ST70 "clicks"

Every so often my oem ST70 makes a mechanical "click" noise. There is no corresponding sound of any kind through the electronics/speakers. It is totally a mechanical sound from the amp itself. It sounds almost like a very small relay but, o course, there is no such device in the amp as it is totally stock other than new caps and a diode in place of the Selenium rectifier.

So what's in there that can "click?" At first I was thinking it was expansion of something - chassis, whatever - due to the heat as the amp warmed up but it seems to happen even after the amp has been operating for a while and has pretty much gotten as warm as it's going to.

I'm out of town so I can't check anything at the moment but thought I'd ask if anyone has had this happen/know what it could be. Again, it's totally a mechanical sound with absolutely nothing being transmitted through the electronics. The amp has run for many hours at a time and functions perfectly so I guess I don't mind if it feels like clicking on occasion but I would like to know what causes it.

Not a real techie here but I do like these mysteries. If there is in fact a mechanical sound from the amp that does not translate into the signal path I can only think it would be the expansion of components from heat or something similar. The only remedy I can think of is to tighten everything down again, trannys to chassis, tube socket mountings etc etc. There is just nothing that I can think of inside that would replicate the sound of, or act as, a mechanical breaker or relay unless a tube or something would actually have a short or arc or something weird, but then you would hear it out of the speakers or have performance issues.

There are many mysteries with these old rigs...ghosts in the machines, as it were. If it sounds OK and you can't get rid of the click consider it a quirk and let it ride, I guess.

Hmmm...It hadn't occurred to me but the upstairs heating/AC system is directly above the room where the amp is located. OTOH, I could understand some sort of transmitted pulse being picked up by the amplification circuit - but again, this is clearly a mechanical click coming from the amp. if it was some sort of arcing in the amp itself, I would think there would be a sound from the speakers at the same instant. But I would expect arcing to have more of a "snap" sound. The sound is exactly like that of a VERY small relay engaging. It seems to be from the left side of the amp - the rectifier tube side.

MexicoMike wrote:Every so often my oem ST70 makes a mechanical "click" noise.  There is no corresponding sound of any kind through the electronics/speakers.  It is totally a mechanical sound from the amp itself.  It sounds almost like a very small relay but, o course, there is no such device in the amp as it is totally stock other than new caps and a diode in place of the Selenium rectifier.

So what's in there that can "click?"  At first I was thinking it was expansion of something - chassis, whatever - due to the heat as the amp warmed up but it seems to happen even after the amp has been operating for a while and has pretty much gotten as warm as it's going to.  

I'm out of town so I can't check anything at the moment but thought I'd ask if anyone has had this happen/know what it could be.  Again, it's totally a mechanical sound with absolutely nothing being transmitted through the electronics.  The amp has run for many hours at a time and functions perfectly so I guess I don't mind if it feels like clicking on occasion but I would like to know what causes it.

Things bolted together that is heat-cycled may build up tension, and if they are not
totally tight mounted they might relieve this tension by sliding.

The most common experience is the "clicks" from a stove when the fire has started, or the
corresponding sound when a large exhaust system cools down.

In your case you have transformers bolted on the chassies, and if you have a cage
mounted there is another potential "clicker". Even the tubes might click.

+1 on the tubes ... those can tick 'n click as they heat. One of my Psvane's do that. I'd think that can be usually attributed to some slight misalignment in the internal structure that stresses it when cold. Doesn't affect operation any ... at least till the tick turns to a crack, or maybe a crumble ... or BOOM!

An arc inside one of the filter capacitors can sound like that but be filtered out by the capacitors farther downstream, so there is little or no audible evidence.

Don't ask how I know.

Also, don't ask how I know that if this is happening, the capacitor will eventually fail completely.

I agree with GP49. I would pay attention if the clicks increase over time.