF caps were installed to beef up the filtering on the output stage ???

If you could link to a schematic and list the part numbers on the schematic, that would help.

[NickH]

I'm working on a 6100 for a buddy. He said it would would in pentode but not triode so I tested the ceramic resistors first and sure enough R252 and R253 were open so I took the board up and damn all 4 of the 220u 400V caps solder joints were broke. What I need your help on is that this amp has some different components than what the schematic calls for??? What is up with that. The amp has in it 5W 1k resistors and the schematic says 470ohm. The caps are 220u 400V but the schematic says they should be 47u 450V. I must have the wrong 6100 schematic but I can't find another one anywhere. Any ideas?

I think I know this one, The 6100 started out life using EL34's and were modified by Marshall to 5881's for a few reason's,(If I remember correctly, it was something like reliability for the US market & shortages of EL34's etc. etc.)

I am in the middle of converting a 6100 back to EL34's so I have the link to the Marshall conversion guide handy, this should show which components were changed for new values and what the new values are..

5881_EL34conversion.pdf

Cheers



N

[drewl]

Wow, nice little document, it explains why I've seen a cap/resistor in series with the feedback tap on certain JCM900's.....but not why.....I'll have to study this further to understand their reasoning.

[StratTone]

Ok, sorry for the long delay. I have been working on this 6100 on and off here for a while. First when I got it, it would only work in pentode. Took the main board off and it had 3 open wire wound resistors, and all large voltage caps were loose and needed to be sucked out and reflowed. I played it for a good hour and not problems. Got it back from the customer and now it is blowing the main fuse everytime. Took the power supply board out and reflowed about 5 bad solder joints and still blowes. So then I started by testing the power tubes. 3 had bad grid leakage but no shorts. So I then took them completly out to see if it would still blow and it did. Then I checked all the caps on the power board, cleaned up and or replaced any coroded wires, checked all resistors on the board as well as diodes and can't seem to narrow this one down. Any help would be appreciated.