Switch on hum/buzz

[synsei]

When I switch on the Valhalla amp, and once the power valves are lit, there is a pronounced hum/buzz coming from the left speaker which gradually fades and changes character (becoming an intermittent and quiet rustling sound) before disappearing completely after about a minute and a half. Is this anything to be worried about?

http://i1019.photobucket.com/albums/af316/Sgtgrash/Hifi%20Stuff/DSC_0028.jpg (http://s1019.photobucket.com/user/Sgtgrash/media/Hifi%20Stuff/DSC_0028.jpg.html)

[worrasf]

I'm not familiar with the layout of your power amp but I had a similar thing with my Croft S4s which was down to a failing 12BH7 tube on the left channel.

Has the hum only just started on switch on or has it always been there?
If the former I'd suggest swapping the small tubes at the front (?12ax7) left for right and see if the noise comes from the other channel. If it does then it's down to that valve - worth swapping them out for new ones as a matched pair.

Just my twopenneth
Steve

[Ali Tait]

Could be down to dirty pins on the signal valves, give them a good clean. Good advice from Stephen though, do try what he suggests, it'll tell you whether it's the valve or the amp.

[synsei]

Hi guys, I have just fitted a NOS, matched pair of Philips ECC88's into the amp. It exhibited the same issue with the Mullards it was fitted with before, although one of the Mullards snuffed it hence the new Philips valves. One of the Svet power valves is dimmer than the others, could this be it? I will try moving it later to see if the issue moves with it. Is it okay to use DeOxit on valve pins and sockets?

[Ali Tait]

Try the signal valves first, then the power valves, see if the dim one is still in the same position or the same valve stays dim.

[worrasf]

Is it okay to use DeOxit on valve pins and sockets?

Well I have but I understand Glenn C disapproves :(

Steve

[synsei]

Same amp André

[realysm42]

Well I have but I understand Glenn C disapproves :(

Steve

Why?

[Ali Tait]

Buy a fibre pen off ebay, good for pin cleaning. Or, this isn't a wind up, stand your valves in a bowl and pour in some Coke till the pins are covered. Leave overnight and wipe dry in the morning.

[synsei]

I have moved the dimmer 6L6 from the left to the right side of the amp but the hum/buzz is still occurring through the left speaker. Again, it is only for about fifteen seconds or so and then it goes completely. I also swapped the ECC88's around with no effect.

The amp was recently recapped and had some replacement resistors fitted and it was hoped that this would solve the problem but it has not unfortunately. The caps needed replacing anyway as a couple of them were bulging. I cannot be more specific than that gentlemen as it was not I who did the work...


Buy a fibre pen off ebay, good for pin cleaning. Or, this isn't a wind up, stand your valves in a bowl and pour in some Coke till the pins are covered. Leave overnight and wipe dry in the morning.

The old Coke trick eh? I cleaned many a penny in Coke in ma yoof... :lol:

[Ali Tait]

What voltage rating are the mains transformers, could be the components are being stressed by too high voltages.

[worrasf]

Why?

Something about leaving a residue on the pins I think - but I did them all anyway :D
Cant say it made any difference though :eyebrows:

Steve

[synsei]

The transformers have been checked and they are spot on for our voltage apparently ;)

[dantheman91]

Hi

Could it be one of your speakers could try swapping them round to see what happens

[synsei]

Definitely not the speakers Dan it is an amp issue. There is no hum or buzz when the T-Amp is hooked up to them and besides, Shane had the same problem when he owned the amp and it was running different speakers and valves back then ;)

[Rare Bird]

Would it be a big job to swap over the output transformers see if the issue still shows running on the right channel?

[synsei]

I wondered whether it might be the output tranny myself although I hope it is not as I am guessing that it would be expensive to replace. Swapping the Trannies over is beyond my ken...

[The Black Adder]

I've found that some amps hum at first then go quiet, some don't hum at all at first then start to hum as the valves start to be driven. I don't think there is anything to worry about so long as it's not really loud then, if that was the case I'd have it checked.