Laney VC30 hum/buzz

[alec-]

Hi all! I've got a troublesome Laney that's puzzling me at the moment. I was having issues with a persistent 100hz~ hum and given it's age figured a cap job would probably solve it. This done and a new set of valves installed I fired it up again to find, guess what, it still hums.

Have read through a number of threads here and at other places but can't seem to figure this one out from any of the advice. Here's the strange thing - the noise stops (or at least is bearably gone) when the chassis isn't installed in the cab and is also lower in volume with it resting in the cab but 'inside out' - but when installed it's loud and seems to push the amp towards popping the HT fuse. Voltages across the amp seem fine (they're a touch high but apparently no higher than Laney expected this model to run at!) Could something be causing the amp to self oscillate when it's in the cab? Any suggestions or pointers for things to check would be helpful and very much appeciated!

[g1]

Does the reverb control have any effect on the hum when chassis in cab?

[alec-]

It doesn't seem to, no, and the noise is there with the reverb entirely disconnected too unfortunately!

[g1]

Do the mounting screws have to be tightened, or does it just have to sit in the cab in correct orientation? Is there any shielding in the cab?

[alec-]

Both with and without the screws in. I don't think there's any shielding in the cab - certainly nothing obvious.

[Mick Bailey]

Does this amp have the Invader speaker or the Celestion?

[alec-]

This model has the Celestion in (it's from 2004-ish, I think?)

[Mick Bailey]

That should be OK. The earlier Invader speaker sits closer to the output tubes and the magnetic field can affect their operation (causing an imbalance in current draw and more hum).

With noise, it's best to determine if the preamp or power amp is responsible. Remove the PI tube and see if the problem persists. If it does, the fault is with the power amp (or power supply), if the fault goes away then it's the preamp or PI where the noise is coming from.

[alec-]

Just double checked and it is still there when the PI tube is removed but it is certainly louder with it installed. Moving the chassis back and forth in relation to the speaker does seem to increase the hum so perhaps could be the same issue as with the Invader speaker?

[Mick Bailey]

I would remove the speaker, remount the chassis in the cabinet and re-connect the speaker (say) a foot away. If the noise stops, then you know its a proximity issue.

[alec-]

Just given it a quick spin with the speaker mounted on the front rather than rear of the baffle (giving about an inch extra space) and that does seem to have made a huge difference, so I think we're getting there! Thank you! Would never have thought to try that!

Would you think that the voltages being high/the power valves somewhat over dissipated would also contribute to the effect? Might give something other than JJs a try, too, just for the sake of it...

[Mick Bailey]

More dissipation does increase the effect, but these amps do run their output tubes hot - did you calculate the actual dissipation?

The issue is that the electron stream inside the tubes is magnetic and is deflected by a magnetic field. If one side of the push-pull output is affected more than the other then the common-mode noise cancellation is reduced and you hear this as hum. It's like having a badly mismatched set of tubes. If the tubes do already have some degree of mismatch then you can change the order of them to improve matters. Another way to improve things is to bend up a piece of steel sheet slightly higher and wider than the output tube set and mount this between the speaker and the tubes. It needs to be thick enough to resist the magnet pull.

[alec-]

Aha, of course. Yeah, they're sitting at about ~14w (120% ish?) and as before the voltages seemed high to me - but Laney claim these should be fine. I'm obviously increasingly skeptical... Could perhaps try dropping them to match schematic values more closely?

[Mick Bailey]

High voltage usually isn't an issue and it's not uncommon to find EL84 amps running what appears to be an excessive voltage e.g., Mesa, Marshall, Traynor. The plate and screen dissipation need to be kept within safe operating limits, though. When checking voltages I also check the heater voltage, as this can give a clue to how much 'over' the amp is running overall.

If you want to reduce the dissipation you could increase the value of the cathode resistor, though I've never needed to do this with this particular model.

[alec-]

The heater voltages were definitely above schematic values, too, around 0.6v overall.

I grabbed a quad of Sovteks to try (as the data sheet suggests they're 'happier' with higher plate dissipation) and although there is a really tiny amount of hum left it does seem to largely be behaving itself (and also running significantly cooler) Thanks so much for your help!

Now I just have to chase a bit out of the grill cloth mount to fit around the new speaker mount, fun! Hah!