Mesa Mark IV two tubes hotter

[diazibon]

Hello,
I have a mesa mark iv 20 years now, it seems to sound ok, but i noticed that the power tubes aren't heating evenly. Twe two tubes to the right (looking from behind) are colder tha the other two. Don't want to break it because of something working improperly. I switched the tubes but it seems to be the circuit, because the tubes in the same sockets are colder now. When the amp isbin standby the difference is also noticeable.
I know is hard to tell, but can you give me some advice? I supose changing the pream tubes will not be the solution... the caps? Provably something in the power source or the transformer?
By the way, is possible to use a power tube probe, like a vht power tube tester or something similar in the mark iv? I don't know if it woild be harmful.
Thaks a lot everybody,
Greetings

 

[DrKev]

I'm not a Mark IV owner and so I may be missing something obvious the experts know (in which please let me know) but...

Let the amp and tubes comletely cool down. Then power on, staying in standby mode only (so there are no functional high voltages on the tubes and they have not been conducting any meaningful current). The only real heat source is the heaters for tube cathodes. Do you notice a difference in temperature?

If the answer is NO, and the temperature difference requires the tubes to have some actual run time to see the difference in temperature, take it to an amptech for a checkup.

If the answer is YES, there is a difference with only heater voltages present...

The heaters for the 4 6L6's are all run identically from the exact same 6V AC supply and they should all be heating similar amounts. Unless there are obvious visual differences in the orange filament glow between the tubes when in standby mode, I personally would say all is probably well and ignore it. Temperature of a tube's glass envelope is a not a reliable indicator of tube or circuit health or circuit function and any difference in apparent temperature could be just more efficient air cooling for hose two tubes.

As I said, if the amp functions well, I'd think you're good to go. If that's not something you are happy to do, and are not comfortable working inside tube amplifiers with a multimeter, I'd take it to an amp tech to have a check up. If the tubes have been in there a long time and have approaching 1000 hours playing time, replacing them may be a good idea anyway.

 

[Fiveband]

Using a probe like the VHT would be a good first step to check bias if you have one handy, but might not show a problem with the heater circuit.