Laney VC30 Porblems

[shadygreentree]

Hello All
I have a Laney VC30 I am trying to fix. It stopped working when the user replaced valve. I found 470 ohm 5W resistor in HT line feeding plate of preamp valves and screen for EL84 open. The amp started working once resistor was replaced but

1. it still has lots of crackling and popping sounds. At times it gives out a hum like one of the supply rail is shorted.
2. The screen dropper resistance shown on schematic is 100 ohms but in this amp it is 82 ohms, I am not sure if it is a mod done by someone or is it design change by Laney
3. The HT voltage is much higher than 290V shown on schematic. It is about 350V

Wonder if anyone can please help.

Schematic can be found here:
http://bmamps.com/Schematics/Laney/L...30_pre2008.pdf

Thanks in advance.

[Mick Bailey]

The parts list shows 4 x 82R 2.5W resistors, so it looks like your amp has factory installed resistors that were subsequently changed to 100R, as the 2008 revised schematic (Electrotanya) also shows them as 100R.

If you're in the US, make sure the transformer uses the correct tapping - it can be wired for 100v or 120v. 100v uses blue + red/grey. 120v uses blue + brown.

When the amp is working, is the volume good and fairly clean?

[shadygreentree]

Hi Mick Bailey
Thanks for quick reply.
No, this is not US version, I am in Australia. No taps on transformer.
I tried plugging in new valves, the output is relatively clean, and its loud. I am still worried about getting the new valves cooked due to high HT and screen voltage.
The voltage on cathode is 6.6V and the schematic says it should be close to 9V.

[Dave H]

With 350V HT there should be more than 6.6V across a 56R cathode resistor. Check to make sure the cathode resistor really is 56R and also measure all four 82/100R screen resistors to make sure that none is open circuit then power it up and measure the screen voltage on pin 9 of each EL84.

[Mick Bailey]

Also measure the plate voltage on each EL84 socket. This is a better measurement than HT and gives you the figure to work out tube dissipation. Measure the heater voltage too, to see if this also high.

Whenever I get unusual voltage readings there's another check I do - my meter battery. A low battery in some meters can cause errors.

[pdf64]

With a shared cathode bias resistor arrangement, all power tubes must be operational / conducting for sufficient current to flow and generate the bias voltage.
If one or more power tubes stop working, the remainder run hot.
Maybe one or more EL84 are dead or otherwise not conducting for some reason?
The resultant lower current draw would go towards explaining the high VB+ and low cathode voltage.

[Dave H]

Maybe one or more EL84 are dead or otherwise not conducting for some reason?

That's what I thought. If say only three EL84 were working it could explain the hum and low cathode voltage. I'd check the bias current on both sides of the OT to see how well balanced it is.

[shadygreentree]

Thanks all for taking time to reply.
I replaced all 4 EL84 and the amp is working better. Not 100% OK but better. The cathode voltage has come to 12V. Checked all resistors etc. There are still some pops and noise without even the driver plugged in. I may try replacing the HT filter cap.

[The Dude]

I'm calculating about 53mA per tube, which is pretty high, IMO. Are the tubes red plating? If it were mine, I'd probably try raising the value of the cathode resistor if you've verified that everything else is working correctly.