Gibson Skylark GA-5T Repair

[zachburns12]

Hello!

I literally just received this Skylark about an hour ago. It looks to be in really good shape, very clean. It has a new speaker in it. However, I plugged it up and unless I turn it up to about 8, it's almost inaudible. It sounds a bit thin on top of that. Now, my first question is this and perhaps a silly/easy to fix one. The amp was shipped to me so the tubes were taken out. I replaced them as I saw fit. But I may have put them in the wrong order. I've attached pictures showing the arrangement. I'm only confident of the second tube from the right because it had a different receptacle than the other 3. I figure it's worth an ask instead of trying all the combinations.

If that's not the issue, i'll continue searching the forum for other issues similar but if someone could quickly point me in the right direction I can get to it. I hear these are a pain to get open, but i'll rip into it. I'm an electrical engineer so i'm good with electronics but i've not dealt with tube amps before.

I can post more pictures they would help, of course.

Thanks!

[The Dude]

Do not just "try all the combinations"! Different tubes have different pinouts. You can seriously damage an amp if you get the wrong tube in the wrong socket!

The schematic here also shows tube location.

[zachburns12]

I didn't, don't worry. I did know that much lol I admit that I let my excitement get the best of me. I tried reason out the correct way. I don't believe I was successful.

[Justin Thomas]

Looking at your pic, you got lucky! Good thing it wasn't one of the Gibsons with 12AX7s AND 6EU7s in the same amp. Tgat said, have you tried new tubes? As if anyone has 6EU7s and 6C4s lying around... :/ damn oddball tubes.

Justin

[zachburns12]

Well that is great news haha I have not tried new tubes. I was told this amp was "recently serviced" so I wasn't expecting this. Are those tubes difficult to find?

[The Dude]

Have you verified that the speaker works? It may have been damaged in shipping. Does the speaker cone move in and out freely? What you describe sounds like it could be that the speaker magnet has shifted and the cone is frozen.

[zachburns12]

The speaker appears to be almost brand new. The cone does move in and out freely. I have just noticed that 3 of the 4 tubes glow when I turn it on, and one does not. I have to assume that's going to be an issue. It's the second from the left, one of the longer ones. Which one is that? I can't find any markings on the tubes. One says 6BQ5 and the markings on the other must have worn off.

[The Dude]

The 2 left tubes should be 6BQ5 (EL84) tubes. The filament pins are 4 & 5. Pull out the tube that does not light and measure the resistance between those pins to determine if the filament is open, or you could just switch the 2 output tubes into each others sockets and see if the problem follows the tube or the socket.

[zachburns12]

Great. My multimeter got lost when I last moved. But I switched the two output tubes and it definitely is the tube. But I did make another discovery. In the original position, there was no noise, just the thin sound. When I switched the output tubes for the test, there was a background noise that took the speed of the tremolo. It's a sort of soft whooshing noise that speeds up/down with the tremolo frequency knob. I'm not sure if that means anything. But it's an observation.

[zachburns12]

Would this tube set do the trick? I wouldn't mind getting a whole set of replacements just to have.

http://www.ebay.com/itm/GIBSON-GA-5-...-/200793650509

[Randall]

First time for me to see a transformer driven tremolo phase splitter config. That's neat.

[g1]

But I did make another discovery. In the original position, there was no noise, just the thin sound. When I switched the output tubes for the test, there was a background noise that took the speed of the tremolo. It's a sort of soft whooshing noise that speeds up/down with the tremolo frequency knob.

Unfortunately, this may indicate a problem with the phase splitter transformer. The good tube should give the same sound in either socket. But work the tube in and out of the socket a few times to make sure it's not a dirty contact at the socket.

[zachburns12]

Unfortunately, this may indicate a problem with the phase splitter transformer. The good tube should give the same sound in either socket. But work the tube in and out of the socket a few times to make sure it's not a dirty contact at the socket.

Well that is exactly what I didn't want to hear. How do I check that? How easy is it to fix? I enjoy fixing things like this but I think I would much rather play the amp so if it's fairly involved I think i'd rather just take it to someone and have them fix it.