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Valve amp humming left channel

grahamfocal

grahamfocal

Member
Hi,
I have a Croft series 5c power amp that has a low level hum in left channel only.
Removed everything else from the chain, pre amp etc.
With only the power amp and speakers connected, turn on power amp, wait 10 seconds, and in comes the hum.
I have swapped all valves, even buying 4 new valves, no change.
Could the rectifier valve cause this issue, as its the only one I have not changed, but don't want to try, if there is no chance its that.
Cheers.
 
It is very, very unlikely to be the rectifier. I presume you've changed the valves in the regulator? Even then it is very unlikely to be this as, AFAIK, this amp has a shared PSU. Impossible to diagnose psychically at a distance but a front end decoupling electrolytic would be my first guess...
Does the hum go with inputs shorted? A passive pre at zero vol should do for this test.
 
Arkless Electronics said:
It is very, very unlikely to be the rectifier. I presume you've changed the valves in the regulator? Even then it is very unlikely to be this as, AFAIK, this amp has a shared PSU. Impossible to diagnose psychically at a distance but a front end decoupling electrolytic would be my first guess...
Does the hum go with inputs shorted? A passive pre at zero vol should do for this test.
Click to expand...

Hi, I don't have a passive pre, but could knock up a short plug/wire to test, yes?
 
The hum is very subtle, and might be acceptable to most, you have to have your ear right to the speaker, it's purely as it doesn't do it in the other rh speaker.
 
There should be silence with the inputs shorted. Chances are you've not got a proper short.
If the hum is that quiet then it's possible that it's supposed to be like that! Is it the channel closest to the mains transformer by any chance?
Some on line pics of it suggest it has solid state rectification....
 
I think the 5c is SS rectification with a single 12at7 regulator.
 
thebiglebowski said:
I think the 5c is SS rectification with a single 12at7 regulator.
Click to expand...

12AT7 is a double triode. It appears to have an LM317T regulator. Vastly better than any valve one anyway! This could be fitted for the bias supply though... just not enough detail in any of the on-line pics to to able to at least partially reverse engineer it and say exactly what is what with it..
 
grahamfocal said:
Will try making a better short plug tomorrow. What will it prove if its silent?
Click to expand...

Probably a waste of time actually but with shorting plugs at the inputs (both at the same time ideally) the amp will be producing its very minimum hum and noise and all present due to the amp itself.

It would seem this specific example may be non standard from my on-line sleuthing... made self/auto bias by Alan Firebottle if this is indeed the same one...

The only schematic I can find, for the original series 5 back in the 90's, is self bias anyway.... it also shows all the double triodes as ECC83's.... not ECC81 as input valve and then ECC83 as phase slitter. Rather similar to Leak Stereo 20 in many ways.
 
Arkless Electronics said:
Probably a waste of time actually but with shorting plugs at the inputs (both at the same time ideally) the amp will be producing its very minimum hum and noise and all present due to the amp itself.

It would seem this specific example may be non standard from my on-line sleuthing... made self/auto bias by Alan Firebottle if this is indeed the same one...

The only schematic I can find, for the original series 5 back in the 90's, is self bias anyway.... it also shows all the double triodes as ECC83's.... not ECC81 as input valve and then ECC83 as phase slitter. Rather similar to Leak Stereo 20 in many ways.
Click to expand...

I do appreciate all the help, but not sure what you are saying, am I being to finicky? I have had valve amps before, but both ch would make a slight noise, this had different noises from both ch.
 
grahamfocal said:
I do appreciate all the help, but not sure what you are saying, am I being to finicky? I have had valve amps before, but both ch would make a slight noise, this had different noises from both ch.
Click to expand...

Possibly... Without hearing the level of the hum in person etc it's hard to say if its normal but from what you're saying up thread if it really needs "ear against woofer" to hear it then probably...
Hum and noise is not inherent in valve power amps though. They can be very quiet and in fact my olde Leak Stereo 20 is silent in operation.
 
Arkless Electronics said:
Possibly... Without hearing the level of the hum in person etc it's hard to say if its normal but from what you're saying up thread if it really needs "ear against woofer" to hear it then probably...
Hum and noise is not inherent in valve power amps though. They can be very quiet and in fact my olde Leak Stereo 20 is silent in operation.
Click to expand...
Best way to describe it, the amp buzzes a little (non speaker) and along with my michell argo power supply the same, the volume is the same. Hence not alot.
 
Arkless Electronics said:
Possibly... Without hearing the level of the hum in person etc it's hard to say if its normal but from what you're saying up thread if it really needs "ear against woofer" to hear it then probably...
Hum and noise is not inherent in valve power amps though. They can be very quiet and in fact my olde Leak Stereo 20 is silent in operation.
Click to expand...

Added short plugs.
Right chan completely silent, left chan still low level humming, but not as loud tbh
 
Last edited:
grahamfocal said:
Added short plugs.
Right chan completely silent, left chan still low level humming, but not as loud tbh
Click to expand...

Well there yer go then, that's the remaining hum and noise of your power amp when all external influences have been removed.
If the level bothers you then there may well be things that can be done to reduce or remove the hum and it's likely that other upgrades could be made to it.
 
Arkless Electronics said:
Well there yer go then, that's the remaining hum and noise of your power amp when all external influences have been removed.
If the level bothers you then there may well be things that can be done to reduce or remove the hum and it's likely that other upgrades could be made to it.
Click to expand...

Thank you.
 
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