Mark III Blowing tubes in simul-class mode?

[James Lee]

Where would start looking to fix this problem?

Is it the transformer maybe?

Thanks in advance!

JL

FYI-This is a friends amp so I am not sure if it is a series 300 or not as of yet.

 

[stokes]

More info would be helpful.Did it blow all the tubes?Did you put new tubes in and have it blow again?Are there any symptoms before they blow?Are you using tubes that were selected for the proper bias range for the amp?Are you capable of opening the amp up and having a look around if need be without getting yourself fried?Give us some more info and lets take it from there.

 

[jamme61]

stokes is right, we need more info. If your not sure about taking out the chassis, then don't waste your time or health on it, take it to a tech. It could be as simple as a bad tube or resistor, but you don't want to get electrocuted looking around, Be careful! If the tubes are real old, you could just buy new power tubes and you could get lucky, but you might fry a brand new tube. I'm not a tech and I know just enough not to get electrocuted (so far LOL) so I know how much this sucks for you, wish i could help more.

 

[James Lee]

As far as I know, only the power tubes assoiciated with the simul-class function are blowing. I will have the amp tonight so I can definately open it up and take a look around. No pre symptoms to speak of. It runs fine when not using the simul-class mode so I was thinking some type of short but I was wondering if the transformer could cause that. I also need to determine if it is a series 300 or not. I guess I could tell that just by placement of EL34's vs. 6L6's. It just seems if it was the transformer it would have problems either way you ran the amp (non simul vs. simul).
I will try to have more info soon.

Thanks for your willingness to help.

 

[James Lee]

Jamme61- I am about the same..... know enough to get myself into trouble.

Thanks to both of you for replying!

Here's the deal:
Replaced all power tubes with new tubes. Plays fine Class A. As soon as you put it into simul-class the inner tube on the left (looking into the back) glows red much more intense than normal I went to turn it back to Class A (it did not look good!) and the tube glowed blue and the fuse blew.

I replaced the fuse and again Class A works fine so only the inner left tube looked to be blown. It gives no warning at all other than lighting up like a XMas tree.

All the power tubes are Mesa's 6L6's the new ones were marked gry and grn marking where the old ones had ylw and grn.

 

[jamme61]

I had this problem with mine and it was a resistor going from the tube socket to ground(I think it was pin 5 to ground not sure) and the connection to the ground was broken. This happened because I was always changing power tubes(experimenting) and when you take them in and out the pins move a lot. Check the socket where the tube is going red and also the other matching socket on the other side. If I remember right it was the tube that wasn't going red, that had the broken connection. Look at them and also use the continuity tester on your DVM to make sure those connections are good. Again I'm a novice but it might be this. Good luck and be careful.

 

[stokes]

In class A only the outer tubes are being used.The red glow in the tube indicates it is drawing too much current.I dont use the Mesa tubes so I am not 100% sure about their matching scheme,but I would think your problem could be as simple as having the wrong bias grade for that amp.If the tubes are the right ones for that amp I would check that you have bias voltage going to that tube and that your screen resistor is okay.It is possible that the tubes didnt actually blow,it could be redplating and blowing the fuse before the tube actually went south.If you dont have the ability to do the 2 checks I think you are better off taking the amp to someone who can.

 

[James Lee]

You guys are awesome!

This has helped give me a starting point. I should be able to look at it this weekend. I will be sure to post results.

Thanks again.

JL

 

[jamme61]

Cool, good luck, and get back.

 

[welshboogie]

I had this problem with mine and it was a resistor going from the tube socket to ground(I think it was pin 5 to ground not sure) and the connection to the ground was broken. This happened because I was always changing power tubes(experimenting) and when you take them in and out the pins move a lot. Check the socket where the tube is going red and also the other matching socket on the other side. If I remember right it was the tube that wasn't going red, that had the broken connection. Look at them and also use the continuity tester on your DVM to make sure those connections are good. Again I'm a novice but it might be this. Good luck and be careful.

+1 on the above, I had exactly the same problem too - resistor to ground - my tubes started to smoke!!!!!

 

[James Lee]

Hey all,

Sorry it took so long to report back but work has put me through the ringer the past 2 weeks. Tonight I finally got to look at the amp.

You guys hit the nail on the head! At first glance everything appeared to look fine but a closer look revealed a broken connection on the component next to the screen resistor.

Stokes- you were correct. All the tubes are in working order still.

I can't beleive how the Mark III sings in simul-class mode! This is the only Mark III I have got my hands on to try out. I never seen one up to now.

Thanks SO MUCH for your help! This board is full of knowledge from great people willing to help.

You guys RAWK!!

JLH