Mark III (new or old) tubes get incredibly hot, sound fades

[inhalingsounds]

So, recently one of my tubes started getting hotter than the other one (I only use two at a time, in positions 1 and 4) so I thought it was time to switch them.

I did, but following a guide on how to position tubes I opted to use positions 2 and 3 instead. When I started the amp, the slo-blow fuse burned out. I tried again, another fuse went down. So I let the amp rest for a while, switched back to positions 1 and 4 and the amp seemed to come back to life.

However, 4-5 minutes after starting to play, the tubes got really hot and the sound faded out. Tried again - same result.

I thought it might be a defective (new) tube, so I reached out to an old 4 tube pack I had as a backup. I put the 4 tubes just to be sure everything was set as it should, and ... tubes very hot, amp fading out again.

Can anyone understand what is happening? Reaching out to an official Mesa Boogie rep here is a bit difficult as they are relatively far away, so any help in tackling this would be great.

 

[BlackBoxy]

could be a bias issue, a tech would easily find it out and fix it

 

[McBarry]

if the hot tube is redplating (google for pics ) chase a bias issue.
Other possibilities for sure, but totally agree with BlackBoxy that it's bench time and likely a simple diagnosis and repair.
Can U let us know so future readers can learn ?? Cheers, DB

 

[inhalingsounds]

I just made another test. Tubes aren't really going red, they just go super yellowish-orange-to-blue and then the sound fades out. I'm not sure if that counts as red plating.

The thing is: I'd be willing to try and adjust the bias just for the sake of not having to drive so far to meet with a MB tech, having it repaired, wait forever for it (I have gigs coming up) and then having to come back again to get it. If it was "just" a bias issue, I think I could handle it.

Does anybody know of any videos showing how it's done on a mark III or a similarly-equiped MB?

 

[BlackBoxy]

hi,

if you have a pot to set the bias, so its relativly easy when you know witch tension to get, with a probe and a well matched quad of tube , have to wait at least 30 mns and let all tensions get steady!

but if no bias pot, means the bias is fixed, you have carbon comp resistors, the issue is that carb comp doesn't like to much long term heat, and with age tend to increase in value , i doubt it would be this anyway, allen bradley are killer resistor on reliability , even with more K in it wouldn't move too much balance, i would look at E voltage or Blue voltage power supply before !

whatever, bring it to a tech, Mesa schematics are quite detailed on check spot and tensions !

good luck

 

[McBarry]

Inhalingsounds,

it's not something that is easily diagnosed, especially by email/forum.

The colours U mention sound like TWO separate conditions, occurring simultaneously.
But also I'd consider screen grid resistors or a coupling cap in position 1 and/or 2 and a dry joint in same area. (don't have schematic in front of me right now...).

Get it to a tech as prev mentioned.

If you're really keen to do it yourself..... get a schematic, then check your voltage vs schematic voltage(s) at ALL the power supply nodes and then downstream. Without tubes.
Check PSU resistor values - some might need lifting one leg to be sure of accuracy. Then check downstream of power tubes - O/T resistance(s), feedback loops if pertinent.
This is all assuming you're competent and comfortable working inside LIVE chassis.
Potentially lethal voltages await the unwary.. even a whack off a charged filter cap is painful.

Let us know your progress..
Regs, DB