Another humming problem!

[alcarmichael]

A few weeks ago I posted in here regarding my power amp and its humming transformers. I rectified that issue with your help. Last night I turned my system on and there was a very loud buzzing/humming coming through the speakers. Initial assumption was the power amp again but this time it turns out to be my Croft 25R/RS pre amp. It does it with nothing connected to the inputs, I did quickly swap over the interconnects from pre to power but no change.

Nothing has changed in my system or even anywhere else in the house.

Please impart your wisdom on me once again....

[Bigman80]

Got any spare valves? Swap them out one at a time, Always worth taking them out and re inserting them too. Could just be a bit of grime on a contact.

[Stratmangler]

Any chance of recording 30 second or so of the noise and posting it up?
It'll give a better idea of what you're dealing with.

[Jazid]

Electric storms last night, could it be related to a power surge?
But for the timing I'd say as bigman above, also you could try shorting an input when selected to see if the hum goes. Just attach an RCA cable and stick the blade of an INSULATED screwdriver across the plug at the other end.

(normally it wouldn't need to be insulated but safety first...)

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[alcarmichael]

I don’t have any spare valves. Will look into ordering some this evening.

I will take the pre back to work with me and connect it into my other system to enable me to try and get a sound clip.

To short the input am I connecting one rca cable and shorting the centre part to the outer part of the connector?

[YNWaN]

Yes.

[Jazid]

To short the input am I connecting one rca cable and shorting the centre part to the outer part of the connector?


Yup that's the thing, won't take a second and it eliminates one issue from the list.

[Bigman80]

Just get the cheapest new valve you can, it's only for checking the good ones.

What type you need?
I don’t have any spare valves. Will look into ordering some this evening.

I will take the pre back to work with me and connect it into my other system to enable me to try and get a sound clip.

To short the input am I connecting one rca cable and shorting the centre part to the outer part of the connector?

[337alant]

A loud buzzing at the speakers could be DC getting through I would check he output of your pre amp and power amp for DC
Disconnect your speaker as you could blow a driver

Alan

[Bigman80]

A loud buzzing at the speakers could be DC getting through I would check he output of your pre amp and power amp for DC
Disconnect your speaker as you could blow a driver

AlanA quick step by step may help avoid a blown driver?

[alcarmichael]

So I’ve tried the RCA shorting thing - no change. Next I cleaned all the pins of the valves with contact cleaner and refitted, no change (except my valves are all nice and clean now!).

I have noticed there’s a very slight hum coming from the pre itself, unsure if this is normal.

I’ve checked the valves and in there I have:

Tungsram ECC83 x2
USA (can’t make out the emblem) 12 BH7A x1
Golden Dragon ECC83 x2
Mullard (?)5A2 x1

I guess the next step is to buy a set of replacement valves and see what happens....?


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[Bigman80]

Hmmm

[walpurgis]

You haven't got the pre-amp too close to the power amp by any chance. It could pick up hum from that.

[alcarmichael]

I normally have the power amp on the shelf above the pre. It’s been fine like this though until Friday night. I now have it at work with me in a different system, wasn’t that close to this power amp and same problem.


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[alcarmichael]

I have ‘filmed’ the hum but don’t appear to be having much fun with YouTube while trying to upload the clip for some reason. Will try again later. Unless there’s a better way to upload a sound clip....?


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[vinyl]

Have you emailed Glenn Croft?

[alcarmichael]

Not yet. I’m sure he will sort it for me but I thought it may be a good idea to ask here first in case it was something simple.


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[Jazid]

I have noticed there’s a very slight hum coming from the pre itself, unsure if this is normal.

I’ve checked the valves and in there I have:

Tungsram ECC83 x2
USA (can’t make out the emblem) 12 BH7A x1
Golden Dragon ECC83 x2
Mullard (?)5A2 x1

I guess the next step is to buy a set of replacement valves and see what happens....?


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Hi Alex
1.- Does the hum come on as soon as you switch on or after a few seconds as the valves warm up? If on start up I doubt it is a valve and I suggest you get it to Graeme at www.ampregen.com who will sort it for you and is relatively local. If it builds up then here's a plan of action:
2.- Keep the power amp volume down so you won't damage the speakers and perform each test just for long enough to get a result. As 337alant says it is just possible there could be a quite high voltage coming out of the pre and so use a modest power amp 'in case' and don't touch the pre-out leads.
3.- Don't remove the two PSU valves at any point. These are the 85A2 (Pretty sure this is what is in there, not an 5A2 which is a diode...) and the 12BH7a.
4.- Unplug from the mains (not just switch off) between each operation and leave for a few minutes for the capacitors to discharge.
5.- Check for any burnt or blown looking components in the pre - they will be pretty obvious ie. blackened or leaking gunk. If so, Graeme again. If not
6.- Note down where all the valves go.
7.- Starting from one side unplug only one ECC83 from the pre. Power up and check if the hum has gone. If yes you might have a culprit. Note which it is and put it back in the circuit for now and continue. If the hum remained with the valve out continue.
8.- Unplug the next ECC83, power up and try again. Any reduction in hum? If yes, write down what happens and progress to 10.- below, if no, replace the valve and continue.
9.- Repeat for the other two ECC83 valves one at a time, noting any changes, replacing and proceeding as above.

10.- At this point you have checked either two (if coming from point 8.- ) or all four signal valves. If any are malfuntioning you might have found that removing just one of them eliminated the hum. If so this is the valve to replace. You may have found that removal of just two of the valves had no effect on the hum, if so these valves are probably good. If removal of more than one of the valves eliminated or changed the hum then these need further investigation, see below.

11.- Make a cup of tea. Tea is good.
12.- Leave the two known good valves in circuit. Power down and wait. Remove the two suspect valves and swap their positions, so both remain in the pre. Power up. There should be hum but it may be different? No matter, power down, take notes, wait, and continue.
13.- Remove one of the two swapped suspects and power up again. Is the hum still there, same or reduced? If so power down, note and continue.
14.- Replace the removed suspect valve to its new position, remove the existing suspect valve and power up. Any hum?
15.- At this point you should have found that either the hum reduced or was eliminated by one of your actions. If, with both valves in and swapped the hum was reduced you will probably find that with only one or other of the valves in place it will have been eliminated. If so the problem is the valve that was not in circuit when the hum was not there.

16.- There are other permutations but at this point you have enough chance to get the culprit that we can wait to see what you find. Assuming it is one valve then personally I'd buy a matched pair. My reasoning is that there are probably one GD and one Tungsram in each channel, and for channel balance and sonics replacing the pair will help. They don't have to be the same make as Glen's orignals however, try something different if you feel like it. Then say for example it was a GD valve that was faulty, I'd replace both GD tubes, keep the known good one as a spare, chuck the known bad one, and enjoy the sounds.

[alcarmichael]

Hi Alex
1.- Does the hum come on as soon as you switch on or after a few seconds as the valves warm up? If on start up I doubt it is a valve and I suggest you get it to Graeme at www.ampregen.com who will sort it for you and is relatively local. If it builds up then here's a plan of action:
2.- Keep the power amp volume down so you won't damage the speakers and perform each test just for long enough to get a result. As 337alant says it is just possible there could be a quite high voltage coming out of the pre and so use a modest power amp 'in case' and don't touch the pre-out leads.
3.- Don't remove the two PSU valves at any point. These are the 85A2 (Pretty sure this is what is in there, not an 5A2 which is a diode...) and the 12BH7a.
4.- Unplug from the mains (not just switch off) between each operation and leave for a few minutes for the capacitors to discharge.
5.- Check for any burnt or blown looking components in the pre - they will be pretty obvious ie. blackened or leaking gunk. If so, Graeme again. If not
6.- Note down where all the valves go.
7.- Starting from one side unplug only one ECC83 from the pre. Power up and check if the hum has gone. If yes you might have a culprit. Note which it is and put it back in the circuit for now and continue. If the hum remained with the valve out continue.
8.- Unplug the next ECC83, power up and try again. Any reduction in hum? If yes, write down what happens and progress to 10.- below, if no, replace the valve and continue.
9.- Repeat for the other two ECC83 valves one at a time, noting any changes, replacing and proceeding as above.

10.- At this point you have checked either two (if coming from point 8.- ) or all four signal valves. If any are malfuntioning you might have found that removing just one of them eliminated the hum. If so this is the valve to replace. You may have found that removal of just two of the valves had no effect on the hum, if so these valves are probably good. If removal of more than one of the valves eliminated or changed the hum then these need further investigation, see below.

11.- Make a cup of tea. Tea is good.
12.- Leave the two known good valves in circuit. Power down and wait. Remove the two suspect valves and swap their positions, so both remain in the pre. Power up. There should be hum but it may be different? No matter, power down, take notes, wait, and continue.
13.- Remove one of the two swapped suspects and power up again. Is the hum still there, same or reduced? If so power down, note and continue.
14.- Replace the removed suspect valve to its new position, remove the existing suspect valve and power up. Any hum?
15.- At this point you should have found that either the hum reduced or was eliminated by one of your actions. If, with both valves in and swapped the hum was reduced you will probably find that with only one or other of the valves in place it will have been eliminated. If so the problem is the valve that was not in circuit when the hum was not there.

16.- There are other permutations but at this point you have enough chance to get the culprit that we can wait to see what you find. Assuming it is one valve then personally I'd buy a matched pair. My reasoning is that there are probably one GD and one Tungsram in each channel, and for channel balance and sonics replacing the pair will help. They don't have to be the same make as Glen's orignals however, try something different if you feel like it. Then say for example it was a GD valve that was faulty, I'd replace both GD tubes, keep the known good one as a spare, chuck the known bad one, and enjoy the sounds.

I'll hopefully try this very detailed - thank you - test this evening.
The very slight hum starts when the light changes from red to green. If the power amp is on this is also when the loud buzz through the speakers starts.
I knew there was something before the 5A2 hence the (?), I just couldn't make out which number it was! Thank you for clearing that up :)


Is that buzzing superimposed on the music or is there no music signal at all?

Both! It buzzes with no music and when music is playing.

[vinyl]

I think you misunderstood the question - I was asking if it works at all, ie a signal comes out, or if it was totally dead bar the buzz. Anyway, you've answered that question.
Is it definitely buzzing, or is it humming? Buzzing would be like a wasp in a jam jar.

Actually, it probably doesn't matter. I doubt the problem is as simple as a dodgy valve and diagnosing problems via forum posts is very difficult. It probably needs to be examined by someone who knows what they're doing.

This (excellent advice Andrew :thumbsup::thumbsup:)

[alcarmichael]

I would say a wasp in a jam jar would be far too quiet and probably too high a pitch so lets go for humming!


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[alcarmichael]

A hum could be caused by numerous things but just as an academic exercise I'll take a stab at it. My guess (and that's all it is) is that the hum is caused by excessive ripple on the power supply due to the voltage regulator having failed.
The circuit is likely to be rather more sophisticated than a run-of-the-mill valve preamp so I'd get Glen Croft to take a look at it - he'll know the circuit better than anyone.

It's certainly looking like that's going to be the plan of action. I just wished I discovered the problem sooner as I was up in Yorkshire all of last week so could've taken it up to Glenn.

I guess I can still try and rule out a dodgy valve with the process provided by James, unless you think it's probably a waste of time and a valve is very unlikely to be the cause of my problem?

[Jazid]

Hmm, just looked back to the original post. I had it in my mind that the hum was in one channel and thought heater:cathode leakage in a signal valve, but it seems it's in both, my bad, I must read more carefully next time. Apologies Alex. Given that the hum is in both channels and there are two sets of ECC83 that are probably not shared across both channels it becomes rather unlikely that two valves have failed, one on each side, simultaneously, so Andrews PSU option is more sensible. If you wish to play around then at least 85A2 valves are cheap and plentiful, they are almost always Mullard, and you can get them NOS from eg. Langrex, Billington and so on for about a fiver each plus postage.