Hum with my AC30/4 on the tremolo channel only.

[pompeiisneaks]

So I've been playing my AC30/4 for some time and love it, but there's always been this annoying 120hz hum on that channel only.  Normal is clean.  I've just not been able to figure out what the hell is wrong.  BTW it's basically identical to Sluckeys' AC15 plus the two added power tubes.  (Which is just a 1960's AC15 with the EF86 Preamp Normal Channel).

I've chopsticked the crap out of it and can't seem to find anything that's making noise.  It doesn't seem to originate from the input itself, I only see the tremolo there, in a nice pretty sine wave at say 10Hz.  I'm attaching screenshots from my PI from side one and two to see the freq is 120hz and the somewhat weird shape of it.  If anyone has ideas of what I can check next, I'd be happy, driving me nuts!. 

Sluckeys layout/schematic for the AC15: http://sluckeyamps.com/VAC15/Vox_AC15.pdf

I've done the mods to the thing to allow a pot to adjust the trem instead of the three switched mode, like he did, but otherwise it's pretty much the stock Vox circuit.

Any ideas?

~Phil

[shooter]

have you tacked in a big 'ol cap on the D tap, 50 - 100uF in || with the 32 that you have?

[hesamadman]

I had this problem. Elevating the heaters did the trick.

[pompeiisneaks]

Heaters are 60Hz hum right?  this is 120Hz.  It's also only on the one channel.  (120hz means dc rectified but not super clean, vs 60 is non rectified ac).

Shooter, I can try, i think I've got a 40uF I could put in parallel and see.   So you're thinking the 33uF may not be enough filtering?

~Phil

[pompeiisneaks]

Tried adding a 40uF which would push me up to 73uF with no change.  :(.

~Phil

[shooter]

no change

easy check anyway, have you tried rolling tubes into the trem?  can you follow the 120hz along the trem signal path with your scope?
typically by the time I get to a "D" tap my ripple is <10mV

[pompeiisneaks]

I can follow it back to the shift network, and then it seems to dissipate, but I'm not 100% sure as my scope seems to be stupid sometimes and it can't find some signals.

~Phil

[shooter]

I'm thinking from in/out V1 to in/out V4 (sluckeys tube designations)

Is the 120 there at the PI when you have trem off?

[pompeiisneaks]

what do you mean by off?  If I have an input plugged in or not, it still had the hum, but no tremolo

Phil

[pompeiisneaks]

And it works just fine if I turn it on too, jus still has the hum

[shooter]

So your source for hum probably isn't the OSC and if it hums with/without input, you should be able to pull V1 and V6 and narrow your hunt.

[pompeiisneaks]

I pulled v1 with no problem, but no change.  V6 is one of the power tubes, which do you mean?  The PI?

Phil

[sluckey]

V6 is the vibrator oscillator/phase inverter. Your power tubes are V7,8,9,10.

[pompeiisneaks]

Yeah I didn't have it in front of me and wasn't doing my math right, it's because the rectifier was V3 and that threw me off :D  I was thinking the last tube would be the rectifier.  SO basically if I take out the oscillator tube and the mod/pi for the trem tubes and the tone is still there, I know it's related to the preamp tube itself?  That's what you're suggesting?  I'll give that a try.  Edit, sorry I just reviewed the schematic again, V1 is the input tube, so I'm guessing you want me to rule out the input being the cause of the noise as well as the oscillator, then it becomes the remaining tube or that area?  Gotcha.

~Phil

[pompeiisneaks]

OH man, I got it.  Removed V1 and V6, still noise.  That meant I could look to a much smaller area, I started chopsticking again, and I found the source, the OT main power side leads were running too close to the tone stack wires.  I just pushed them back a way towards the back corner and bam, noise gone!  (or so much more minimal that at max volume it's almost non existent.

Edit: See in the picture bottom right corner under the tone pots, there's the main OT wiring going across but right under the wires of the tone stack, I pushed it back into the edge corner and the noise is gone :)

~Phil

[sluckey]

Do I see your OT secondary common wire connected to the pot buss ground? If so, disconnect it and connect it to the power ground as per the ground scheme drawing.

[shooter]

I found the source

when you eliminate all the good things only the bad remains

[pompeiisneaks]

Do I see your OT secondary common wire connected to the pot buss ground? If so, disconnect it and connect it to the power ground as per the ground scheme drawing.

Nope, if you look the black wire goes under the chassis with the other leads over to the three way switch and connects to the jack only.  I know I've heard that's not the best because the jack could wiggle loose and you lose ground, so I guess I could run a jumper from there over like you have in the layout, but I just buttoned it back up and put it back on my stack hehe... I'll have to remember to do that from now on.

~Phil