Gibson GA-20 Hum problem

[stompjunkie]

Hey Everyone,

I've been re-building a Gibson GA-20 (the older octal version) it's been re-capped, and most of the resistors have been replaced for being way too far off spec. Everything on this amp seems OK, except for a huge hum problem. I've done everything I can think of ie tube swapping/testing, replacing every lead that doesn't have good continuity, making sure the bias is set within a descent range, voltages are reasonable etc...

The hum doesn't change with the controls, and persists when all but the rectifier and output tubes are pulled.

the OT had a short in it so I replaced it with a MM trannie. It's hum doesn't change when I swap the primaries which seems really odd to me.....

In my googleing I've found that quite a few people have had hum issues with these. I was wondering if there was anything that you might know of that would be a problem.

I really appreciate any help with this. I'm a bit stumped.

[Phil_S]

Does it hum when you put the amp chassis back into its housing? Or are you reporting this with the chassis on the work bench?

Got a picture to show us?

[tonestack]

Does the power transformer hum?

[selloutrr]

the tubes are known good and not just new?

the filement windings are referencing ground?

the wire dressing is neat and as short as possible?

signal wires dont interact with power wires unless nec.?

chassis is grounded to your AC ground. ( 3 prong )

faulty input jack? not closing

[selloutrr]

could also be a bad internal ground ( cold solder ) in the grounding back of pot, chassis, just an over sight on an eyelet.

check continuity to the chassis on all grounds.

[Phil_S]

The hum doesn't change with the controls, and persists when all but the rectifier and output tubes are pulled.

Are you saying the hum persists when only the power tubes and rectifier are in circuit? I think that's what you said. IMO, we need a good look at the power section grounds.

Many old Gibson amps had floating grounds. You may want to terminate whatever passes for a ground buss on the chassis. Not sure if this applies.

According to the schematic, the filament winding is center tapped with the CT elevated at the power tube cathodes. I am amazed to see this on a Gibson schematic. Given what you describe, I might be tempted to move it directly to ground.

More "drastic" steps, only inasmuch as they deviate from the schematic, include putting stoppers on the grids and screens. I know this isn't usually something you do to address hum, but they are "missing".

I know you did a cap job, but maybe you just got a bad cap? Swap out the 20uf that's on the rectifier?

These are supposed to be sweet amps. You'll get it running soon.

[selloutrr]

quick way to test filter caps in ciruit after visual inspection without a fancy cap tester. Basic good bad test...

drain the filter caps, confirm the correct polarity and meter the ohms between the possitive and negative lead of each cap it should be greater then 15K if not it's bad.

[stompjunkie]

Thanks for the replies!

Phil... The hum is the same in on the bench as it is tucked back in the cabinet across the room...

tonestack... I haven't noticed any unusual hum in the PT, but I'll check it tonight.

selloutrr... I've used many tubes both known good and new. Currnetly running a set of known good ones.

The filement windings are grounded

The wire dress is a mix match. I've replaced the bad leads and routed the best that I could. I've moved them all around with my trusty chopstick with no luck. I've tried to make the signal and power wires mix as little as possible.

The 3 -prong was the first change to the amp. The ground makes good continuity.

I've checked the input jacks, I even used alligator clips to make sure they were making good contact.

I'll triple check the all of the grounds tonight.

Phil... Yeah, it'll hum with just the rectifier and power tubes in. I tried moving the filement ct to ground yesterday as I was suprized that they would have it on the cathode, but it didn't seem to help any.

The stoppers are a good idea, I can try that too while I'm at it. I'll try replacing the filters...as long as I have some waiting around.. I'm sure it wont be too long before I order more parts....

selloutrr... thanks for the tip I'll give that a try before I swap them out.

Many Thanks for all of the help. I'll give those tips a try and report back!

[selloutrr]

just a thought try bussing all the grounds to one location with alligator clips.

[Firestorm]

Make sure the heaters on the output tubes are in phase (pin 2 connected to pin 2, pin 7 connected to pin 7), otherwise the hums add instead of cancelling.

[labb]

One way to check to see if the hum is coming from the heaters is to disconnect the heaters from the PT and power them with a 6 volt battery. (one of those big lattern batteries used in the large spot lights.) That way you can at least rule out or confirm one potential problem source.

[Phil_S]

just a thought try bussing all the grounds to one location with alligator clips.

My instincts lie here, too, though doing the battery check on the filaments is worthwhile and no stone should be left unturned.

You don't say if this amp has one of Gibson's "famous" floating ground schemes. I've got two Gibson amps, both with floating grounds. On one, a cap job fixed the hum, so I didn't mess with the grounds. On the other, I had to rework the whole ground buss to get it to stop humming.

One other thought is that the hum is transformer induced. If it is not too troublesome to unbolt the output transformer so you can rotate it, that might be worth a try.

[stompjunkie]

I didn't have much time last night, but I bussed all of the grounds to one central ground with no change in the hum. I will hopefully be able to do a bit more tinkering this weekend.

As far as a "floating" ground, I may be a bit unfamiliar with those schemes. The PT is grounded to one central point on the chassis, and the signal grounds trace back to the chassis through the input jacks. As far as
I can tell it's all pretty much the same as the standard octal version schematic.

I have rotated the OT with no results as well.

I'm going to try the filement battery test and test/swap the caps and see what happens next. I'll do my best to keep you all posted.

Thanks again for the help!

[Phil_S]

Floating ground means there is no conventional chassis ground point like we are accustomed to seeing. I think they rely on the input jack contact with the chassis. Yuk. Sounds like you don't have that.

[Phil_S]

Show us some pictures if you can. Other eyes may see things. There's got to be a solution and this amp doesn't have that many parts. Besides, knowing it hums with preamp and PI tubes pulled pretty well confines the problem to the power amp section or grounds.

Reading back through, you say you've swapped tubes. You aren't specific and I wondered if you have swapped the rectifier tube. Also, can you give us voltage readings with all tubes in circuit, both tube socket readings and at each of the 3 filter caps? I keep thinking this is something simple, if you will, hiding in plain sight. For that reason, going through the basics may prove to be worthwhile.

Is the hum 60Hz or 120Hz? That would also tell us a great deal. Low E on the guitar is 82Hz and 5th string open A is 110Hz. 60/120 is approximately A#/Bb.