Airline GIM-9131A knowledge/repair question

[btah]

I am hoping someone might be able to give me a little direction. I have done a bunch of searching and haven't come across anything that might help me. I am a newb to the forum and sincerely appreciate any comments I get.

Here is what I have. An Airline 9131a. Actually found it in the recycle bin about a year ago. Cosmetically in excellent shape. I also think internally is in, for the most part, in good shape. Clean and appears well taken care of. However, it does have a few problems and I am not sure which direction to go. Also, I am not a musician at all. However, I do have a guitar in the house from when the kids were small. A very cheap one, but it is a guitar...

The amp has two channels. Each channel has two input jacks. One channel has volume and tone. The other channel has volume and tone but also appears to have the reverb and tremolo hooked up through it.

To the point: If I jack in the guitar to channel 1 and hook up a bookshelf speaker to the output things seem OK. I had one of the kid's friend play through it and it sounded decent to me (for what that is worth) but at random times the amp will just start squealing. Not feedback, but a high screeching noise. Turning the amp off and immediately back on will allow it to be usable for some random amount of time and then it squeals again.

For channel 2, if I turn the volume up but the reverb down, it isn't very loud. As I turn the reverb up it gets louder but also starts to hum. Not sure if the reverb has to be turned up with the volume or not. But, again, at random time periods, the amp will start to squeal.

My thought was that since I can turn the amp off and immediately back on and it will go for a while (a minute, 10 minutes, 20 seconds... it is random, no pattern) I think the tubes might be OK. I have looked around locally and haven't found a place/way to get the tubes checked.

Anyone have a thought of what I can look at? Oh, I have cleaned all the pots several times. It seems like there isn't much but tubes and capacitors.

If you got this far, thanks...

[btah]

I should have also said I have a little electronics experience, not a lot, but a little. For electrical testing I have a few DVMs and a capacitor checker.

[btah]

Dang, I should have also included, if I hook up a speaker any bigger than a bookshelf speaker, it will immediately start squealing. No amount of turning it off and on will stop the squealing. Problems happen whether the amp has just been turned on or has been on for an hour.

[Jazz P Bass]

Ther are a few things that you can do to help narrow down the problem.

First off, you need to identify, according to the attached schematic, which tubes are which.

Once you have that information in hand you can intelligently troubleshoot the issue.

1: pull one output tube. See if that gets rid of the squeal.
No?
Put it back in and pull the other one.
No?

Move on to the preamp.
There are two separate channels.
One at a time pull the first input tube to that channel.

What you need to do is narrow down which circuit is causing the squeal.

Of coarse the health of the power supply should be verified but that involves opening up the amp.

Wards_Airline_GIM-9131A.pdf

[The Dude]

It should also be noted that the amp is a bit dangerous as is. The death cap should be removed and a grounded AC cord installed.

[btah]

I looked at the schematic included in the response and have a question. The amp I have has 6 tubes total. 2-6l6 and 4-12ax7. The schematic seems to indicate there are more than 6 tubes. However, I could be reading the schematic it totally wrong...

[The Dude]

The schematic shows 7 tubes. Perhaps yours is a slightly different build?

[btah]

I was wrong. I see 7 tubes on the amp. I suspect by looking at the schematic when it shows 12ax7v2a and 12ax7v2b it is really talking about a single tube. Is that correct? The schematic is just pointing out different pins for the same tube?

[loudthud]

Yes, the 12AX7 is a Dual Triode meaning it has two independent tubes inside.

[The Dude]

Yes, you are correct. The 12AX7 is a dual triode tube. There are basically 2 triode tubes in a single package.

(simulpost)

[btah]

Thanks for all the responses.

First test:
pulled one of the 6L6, no squeal
put it back in a pulled the other 6L6, no squeal
put it back in (they are both in), squeal
just for completeness, I swapped the 6L6(s), squeal

Does that point somewhere?

Also, just to make sure I say it. It doesn't matter whether I have the guitar jacked in or not.

Also, also. Let's get an equipment thing out of the way to make sure I am not fighting that. Since I don't know anything about guitar amps, let's make sure I don't have an issue with my cable I am using to connect to the speaker. It has the quarter inch plug that jacks into the amp. However, the wire hooked to the plug is heavy speaker wire. I have used this core for a while hooked up to a receiver that had quarter inch plugs so I believe the cord is good. It is well built and the plug is a Conquest Sound plug. Just wanted to bring this up in case there is some weird grounding thing that can happen with the setup I have.

[The Dude]

I would start at the front end. And, the speaker cable should be fine.

[btah]

I pulled all other tubes out, one at a time, and the only time the squealing stops is when I put the V3 tube. Which appears to me to cut all signal to the 6L6s. So, with the V1, V2, and V4 12ax7(s) pulled as well as the single 6V6, there is still squealing. Did I just narrow it down to something between the V3 and the output 6L6? Also, I did try some of the other 12ax7(s) in the V3 slot and the squealing still happens.

[Jazz P Bass]

I am thinking that your Negative Feedback may be Positive.

It will take some moxy to resolve that.

The high tension (voltage) leads that go to pin #3 of each output tube would need to be reversed.
They are typically Brown & a Blue wire.

If someone was In the amp over the years, and got the wires at the wrong pin, or maybe the output transformer may have been replaced & the polarity is incorrect for a negative feedback circuit.
Reversing the plate leads would resolve it.

Then again, the amplifier may be oscillating.

Start pulling preamp tubes.

[The Dude]

1) Has the amp been worked on recently or any parts changed? Is there any chance the output transformer primary is hooked up out of phase?
2) Try unplugging the reverb tank and see if the oscillation is still there.

(simulpost again!)