Click, pop..........?bang

Right, this is a simple q, it concerns my second system. This is an arcam cd72 hooked up to an AVC el34 push pull valve amp (gec5670 preamp valves and SEDwingedC EL34 valves). All single wired to Tannoy M15 speakers.

With everything on, I get a really loud hiss from the left speaker. when i play something, the hiss dissapeaars by and large, but i start getting occassional clicks and pops everywhere in the sound. The valve amp looks ok, has been correctlky re-biased by me, and doesn't buzz/hiss or smell funny/burnt.
also, everything was working fine up till this week when i noticed it, and i hadn;t fiddled wiht anything prior to this.

I guess this means I need new valves, or am i overlooking something?

Cheers for any help.

Gaffer

PS valve replacement long overdue anyway, so that's what i've assumed (had good use over 3 years)

 

try swapping the tubes round to see if the sound follows the tube so to speak. If it doesnt could well be a resistor though.

 

or a cap but yes, def swap them round first. When you say 'everywhere in the sound' do you mean across both channels or something else? If its in both channels its pretty likely theres something gone wrong in the PSU- does it have tube rectifiers, sounds like it doesn't, but if it does, have a look at those, if not open up and have a look round the psu circuit. Watch out for big electrolytics- they hold charge.
If you meant something else and the fault is Left channel only, its probably in the left channel

 

Thanx guys, will fiddle some more later. I re-biased again and found the valves to be hovering at around -44V in my folks housse

previously, in my flat , they had just been set to -40V.

I re-biased to -37V but hiss still there. Will do some swapping later, but notice a resistor on the left channel (on one of the valve pins) looks ?slightly charred.....doesn't smell burnt though.

will prob need replacing.

thanx for your help

 

is it charred- you sure its not just solder flux? Pull the R out and test its resistance before you do anything else, you may save yourself the cost of a tube and it'll only take you five mins.

 

Always check the main voltage before biasing, it may have been high at your folks house and the resistor as Anex said (set your multimeter to one of the lower setting and test it BUT try all the settings before establishing its a duffer, or if you've got a signal test function use that).

 

nope, resistor definitely charred and reading 9.5ohm rather than 10ohm of the other three (one per tube).

 

is that out of ckt? It probably is that by the sound of it but if you read in ckt you could be reading parallel R's too so the prob may be elsewhere. Worth checking anyway but does sound like that one

 

Is that not the cathode to ground resistor your reading thats used to take the bias reading from. If so why would that cause the hissing? It would give you a different bias reading but you just re-do your maths. I maybe wrong though.

 

Figured it out at last. I swapped one of the pre-amp tubes from the noisy left to the qiueter right channel, and now both channelsw have equal (and low ) hiss.

Looks like i need new tubes.