Loud cracking sound when switching tube amp from "standby" to "on"

[elgravos]

Trying to figure out what is wrong - if anything with my amp (Boogie Mark 5-35)

In the last few weeks, I've had two instances where when switching the amp from "standby" to "on", I've heard a large cracking sound coming from the amp.  Amp wasn't cold either time and had been running a good five minutes at least on standby before I threw the switch.  My first impression was that a tube had exploded or something along those lines.  It's not an amplified noise being output by the amp, it's the mechanical sound of a glass or metal component "cracking". 

First time round, there was a hot glass / metal smell that went with.  Turned the amp off straight away and let everything cool down.  Checked the tubes and eyeballed the components but nothing appears broken or burnt.  Tried it again later and everything was back to normal.  Happened again several days down the road, though no hot glass / metal smell that time. 

It seems pretty worrisome, but strangely, the amp seems to work fine.  There is no extra noise in the circuit and all channels / modes work as normal.  Any thoughts on what this might be?  Had the amp for about 18 months now and have not yet changed the tubes - haven't seen a need to as the sound is still fine.  I do drive them pretty hard (attenuator is on full time with master around 4 and gain is usually on the high side) and it runs a couple of hours a day.  Could it just be that the tubes are getting old and need replacing at this point?  Appreciate any thoughts on this.  Cheers.

 

[mayfly]

I'm willing to bet that it's arcing within the standby switch.  Depending on the design, this switch can undergo a lot of stress, leading to eventual failure.  I used to put a big old 1Kv ceramic cap across it to help protect the switch from rapid current changes.

I'll have a look at the schematic and get back to you.

 

[mayfly]

Had a look at the MK V schematic.  Yep - check the standby switch.

Also - there are no cathode followers in this circuit, and there appears to be a tube rectifier.  Because of that, you don't need to worry about the standby switch. If you want to, you could just leave the standby switch on.

 

[elgravos]

Thanks for the insight, will definitely look into it...  I don't particularly like messing with high voltage stuff so may have to take this to a pro!  Arcing sounds a little dangerous so may reach out to Boogie as well and see what they suggest!