Mark III issues - need your help

[arnek85]

Hey guys,

after having 10 great and satisfying years with my reliable Mark III (Blue Stripe, Simul Class) it seems to start developing symptoms of old age.

Sometimes at switching on the power switch, it immediately blows a fuse. After changing the fuse and a successful power-on, it operates well and it always stays on. So the problem occurs just at powering on. (power switch, not standby switch)
Time to change all power tube. I'm using 4x5881's from Mesa. After swapping the tubes it seemed working fine. But few days later it blew just another fuse. In class a mode and simul class mode, doesn't matter.
Anyone there who had the same issue or an idea that could help me isolating the cause? Bad caps or resistors or anything typical?

Cheers!

By the way, there is an old thread about that issue. At first the amp didn't blow the fuse, what led to burnt resistors. But I already changed these damaged parts.
http://forum.grailtone.com/viewtopic.php?f=27&t=72021

 

[darkbluemurder]

The symptoms you describe could be bad electrolytic caps in the power supply. Given that the amp is now in your hands for 10 years it is definitely time to check these. If you are not familiar with tube amps, take it to a qualified technician. Visual signs of bad caps would be bulbs on the positive side.

I have done a cap job on mine just recently after having it more than 10 years - it still had the original caps which were by that time 30 years old and way past their expected service time. They have not caused any problems but I am not taking chances.

With the simul class amps you would need 4 220uf caps @350V and 3 33 or 30uf caps @500V or greater.

Cheers Stephan

 

[treedroppings]

Can also have blown screen resistors if a power tube got loose in the socket!

 

[anngriffin]

Sorry, I even don't know how I can help you! The situation is a bit confused! Try to check out tube amps, first of all!