Power tubes red hot, I need help!

[mrboogie]

I just picked up a used Nomad 45 2x12 from Elderly Instruments and I love the amp. I do have a problem however and the far left pair of EL84's turn red hot even in normal mode with the volume at 9'o clock. I haver moved the tubes around and tried a set I thought was good but the second tube from the right gets red hot and then its partner does the same thing. These tubes are Mesa tubes and sound good until the tubes get red and then the amp breaks up too much. I have new tubes on the way (Tung Sol 12ax7's and Sovtek EL84's) but I think the amp may have a problem that needs fixed.

Any ideas? Any good tech's in the North Eastern Indiana area?

Thanks for the help!

 

[jamme61]

I had this happen to a MKIV I had and it was a broken ground at the socket where the power tube was. If it's always in the same socket with different tubes it could be that. Did you open up the amp yet? Please be carefull if your not sure bring it to a tech. You could get killed if you don't know what you're doing, (NOT KIDDING) Maybe bring it back to the store? I'm sure they could fix it. good luck.

 

[94Tremoverb]

If it's always the tube in the same socket that gets hot first regardless of what tube is in it, that socket has a bias voltage failure. (It then causes the other tube on the same side to run too hot due to it being coupled in parallel with the overheating one.) Chances are it's either a bad contact between the tube and the socket caused by the socket contact being a little too opened out - this can happen if you push in a tube with bent pins - or the grid resistor is bad, or there's a crack in the circuit board solder joints, or some other problem like this. If you're not familiar with working on tube amps, best to take it to a tech - even re-tensioning the sockets is tricky if you haven't done it before.

 

[Heritage Softail]

There are life threatening voltages in the amp. You only get to die once. :idea:

 

[mrboogie]

thanks for the reply.

I am aware of the voltages stored in the caps and know not to touch the wrong thing. I am still trying to find a good tech in my area since I am new here.

I am 2 hours from Elderly so I doubt if that is an option.

 

[94Tremoverb]

Was it like that when you got it? If so I would take it back anyway - they need to make it right, you can't run it like that. And for free if it was like that when you bought it. Worst case is that it may need a new power tube socket PCB, although it's unlikely.

 

[mrboogie]

Well I would take it back to Lansing but I could pay the tech I found in town for two hours and still save cash compared to driving 300 miles to have a minor repair done on the amp. I received my new tubes today and the same two tubes are getting red hot. Therefore the amp needs service.

I have found a good tech local and I was thinking of having him to a bias mod to my amp so I can adjust the bias to achieve a better tone. I am happy with the tone now although it could be less fuzzy on the overdrive, but I would like to have the option of changing the bias in case I need to.

Anybody out there have an opinion on the bias mod? Is it really going to help my tone if the bias is set different or is it overkill and just a hassle since I will need to rebias the amp after changing tubes?

 

[mrboogie]

All better now. I guess it was one of the pin holders was pushed up through the board and clamped down on a resistor grounding it out. I guess was the guess of one of the replies. Great job!

New tubes and the amp sounds fantastic! I love channel one on the modern setting for an overdriven Bassman sound. this is an awesome sound for blues if you like just a little dirt that cleans up when backing off the volume on your guitar. I also have no trouble with the reverb like I have seen a lot posted on here? I guess the amp hasn't been used much.

I am curious as to what is the best mod I can do to this amp. I guess the power tubes are running plenty hot so a bias mod is out. How about the parallel FX loop? I am running a 7 band eq pedal through the loop just to give me more control over the eq. So far so good on the amp and all this for just $700.