Can I enlist a little help, crackle in valve amp

toms wait

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Hello all
My AI500 has a small left channel crackle.
Slight and very random, not continuous.
I have tried valves out tests.
centre pre-valve out gives louder crackle.
left channel out the crackle disappears
volume control doesn't affect it
source selector doesn't affect it
with driver valves out it is still there
I have swapped driver valves over and it stays in the left
I have changed all the small valves and it is still there
I have checked loose, broken missing solder and damaged wires
It is mainly noticeable at switch on and after a while it settles and comes back when hot.

Do any of you clever valve people have any ideas that I have not tried

thank you all
 

Lawrence001

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When you say driver valves (as opposed to "small" valves) do you mean the power valves? I've not heard of a test with valves out, but if it still crackles I wonder if it's power supply related does the amp have a rectifier valve?
 

toms wait

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When you say driver valves (as opposed to "small" valves) do you mean the power valves? I've not heard of a test with valves out, but if it still crackles I wonder if it's power supply related does the amp have a rectifier valve?
Yes power valves, no rectifier valves on this.
I wonder if its a loose wire on the underside of one of the circuit boards I have not looked under yet.
 

audio_PHIL_e

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Could it be temperature-related, in a (eg, coupling) capacitor? Hope you get to the bottom of it soon.

PS I don't know if there are any inductors in your kit, but I have heard one making what I described as a "grumbling" noise (which went away when the inductor was replaced) but could be interpreted as a kind of crackling. It too was worse when the circuit was "hot".
 
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slavedata

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What amplifier is it Make & Model no? My Fisher500C crackled when warming up when the Output Valves were on the way out.
 

Disarmamant

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Probably one of the 0.47uf coupling capacitors between the ECC88 anodes and the EL34 grids. Did it get worse when you tried it without the driver valves in place?
 
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rabski

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I hate things like intermittent crackling, because there are too many potential causes. Most are noted above, but I'd add to the list: dry solder joints (the PCB around the higher-power resistors is worth checking) and cathode resistors and bypass capacitors. Also, all the HT lines/board and especially some of the relevant resistors.

Really, in view of the age as well, it's worth a proper strip down with a view to replacing some things that are subject to heat over time.
 

toms wait

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I am going to have to work through all of the above over the next few days.
It has been checked/serviced and upgraded recently so some of the things above won't apply.

Unfortunately for you all and I, I don't know what many of the above are! But will work it out from positions/circuit diagrams and elimination with it off and drained of power of course.
This is bad for me because I may miss the obvious, bad for you because you may have a long wait for feedback/a result!
 

toms wait

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What amplifier is it Make & Model no? My Fisher500C crackled when warming up when the Output Valves were on the way out.
Audio Innovations 500, have swapped output valves as a quartet and swapped them around as channels. To no avail.
 

JANDL100

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Whenever I've owned a valve power amp I think that my deep down subconscious was always a bit on edge waiting for the crackling / rustling to start. It was always one of the valves on the way out.

Whenever I sell a valve amp, I think Never Again.
...... Until next time. :D
 
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Disarmamant

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I am going to have to work through all of the above over the next few days.
It has been checked/serviced and upgraded recently so some of the things above won't apply.
If It were me, I’d check the voltage rating of the 0.47uf capacitors I referred to. IMO they should really be 630volt types.

If the schematic I’m looking at is to be believed, those capacitors are rated at 400volts. However, since the amplifier has a solid state rectifier, the voltage on those capacitors could well be at or above 400 volts at switch on in the period before the valves heat up and start to draw current.
 

toms wait

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If It were me, I’d check the voltage rating of the 0.47uf capacitors I referred to. IMO they should really be 630volt types.

If the schematic I’m looking at is to be believed, those capacitors are rated at 400volts. However, since the amplifier has a solid state rectifier, the voltage on those capacitors could well be at or above 400 volts at switch on in the period before the valves heat up and start to draw current.
I have a volt meter but will check the dry joints etc before I start poking in there with it on.
But thanks for the advice.
 

Hifimad1

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I have a volt meter but will check the dry joints etc before I start poking in there with it on.
But thanks for the advice.
Best to check when not powered up. If you have to live test be very careful . A quick way to check dry joints is with it switched on and connected to your speakers (at Low volume) with the amp plugged into an rcd trip. Use one hand only ! ( other arm behind your back) tap around all joints with a long wood chopstic & listen for noises through speakers.Do not attempt this if you are not experienced with electronics !
 

toms wait

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Best to check when not powered up. If you have to live test be very careful . A quick way to check dry joints is with it switched on and connected to your speakers (at Low volume) with the amp plugged into an rcd trip. Use one hand only ! ( other arm behind your back) tap around all joints with a long wood chopstic & listen for noises through speakers.Do not attempt this if you are not experienced with electronics !
not for me that one then!
 

Disarmamant

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I have a volt meter but will check the dry joints etc before I start poking in there with it on.
But thanks for the advice.
I was not suggesting you check the actual voltages in the amplifier, just that you take a look inside the amplifier with no power and check the voltage rating of those capacitors which will be written on the capacitor body.
 

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