Gibson Ranger GA20-T Rebuild

[TomCarlos]

Hello Friends...

A friend handed me an old Gibson Ranger GA20-T amp for restoration. He recently bought this chassis and besides being old, it is missing the Power Transformer (and who knows what else).

We purchased a new Mercury Magnetics PT GA20-T-P. It's supposed to be an exact replacement. I wrote to Mercury to get a spec sheet because the wiring colors do not seem to be an exact match to the amp. So while I wait for that...

I'll be taking my time on this one... lots of cleaning, cap replacements, will check everything from top to bottom.

Step 1 - We will be installing a 3 prong electrical plug. I'll ground the line right at the screw holding the transformer.

On the schematic, I see a .022uf cap on the switch side of the transformer. I don't think we'll be needing this cap, will we? I thought this is an old wiring scheme for providing some type of ground to the chassis- true?

Step 2 - The two big caps you see in the photo will be replaced. One cap is the 10/20 cap that appears on the left side of the schematic pic shown here. The other cap is a 10/10 in parallel that serves as the 20uf shown on the right side schematic (connecting to the 10K). I guess I will buy two 20uf and a single 10uf to replace these.

Thanks, Tom

[52 Bill]

On the schematic, I see a .022uf cap on the switch side of the transformer. I don't think we'll be needing this cap, will we?

Yes, that is a ground cap or "death cap". With the 3-wire ac cord, you will not need it.

I guess I will buy two 20uf and a single 10uf to replace these.

Yes, that will work fine and will take up less space as well.

[TomCarlos]

Thanks Bill...

I found another schematic for this amp and the cap is not there. So on to the rest of Step 2 and mounting the new PT. The Mercury transformer mounting clamps do not fit exactly over the existing holes in the chassis so I'll need to drill a couple holes. A good stopping point would be to get the incoming AC taken care of and connecting the 6.3 volt taps. Always nice to have a power light working! I'll hook up the secondary connections once I get to Step 2 and make sure I properly map the windings to the existing circuit.

Tom

[Chuck H]

What Bill said, and...

If you find it's easier to get 22uf (a more standard value) that should work just the same as any 20uf of the period. Modern caps have much better tolerance than older and the 22uf value is well within the +/- 20% of the day. 10uf should be easy enough to acquire.

WRT old Gibsons... The only one I've ever worked on had a leaky coupling cap. I've read MANY threads here where an old Gibson had a leaky coupling cap. I'm not suggesting that you change all the coupling caps, but since you'll be doing extensive work anyway I would suggest that you test the caps that are there to avoid problems.

[TomCarlos]

Gents...

I am starting to compile my shopping list and will come back to ask some questions about the caps. But first...

I am trying to map the wiring for the new transformer. The old PT was not on the chassis when purchased. I see 7 original wires that went to the old PT. It looks like it went something like this...

2 wires (appear to be Red) to the 5Y3GT Anodes, (Pins 4 and 6)
1 Wire (appears to be Yellow) to the 5Y3GT Heaters (Pin 2)
1 Wire (appears to be Yellow) to the 5Y3GT Cathode / Heater (Pin 8)
2 wires go to ground (hard to tell the colors) - The empty pin 1 of the 6V6 tube was used as a ground point.
1 Wire (looks like a Green/Yellow) goes to the 6V6s Pin 8 (Cathode / Beam Plates)
Total = 7 wires

The new Mercury Magnetics PT GA20-T-P has the following:

2 Green wires on the Primary side that are for the filaments - Measured on all tube sockets!

On the Secondary side, there are also 7 wires.

I am guessing the Reds will go to the 5Y3GT Anodes, (Pins 4 and 6)
Looks like the Yellows will go to the Cathode / Heaters (Pin 2 and 8)
That leaves us a (1) White, (2) Red with Yellow, and a (3) Green with Yellow.

You'll notice the Green with Yellow has a red sticker- that says "If this Center tap is not shown on the schematic, do not use it." PERFECT, I believe I have identified the Center tap that is shown next to the tube name 5Y3GT. That one should go to Ground.

So that leaves me with the (1) White and (2) Red with Yellow.

One of these must go to Ground, the other will go to the 6V6s.

I wrote to Mercury to ask for a Spec sheet but I'm not sure that will answer my question.

Any "intelligent" guesses out there?

[52 Bill]

Standard American transformer color codes must be out there on the web.

Yellow is always for the 5 volt rectifier filaments, Green for the 6 volt filaments with a Green/Yellow center tap, Red for the high voltage with red/yellow for the center tap. I will assume the white to be the internal electrostatic shield.

The center taps can be checked with an ohm meter, but use caution, because when you connect and then disconnect the meter leads you can induce a pulse in the windings that will cause a high voltage to appear on the other windings.

[TomCarlos]

Yup... I think you have it Bill... thanks!!

I am attaching a drawing showing what the wiring should be... NOT UNLESS anyone weighs in otherwise.

But it makes sense now.. you have Red Wires, the Red/Yellow is the center tap. You have Green wires, the Green/Yellow is the center tap. It's the Green/Yellow that goes to the 6V6s.

For the Greens, I know they are typically the Heaters. But I didn't realize the 6.3 volts could have a center tap- lesson learned.

And the White wire has not connectivity to the others, so it must be the electrostatic shield.

I guess I am still learning how all this works. But I wanted to make sure I understood the wiring before I connected anything!!

[52 Bill]

But it makes sense now.. you have Red Wires, the Red/Yellow is the center tap. You have Green wires, the Green/Yellow is the center tap. It's the Green/Yellow that goes to the 6V6s.

For the Greens, I know they are typically the Heaters. But I didn't realize the 6.3 volts could have a center tap- lesson learned.

With the filament center tap connecting to the output tubes cathodes, the filament voltage will be raised by the dc bias voltage of the output tubes. This will aid in lowering the hum from the tube filaments.

[TomCarlos]

Ok... next thing I need to look at carefully are the ground connections....

I am attaching the schematic for the GA-20T amp. Keeping in mind that I now have a 3 prong electrical wire and the chassis is grounded... Is this where I would use what I learned about Star Grounding and bring all ground connections back to a grounding block?

I'm sure there is probably a strategy of where and how to tie the grounds together.... High Signal vs Low Signal vs Input grounds... etc etc etc. The Input jacks are grounded through the chassis.... I get that. But I want to look at all the other ground connections running through this amp.

Any advice would be appreciated.

[Enzo]

Changing from a two wire power cord to a three wire grounded cord doesn't change anything in the amp circuitry. I would not recommend wholesale changing of the grounding unless you have a good reason to. Don't do it just because it "ought to be better this way". You often will run into the old unintended consequences.

[Chuck H]

Enzo is right about unintended consequences. You could just see to it that no preamp grounds are daisy chained with power supply grounds. They probably aren't now, but who knows. Beyond that, why fix what's not broken? Ok, now to go against what I just said...

I might try a redesign on that trem circuit intensity control. I don't know what sort of pots Gibson used, but I wouldn't trust a pot that old or a modern built pot to carry HV. It's probably possible to hardwire the plate circuit and variably couple the cathodes instead.

[TomCarlos]

Thanks Enzo and Chuck....

All ground connections from the transformer go to the unused Pin 1 of the 6V6- including the electrostatic shield (I guess the old transformers had one too). So I will probably leave that as-is. I am debating on whether or not to splice the new PT to existing wires or to remove all wires from that connection point and resolder. With such a tight squeeze, I am hesitant to do that.

I think my bud would prefer that I not alter this amp (if we can avoid it, safety issues an exception). So I'll look to see if there are examples of how to redo the wiring Chuck mentions.

[Tom Phillips]

... I am debating on whether or not to splice the new PT to existing wires or to remove all wires from that connection point and resolder...

My vote is to remove the old wires and connect the new transformer leads directly to the intended original terminals. The splice method is a sure sign of a quick hack job and I'm sure you don't want your careful work to look like that.

I recommend that you retain the original ground scheme to start. Get the amp working and then decide if improvements are necessary. Best not to flit with unintended consequences as Enzo pointed out.

Cheers,
Tom

[soundmasterg]

I rebuilt one of these amps for my uncle about 10 years ago. My brother has the amp now. All coupling caps leaked DC badly and had to be replaced except for the flat mica cap. It had bumblebees and Astrons in it originally. All electrolytics were replaced in the amp as they had physically leaked inside the chassis and obviously didn't work, and really those should be replaced anyway. A 3 wire cord was installed and the noisy carbon comp plate resistors were replaced with new ones. There were a couple other resistors that were replaced too because they had badly drifted but I forget at the moment which ones. The Gibson board is annoying as they have the caps on the bottom and the less often changed resistors on top. The whole board has to be pulled to work on the amp properly. The grounding is to a terminal on the tube sockets as you mentioned. It isn't technically correct but works well in that amp. The preamp sockets are also shock mounted which is a nice touch. I still have info around here somewhere about that amp, and it is across town if you get stuck and need some info, though I am very busy with last term of an EE program in school at the moment. Hope the info I gave you helps!

Greg

[TomCarlos]

Thanks Greg... Yes, that is helpful.

I'm going to creat my shopping list soon. I guess the questions I have center around the types of caps I should get. I see the bumble bees and the ones I call "the bananas." Based on the experience of others, i am wondering if Orange drops would work ok in some spots but axials in others.

Coming off the power supply, we have the two 10k resistors. One measures at 10.5k and the other at 9.5k. So I will strat there and continue on.

Tom