Laney GH50L loud hum from the power amp

[Palantine]

Goodmorning forks!
Yesterday I received parts. I swapped the old sokets for the new ones and added an Allen Bradley 100 ohm, 2W pot.
Disconnected and insulated the centre tap and voilà: amp is still humming.
The humdinger works: in fact, if I rotate it, the hum increase (or decrease) but in a different way.

Now I’m pretty convinced the hum comes from the pre-amp section (I know Enzo....you already told it to me).
I swapped all the pre-amp tubes one by one starting from the PI to V1, as mentioned above and that's what I discovered:

The amp hums WITHOUT V1; what does it means? That hum starts from V2, right or wrong??
What do I have to check? Do I have to check voltage on P10 (220V) and P16 (120V)?

Carlo

[Jon Snell]

Correct.
With V1 removed, if you remove V2 and it is then silent, that is the section to work on.
There are only a handful of connections to worry about.
Remove C10, does it go silent? If it does and then the issue is from V2a.
If you ground P9 and it goes silent, it is coming in from the gain control.
If you ground the top of the gain control and it is silent, it may be from the switching or V1b.
If you remove the 22n coupling capacitor from P5 and it is quiet, V1b circuitry is the cause.
If you ground R4 and it stops, V1b is generating the hum but only when the drive switch is selected.
Have you bridged C8 with a 10u 350v capacitor; did it make any difference?

[Palantine]

Long story short; the thread title must be changed....as the HUM did not come from the power-amp BUT from the pre-amp and more than one person put me on the right track.

After checking EVERY possible source of hum (cables, resistors, capacitors) after measuring voltage and also verifying the FX loop, I simply swapped V2 (which was a TUNG SOL ECC803S for a JJ ECC83s and immediately solved HALF my issue.
Then I swapped V1 for another JJ ECC83S and solved completely the HUM issue.

Now I have one of the quietest amp on the planet. the A/B comparison is ridiculous!

So: I wanna thank everyone who contributed to help me debugging the problem; in hindsight I can say this was an easy fix, but in the meantime that makes me crazy!

Good things: I installed an hum balancer successfully (with YOUR help); on my first attempt (few years ago) I burnt two 17€ multi-turns pots!
I checked and replaced old octal sockets and resistors
I checked and resolded old solder joints and setted a correct bias (40mV with 427 plate voltage per tube)

So, again, thank you all.

P.S. could someone explain me how to edit the thread's title?

[Mick Bailey]

Changing the title then alters the relevance of the replies - people are basing their responses on your presumption that it was the power amp at fault.

[Palantine]

Changing the title then alters the relevance of the replies - people are basing their responses on your presumption that it was the power amp at fault.

At first I thought it was necessary to change the title (but for me, because I wanted amend my mistake in some way), but now I agree 100% with you.
There are probably sereval people starting from a wrong assumption who can find useful this thread exactely as it is.

[Enzo]

REmember, this is just a conversation, this is not a reference library. Changing the title is not necessary. Anyone doing a search will be looking for pretty much just the model number anyway.

[g1]

So you said the problem was 2 tubes. One was the Tung Sol ECC803S. What was the other hummy tube that was in V1 position?

[Palantine]

So you said the problem was 2 tubes. One was the Tung Sol ECC803S. What was the other hummy tube that was in V1 position?

Sorry for the incredible late answer....the humming tube in V1 was an EH 12AX7.
As far as I know, Laney amps demand only JJ or Chinese tubes.....they don't like Russian tubes except for the PI slot (Sovtek 12ax7 LPS)