Laney GH50L loud hum from the power amp

[Palantine]

Hello everyboby from Italy, hope you stay at home safe and secure.
Having (unfortunately) lot of spare time due to the lockdown, I'm keeping one amp a day and check if everything is ok....reset bias, change tubes, speakers, try new combination of pre amp, etc
Yesterday was the turn of my loved/hated LANEY GH50L made in UK; I own this amp from long time...bought second hand, rarely used coz is loud and not versatile.
I changed the power tubes (just to try another duet) and rebiased the amp with a bias probe (after reading the plate voltage on pin 3); it was 58mV per tube!! I re-set to 40mV with a plate voltage of 422v circa.
Doing this procedure I noticed that one socket is (or always was) defective.....no good contact on the heaters (at least pin 2 and don't know how many others).
I managed to restore good contacts on both sockets and after that, both the two EL34 are now glowing.
The problem is: I don't know over the years how many hours/mintes the amp has played just with one tube!

Now I have the amp working, with full volume, but with a LOUD HUM coming from the power amp;I say that because I putted a cord on the RETURN (in series mode) and the HUM is still there; GAIN at zero, just opening the volume, the HUM starts. Not hiss or white noise......just HUM. Hiss and white noise starts when I switch the second gain stage, but I can live with it.
So, for what I know "Tubes will make a myriad of strange and seemingly inexplicable sounds. The one thing they won’t do is hum. It can, however, seem that way if one or more power tubes go out. When this happens, the natural hum from your power transformer is no longer being phase canceled by the tube that just died. This makes it seem like a bad tube is humming, but in reality, it just isn’t silencing the hum anymore. That’s pretty rare, but if it does happen you’ll need to replace the power tubes."

Do I have to replace the OT? How could I check if the OT is correctely working? I guess working only one half of the power amp for few minutes at a time - but for many years - it may have damaged the OT.
HT fuse is ok; volume seems OK, tubes are glowing with the same intensity
Do I have to replace power tubes (I checked and they work...no shorts)?
Any advice helping me to figure out this HUM, would be very appreciated; as far as I remember, this amp always humming loud but few years ago I was less demanding.....that said 90% of the time, this head has not working (turned off) in my storage room.
Thanks in advance and take care!!!

Palantine

[The Dude]

Your schematic link is dead, so here ya go.

laney_gh50l_sch.pdf

It's unlikely your OT is bad. Although possible, that is not a very common failure. Do you have a bias probe or scope? Because a tube lights up or has filament voltage does not mean the tube is working. I'd check all of your screen resistors first and/or make sure you have screen voltage at all of the sockets. If you have a bias probe, make sure all tubes are conducting. If you have a scope, check for ripple on the B+ supply. If not, check ACV with a DVM. You could have bad filter caps. If you want to test the OT, here's a method.

https://www.premierguitar.com/articl...ormer-tester-1

[Palantine]

Your schematic link is dead, so here ya go.

[ATTACH=CONFIG]57965[/ATTACH]

It's unlikely your OT is bad. Although possible, that is not a very common failure. Do you have a bias probe or scope? Because a tube lights up or has filament voltage does not mean the tube is working. I'd check all of your screen resistors first and/or make sure you have screen voltage at all of the sockets. If you have a bias probe, make sure all tubes are conducting. If you have a scope, check for ripple on the B+ supply. If not, check ACV with a DVM. You could have bad filter caps. If you want to test the OT, here's a method.

https://www.premierguitar.com/articl...ormer-tester-1

I just have this kind of "bias probe" and a Multimeter......I need a more explained help, sorry.
What do you mean with "check ACV with a DVM"?
As fot the caps, the amp was recapped with F&T and JJ electrolitics no more than 5 years ago, but as I told in the main thread, I use the amp very seldom (i.e. never).

Nota bene: I have 470 ohm screen resistors (coz the amp can run also 6L6) unlike you can read on the schematic and they measure correctely.

[diydidi]

Make sure pin 5 's of power tubes are getting negative bias. Also check to see if the 470ohm SG resistors are good. If one is bad, that can also indicate a faulty tube.
Next measure plate current through each tube with your bias tube. They should be similar.
If these are all ok, next check the heater supply. Sometimes heater ref resistors are used. One of these could be open too.

[Mick Bailey]

I would remove the phase inverter tube (V4 on the schematic) and see if the hum is still there. Leave it removed while carrying out the checks mentioned. You say that the volume control and channel switching affect the hum and removing the phase inverter will isolate the power amp for testing purposes.

[Palantine]

All I can say at the moment is: both screen resistors are ok (measured 470 ohm); bias is 40mV per tube (quite balanced...one is 40mV, the other is 41mV). There is only ONE bias trimmer.
Changed the phase inverter and the hum is still there; no hum with the PI pulled out.
As far I can understand tubes are ok (brand new); I guess the problem comes from the HEATERS.
Old Fender amps had a pot for hum balancing....I can't explain because I'm a "follow the instruction" guy but there is a "non cancelling problem with heaters and the black wire.
Read this please: https://guitar-dreamer.blogspot.com/...0r-repair.html and tell me what do you think.

I tried THIS method years ago but with very bad results.....the very expensive pot I bought was smoking and burning after few second, so I restored the stock configuration and forget the amp....

This amp has always been a nightmare for me!

[Enzo]

The article you cite shows the guy installing a hum balance pot on the heater supply. But he also shows the original schematic which has a grounded center tap on the heaters. If you install a hum pot you can't leave the center tap. You can't have both. I'd wager that was why your attempt burnt up the pot.

If pulling the PI tube kills the hum, then the problem is NOT in the power amp.

The FX loop has a bypass switch? Great, turn it to bypass. Hum remain? Does turning the MASTER volume down turn the hum down?

With master up, does pulling V1 kill hum? How about V2? and V3?

[Palantine]

The article you cite shows the guy installing a hum balance pot on the heater supply. But he also shows the original schematic which has a grounded center tap on the heaters. If you install a hum pot you can't leave the center tap. You can't have both. I'd wager that was why your attempt burnt up the pot.

If pulling the PI tube kills the hum, then the problem is NOT in the power amp.

The FX loop has a bypass switch? Great, turn it to bypass. Hum remain? Does turning the MASTER volume down turn the hum down?

With master up, does pulling V1 kill hum? How about V2? and V3?

Dear Enzo, I can now reply to half your questions. In the next hours I want to check all the GROUND connections (there are solders but also "ground screw"), then I'll check every preamp tube.

Yes, pulling the PI kills the hum and any other noise.....dead quiet.
I remember hum was there with the FX loop in bypass, then I switched to INSERT in order to plug a cable to bypass the preamp BUT the PI is part of the POWER amp and hum was still there
Also I know for sure that having the MASTER turned all the way down kills the hum; the hum increases as the MARTER volume increase and vice versa.

[Enzo]

Also I know for sure that having the MASTER turned all the way down kills the hum;

OK, there that shows the hum comes from before the master control.

Step back a couple feet and look at the bog picture. In this game we don't guess, we use a systematic approach to isolate the problem. Pulling the PI showed us the problem was not after it. We now know the problem is before the master. Pulling tubes one at a time starting with V1 is not to test the tubes, it is to determine if the problem is before or after V1 - or right in it. If we pull V1 and hum remains we know the hum is after V1. Once we know WHERE the problem is, it is a lot easier to find WHAT the problem is. Just guessing maybe it is this and maybe it is that and going after this and that with a soldering iron is just not efficient.

[Palantine]

OK, there that shows the hum comes from before the master control.

Step back a couple feet and look at the bog picture. In this game we don't guess, we use a systematic approach to isolate the problem. Pulling the PI showed us the problem was not after it. We now know the problem is before the master. Pulling tubes one at a time starting with V1 is not to test the tubes, it is to determine if the problem is before or after V1 - or right in it. If we pull V1 and hum remains we know the hum is after V1. Once we know WHERE the problem is, it is a lot easier to find WHAT the problem is. Just guessing maybe it is this and maybe it is that and going after this and that with a soldering iron is just not efficient.

Can I pull safely ONE tube at a time with the amp running? I mean stand-by on and a charge (my 2X12 8 ohm total) with master on?
Or is better shut off the stand by - pull the tube - and turn it on, for every tube?

Sorry, probably a dumb question

[Enzo]

yes, but why not flip it into standby for the few seconds it takes to yank the tube. Or the few more seconds it takes to replace the one tube and pull the next.

[Palantine]

Yesterday (I'm a headstrong, I know) I decided to check connections on the power tubes sockets AND on various GROUND points all over the chassis.
I found a weak wire coming from pin 1 + pin 8 to ground from ONE EL34 (just touching the wire actually it remained in my hand) AND a defective pin 8 in the other EL34 (same story of the wire.....desoldered the pin and the "tine" or "prong"...(I don't know how do you call it in English) has fallen away.
Both are cathodes - correct me if I'm wrong - and I couldn't have had 40mV on each power tube if there hadn't been contact.
So, these are most likely NOT the causes of the loud hum (we know now it comes from the pre-amp section) BUT I had defective sockets (now broken) and I have to replace them.
So, I ordered new sockets and I'll report back the results of the pre amp procedure, once I'll have the amp working (at least the power section restored).
Happy Easter to everyones and stay safe!

[Jon Snell]

If only one EL34 was conducting then the amplifier will have an unbalanced output stage and it will hum quite badly.
I would replace the valve bases and wire the new ones in.
Take photos of where all the wires come from, it makes it easier to get it right first time!

[Palantine]

The article you cite shows the guy installing a hum balance pot on the heater supply. But he also shows the original schematic which has a grounded center tap on the heaters. If you install a hum pot you can't leave the center tap. You can't have both. I'd wager that was why your attempt burnt up the pot.

If pulling the PI tube kills the hum, then the problem is NOT in the power amp.

The FX loop has a bypass switch? Great, turn it to bypass. Hum remain? Does turning the MASTER volume down turn the hum down?

With master up, does pulling V1 kill hum? How about V2? and V3?

Enzo, please, very clear but in this configuration where the humdinger litterally 'REPLACE' the center tap, what happens to the originary center tap AND T5A fuse?
As for the procedure: I will CUT a little portion of insulation of the HEATERS wires and I will solder them on pin1 and pin3 and then to the tube sockets.
On the wiper I'll solder the GROUND wire.
And I'll remove the center tap and insulate this wire.
Correct?

[g1]

Yes, the center tap of the heater winding should be disconnected and insulated.
You don't need to strip insulation from the heater wires, just connect pin 1 & 3 of the pot to the nearest tube socket heater pins.
Pot wiper goes to ground.