Gibson GA-15RVT How can I eliminate hum noise?

[taku319]

Hi,there.

I'm posting this topic from Japan.

Recently,I got this amp as a broken,and I think I fixed it.
I replaced filter cap (20uf ATOM)/resistor(270ohm,130ohm)/diode.

But there's still big hum noise!!Very annoying.
Hum is always big,not depend on the loudness(or volume).

Does anyone can solve this problem?Plz!!!

[Structo]

It doesn't sound too bad on the clip.

Does it seem distractedly too loud when playing?

Make sure the heater wires are away from signal carrying wires.

Does the heater winding on the power transformer have a center tap or are you using two resistors as an artificial center tap?

In most cases I think the resistors sound quieter than the center tap because the center tap many times is not balanced perfectly.

So you take two 100R resistors and run one from each side of the heater wires to ground.

Fender amps used to use something they called the "Humdinger" control which is a wirewound pot that is connected across the heater circuit with the wiper to ground.
Then you dial in the least amount of hum.

So if you go through your amp and it is not the heater circuit causing the hum, then check the rest of the lead dress to make sure wires are neat and orderly.

[Structo]

OK, I'm looking at a schematic.

Does your amp have one side of the heater circuit to ground?

http://www.el34world.com/charts/Schemat ... -15RVT.pdf

If it were me, I would disconnect that and run one 1watt 100 ohm resistor from each side of the heater circuit to ground for a center tap.

That should quiet it down.

[taku319]

Structo

Thank you for advice!
I bought 100ohm*2 and try center tap,,,,,,no difference,,,,lol

Also I tried change the electric/sound lead line,and,,,,,no difference.

Maybe this hum is nature of this amp?

Now,I'm bewildered~~

[Cliff Schecht]

Make sure ALL of the ground connections in the amp are solid. The first place I look when I'm getting bad hum is for a bad/missing ground connection.

Check that the tubes are making good contact to the sockets. Retension sockets if needed.

Check if all of the resistors are good in the circuit. Look for physical signs of degradation or measure each value if possible (remove all tubes for this). Some resistors can't be measured in circuit because other resistances have a path to ground that parallel to the resistor you are measuring and give an incorrect reading, get a schematic and use your judgement here if needed.

Are the tubes known to be good? A bad preamp or power tube can easily cause all sorts of goofy problems.

[rdjones]

Structo

Thank you for advice!
I bought 100ohm*2 and try center tap,,,,,,no difference,,,,lol

Also I tried change the electric/sound lead line,and,,,,,no difference.

Maybe this hum is nature of this amp?

Now,I'm bewildered~~

It looks like the power transformer may have been replaced with one of a different mounting style.
[img:400:267]https://tubeamparchive.com/files/thumbs ... 38_175.jpg[/img]
Was the the transformer oriented for lowest hum induction ?
Is the hum still there with the output tubes removed ?
Show us a picture of the other side of the chassis.

reddog

[Lynxtrap]

I can't download the audio for some reason.
Is the hum ac or dc? That's the first clue to finding the source.
Are the heater wires twisted, run as a "twisted pair"?

[Phil_S]

Try moving the ground for the CT of the interstage tranformer to the place on the terminal strip where the 130r cathode bias resistor is grounded. I think this is the cloth yellow wire at eyelet #11 (trace it back to the tx which has 2 green and one yellow on the secondary). You can accomplish this by moving the wire to eyelet #21.

I also see an orange (maybe brown?) wire at eyelet #15 that looks like it goes the terminal strip ground. That's wrong. There is a "virtual" ground buss on the board that snakes through a number of eyelets in the middle row. You've got to find where it should connect a the next eyelet.

I am not sure #15 is a ground, as it is hard to see. Can you provide a close up picture of this and the terminal strip where you have the filter caps? If it is the B+ supply to what might be two 100K plate load resistors, then the wire is still placed incorrectly. Run it next to the board (pot side, not tube side) on the chassis floor and make a right angle turn to get it across to #15 so you minimize B+ bleed to the the signal path.

In the end, you might find a bit of chopsticking of wires will address any of the many lead dress issues that might exist. How about a picture of the tube sockets? I'm thinking maybe someone has rewired those without understanding how unforgiving this amp is for lead dress. V2 looks like its a mess for lead dress, but the view isn't good enough to be sure.

If any tube shields are missing, please install them. You can tell from the sockets whether one should be there. This will make a difference, too.

I am suspecting multiple issues, but I'd do the interstage transformer CT change right away. That one is at the top of my list. Maybe you get lucky and can stop with a single change.

I've had lots of experience fixing the hum on my GA20-RVT, which is a very similar amp. Those aren't guesses above. They reflect my experience in fixing hum on this particular design.

[taku319]

Thank you everyone!I really appreciate.

Phil_S
"Try moving the ground for the CT of the interstage tranformer~"
This reduced some hum,but still hum noise noticeable/annoying,,,

Lynxtrap
"Is the hum ac or dc? That's the first clue to finding the source."
I'm not sure,,,but the noise sounds like [vu~~~~n].

rdjones
"Was the the transformer oriented for lowest hum induction ?"
I don't know,,this trans was already there when I bought it.

"Is the hum still there with the output tubes removed ?"
No,when remove V5/V4,there are no hum/hiss.

[Phil_S]

Taku,
If it helps, here is a picture of what I believe to be an original GA15-RVT.

I can see the B+ rail connection to #15. It goes from #15 to #17, then, I think, to #21, and from there to the filter cap. Notice this is tight against the board, keeping it away from the signal path. Now that I see a close up of yours, I don't know how I'd deal with that orange B+ wire. I might try to slip it in between #25 and #26. I can't say for sure it is a problem.

Look at the tube socket lead dress. It is done very carefully and directly. I think all those red crossed wires are part of the problem. Probing them with a chopstick or other non-conductive tool and moving them around a bit may show you one or more lead dress problems.

Phil

[taku319]

Hi,everyone.

Finally,I found the cause of hum!

It was　because of the power trans's direction.

Now,I'm wondering whether I buy a new trans or dispose this as a junk amp.mmm,,,it's difficult question..

[martin manning]

Can you rotate the transformer 90 degrees? That might improve things.

[Phil_S]

I think the original on that amp was rated 230-0-230. Not sure what you've got in there now. With today's US line voltage above 120, the amp will produce plate voltage at around 315VDC. If there is nothing wrong with that PT except its orientation, just rotate it and see if you can make it work.

[rdjones]

Can you rotate the transformer 90 degrees? That might improve things.

I wonder which would be more susceptible to induction, the OT or IT ?
In both amps I have that use a transformer inverter, both are mounted inside the chassis presumably shielded from the PT.
Then there's the question of the output leaking back into the interstage ...
(engineer brain at work)


reddog Steve

[Phil_S]

The interstage tx on this amp is in a relatively good spot. Also, to R&R anything but the PT requires removal of the board to get to the mounting bolts, a huge PITA. While you have a very good suggestion, the most practical thing to do is to work with the PT, which really is far enough from the OT not to make such a problem. I am actually surprised that Taku is having trouble given the distance, but there's no telling with a replacement item.

I have some pix from someone I know from another forum. Here's the tx layout. The OT is closest to the PT. The interstage tx in next to the OT. The reverb is on the end. I think the 1/4: phone jack is a hack job of some sort for an external speaker.

Curiously, as I look at it, even with the replacement PT there should be plenty of room to mount a shield plate next to the PT to screen it from the OT. I don't know if that's a good idea or not. I don't know enough about it.