Hum in Fender Vibro Champ

[Axtman]

I have a 1968 Fender VibroChamp with a hum. This hum is 120 cycle and is not affected by the controls.

Here is what I have done to troubleshoot:
1. Removed both preamp and vibrato tubes.
2. Checked all ground connections.
3. Swapped out can capacitor.
4. Checked all capacitors. They had already been replaced.
5. Touched up some cold looking solder joints.
6. Swapped out rectifier and power tubes with known good ones.
7. Swapped out rectifier tube with solid state replacement.
8. Checked voltages. They are a bit high since my electricity is 121 volts.

It still has a hum and I am very puzzled. I suspect a bad power transformer, but that is just my hunch.

HELP!!

[Jazz P Bass]

O/k.
You have specified what you did to troubleshoot the hum but you did not indicate what any of the outcomes were.
Does removing the phase inverter tube have any affect?
Same with the preamp tubes.(one at a time)
Did you monitor the power rails of the different stages for excess Vac ripple?
What is the heater voltage measurement?
And, with no input to the amp, what is the amplitude of the hum (measured at the speaker terminals, Vac)
Try that again with the input jack grounded tip to ring.

[trem]

Does you can cap have an un-used 20 mic (or any value) tab on it.?

[Axtman]

Jazz P Bass,

None of the 8 items I mentioned above made any difference in the hum.

The amp does not have an inverter tube. Just a preamp tube and a tube driving the vibrato. I removed those and get a hum. Of course if I pull the single 6V6 output tube or 6Y3 rectifier tube the amp goes quiet.
The hum is about 55 millivots.
The filaments are about 6.5 volts.
Grounding the input has no effect on the hum.
The hum is not affected by the controls.

Trem,
There is no spare 20 uf tab on the can cap.

Thanks

[pontiacpete]

Did you clean the input jack?

[Jazz P Bass]

The next step is to find where the hum is being injected.
Get on the 6V6 grid pin & see if there is any volts ac ripple on it.
And maybe temporarily lift the 330pf cap on the grid to cathode.
If the grid is not seeing any ripple then the next suspect would be the output transformer.

[Bob M.]

Using a twisted pair heater string with ground reference resistors is never a bad idea in these amps although it's a bit of work. Some guys inject the cathode voltage from the power tube to float this heater string above ground.

Bob M.

[Zouto]

Using a twisted pair heater string with ground reference resistors is never a bad idea in these amps although it's a bit of work.

Bob M.

Yeah Bob, heater rewiring plus a complete cap/resistor job (with single 22u/500V caps and metal oxide resistors) removed all the hum in my SF VibroChamp, so it's quite a good idea. You'll get what you worked for...
Zouto

[Bob M.]

I have one Champ amp with DC heaters; very, very quiet. It can be a big implementation but works just great, kinda the ultimate. I also like using a Pi filter prior to the output transfx center tap in Champ amps and their brethren. This also works very, very well and reduces ripple a great amount. Because in a single-ended Champ you don't have that natural hum-cancelling that comes with push-pull circuitry, you have to pay attention to a bunch of other little things that can make a big difference: grounding, heaters, filtering, lead dress, it all adds up. These amps are ripe for improvement.

[Rick Erickson]

The best way to silence a Champ that I have found is to add a choke and an extra filter cap between the rectifier and the output transformer primary. [Rect - Filter-Choke-Filter-OT...and on to driver & preamp] This smooths out the ripple in the power supply and knocks the residual hum right out of the power amp. I think I used a Deluxe Reverb choke. It was several years ago and it is still working fine. Balancing the heaters isn't a bad idea. I probably did that too, but I don't remember. I don't think DC heaters are really necessary but it is always an option if you are so inclined.
RE

[Steve Conner]

I agree, I think balanced heater voltage (maybe also elevated using the power tube cathode voltage) and a choke for the power supply are the two biggest things you can do to quiet any Champ-type amp. They certainly made a huge difference when I tried them: the hum went below the noise floor.

[Enzo]

A long time ago, Fender issued a service bulletin on the Champ. They told us the hum is normal, and stop trying to "fix" it. The fix would be to do as you have done, add filtering stages to B+. That was not something warranty would cover.

[ornjon]

Choke on the Champ

The best way to silence a Champ that I have found is to add a choke and an extra filter cap between the rectifier and the output transformer primary. [Rect - Filter-Choke-Filter-OT...and on to driver & preamp] This smooths out the ripple in the power supply and knocks the residual hum right out of the power amp. I think I used a Deluxe Reverb choke. It was several years ago and it is still working fine. Balancing the heaters isn't a bad idea. I probably did that too, but I don't remember. I don't think DC heaters are really necessary but it is always an option if you are so inclined.
RE

Hi,
As I'm a novice in this world I was wondering if you or someone could be a bit more descriptive as to how to install the choke and what sort of cap I should use with it? Where is the rectifier and the transformer primary (pin number or?)?