Low frequency hum on a hand-wired Pro Jr.

[Pmiguy]

Hi everybody,
I ditched the circuit boards in my Fender Pro Jr. and hand wired the circuit using the Hoffman project plan.  I've been playing the amp, making mods, etc. but there is always this annoying low frequency hum.  It's a basic schematic with a 12ax7 preamp, a 12ax7 phase inverter and two el84 output tubes.

I am a retired electrical engineer with good knowledge of electronics but still getting my feet wet in the world of tube amps.  Unfortunately, I don't have an oscilloscope available.

Here's what I've found
-low-volume hum at all times
-adding a 22uF capacitor to the B+ supply has no effect (47uF->69uF)
-Volume control has no effect
-Tone control has no effect
-Pulling V1 has no effect
-Pulling V2 (phase inverter) eliminates the noise

I've looked up and down at the wiring and construction.  I've tried swapping tubes. The heater wires are twisted pretty well and run next to the frame.  It seems if the heater supply had noise then V1 would be picking up the noise also, but that's not the case. 

I've seen on YouTubes and forums that the transformers in this amp are cheap, too small, too close together, may not be well shielded, etc.  Again, though, if it were transformer noise then V1 would pick up the noise also, as well as V2.

Can any of the veteran builders here give me some clues ?  It looks like this phase inverter is picking up hum.

Thanks!!! It's nice to have a resource like this.

[Dave]

It's definitely in the PI. Double check everything connected to the PI. Make sure the plate resistors measure close to nominal. Check the filter cap that services the PI.


Dave

[shooter]

this it?
https://el34world.com/Hoffman/files/Hoffman_Pro_Jr.pdf


measure VAC at the 3 PS taps
i would expect something along the lines of;


~4vac, 1-2VAC, >.5vac
if you've been moding anyway, I would consider adding another TAP in the PS for just the pre, 3 gain stages on one tap can be problematic with ripple sneaking around

[Brownie]

Are the 47ohm resistors still there for the artificial center tap of the 6.3VAC heater winding?  There has to be something if there's no center tap on the winding.

[Pmiguy]

Thanks all,
I only had a little time to look at this today.
Yes, that is the schematic I followed.
Yes, both 47 ohm resistors to ground are in place for the heater feeds
I ohm'ed out the entire power supply section.  No bad solder joints found.
Measurements: B+=317V, Y=293V, Z=221V
Plate resistors measure very close to spec
Measured/Calculated plate currents match very closely between the sides (78uA vs 75uA)

Questions- how do I measure the AC voltage on a high voltage rail? I'd love to do that. If I put one meter lead on the rail and the other to ground then there will be hundreds of volts across the DC blocking capacitor in the meter.  Won't that possibly blow the meter, potentially shorting out the amp's power supply caps through the meter?  I wonder if there is some type of spec on this type of thing.  I've thought of adding my own DC blocking cap, maybe 100pF, specced for hundreds of volts, in series with the high voltage lead.  Maybe I worry too much??

I'll think about adding another PS tap.  Probably just another 22K resistor and 22uF cap?

Could it have something to do with the feedback resistor?

Thanks so much everyone!  I'll do some more investigating tomorrow.

[AlNewman]

Could it have something to do with the feedback resistor?

I was going to suggest looking at your feedback supply.  Perhaps disconnect it and see if it makes a difference.

All your voltages around the PI line up with the schematic?

[shooter]

B+=317V

Won't that possibly blow the meter

yep, but most meters, n you'll have to verify are 600vdc


I make those measurements with a Fluke 87, or my Tektronix scope.  SE amps that I built are picky about to much AC on the DC

I am a retired electrical engineer

I made the assumption you'd have a scope n could use AC coupling

[acheld]

See:  https://www.valvewizard.co.uk/heater.html for a concise discussion of this problem. 

For my builds, I use 100R resistors in place of the 47R (not sayin' that is better).   If there is heater hum, my next step is to put in a humdinger, very easy to place, and solves the problem maybe 80% of the time (though I am very compulsive on heater lead dress).   

IF the humdinger doesn't solve the issue, then I start considering all the more complicated solutions. 

I'm assuming you're sure this is a 60Hz hum, not 120, right?   That can fool even an old crustacean like me.  A frequency analyzer on your phone works like a charm to be sure.

[tubeswell]

Pics of your handiwork might be useful for us. TIA

[Pmiguy]

Thanks for all of the input so far.

I lifted the feedback line and the noise got worse.  I also noticed that the feedback resistor is burnt.  There must be a wiring issue in this area.  It will take me a while to trace it all out and I'll let you know what I find.
 
I've ordered a Fluke meter with 600V safety on both AC and DC. It will be great to see the amount of AC noise on the power rails.
Thanks for the tip on the frequency analyzer app.  I downloaded Decibel X and am anxious to try it out.

I haven't bought an oscilloscope yet because I don't know how deep I want to go into this hobby.  This is my second project. I enjoy it but I have enough amps already and have no need to build another.  I've fixed two amps for friends and now somebody I don't know wants to bring his amp by. I live in a fairly big town but somewhat isolated and there is a need for a repair guy.  I don't want to be "that guy that fixes amps".  Not a bad problem to have, I guess.

[AlNewman]

There's a lot of connections around the PI...  It's easy to make a mistake.  Turn it off and go through everything, there could be an aha moment.

[Pmiguy]

I was able to fix the hum problem with better shielding around the preamp and phase inverter areas.  I noticed a big increase in noise whenever my finger was within several inches of these tubes, especially the lines to the grids.  This amplifier chassis is not in the original cabinet so the new cabinet may not have the shielding foil of the original.

I'm thinking of adding grid stopper resistors to the 12AX7 grid pins.  68k ohm seems to be a popular value.

Thanks for the tip on measuring AC on the power supplies.  I have a new Fluke meter with 600V safety rating.  The noise measures:
B+   3.0V
Y     80mV
Z     10mV

Thanks for the tip on the frequency analysis app.  I found that the noise peak was around 190Hz.