Laney GH50L hum

[jokn]

Hi folks! I'm a complete noob when it comes to electronics, but I'm eager to learn. I recently picked up a Laney GH50L cheap, as it was being sold with a description of "crackles and pops, tubes missing". I replaced all tubes and fired it up: after biasing it sounded pretty much as I remembered (I used to have one years ago), but there was noise, like various types of it. After much Googleing and poking around, I managed to get rid of some of the noise and general fizziness.
Here's what I've done:
-replaced the reservoir caps (50+50uF on the schematic, were actually 100uF) and smoothing caps(C12, C11, C10, C1).
-installed a 220R humdinger pot in place of the heater winding CT
-replaced heater wiring for V1 and V2 with a twisted pair
-removed C9 (based on reported reduced hiss by someone on AX84)
-replaced C5 and C6 with a single 2p2 (same as above)
-replaced V1a and V1b grid stoppers with 2W metal films, went to a 180k on V1b
-moved the power tube cathode ground connection to the point where the caps and the PCB are grounded.
-tried several known good tubes in all positions.

As it stands, I'm really happy wih the tone; the fizzy top end is gone and the hiss associated with the additional gain from V1b is just the price you pay for the gain, I guess. However, a hum remains. It's very close to a low G# on the guitar, so I think it's 100Hz.

- the hum is present with or without input, and is pronounced enough to be a nuisance at bedroom master volume levels, if the gain is above 10 o'clock
- the hum volume responds to both gain pots and master volume
- it fades away within a second of switching off HT

Any and all help in tracking it down would be greatly appreciated. I only have a DMM at the moment, although I'm on the lookout for an affordable 'scope.

Sorry about being so long-winded

http://www.schematicheaven.net/newamps/laney_gh100l.pdf

[The Dude]

Firstly, welcome to the place. There are numerous problems that can cause hum- everything from input jack issues to bad filter caps. I would suggest you start here:

Tube Amplifier Debugging Page

Click on the "Hum" category and follow RG's tips. If you have further questions or problems, come back and ask. There are many here willing to help, but I would try narrowing it down a bit with those instructions first.

[jokn]

Thanks for the link! I've gone through the list now, and so far no joy. I did measure voltages, and there's a discrepancy I don't understand: I'm getting 308V across P49 and P50 which is to spec, but I'm only gettin 407V at P51, while the schematic shows 460V at that point. I did also notice that the schematic is for main PCB 9050-3, and what I have is 9050-5. There are caps labelled CX1-CX4 next to the B+ diodes that I'm not seeing on the schematic... I'll have to get in touch with Laney and get the correct schematics for my amp.

[lowell]

P51 will change a fair amount depending on the bias of your power tubes. Though 53v is a fair amount admittedly. If Laney biases on the colder side, and your amp is biased hotter. P51 will read lower since the tubes are drawing more current and pulling on the HT. Have you checked the bias? And since you're "new" to electronics you should know that MEASURING THIS CAN KILL YOU if you slip. So be extremely careful. You should clip you meter in place with the amp unplugged first. Then fire it up. This is of course if you use the transformer shunt method... which is really the only way to do it other than desoldering an OT primary. I prefer not depend on 1ohm cathode resistors. Just my own personal preference.

I'm guessing these are EL34s?

Fixed bias amps should be biased to 70% max dissipation. JJ EL34 are 25watts max. 25x.7=17.5
P(watts)=V(voltage)/I(current) or easier said in this application - I=P/V. So X=17.5/407 = .043 = 43ma PER TUBE.

In a 4 power-tube power amp in class AB there are 2 tubes in parallel on each side of the output transformer. So when you do your calculation, double the result - 43ma x2 = 86ma. You'll want to dial in the bias current so your meter reads 86ma.

Before you do this just make sure the 1k5 screen resistors measure correctly with the power off. If one of those is bad, you could potentially have only one tube working, and dialing in the bias current to DOUBLE could result in red plating the one working tube. Although you'd hear a good amount of hum if this was happening. I THINK because the power supply noise rejection isn't working as it should in a normal working AB output.