1953 Gibson GA-40 with background noise and tremolo thump.

[Pinball]

Just recently restored a 1953 Gibson GA-40 amp. It had been sitting unused for a long time. When I got it, I didn't power it up I went directly into ordering the caps. It had mostly wax caps with a few paper caps and two electrolytic cans. One of the cans had been leaking. Anyways replaced all of those and a few resistors that were out of tolerance. Replaced the original power cord and tremolo cord. Checked the transformers, and tubes. They all passed. Slowly brought up the amp with no tubes on a variac and dim bulb tester. All passed. Then powered up slowly with the rectifier tube. All passed. Finally loaded all tubes and powered up slowly and it kinda passed. I have a good guitar sound in both the microphone and the instrument inputs. Volumes and tones work great. Breaks up great in both inputs. More so in the microphone as it should.

I'm running the amp around 114VAC

The issues:

1. I have a lot of background noise. More than should be. This is not a hum. Sometimes even an occasional slight pop and hiss. That is usually only in the first 30 seconds. The noise doesn't increase or decrease with adjustments of the volume, tone, or tremolo. It doesn't change when I plug in an instrument. It doesn't change when I switch around any of the preamp tubes. The sound doesn't change over time. I left the amp on for a couple of hours with no change. The tubes are an even glow. No red plating that I can detect.

2. When I engage the tremolo footswitch, I get a audible oscillating thump. This is a strange circuit as there is a three way switch settings for the speed of the tremolo. If the footswitch is not engaged, of course you hear no "thump". When it is engaged the guitar sound is oscillating and sounds ok but has a background thump along with it.


I have taken a wooden chop stick and moved things around within the amp while it is on to try and pinpoint a possible contributor to the noise but haven't found anything.

[PRR]

Is there a schematic a lot like this one? (Gibson changed schematics at whim.)

I fear you will be replacing many/most resistors. Even in-spec ones. Also some tube-swapping. And a hundred hours, supervised!!, of just sitting fully-live to burn out moisture and to burn-out last-leg parts.

Background thump is very common on the silent test bench. There may be some reduction possible. But I think a lot of thump can be tolerated when the crowd, the bass, the drums are all loud. Can you "play over it"?

[sluckey]

Is this the schematic? Can you verify your tube lineup matches this schematic?

     https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga40_2.pdf

[Pinball]

Yes that is the schematic. Here is the pdf of the original schematic that was in the bottom of the amp.

[Pinball]

Here is some more info. So it pops and cracks pretty load for about 5 seconds as soon as the tubes warm up and then goes away. If it turn off the amp and then turn it back one within a few seconds, it really sounds horrible for several seconds before that quietens down again. It always has noisy background but the early popping and cracking is really loud and kinda scary.

[shooter]

early popping and cracking is really loud and kinda scary.

typically a bad solder connection, loose something


pull V1,2,3
power up
does the background noise go away/change?
did it pop n crackle?

[Willabe]

early popping and cracking is really loud and kinda scary.

Typically a bad solder connection, loose something

Sounds like a bad cracked carbon plate R's. They absorb moisture as they age. 

[Pinball]

So I pulled the tubes in following with the results listed.

1. V1, V2, V3 removed. - No startup noise
2. V1, V2, V3, V4, V5, V6, V7 removed. - No startup noise
3. V1, V2, V3, V4, V7 removed.  - No startup noise
4. V1, V2, V3, V4 removed. No startup noise
5. V4 removed. No startup noise.
6 Added back V4. - Startup noise is back. This is the Phase inverter/tremolo. It’s a 6SN7 tube. The tube tested really high on two different testers. I don’t have another tube right now to swap in.

[Willabe]

Edit: He has an older GA40, with a different tube line up.

How are you numbering these tubes?
It should go, 1/2 = preamp 5879, 3 = LFO 6SN7, 4 = PI 12AX7, 5/6 = 6V6 Power tubes, 7 = 5Y3 rectifier tube.

If that's the way your numbering them, then with V7 the rectifier tube pulled, you will have no dcv, so no other tubes will turn on, so no noise.

5. V4 removed. No startup noise.
6 Added back V4. - Startup noise is back. This is the Phase inverter/tremolo. It’s a 6SN7 tube.

No, 12AX7 is the PI tube, the 6SN7 is the trem LFO (low frequency oscillator) tube. Counting forwards from 5879 or backwards from 5Y3 it's still V4.

Try a different 12AX7. 

It could be a dirty tube socket, the tube, or a bad tube plate R.

Have you cleaned the tube socket and re-tension'd it?

That's an old amp, you should clean and re-tension the all tube sockets.

After you get V4/12AX7/PI fixed, I'd still do this too;

Start with all the tubes in. With amp off, pull V1, turn amp back on, listen, then turn off amp, pull V2, turn amp back on, listen, turn amp off, pull V3, turn amp back on listen.

If 1 pulled tubes quiets the amp, then work backwards, start with V3, to find which tube section is the problem. Turn off amp, pull V3, listen. Then turn off amp, put V3 back in, pull V2, listen, etc..... It might be more than 1, even all of them.

Plate R's from the mid 50's were cracked carbon. They actually develop cracks in them from years of going through the heat/cooling cycle of the amp being played, heating up, then being off. Because of this they absorb moisture over the years and hiss and pop. 

[Pinball]

How are you numbering these tubes?

It should go, 1/2 = preamp 5879, 3 = LFO 6SN7, 4 = PI 12AX7, 5/6 = 6V6 Power tubes, 7 = 5Y3 rectifier tube.

If that's the way your numbering them, then with V7 the rectifier tube pulled, you will have no dcv, so no other tubes will turn on, so no noise.

Looking at the schematic, I number right to left. There are a total of 8 tubes

1 Mic preamp 6SJ7
2 Instrument preamp 6SJ7
3 Driver 6SJ7
4 PI /tremolo 6SN7
5 Output 6V6
6 Output 6V6
7 Tremolo 6V6
8. Rectifier 5V4G



This amp doesn’t have any 12ax7 tubes

It matches up with the schematic I attached in an earlier post.

From what I tried I determined if all tubes are in except the PI/Tremolo 6SN7, there is no popping, Once I put that back into place the popping comes back.

[Willabe]

I'm sorry, my mistake. (I don't recall seeing that GA40 schematic before.)

Clean V4 tube socket and re-tension it.

If that doesn't get it try a different tube. You might need to buy a few before you get a good 1. See if you can find a reliable tube dealer, who will test it for noise in a circuit. Not just in a tube tester for the tubes strength.

[Pinball]

No worries. Yeah there a few versions of the GA-40. The. Tube sockets are different than normal as well. They are basically a single tensioned wire that the tube pins make contact with. You can’t tighten them like normal. I did clean them though and everything appears to be tight.

I’m going to order another 6sn7 along with replacement resistors especially the plate resistors. They are close to the heated areas. I was pecking one them with a chop stick while the amp was powering up and a couple are suspect.

[sluckey]

V4b and V7 should be eliminated from the suspect list before diving in too deep.

[Pinball]

Also it makes a loud squealing sound sometimes. That happens especially if the amp has been on a while, you turn it off and back on within a few seconds.

[Pinball]

Update:

Just swapped in a NOS RCA 6SN7. The loud squealing and popping is gone!

Still have a lot of background noise. Same amount of noise regardless if the guitar is plugged into the input or not. Doesn’t change with adjusting any of the volumes or tone.

[PRR]

> Still have a lot of background noise.

Swap V3 with V1 or V2. Replace V3 plate circuit parts.

[Pinball]

So I wanted to give an update on this amp. I finally got all the hiss and popping out of it. The popping was the inverter tube. The hiss was corrected by replacing a lot of the old carbon resistors. Not sure exactly which one because to get down to the 10k for the voltage reducer, I had to remove several others. It was definitely a resistor tied to the inverter tube though. I replaced all the plate resistors on the preamps and that didn't change anything.

Thanks for everyone's help on this. The amps sounds great now. The cleans are good and the amp breaks up extremely well.