Laney VC30 hum/buzz

[Mick Bailey]

Happy you got it fixed. At one time I used to buy Sovtek EL84M tubes for amps that ran at overy high dissipation, but the price of them is now ridiculous here in the UK.

[alec-]

Yeah >£20 for a single is fairly eye watering!

Out of idle curiosity this evening I swapped out the preamp/PI valves for some Tungsols I had laying about and that seems to cause some further 'interesting' behaviour - our friend the hum returns and it looks like that too causes there to be too much current through the power valves. I wonder if this is just a gain thing or is it more likely that there's some voltage from those stages getting to where it shouldn't be?

[Mick Bailey]

Does the amp behave as before - when you take the chassis out the hum stops?

[alec-]

No, doesn't seem to make much, if any, difference oddly!

[alec-]

Having another poke around this afternoon - it's practically inaudible with the PI tube pulled but still grows in volume a fair bit when the standby is switched off.

Also testing again - it does seem to drop very marginally with the chassis at 90deg to the cab.

[Mick Bailey]

You may just be dealing with an amp that's sensitive to tube types. I've just had a Laney LC-30 (similar topology) that's really picky about tube brand - almost everything I tried had some degree of noisy hum, until I finally ended up using a NOS Brimar as the final cure.

When you're measuring the dissipation, are you subtracting cathode voltage?

[alec-]

Yeah, had subtracted that when doing the calculation.

Interesting to hear about the LC-30! With them having similar layouts do you think there's any great possibility of the noise being a coupling issue somewhere? From the PT maybe? Or just the preamp chucking out too hot a signal with some valves?

Thankfully it's sitting fairly quiet still with the JJs in the preamp but will have to maybe pick up some other brands and investigate!

[Mick Bailey]

The PT is toroidal so the magnetic field isn't as much of an issue and is far enough away from the preamp tubes. Tube gain amongst brands is fairly consistent - certainly not enough to produce dramatic gain changes in the amp itself, but the construction varies considerably if you look at the internals and electrode spacing.

[alec-]

Was having another play around with the amp today trying dropping heater voltage etc. to see if that made any difference - which interesting did to but a probably negligible amount.

The other thing I noticed is that while it was sat on standby (with heaters stock etc.) that there was a very faint hum still there and it does seem to be coming out via the speaker too. So now I'm really wondering if there's a PT or heater supply issue too - or if at this stage I should just admit defeat!

[Mick Bailey]

Sometimes the mechanical buzz from a PT gets transmitted through the cabinet and sounds as if its coming from the speaker cone. Another reason is the OT may be picking up the stray field from the PT. Was the amp not fixed in post #15?

[alec-]

Hi Mick, yeah, things are/were definitely at a usable level after those changes. I think I'm probably being a touch fussy at this stage - it's likely one of those 'now I've noticed it' kind of deals but would be nice to get rid of as much of it as I can (although I guess good to head off any other potential issues it might cause?)

Have tried unbolting the PT from the chassis and that doesn't seem to make any difference.