GA-30 restore with odd schematic

[shaun]

Hi All. I have been asked to restore a lovely old GA-30. The cabinet etc is in great condition, but someone has monkeyed around with the tube layout, trading the 6SC7 for a 6SQ7, and they also rewired one of the preamp 6SJ7s for a 6SQ7. The 6SQ7 is a dual triode, as far as I understand the datasheet, so this seemed like an odd move.

But I also seem to have a Gibson amp model that does not match the GA-30 schematics I've found online. For example, the schematic (posted here) has two volume pots, while the amp I'm working on has only one, plus a single tone pot. It has a total of four inputs, but a single volume pot.

I'd like to find the correct schematic; perhaps someone knows where I might look? If I can't find it, I'll restore it according to this schematic and hope for the best.

UPDATE: Just found a GA-25 schematic - attached. It looks very similar to the amp I have. Strange. The tube layout in the amp I'm working on is the exact same as the GA-25 schematic.

Thanks, as always, for the help.

[sluckey]

What does the inside of the chassis look like? Is the speaker a field coil? Is the OT mounted on the speaker?

[bmccowan]

Welcome to the strangeness of Gibson amps. They changed circuits and mixed model numbers at random. I have that exact amp. A GA-30 that better matches a GA-25 schematic. The tubes are 6SJ7, 2x6SQ7, 2x6V6, 5Y3. I think it is basically the GA-25 with the added "tone expander." Shallow cabinet with wallpaper thin covering; one 12" and one 8" speaker. They also moved around the placement of the tubes, so be careful to not be caught off-guard by that. Many years ago I worked as a carpenter. A common jobsite expression was, "can't see it from my house." I have a feeling that might have been said a lot in the Gibson amp shop. The good news is that once you are done, it will sound great for jazz, blues, and early R&R. And, the knobs are so damn cool!
I will look to see if I have a schematic that matches.

[shaun]

What does the inside of the chassis look like? Is the speaker a field coil? Is the OT mounted on the speaker?

The OT is mounted on the speaker - not a field coil speaker. bmcmccowan describes it perfectly. Shallow cabinet with two speakers and the "tone expander," although it seems to shrink the tone rather than expand it:).

6SQ7s are installed where the GA-25 schematic calls for 6J5s, so that has me perplexed because I can't find a schematic that indicates 6SQ7s as correct in those locations. And they don't seem to be equivalents. I'd be grateful if it turns up. Thanks for the info, bmc. And yes, coolest knobs ever. I guess they can be seen from the house. 

FURTHER UPDATE: Here's some wiring snaps. It seems V2 and V3 are wired for 6SQ7s, with diodes on pin 4 and 5; I'm not familiar at all with these tubes. In addition to the caps that have been replaced by an unknown tech, there is a 10uF cap and a 150k resistor - both new - that are connected to absolutely nothing. The old 1M grid leak resistor on V1 reads 78k, so that'll need to go.

Sure hope I can find a schematic with 6SQ7s in V2 and V3. The work that has been done recently bears little resemblance to either the GA25 or GA30 schematics I have so far, and it seems suspect, but I hate to fly blind on a vintage piece like this.

[PRR]

> The old 1M grid leak resistor on V1 reads 78k, so that'll need to go.

Have you allowed for the shunting resistors on the input jack? 78k is not wrong, though not what I figure from the alleged values.

[shaun]

> The old 1M grid leak resistor on V1 reads 78k, so that'll need to go.

Have you allowed for the shunting resistors on the input jack? 78k is not wrong, though not what I figure from the alleged values.

Ah yes, I wondered about those. I was going to lift one leg of the 1M first and test it, but you are right of course - I need to do more math, which would save me work and keep the amp original. And also, instead of a 22k, the amp actually has a 33k (must have run out of 22k on the day), so that might count for the difference. Thanks, PRR.

[bmccowan]

Shaun, I searched my files and do not have a schematic. I do recall searching for a schematic at the time I worked on mine, and not finding it. I did at that time find on various forums, a few others that had that version. They jokingly called them GA-25 1/2 models. Mine had not been touched from new, so I simply replaced components as needed. I see from an old thread here, that I replaced half of the resistors and all of the caps. I just did a quick search and these guys https://www.musicparts.com/products.asp?Company=Gibson claim to have that model schematic for $15. Well worth it if your amp has been altered enough that you are unsure how to get her back running. I still have mine and when I have more time would be happy to open it up and confirm anything that schematic does not clarify. This is a very cool model amp, so I am glad you are interested in returning it to stock form.

[shaun]

Thanks bmc! I'll look for the GA-25 1/2. That could be the one. I checked the link you gave me, and their schematic does list the two 6SQ7s but also states two volume pots and one tone pot. So something is different. But I guess it might be worth the $15 gamble.

[bmccowan]

Yup - I think if that schematic gives you the connections for the 6SQ7s you will be OK. The pentode 6SJ7 has been configured the same way in every Gibson I've seen, and the cathode biased 6V6s will be standard too.

[shaun]

Thanks.
I bought the attached schematic online from the link you provided. It is apparently known as GA-30 1949 version.

The only question left is how the diodes function. But I can probably keep the wiring in the amp, which simply has pins 4 & 5 connected, and they are the diodes. I know what a diode does, but I don't have a clue why they are there, and perhaps I'll never need to know:). They are not shown in the schematic.

[bmccowan]

I do not think the diodes in the 6SQ7s are used in the circuit. I suspect they are grounded. But there is no schematic in your post - just part of a tube data sheet.

[shaun]

I do not think the diodes in the 6SQ7s are used in the circuit. I suspect they are grounded. But there is no schematic in your post - just part of a tube data sheet.

Sorry - I guess it got sent to Pluto. Now attached.

[bmccowan]

That's your amp. I do not see a second Volume control in that schematic? - just a 500K volume and a 2 meg tone pot. The two 6SQ7s are used a phase inverter. The "tone expander" grounds the negative feedback through a 10 uf cap. The red 2x10 firecracker filter cap will no doubt be bad. An F&T 2x16 double cap makes for an easy replacement. As you will see in the schematic there is no connection for the diodes in the 6SQ7s.
You can also see in the schematic that Gibson did not let many components get in the way of the raw sound coming through.

[shaun]

It was the listing where I bought the schem that said it had two vol pots - they were incorrect in that. Yes, it is the right schematic, and you were right - the diodes are simply sent to ground. Boy, it's so much easier with a good schematic.
Thanks for your help.

[bmccowan]

Happy to help. Keep us all posted please.

[shooter]

Boy, it's so much easier with a good schematic.