Hum issue with AA1164 Princeton Reverb build.

[Anthon]

So I made this AA1164 Princeton Reverb, but it makes pretty bad hum.
I'm almost certain it has something to do with my sketchy grounding layout.

Some explanations how the hum behaves:
- Reverb level pot adds very much hum, when you increase it.
- Volume pot reduces hum somewhat, when you increase it. It also makes a tiny click around 2 o'clock
- When every pot is at 0 and volume at max, there is hum, but much much less. I don't consider it acceptable though.

I've placed 1 ohm resistors between cathode and the ground, on a jack where the ground pot should be - so I could read the current of the tubes without removing it from the cab, for easier bias.

Here is a grounding layout that I used.  I would appreciate if somebody could correct it.
http://oi68.tinypic.com/2ztlngw.jpg
http://oi64.tinypic.com/s2tz52.jpg
http://oi64.tinypic.com/mtqy6w.jpg

 

[MagnetoSound]

Layout looks ok, the crucial thing is that your filter caps are grounded close to your CTs and your star point looks ok from here.

A few things to check ...

Is the green/yellow wire the filament CT (and hence why you have no 100-ohm balancing resistors)?

Regarding the reverb, double check the leads are the right way round. It will often hum (and have low signal) if they are reversed.

You might find that a wire between the ground lug on the input jack (or the back of the volume pot) and the star ground helps things. That is quite a distance across the chassis at the moment.

Check the volume pot is not broken. That 'click' you describe might indicate a fracture in the carbon track.

 

[Anthon]

The ground of the multi-filter cap goes right to where CT and the star ground is.

I grounded the filament CT, so I removed the 100 ohm resistors.

Tried switching the reverb tank out/input, but then I got not signal at all.

 

[Anthon]

I've lifted everything but the filter cap and CT's from the star ground, and connected it to the input jack ground.
Much better now.

However, I still get the hum when I adjust the reverb. Perhaps wrong reverb drive transformer or reverb tank?

Specs of the reverb driver:
Fender Reverb Driver 125A20B / 022921 Single-Ended
Pri: 22800 Ohm
Sec: 8 Ohm
Power: 3,5 Watt

Specs reverb tank:
Model 4AB3C1B
Input Impedance 8 Ohms, Output Impedance 2250 Ohms
Input Insulated/Output Grounded

 

[MagnetoSound]

Yes, interesting, When you say 'everything but the filter cap and CTs', what does that include?

A lot of the early Fenders connected the tagboard grounds to the front panel pots.  Sounds like perhaps you have done almost the same thing.

I take it the volume pot is fine, then.

The reverb hum might be a ground loop between the two phono jacks ... if you unplug the send to the tank input does it go away?

 

[Anthon]

I've built quite a few of earlier Fender amps (Tweed Champ, Princeton, Deluxe, Bassman) - and I had the best results by grounding CT and Filter caps to the star ground, and the pots to the input jack. In and output jacks are not isolated, so they are always grounded. So I did the same with with this build.

Now everything goes to ground on the input jack, except CT's and the filter cap (grounded one of the screws of the transformer), the cathodes of power tubes, RCA connector grounds (not isolated), and the Reverb transformer (grounded to the ground of the RCA jack), and OT (grounded to the output jack).

I was thinking about a ground loop with the reverb tank. I've tried unplugging the send, but it doesn't affect the hum.

 

[CJ]

pull every tube but the power tubes and rectifier tube if you are using one,

then replace hem one by one and listen to hum,there is always hum on a Fender when you turn up the reverb,  the tank is like an antenna,  try a piece of sheet metal under the pan,

try rotating the reverb xfmr, notice how it is at a 45 degree angle on a stock Fender/ they do that for a reason.

are the power tubes matched?

 

[Anthon]

... there is always hum on a Fender when you turn up the reverb,  the tank is like an antenna,  try a piece of sheet metal under the pan,

I didn't realize it acted as antenna, and I placed it right under the power transformer on my workbench  :
I moved it away, and it improved the hum ratio dramatically.
There is still some hum though, but not very much. This is the first spring reverb I'm making, so I'm not sure what to expect.
It doesn't bother me when I'm playing, even on very low levels - so it should be good I guess?

The pot still makes a click when I turn it. I'll replace it tomorrow, and see if it solves the problem.

 

[MagnetoSound]

I didn't realize it acted as antenna, and I placed it right under the power transformer on my workbench  :
I moved it away, and it improved the hum ratio dramatically.

Doh!

 

[Tubetec]

Was going to chime in about the reverb ,but Cj got there first ,
Tanks are notorious for picking up hum from the mains transformer ,position and orientation of the tank can make a huge difference,
Experiment around for minimum hum , but its unlikely it will dissappear completely. A humdinger pot was usual on most of those Fenders ,thing is because you have several sources of hum adding and subtracting ,minimum hum will only be achieved at one particular setting of the controls on the front panel. 

 

[Tubetec]

I do notice from your diagram you have the grounded sides of the pots soldered to the pot casing ,which will mean there earthed to the chassis at that point too ,so there loops going on ,its the way fender did things too ,sometimes you can be fortuitous and a hum injected in one stage can be canceled on the next , its very hard to give you a concrete answer ,no two situations are ever the same so trial and error is the only way to improve things , Id suggest allowing the pots to ground via the mounting nut and back them with a toothed washer ,and disconnect the wire from the pots to the star ground ,might end up worse might end up better ,no way of knowing until you try it out .