Mark IV crackling noise during warm-up

[avoelp]

I have a Mark IV (B) that I have used regularly and that has never failed on me. Lately, I haven't used the amp for 1 1/2 years or so. After taking it out of the flight case, switching the power on, letting it warm up for about half a minute, and then switching it from standby to on, the amp makes a crackling noise and the guitar sound is thin, scratchy, and low-volume. When the amp has warmed up for another 2-3 minutes, the noise is gone, the tone is perfect, and everything is fine again.

This happens every time when I turn on the (cold) amp. None of the controls on the front panel (including the output volume pot) has any effect on the crackle, and it also does not matter whether I play in tweed or full mode, Class A or Simul Class, or in triode or pentode mode. I have removed all the tubes and cleaned the contacts, and I have also swapped the inner and outer power tubes; none of this had any effect.

Any suggestions what the cause and the cure might be?

Best, Andreas

 

[user 21540]

It could be a tube going bad that is fine when it gets up to the correct operating temperature. I would try the tube troubleshooting method in the first instance although with your described problem you only have 90 seconds to test before the problem stops each time. You would have to cool the amp down and test again. This is a video of content they supply in their manuals:
https://www.youtube.com/watch?v=wSl-l9aWDHw

V1, the phase inverter and the reverb tube would be the most obvious culprits to check. The manual tells you the tube assignment on pg 17:
https://mesa-boogie.imgix.net/media/User%20Manuals/Mark%204.pdf

It may also be a cold or slightly broken solder joint somewhere that makes a better electrical connection as the amp warms up but you would need a tech to verify that.

 

[stokes]

Could be a lot of things.Its very possible its just a dirty, oxidized tube socket.Or a bad tube, as wildschwein says.Another likely cause is plate caps.You say the amp has sat unused for over a year.This practice is not good for your power supply filters.They will actually wear faster when not used.When the amp goes a long time not being powered up the caps electrolyte dry up,they need to pass current to stay viable.You say the amp "sounds thin, scratchy and low volume" this sounds to me like the caps are taking a long time to fully charge.If the amp is 10 years or more old, its likely time to re cap.

 

[avoelp]

... agreed. I have meanwhile exchanged the tubes for a test, and it had no effect. So, likely, one of the caps. As long as everything is fine after two minutes or so after power-up, this is not much of a problem. Otherwise, I'll probably have to send the amp to a tech ...

The amp is from 1994, so that's definitely more than 10 years

 

[user 21540]

Yeah you're definitely due for a cap job by now. 20 years is a good rule of thumb provided the amp is used regularly.

 

[avoelp]

... my 1978 Mk II 'A' is still all original, without any issues whatsoever. Maybe Mesa has used caps for the Mk IV that do not last that long ...