Gibson GA8T Power Supply Issue

[jspartz]

Hello. I have a problem that I can’t seem to figure out. This is a 1962 Gibson GA8T with 6BM8 power tubes and a 5Y3 rectifier. The power supply was previously updated to a grounded power cord. I pick up the amp for a good price because “it has issues.” I just swapped out the power transformer with a new replacement.

Now, when I power up the unit without any tubes in the sockets, I am getting:

• 120 volts on the heater/filament for the 12AX7 and 6BM8 power tubes. As you know, this is much higher than the 6.3 volts I expected to see there.
• Also, pin 2 on the 5Y3 if showing 8 millivolts AC / 1.5 volts DC. The other filament lead on the 5Y3 (pin 8) holds steady at 5.8 volts which is expected.

All in all, I am missing where the problem is at and would appreciate some guidance.

Here is the schematic: https://elektrotanya.com/PREVIEWS/63463243/23432455/gibson/gibson_ga-8t_schematic.pdf_1.png

Thank you!

 

[tony hunt]

It reads initially like a wrong connection. Difficult to diagnose online. You may have done the "Divide and conquer." steps below already,
But answers, in this order, will help get more forum input and maybe help us not go on a wild goose chase:

• do you know what the "issues" were before aquiring the amp?
• what issue did the original PT have that made you change it?
• Did you check the old and the new PT unconnected readings against each other before fitting the new? Can you share those readings?
• Double checked your new connections?
• The voltages above are with or without tubes in?
• Do you see any signs of previous fixes? Look for new parts, new wire,

It could be something really simple. The repair books from Jack Darr are very good for these early amps.

 

[tony hunt]

oh, and are you using a light bulb limiter, if not why not?

 

[rock soderstrom]

Some good photos would help. Do you have a test tube that you could sacrifice in case of need? Does the heating voltage remain so high under load? With what do you measure the voltages? Is the measuring device working properly?

Really good sounding amplifier when it works again! Killer tremolo...

 

[pucho812]

The easiest explanation would be that you changed the power transformer and it is wired incorrectly.
That does not mean it is the right answer but it would make the most sense to me without seeing photos, etc. plus it was the part removed so very easy to flip wiring.
As others have asked what made you swap out the power transformer?
Do you have the data sheet for that transformer?
I expect a higher voltage without a load but not that high.

 

[Tubetec]

You' ll most likely do serious damage by putting in the tubes with wrong voltages present .
A check for continuity across the windings should reveal the issue right away .
Also just because you read only low voltage ac at the rectifier doesnt mean there isnt hundreds of volts dc present , be extremely carefull.

 

[rock soderstrom]

I once had an old transformer that built up a high static charge on the heating winding without load. With the lowest load, everything was then back to normal.

If the 120V are really real and consistent, they are certain death for the tubes mentioned.

As I said, a few photos would show relatively quickly if it was a wiring fault or whatever.

 

[jspartz]

Thanks for the comments. I can say that I did not reference the output of the old power transformer and that is not an option at this point. I have not plugged in any tubes yet. I have a current (lightbulb) limiter circuit powering with the GA8T to monitor for issues. No problems, yet.

I did find an inconsistency with documentation for the new power transformer. I am reaching out for clarification and to raise awareness of what they sent me. I am double checking the wiring and existing parts (yes, some capacitors have been replaced prior and I am confirming (again) they are correct in reference to the schematic). I should have a better update on the situation in the next day or so.

Thanks again!

 

[jspartz]

Well, the PT documentation has a center tap (CT) for the filament (6.3v) on the secondary (grn/wht). This was incorrectly labeled with a sticker saying "DO NOT USE IT!" and the sticker should have been on shield (grn/yel) lead. I overlooked this initially. Grounding the center tap brought the filament feed into perspective and it is not reading 120v anymore.

I am moving on to the other issue. Pin 2 on the 5Y3 socket is showing up as 8 millivolts but should be 5.8 volts. Pin 8 holds steady at 5.8 volts. I am not sure why. Pin 2 also has the 4 watt 250 ohm sand resistor lead connected to it. The resister is actually measuring 268 ohms so it has drifted over time. I have a replacement resistor ordered but I am curious to suggestions on what else to check.

Schematic for reference

Here is a picture for reference of the chassis on my bench:

 

[tony hunt]

If that was on my bench I would be questioning the workmanship on everyone of those non original "repairs".

What really sticks out to me is:

1. Why do you have a "sand" resistor on pin 2 of the rectifier? Where is that resistor going to? (Pin 2 should just have the one 5v heater wire and nothing else.)

2. The amount of wire and components at the rectifier socket does not look original. These old Gibsons can be a kind of a rats nest compared to a Fender, but not this crazy.
It looks to me as if the "repairman" has used the spare lugs on the rectifier socket to mount additional components. Together with the clusterf**k of new electrolytic caps, it points to a suspect zone where a mistake or handy-work mishap may be lurking.

Could you draw what is happening from the PT - though the rectifier to the first and second filter caps (10uF in the schematic)?
Compare that to the schematic.
Back that up for us with a clear photo of the rats nest at the rectifier.

 

[Admiral-dk]

Are you sure that the 'Sand Resistor' isn't a (Slow Blow) Fuse - those would have been required by Law here in Scandinavia for decades (before the EU regulations) in series with all the windings on the Transformer ...!

Per

 

[JW]

I had to give up on this exact model amp at some point. I was trying to repair for a friend and I determined the power transformer was bad, replaced it, and it stopped blowing fuses, but had crazy hum. My friend finally took it to a tech, and I pried at the tech to figure out what the hell I missed. Turned out I had swapped the 5V and 6V heater wires. Gahhhhh.

Anyway, stick with it. I see it's the tweed version. It's one incredible amp when working well.

 

[JW]

BTW, I found the unused rectifier tabs to have been used as well for convenient component placement. Gibson probably did this themselves. They seem not as tidy as Fenders.