120hz hum only in phase inverter

[thetragichero]

i have been tearing out my hair (what little is left) chasing down a hum in what is essentially a fender vibrolux reverb (both channels mix before the reverb stage and bias wiggle trem instead of a roach).
the hum is totally killed removing the phase inverter, or shorting the phase inverter grids together.
i've completely ripped out the grounding scheme and gone with a star grounding (only connects to chassis by input jacks. reservoir capacitor, bias ground, and ht center tap are on one node. power tube screens, filament center tap, and local grounds the connect to their respective filter cap negative are on another and connected to the 'dirty' node by a wire. all pot grounds and cathode grounds connect to their respective nodes).
i've tried adding another 20uf filter cap in parallel to the one on the phase inverter stage with no change
if anything, it's gotten WORSE. what am i missing?

edit: also almost every single grid (besides reverb send and output tubes) is in shielded wire

[sluckey]

Is the OT secondary grounded with power ground or PI ground?

[thetragichero]

phase inverter ground since it there is negative feedback to the phase inverter tail

[PRR]

the hum is totally killed removing the phase inverter, or shorting the phase inverter grids together.

So it could be from before the "phase inverter". (A long-tail?)

[sluckey]

Disconnect the NFB wire at the OT secondary. Any joy?

[thetragichero]

also tried disconnecting the negative feedback and it didn't help. stage right before phase inverter is a standard 'fender' stage (100k/1k5/25uf) with a big 68nf cap off the plate followed by a 47k resistor to ground and then a 1nf cap to to the phase inverter grid. i've tried different plate resistors on the phase inverter (12at7).
while rewiring the grounds hum was about to be expected (tons of led lights in the shop) until attaching the preamp node to the star grounding point
i've tried clipping clip leads (with the preamp node disconnected) to various points on the preamp buss to various points connected to the star ground point (phase inverter, trem, reverb, star ground point) with no decrease in hum

[pdf64]

So how about if the signal is muted before the LTP?
eg short the hot end of the 47k to the 0V common return of the LTP tail?

[thetragichero]

i had tried grounding the 'hot' side of the 47k already but i wanted to check again. still hums, regardless of grounding it to the phase inverter common or preamp node

[pdf64]

Sorry to ask this, but are you sure the hum isn't due to the valve being bad? 

[thetragichero]

I've tried a different 12at7 in there and it still hummed
now you know why I've been pulling out so much hair!

[shooter]

do you have 1ohmer's to check current in the PA?  If so what's the tubes running at?


try clipping a 100uF filter cap in parallel with the one that feeds the PI, any change?

[thetragichero]

6l6gc 45ma cathode current 420 plate voltage

haven't tried a big cap I'll try that next

[thetragichero]

80uf cap (what i had handy) in parallel, no change in hum

[tubeswell]

Leave the PI tube in, but pull out all the tubes before the PI. Does it still hum?

[thetragichero]

all preamp tubes besides phase inverter out, still hummin'

[thetragichero]

should my next step be to try to shield the power tube grid wires? they're about the only grids not currently run with shielded wire

[PRR]

...tons of led lights in the shop...

So unplug them.

[Leevi]

Is it  background hum that does not increase when turning the volume pot?


Are you sure it is 120Hz?


Have you tried to improve filtering of the negative bias voltage?


/Leevi

[thetragichero]

...tons of led lights in the shop...

So unplug them.

last test (with all preamp tubes but the phase inverter out) was with all the lights off and still hummed

i can try piggybacking more 50uf caps on the bias filters and see if that helps

[shooter]

Are you sure it is 120Hz?

how are you measuring the 120?
at what test points are you seeing the 120?

[pdf64]

Is the decoupling cap on the LTP’s common grid section definitely good?

[thetragichero]

measured as 120hz with a frequency counter app (also the pitch sounds like an octave higher than 60hz)

Is the decoupling cap on the LTP’s common grid section definitely good?

there was a dipped high voltage ceramic there but it was slightly microphonic (tapping could be heard out the speaker) so i replaced it with a metal film cap

[Bieworm]

On my recently finished 64 PR build I had a hum that wasn't traceable either. Turned out to be the footswitch. It's a generic fender type dual switch (vib/rev). When i swapped the connectors on the amp the hum was gone. I opened the lid from the footswitch and saw that the shielded cable was used for the trem. Should have been on the reverb button. So I switched the buttons inside the switching shell...

[Backwoods Joe]

If you have a ltp, maybe you should try elevating the filament center tap?

[SILVERGUN]

just curious..
Was this a working amp with no hum that then developed a hum or a new build that has hummed since you fired it up?

[thetragichero]

it seemed to be working fine, the guy who owns it just asked (after having it a couple of weeks) if there was a way to decrease the little bit of hum when i went in there to clip the bright cap and replace a carbon composition resistor that was making excess noise. it seems to be humming worse now that I've messed with it and done things "right" than before

filament artificial center tap is now at 40vdc (sorta just guessed with resistor values i had on hand in metal oxide) and it doesn't appear to have reduced the hum. guess it's scope time

[thetragichero]

this is what I'm getting at the output jack junction to the negative feedback resistor, 10 mv/division

[thetragichero]

coupling cap from inverting section of phase inverter



coupling cap from non -inverting section



both of these at 100mv/division (edit: read .1v as 1v)

[sluckey]

both of these at 1v/division

But the red variable knob is not in the calibrated position so V/div is meaningless.

[thetragichero]

this is at the output with negative feedback disconnected

[thetragichero]

removed the grid wires to the power tubes and this is on the phase inverter coupling caps





and here's the coupling cap before the phase inverter (lead connected to preamp, not the phase inverter grid)

[thetragichero]

phase inverter grids and cathode

[shooter]

Was the scope grounded to the same point in all measurements?
your images lead me to think ground is somehow not ground at the different test points.


for fun, ground the scope probe to chassis, now measure AC volts (probably <2vac) at different ground points, like input jack, bottom of the long-tail, speaker jack, maybe a preamp tube where the cathode R meets ground

[thetragichero]

yes the scope was grounded to the chassis (alligator clip clipped nicely to one of the mounting holes). not exactly the circuit ground (only input jack ground attached to chassis) but at least constant for all of the measurements

[shooter]

ya, that's the point a common reference
all the other "commons" should be at some very small DC, there should be no AC above "noise".  If the common isn't common, it's a great place for AC to sneak into an otherwise great system, hence the suggestion above.

[thetragichero]

maybe making some headway: set my meter to to vac and the only things not giving me a zero reading were the reservoir cap negative lead and the negative lead for the tremolo oscillator filter cap (like half a bolt ac) touching the tremolo filter cap positive lead made a sound out of the speaker.
enough chasing gremlins for the day

[thetragichero]

alright, after replacing a number of components and wiring i tracked out down to an under board b+ wire for the trem. moved it and even in the noisy led light of the shop it's no longer humming

so: poor board design on my part but not a mistake I'll replicate again

[thetragichero]

although now i have motorboating

edit: 220pf "enhance cap" on the phase inverter was causing the motorboating and the remainder of the hum