Marshall 8100 valvestate tube problem help - diyAudio
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Old 16th September 2011, 05:18 AM   #1
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Default Marshall 8100 valvestate tube problem help

Hello there, i have a Marshall 8100 head and i have a few questions about it. I bought this thing used and the only problems where that 2 pots where broken but they still worked. i only couldnt turn them. It was the treble in the overdrive section and the volume in clean i think. so i replaced those but my sound didnt sound the same. It changed a bit. Well i tried today to replace the 12ax7 tube with a tungsol 12ax7 tube and the overdrive channel volume dropped completely. Could barely hear it. worked fine on the clean channel but not in the overdrive channel. I put in my old tube and the volume went back to normal. I read some forums online about this particular problem but they didnt help much. But what i have noticed is that my amp makes crackling noises sometimes and my distortion is not as great as when i got it. So i though i would replace the old tube. Can it be that tube is no good or that my amp doesnt work well with tungsols? i read on some forum that some guy tried the tungsol and got the same problem as me but then tried a MESA tube and worked great. Also, i noticed that the big block resistors near the tube(heater supply) get very hot. using both tungsol and original Marshall tube. im not even playing and have the volume all the way down and the resistors get very hot that i cant even touch them. is that normal?

or is there something wrong?
the original marshall tube glows ...seems normal as i seen on other amps with tubes...but the tungsol does not glow...

any help on what to check or do would be helpful...thanks
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Old 16th September 2011, 07:34 PM   #2
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i forgot to mention. the LEDs 6 and7 light up when i am in the overdrive channel and playing. but LEDs 4 and 5 never light up. not even while on clean or crunch.

I dont know if that is related to any of the problems i am having that i mentioned before.
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Old 16th September 2011, 11:09 PM   #3
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
When the only difference between working and not working is the tube in the socket, then the tube is not working. The only thing I see is that the second triode is a cathode follower, which leaves a relatively high voltage on the cathode. SOME 12AX7s are not happy over a certain voltage difference between cathode and heater.
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Old 17th September 2011, 02:59 AM   #4
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Thanks for the reply. As for my clean channel, it is not working correctlly. does not sound clean at all and making alot of noise. switch from clean to crunch does not seem to do anything. And as mentioned before the LEDs 4 and 5 are not working. Any idea on what the problem is?
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