135 watt Twin Reverb loud hum

[nosaj]

I am a novice, but is the hum balance pot tied to an elevated ground ? If that connection is bad, you won't get the same degree of cancellation.

But take what I said with a grain of salt if it was never tied to ground in the first place. Unlike others here, I am still taking shots in the dark...

Don't kid your self. You've shooting straight thus far in your posts.
A bad solder joint can make a bad ground.
nosaj

[g1]

Does it still have the original 'output matching' pot wired up like stock? Does adjusting it affect the hum?
Obviously, the previous shop used the 'shotgun' approach and it didn't work. Lack of troubleshooting skills have already cost the owner hundreds, I would highly recommend against continuing down the 'shotgun' road.

[nosaj]

How about some hi-res pics?

nosaj

[Helmholtz]

I suggest to find and post the appropriate schematic.

[nosaj]

I suggest to find and post the appropriate schematic.

http://ampwares.com/schematics/twin_reverb_sf_135.pdf
I'd suggest taking the hum balance pot out of circuit and connecting it to a meter and doing a full sweep to check the ohm range and see if there are any descrepancies. I'd also use an analog meter.
nosaj

[66 Kicks]

I am a novice, but is the hum balance pot tied to an elevated ground ?

It is tied to the chassis right at the pot itself.

[nosaj]

It is tied to the chassis right at the pot itself.

Maybe in a stock one by the schematic, but at this point and already shotgunned amp we really do not know.
We can only speculate.
nosaj

[66 Kicks]

Maybe in a stock one by the schematic

More than just by the schematic. Every one I have ever gotten my hands on had the wiper connected to a solder lug held to the chassis by the pot mount.

[Enzo]

Does it still have the original 'output matching' pot wired up like stock? Does adjusting it affect the hum?

I don't recall an answer to that.


Hum from tube balance will be 120Hz, hum from the bias supply will be 60Hz.

[nosaj]

More than just by the schematic. Every one I have ever gotten my hands on had the wiper connected to a solder lug held to the chassis by the pot mount.

Point being someone has already tried working on this one for the same problem, we do not know what has been attempted without pics.

nosaj

[66 Kicks]

Point being someone has already tried working on this one for the same problem, we do not know what has been attempted without pics.

nosaj

In post #15, he asked if the Hum Balance was tied to an elevated ground. You can interpret this as tied to an elevated ground originally or tied to an elevated ground presently. Given the level of ignorance displayed here about the original condition, I went with the former.

So you came back with the statement, "Maybe in a stock one by the schematic,". The comma makes that a statement by itself. Well here you are questioning what I said. Maybe a stock one is like that or maybe not. And even then only if they happened to wire it like the schematic. It also strongly implies that I am making my assertion based on the schematic.

You have a valid point after that, but you should have started with something like, "Yes in an original one, but...". Do you see the difference?

[Enzo]

I apologize for having the wrong interior layout in mind.

We can see the schematic, but we need to know how the OP's amp is wired. Did someone elevate it or is it grounded? And I still want to know about the bias balance control.

[Chuck H]

I apologize for having the wrong interior layout in mind.

We can see the schematic, but we need to know how the OP's amp is wired. Did someone elevate it or is it grounded? And I still want to know about the bias balance control.

Well certainly something is wrong, regardless of everything being fixed and there being nothing wrong , because I'm betting that amp and the thousands like it that left the factory didn't all hum.

[lo-watt]

Thanks everyone for the responses,sorry I didn’t check these earlier. The hum balance and output matching pots are stock and unmodified. As far as I can tell what has been changed so far was changed correctly and properly grounded. Both of those pots test within tolerance and are properly grounded tested with a Fluke 87/3. No modification from stock have taken place,just replaced parts as deemed to fix the hum. Once all of the normal parts that could cause a serious hum were chucked,not much else was left. In the Power Supply anyway.

[lo-watt]

I actually used a massive soldering iron to make sure all wires from the PT were freshly soldered directly to chassis instead of via mechanical connection like a transformer screw. All of the grounds from fiberboard to brass strip were checked too.