JCM 2000 dsl-50 head repairs-buzzing sound, cutting out, shrill noises

[discocorco]

I know there are a number of JCM 2000 threads up here, however, after searching for over an hour, I couldn't find a thread that answered my questions.

So, when I plug the guitar directly into the FX loop, the guitar sound doesn't make the shrill noises or cut out, however, there is still a continuous hum.

the Hum
persistent, no matter guitar cable or speaker cable, or whether or not the guitar is plugged into the amp or not.

Shrill noises
Intermittent, only when plugged into the input, sometimes the sound drops out, then comes back

Bias readings
It adjusts both tubes were sitting at 72ma
However, It seemed like the bias was dropping out completely and coming back. Not sure if it was my own clumsiness with the voltmeter or if it was actually cutting out.

All the pre amp tubes light up.

So, what else do I need to check for before I just go buy new pre amp tubes? Do these symptoms describe the need for a new printed circuit board? Could it be just the input jack? How would I best test the input jack?

 

[discocorco]

Everyone can rest easy, I figured it out. I appreciate all the input.

1. I found a footswitch to plug into the the reverb selection switch. Apparently, turning the reverb down to zero doesn't shut the reverb channel off. The switch does. That is where the humming was coming from.

2. I took a pencil and tapped the top of the pre amp tubes. I don't know why this helped. But when I tapped the last one from the left, the persistent buzzing ended.

3. I was playing a bass thru the head for about five minutes then the sound cut out again. Then I put a patch cable between the fx out and fx input. Suddenly, it wasn't cut out anymore.

Once again I would like to thank everyone who took the time to read my post and give their feedback.