1998 Marshall JCM900 Head with an AC hum

[mmmguitarman]

I have a 1998 Marshal JCM900 100 watt Head that I have used for over 20 years with no problems. It has been in storage in a nice dry temperature regulated area for over 6 years not being used at all. I brought it to operating temperature with a Variac slowly and there is a very noticeable, what seems to me, to be a loud AC hum. I plugged my guitar into the head and played through a 1999 JCM900 1936 Marshall 2 by 12" cab and through the hum it sounded good. I tested all the tubes on a Model 707 Dyna-Jet Tube Tester and two of the four power tubes are very weak, Two of the 12AX7's were weak as well. After all, this is an original 1998 Head and hasn't been service since new! I have ordered a set of quad matched 5881 (which is currently in it from factory). Also three new 12AX7's. I also ordered two new LCR like Blue Filter Caps as well. I have a knowledge of tube amps and how they work and have repaired many over the years. Just looking for some input from the Forum as to perhaps that reason for this, what sounds like to me, as a pronounced AC hum. Any suggestions would be helpful. Thanks all!

 

[Matthews Guitars]

It's probably bad caps. Storing the amp unpowered for six years when it's already that old is pretty much the recipe for cap failure.

But why oniy order two caps when all six of the blue filter caps should be replaced at once?

It's a sure bet you did not order LCRs. They stopped making them in 1999. Nor would you want to use NOS caps made that long ago.
You'd have ordered ARS or F&T or Ruby caps, most likely.

Filter caps last longest when they are constantly kept operating at their rated voltage. They go bad quickest when stored.

 

[Joesatch]

try running the amp with 2 power tubes. first remove inners then try with outers removed. This may help isolate a bad filter cap or something else.

 

[mmmguitarman]

It's probably bad caps. Storing the amp unpowered for six years when it's already that old is pretty much the recipe for cap failure.

But why oniy order two caps when all six of the blue filter caps should be replaced at once?

It's a sure bet you did not order LCRs. They stopped making them in 1999. Nor would you want to use NOS caps made that long ago.
You'd have ordered ARS or F&T or Ruby caps, most likely.

Filter caps last longest when they are constantly kept operating at their rated voltage. They go bad quickest when stored.

Thanks for your input. Once I install the two replacement caps (F&T's) and new tubes, I will variac it up to power to see if that does it. If not, then I will replace the others. Didn't think about replacing the others. My duh! Thanks again.

 

[mmmguitarman]

try running the amp with 2 power tubes. first remove inners then try with outers removed. This may help isolate a bad filter cap or something else.

Thanks for the input. Good idea. I'll try that as well.

 

[anitoli]

If two of the power tubes are testing very weak they have prolly drifted away from the other two. If the idle current draw between the pairs gets too wide the amp will get noisy. Do a bias check and see what the current draw is for all four tubes, i'll bet it could be all over the place. If not something else is going on.

 

[mmmguitarman]

If two of the power tubes are testing very weak they have prolly drifted away from the other two. If the idle current draw between the pairs gets too wide the amp will get noisy. Do a bias check and see what the current draw is for all four tubes, i'll bet it could be all over the place. If not something else is going on.

Thanks for the advice! I'll check that out as well.

 

[mmmguitarman]

So, I replaced all preamp tubes with JJ's. Replaced the Power Tubes with a matched quad 5881 Sovtech's that were Apex matched. Replaced the Filter Capacitors with F&T's. Used a Bias Master to set the bias and set all power tubes between 36.2 and 40.3. Variac used to bring up to temp. Still 50-60 cycle hum is evident. What is crazy to me is that, with a guitar plugged into the head, it sounds great! Clean channel crystal clear and Lead channel has a wonderful overdrive. When using Channel 1, the 'buzz' is equal the volume output. Same with the Overdrive channel. The hum is louder but the grind sounds better than it has ever. I pulled the three preamp tubes and progressively install them with the power on and standby on. Of course, no output until I added the last pre amp tube. But the elusive hum was still there. This is something I am willing to live with, but bugs the OCD in me!! It was amazing that this amp, 6 months earlier was perfect...then I powered it up six months later....bang....50-60 cycle hum. Strange...but that is the way things have been going for me lately. Any ideas...not experienced to delve into PC boards. Thanks!

 

[mmmguitarman]

Success! I replaced all the caps that I could see on the pcb's. I also cleaned all the pots and electronics. Brought it up slowly with the variac. Plugged into the cabinet and wormed it up. Clean channel quiet as a mouse and it hasn't sounded this good in years (go figure...huh?). Switched to the Lead channel and I could peel paint. Thanks for everyone's help with this. I am a happy player now!