Rebuilt Gibson GA-8T Loud Hum at 120hz.

[g1]

the filter caps neg. side is connected back to the B+ ctr. tap, which also is not grounded, as per the factory wiring

That CT and the caps have to be grounded somewhere.
Can you post the proper schematic? If you don't have it, what is the tube complement?

[One10]

That CT and the caps have to be grounded somewhere.
Can you post the proper schematic? If you don't have it, what is the tube complement?

Here is the schematic that came with the amp, I got it and the amp from the original owner. As wired from the factory, Gibson did not ground the the center tap, nor the caps the traditional way. The caps negative lead went to the B+ center tap at pin 3 on V4, see photos.

I was waiting to fix the water stains on the schematic before posting, but all the others on the internet seem to be very low quality and quite blurry, boardering on illegible. I was only allowed to upload a lower quality jpg.

Gibson's schematics are at best a close aproximation of what ever amp they claim to represent. I have noticed over the decades, that they have left them uncorrected long after several versions of the amp came into being. They almost always have incorrect numbers & figures. They have the wrong pin number showing A.C. voltage (pin 3, V4)for the 5y3, wrong voltage (48v) shown on pin 1, V2, of the 6BM8, should be around 24v, it cannot be 48v as built.

[Enzo]

From Schematic Hell, maybe a little better? I refer the PDF myself.
ga8t.pdf (schematichell.com)

Pin 3 on a 5Y3 is an unused pin, so they used it as a convenient terminal for connections. How does it find its way to ground from there?

[One10]

From Schematic Hell, maybe a little better? I refer the PDF myself.
ga8t.pdf (schematichell.com)

Pin 3 on a 5Y3 is an unused pin, so they used it as a convenient terminal for connections. How does it find its way to ground from there?

Thanks for the link ENZO, that one is a little too blurry for my use and shows different parts values than what is in my amp. The one that I posted, came with it and is the closest to what they did. I had to take a picture of it as I couldn't get a good scan, the poor scanning software doesn't work with my mac. I would also prefer PDF instead.

The negative wires, B+ ctr. tap & heater ctr. tap are lifted from ground. everything negative is connected to pin 3 and the B+ center tap except for the two inputs - they are grounded directly to the chassis.
Only one wire is connected to ground from the power transformer, it is the White/Red line wire. That wire is connected to a lug attached to the bolt of the power transformer. How it was wired is mentioned several posts back, a couple of times.
I posted the wiring for the power transformer awhile back and just little while ago photos. These posts get long and then no one can make heads or tales from them. (I am talking about how much information I posted, not anybody else).

[Enzo]

Maybe I missed something, but you are chasing 120Hz hum? And your center taps and cap grounds do not reach chassis? That right there could be a problem. Power off and what resistance to chassis do you measure from that pin 3?


And don't feel bad, I have one model in my Gibson files, with SIX distinctly different schematics.

[Helmholtz]

What is odd though, now the 'other' hum at 59hz. dropped to -89dB with the spraque atoms!

That "59 Hz" line might just be the PT's magnetic field coupling directly into your phone.
Try different phone positions.

[One10]

Maybe I missed something, but you are chasing 120Hz hum? And your center taps and cap grounds do not reach chassis? That right there could be a problem. Power off and what resistance to chassis do you measure from that pin 3?


And don't feel bad, I have one model in my Gibson files, with SIX distinctly different schematics.

. Hmmm....Gibson must have put all their money into quality, not accuracy...

No, you are correct, I am still trying to reduce the 120hz hum. The center taps & grounds do not reach chassis. I was told Gibson did this as a cheap way to reduce hum?!

it is 0.40Ω measured from V4 pin 3 to chassis.

Maybe the pwr. transformer white/red lead is a 0v / ground return wire? (that is grounded at one of the tranformer bolts) I have misplaced the figures I wrote down on all the leads when I had everything detached.

That "59 Hz" line might just be the PT's magnetic field coupling directly into your phone.
Try different phone positions.

Ok, that is fascinating how things interact. I will try that later today, after I get the amp back together

[Enzo]

. Hmmm....Gibson must have put all their money into quality, not accuracy

Let's be fair. As I said before, I have six different schematics for one model type. They often made changes to amps through production, and they DID "update" schematics. But just not issue a new one for each change as it happens. No, that does not forgive glaring errors and typos.

[One10]

Let's be fair. As I said before, I have six different schematics for one model type. They often made changes to amps through production, and they DID "update" schematics. But just not issue a new one for each change as it happens. No, that does not forgive glaring errors and typos.

That was joke, humor - not to be taken seriously.

[g1]

If there is no wire from CT & cap grounds to chassis, yet it measures short with your meter, then the transformer white/red wire must be connected to the CT. Seems odd though.

wrong voltage (48v) shown on pin 1, V2, of the 6BM8, should be around 24v, it cannot be 48v as built.

Getting a bit off topic, but (if pin9 voltage correct) I calculate somewhere in the ballpark of 50V on that cathode (pin8), so grid volts should be within a couple volts of that. Why 24V ?

[g1]

I have kept the specs as from the factory, with a 20uf 450v capacitor, but instead of a 3-in-1 cap can, I chose to replace with 3 separate capacitors, that is the only change from original.

The original was multi-cap can & had a clamp, can't re-use it, it is permanently riveted to the old cap.

Was that 3 in 1 can clamp soldered or bolted to the chassis? That is the usual method. The can cap negative is usually connected to the can casing. Do you have the original to check if negative of can goes to casing?
If so, that is how the CT and cap grounds were chassis grounded. You would need to restore that connection.

[One10]

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[One10]

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[Helmholtz]

If all circuit points showing the ground symbol measure ~zero Ohm to chassis, they are effectively grounded .
There needs to be only one connection to chassis and that seems to be at the input.
Actually a single chassis ground is best as this avoids ground currents in the chassis, so it doesn't matter where the chassis is grounded.
Any additional chassis ground will cause a ground loop and is likely to make things worse.

I don't trust acoustic hum level measurements with a phone.

Balance the power tubes and see/hear if this silences the amp.

[One10]

If there is no wire from CT & cap grounds to chassis, yet it measures short with your meter, then the transformer white/red wire must be connected to the CT. Seems odd though.

It is just a return wire, o volts, or an internal screen wire between the primary and secondary, the amp came to me wired that way, so i accept it and it has always worked this way before.


[QOUTE=G1;N943100]Getting a bit off topic, but (if pin9 voltage correct) I calculate somewhere in the ballpark of 50V on that cathode (pin8), so grid volts should be within a couple volts of that. Why 24V ?[/QUOTE]

Yes pin 8 is 45v (48v on schem); while pin 1 is 25.5v, not 48v as written in schematic, as it has a couple of resistors before it. I can see where the voltage drops and it is because of the resistors. I did actual measurements before a certain point and after. 25.5v is on pin 2.

Was that 3 in 1 can clamp soldered or bolted to the chassis? That is the usual method. The can cap negative is usually connected to the can casing. Do you have the original to check if negative of can goes to casing? If so, that is how the CT and cap grounds were chassis grounded. You would need to restore that connection.

They bolted them. The outside of the canister is cardboard and the isolated from any outside influence, i.e.: current, voltages, etc. As shown in several pictures and explained, the negative lead (blk wire) was attached to the canister on one end together with the red +, ylw + & blue + wires. The blk - goes to pin3 V4 as shown in the pictures.


Every amp I have ever worked on that has used a multi-cap can, had the all the positive wires and negative wire(s) issuing out one end. The body has always been isolated from any of the wires physically as well as electrically. I have never had one where the clamp was used for a negative ground. I am not saying there are not other kinds, or ways, this is just what I have encountered.