Suspect Problem with EL34 tube(s) on a MA50H

[txslowhand]

Hey all,

I have a MA50H that I've had for about 6 months which just developed a problem today. I turn the head on to warm up, no problem, no hum. When I take it out of standby the head develops a really loud hum after 15 or 20 seconds (after the power tubes warm up).

Question 1 - Sounds like I have a power tube issue to me. What do you think?

Question 2 - Can I take out each tube individually then run the amp long enough to determine if the hum is eliminated thus identifying the bad tube? I know this is a common practice in other amps, just wanted to make sure it's ok to do with the MA50H.

Question 3 - I notice there are two trim pots located in the area of the PCB were biasing is adjusted. Is there a trim pot for each pair (1 trim pot for the 50 watt model and 2 trim pots for the 100 watt model)? If one trim pot, then which one do I use to adjust the pair of EL34 tubes? I'm sure I can figure it out but I wouldn't mind taking the mystery out of which one to use.

Note: I'm familiar and have experience with adjusting the Bias on an amp and know the precautions to take. Somewhere in my notes I have the current to shoot for, I think it's in the neighborhood of 35mA or 37mA. Can someone confirm this?

Thanks in advance to any and all advice.

[jagermike66]

definitely sounds like a power tube problem to me. The 2 trim pots adjust each individual tube bias, IE 1 trim pot per tube. 35ma to 37ma is spot on for EL34 type valves.

Sounds like you have a dead tube. When the amp has warmed up, can you describe the appearance of the tubes? Is either of them glowing red?

[txslowhand]

definitely sounds like a power tube problem to me. The 2 trim pots adjust each individual tube bias, IE 1 trim pot per tube. 35ma to 37ma is spot on for EL34 type valves.

Sounds like you have a dead tube. When the amp has warmed up, can you describe the appearance of the tubes? Is either of them glowing red?

Thanks jagermike66.

I haven't pulled the back off the MA50H for a closer look so I'm not sure what the tubes look like when energized. I'll let you know. Will tackle the amp this weekend so I'll let you know.

Glad to hear that each tube is biased individually. In theory this allows you to replace only one tube if need be without having to replace the pair (assuming you are replacing with same make of tube) but I tend to replace both if one is bad. That's just me anyway. Secondly, individual biasing allows for more leeway in the matching of a pair of tubes. This is a treat having an amp with individual biasing, my past amps would only allow me to bias the pair together which is ok but it means the pair must be pretty much matched.

Thanks again for the tips, I'll keep you posted.

-- Tue Sep 27, 2011 5:43 pm --

definitely sounds like a power tube problem to me. The 2 trim pots adjust each individual tube bias, IE 1 trim pot per tube. 35ma to 37ma is spot on for EL34 type valves.

Sounds like you have a dead tube. When the amp has warmed up, can you describe the appearance of the tubes? Is either of them glowing red?

Hey jagermike66,

I got around to looking at the tubes and both glow pretty much the same. While turning the head to get to the back the speaker cable came unplugged from the speaker (really short cable). Well, I rerouted the cable when plugging it back in and fired up the head. Hmmm, no more god awful hum or at least down to where it was before the trouble started (it always a little hum but not enough to make a difference for my bedroom usage). So, looks like the speaker cable might be the culprit. The cable is a cheezy cable that I borrowed from my Blues Jr. Lesson learned: don't mix Fender and Marshall equipment

....or maybe invest in a decent speaker cable.

Thanks again for your help.

[jagermike66]

haha! wev all done a similar thing I think!

A decent speaker cable is definitely worth the money. The more beefy, the better! My test is always if you turn the amp on, tap the speaker cable. If it makes a noise through the speakers or starts a nasty hum, then the cable's on its way out. If there is minimal noise through the speakers when you tap the cable, then it is all good.

Glad its all fixed!

M