Marshall JCM 900 4100

[jmcgeorge]

I have a JCM 900 I put new tubs in it and rebiased it and eveerything was going good. I was at practice and it was working great the I hit my EQ that was pluged in through the effects loop and half way through my lead the valume seemed to drop about half. I checked the fuses to see if I blew one but the fuses where fine. Can anyone help?

[Chuck H]

Just because you biased it doesn't mean the tubes were all drawing correct current. Just that the total current read correct. That is, unless you checked current on each tube.

A few things are possible. One or more of your new tubes may have failed, you could have dirty socket pins, you may have blown a screen resistor, etc.

Some info that may help here would be, Why did you replace the old tubes? and How did you bias the new ones?

Chuck

[jmcgeorge]

I bought the amp off Craigs List. The tubes that where in it where not mached and they sounded bad. I spoke with Bob from Eurotubes and he said that it sounded like I had bad tubes. 2 of the tubes where Groove and 2 where Sloviks. I replaced them all the pre-amp tubes and power tubes there all JJ The power tubes are 6L6s. I checked the plate voltage and it was 480 and set the bias to 36mA. I check it in one outoutside tube and one inside. I was told that they where paired inside and outside.

[Enzo]

You have the three little bias test point pins poking out the hole just below the power tubes, right? Witha little adjust control on either end?

If you look at the rear, the pin and control on the left work the RIGHTHAND pair of tubes. The pin and control on the right are for the LEFTHAND pair of tubes. The two left tubes are wired together, and the two right tubes are wired together.

Pull all four power tubes. Plug ONE tube into the end socket. Use the appropriate control to bias the one single tube up to whatever current. Remember the reading should be half what it would be with all the tubes. Now remove that tube and try the next.

FOur tests later you will have checked each of your power tubes. If all four work about the same, they are OK. If one is way off or won;t work at all, then it is bad.

Now, take one good tube from the set, and do the same thiong in the next socket over. Work OK? Then try the third socket, then the fourth. You have now tested the circuit of each socket.

These two procedures will - in a very non-technical way - tell you if all your tubes are OK and the sockets as well.

And remove the EQ pedal and its cables. Still the same? We need to eliminate external factors like that.

If your amp does not have the test points, and you are using a bias probe, do the same tests using that. Your tubes are still wired together the same way as above - two on the left together and two on the right together.

[jmcgeorge]

Thanks, I'll try it this evening, but I think there's only one bias trim pot I might be wrong and I was told that with the power tube that they where paired by the two outside and the two inside.

[Enzo]

OH, I get the models all confused in my head. Maybe I am picturing a TSL100 or something. If you have one bias control, then so be it, but you can still test all your tubes and sockets. The procedure I suggested is very simple and doesn't require working in the amp. here in the shop I woulod use different methods.