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KT88 Monoblock amp makes a spitting sound at times

I have a pair of Quad II Forty monoblock amps. Was thinking of selling them, to go the low power SET route, but would have to get more efficient speakers. So thats been shelved as money is an issue . Anyhow they sound great when they aren't acting up.
Having said that , one has started to make an occasional spitting, kind of phono stylus touched by finger sound once warm. Only after warm and only sometimes. Is this symptom of some kind of imenent failure?
The other was making a mild hum,remedied at first by applying light pressure to the top of tube cage. Opened it up and looked for some sign of a bad wire route etc. but has since stopped mystriously. .Any ideas as to this one ?
Have been planning to put fresh KT88's in and perhaps new rectifier tubes, 5U4G in both just to be sure.
Kind of new to the whole tube thing, but happy when the amps follow suit. Any suggestions welcome. Types of tubes and fixes.
128x128montyoldboy
montyoldboy's avatar
montyoldboy OP
14 posts
 
Cheers for ideas to my sonic maladies,

The amps were bought as demos. I have no idea as to the hours on any of the tubes. As long as the circuit is good as I tend to believe, I have no problem with valve refurbishment even in light of my dodgy employment situation,
I will look at all valves as suspect.
As I listen to it now all is good, till the next sonic hiccup.

Again after listening for an album/CD right amp will fill with the finger on the stylus sonics, albeit very briefly. ( scratch, scatch, silence) argh!
The left amp has stopped the low level humming for now.

I now fear as to the recent performance, that there may be a thermal element, gee I hope not. As to complicate my diagnosis.
zaikesman
3,017 posts
 
Swap the tubes, one channel for the other, one tube position at a time, to identify whether the problem follows an individual tube. (If the symptom is only intermittent this may have to be done over a matter of days.) I would start with the input tubes first.
montyoldboy's avatar
montyoldboy OP
14 posts
 
Thanks for the response.I am going to get 4 new 6SH7 tubes to start.
The other amp has started doing the ground hum thing again.

The fix still is light pressure on the tube cage. Will I be best off removing the cage?
This hum is intermittent, but shows up more often after the amp has played through a couple of CD's.

I am aware that there may be difficult to diagnose .Help?
zaikesman
3,017 posts
 
The hum may (though I don't know and am not personally experienced with that amp) be caused by the power transformer inducing a resonance in the tube cage after the temperature has risen in both. Yes, the simplest fix in that case would probably be to just remove the cage. Tube cages always add resonance to the amp chassis to some degree anyway, just from airborne vibration (similar to a turntable cover in that respect), so sound can sometimes be improved a bit by their removal, and they can also make it harder to notice early visual evidence when something may go wrong with a power tube during play.
montyoldboy's avatar
montyoldboy OP
14 posts
 
Thanks to all for offering advice to my sonic woes.
I am waiting for a special screwdriver from IAG in England to remove the tube cage. I hope this will stop the annoying hum in the one amp. I still plan to retube these from top to bottom. That way I will know the ages of the valves and such.
I appreciate all tips.
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