Gibson GA30 Switch Replacement Questions

[neddyboy]

The triple throw power/standby switch on my Tweed Invader has finally crapped out. My plan is to replace both the power/standby switch and the polarity switch and have the more traditional separate standby and power switches. However, once I opened it up I was very confused.

Here's a schematic of the amp: http://archive.gibson.com/Files/schematics/ga-30b.gif. I can't make heads or tails of the switch diagram either.

Can anyone tell me how to rewire the current system into 2 switches? There's a shielded cable running from the power/standby switch to the 12AU7, and one of the positions of the switch seems to short the shield to the center wire. The switch is wrecked enough that I can't test it -- everything shows "on". Could it be as simple as wiring the standby switch to deliver 120v into the black lead of the PT, and wiring the shield & center of the coax to the power switch?

This is a totally new way of doing standby and I can't get my mind around it. Can anyone help?

EDIT: I figured out the power switch. Pretty straightforward. I'm still confused about the coax. I'm used to standby switches breaking the high voltage circuit near the rectifier. The only 2 leads left for the second switch are the shield & center of the coax. The switch would short triode 2 to ground when on. That seems pretty weird to me!

[Enzo]

The three gangs of the switch are all shown OFF. The next three positions are standby, on1 and on2. The two on positions simply chose which side of the mains the cap to ground connects to.

The left one is the standby. It shows the two left positions grounding the signal path at 0.05uf cap leaving V2b. That kills the signal, which is the purpose of a standby switch. So in off and standby, the signal path gets grounded off. positions 3 and 4, the two ON settings leave the signal path unburdened by this.

The switch gang on the right is the power. SImple enough. the left most position is off, the other three positions complete the mains circuit to the power transformer, powering the unit.

The center switch section is the ground switch. It is there to find the setting with least hum. IN standby and off, it doesn;t matter what it does, so they just wired them to the third. Now in the third position, on1, the cap couples the left mains wire to chassis, and in on2, the fourth position, it couples the other side of the mains to chassis.

But why worry what they did, you are wiring up something different anyway. So do it like any Fender or other common amp.

I hope you intend to put a three-wire power cord on it.

Wire the incoming neutral wire to the power transformer primary directly. The incoming hot wire should go to the fuse holder first, then the power switch from there, and then on to the other primary wire on the transformer.

With a grounded power cord, you won;t need the cap any more, so no ground switch.


You COULD break into the B+ with a Fender style standby switch, but why bother? Use the existing standby wiring - that shielded wire from the 12AU7. Just wire your new standby switch to the shield and wire. Shorting the wire to its shield mutes the amp.

[neddyboy]

Hi Enzo, you've helped me out before! Your description matches my understanding of how it works. I've already wired it as you describe above, but I wanted some confirmation I had it right. And yes, the amp has had a 3-prong plug since I overhauled it 10 years ago. Thanks so much!

[neddyboy]

I wired it up as we discussed, and the standby switch works fine, but flipping the power switch does nothing. I'm happy there was no smoke, but guitar noises would be great! I don't see any thing else that's not connected. I used the two black wires off the PT for the 120v. Any ideas why I'm no hearing anything?

[Randall]

If the power switch does nothing, how do you know the standby switch is fine?

[neddyboy]

Sorry, I switched the two terms. I can switch on the heaters & lamp, but when I turn on the high voltage, nothing happens, no sound. I need to check the high voltage pins to see if they're getting anything.

[Enzo]

So you changed the standby switch rather than leave it intact? The existing standby circuit had nothing to do with high voltage.

[neddyboy]

I'm giving you lots of info because I'm not sure what's critical, so pardon the novel-length description!

Before I started, there was a polarity switch and a 3-position switch -- off/standby/play. The 3-position switch is kaput. It switched on the lamp and the heaters in the middle position, but when I switched to "play" I got nothing. It's been intermittent in the "play" position for about a year. I replaced both switches with SPST switches, and wired one in what I consider a "normal" standby switch. That switch acts like I'd expect: the heaters and the lamp light up. It's the second switch that isn't working as expected.

You said "The existing standby circuit had nothing to do with high voltage." That's the problem I think. What does that shielded cable going to the 12AU7 do? Is that just dump signal to ground? How does this system work? Again, the only standby system I've ever encountered is the heater/high voltage type. Is it different on this amp? It seems like the first switch is a full on/off switch and the second switch just allows signal to pass. This wouldn't protect the tubes at all, but would just mute the audio in the middle position. Is that right?

[Enzo]

From post #2:

The left one is the standby. It shows the two left positions grounding the signal path at 0.05uf cap leaving V2b. That kills the signal, which is the purpose of a standby switch. So in off and standby, the signal path gets grounded off.

That switch grounds the signal. That mutes the amp. Muting the amp is the purpose of a standby switch. there are lots of ways to do that without turning off the high voltage. The standby isn't there to protect tubes, it is there to allow the amp to remain warmed up but silent. Like during set breaks.

Your schematic shows a four position switch doing all the functions. If you have a separate ground polarity switch, fine, it won't be needed with a three wire powr cord.

You get no sound with the power switch in the run position. How did you decide it was the switch's fault? Is high voltage present?

[neddyboy]

I haven't checked yet Enzo, I'm back to work today! I'll check after work though. Here's a sketch of how it's wired now.

[Enzo]

measure resistance across that piece of coax, make sure the inner conductor is not shorted to the shield. Do that at the switch for ease.

[neddyboy]

The resistance is 475 megaohms.

[Enzo]

Well then, start at the start. The tubes light up? There is high voltage at the tube sockets where it belongs? The speakers and their wiring works? Amp is utterly silent or is there any little background hum or noise?

[neddyboy]

Good old tube amp debugging! Yeah, I know what I need to do now. Most of my initial confusion comes from not understanding what "standby" really is. My small understanding of the standby system comes from building an 18 watt Marshall. It's really different!

I haven't had a chance to check the high voltage yet, but yes, the heaters and lamp light up. Speaker wire is intact, and dead silence, no output tube noise.

I think I'll be OK from here. Checking the high voltage will tell me a lot, but this amp is kinda fragile: it's quite possible I bumped something when I installed the switches. Thanks for the help!