JCM900 Volume Fades

[misterc57]

Just bought JCM900 Dual Reverb 100 watt head a couple days ago.

After playing for a while, the volume drops, sometimes fades to nothing, eventually comes back.

I have noticed this problem in both power modes and in both channels.

It came with new matched JJ EL34 power tubes. The sticker says they were replaced in March 2009 and biased at 33V.

Today I replaced the likely original Marshall preamp tubes with new JJ ECC83 tubes. Afterwhich the volume drop still occured.

The amp has a sticker saying it came equipped with 5881 power tubes and only replace with the same. Is it ok to use the EL34?

I later jumped the effects send/return jacks and played for about 20 minutes. So far the problem has not returned (time will tell). Hopefully that was the issue.

Wondering about the effect level knob next to the jacks. Does it matter where it is set?

Thank you. MC

[acorkos]

Wondering about the effect level knob next to the jacks. Does it matter where it is set?

it matters if you are running effects in the loop and need to adjust the signal level hitting the effect input.

spray some spray cleaner into the effects loop jacks and insert/remove/reinsert your cable plug a few times

[txstrat]

The effects loop jacks can gather dust with time and lose their function. They are usually fitted with switching contacts to bypass the level pot when no cable is plugged in. When you jump the loop the pots setting should matter. Clean the jacks and everything shoud be fine. Pull the power cord before you do...