50Hz hum

[txstrat]

The PI stage is grounded to the bus on the back of the pots leading to the input jacks. The pots are NOT insulated - so it's not only grounded at the jacks rather than grounded by all pot housings too.
I'll take the PI ground from this bus and put it to the secong caps ground.

I've tried 3 sets of matched power tubes. None of them made a difference. Pulling only one tube neither made no difference.

How could I handle the bias supply? Would changing to full wave make a difference?

The hum is very deep and I'd say it's like E-String 3rd fret on a bass guitar. Should be 50Hz then.

One more thing, I run a 1 ohms/2w resistor from the power tubes cathodes to the reservoir caps ground.
Would it make sense to run a separate wire from the resistor to the grounding spot?

[Enzo]

One fix at a time. I doubt the bias supply is involved. If it were, then pulling the PI wouldn;t kill the hum.

[txstrat]

OK, thanks Enzo.
Here's my next changes:

1. changing the PI ground - if that doesn't help
2. changing to a separate wire for the power tubes cathode resistor - if that doesn't help either
3. changing to insulated jacks all around - if that doesn't help
4. burn all my equipment and start playing drums...

Just gimme time. I'm at work right now.

[txstrat]

1. Different PI ground decreased the hum a tiny bit, but it's still there.
2. Separate wire to cathodes resistor didn't help
3. let the speaker jacks float and clipped a wire to the PT bolt - no change.
4. either I have to burn my stuff now or I'll rewire it from scratch again.

[dai h.]

(I'm just throwing this out there--I haven't looked over in detail so this may not be a solution, but), a) you have a sort of semi-(or incomplete)looking pi filter for the bias (looks like there should be another filter cap), and b) the ground for the bias looks like the returning AC ripple (back towards the source which is the PT) travels through the noisiest part of the B+ ground line.

[txstrat]

Thanks for your thoughts dai,

I'm using only one filter cap in all my amps' bias feeds with no problems so far. But I'll try adding another one after the dropping resistor/voltage divider. Thanks.
I've changed the ground wire from the PT to the PT bolt to no avail.

travels through the noisiest part of the B+ ground line

What do you mean by that?

[dai h.]

the filters in the B+ line and Ls and Rs help "smooth" and "purify" the DC after going through the diodes (AC to DC). The early portions (right after the rectifier) still has lots of residual AC (ripple) and the returns are the "dirtiest" parts, so they need to be connected (to the ground line), of course, but in a way they don't introduce noise (get into the audio). Look on GEO(R.G. Keen's site) for example, there is a good explanation, diagrams, etc.

[txstrat]

Here's my thoughts for now:
I have disconnected all preamp stages, including the filter caps etc.
Amp runs with PI and power stage only. PI input is grounded before the preceding coupling cap. Still hum.

What makes me wonder is, that all things I tried to reduce hum - which might have decreased hum in other amps - didn't help. To me there must be some fundamental thing which I haven't considered yet, a faulty part or poor connection or whatever.

After reading the article of R.G. on the geofex site I'm about to try some different grounding schemes.

Any suggestions?

[dai h.]

just general ones. Look it over end to end. Could be the grounding scheme, but other things can also cause hum. Layout, something that should be grounded that isn't, (occasionally) a bad part (tube, filter cap, etc.). I suppose the entire chain should be considered (not just the amp but the gtr. as well).

[pdf64]

Sorry if you answered this earlier but have you tried a different PI tube?

[txstrat]

Sorry if you answered this earlier but have you tried a different PI tube?

Yes I did to no avail.

just general ones. Look it over end to end. Could be the grounding scheme, but other things can also cause hum. Layout, something that should be grounded that isn't, (occasionally) a bad part (tube, filter cap, etc.). I suppose the entire chain should be considered (not just the amp but the gtr. as well).

Dai you hit the bullseye.
I clipped another filter cap across the bias circuit (now there's one before and one after the dropping resistor) and the hum decreased.
It's now less than half as loud as before. I think I can easily live with it. I'll build the amp up again and maybe take care of the bias circuit a little more when I'm done.
Thank you so far guys.

Matt

[defaced]

Sorry if this has been asked already, I'm trying to follow along at home, but on your layout there are no ground wires for the 22uf and 8uf filter caps in the preamp. Where are they grounded in your build?

Another thought, because the PI is grounded through the pots bus wire and not directly to its filter cap negative terminal, you have PI ground current going though the chassis. I know there are alot of different ground schemes, and this one has worked for you before, but I'd change that. I'd put the PI ground to it's negative filter cap terminal, and ground the pots/input jack bus elsewhere. Also, because the pots aren't insulated from the chassis, each pot/bus wire section is making a ground loop. Might not be killing you, but then again, it might.

[txstrat]

Sorry if this has been asked already, I'm trying to follow along at home, but on your layout there are no ground wires for the 22uf and 8uf filter caps in the preamp. Where are they grounded in your build?

Another thought, because the PI is grounded through the pots bus wire and not directly to its filter cap negative terminal, you have PI ground current going though the chassis. I know there are alot of different ground schemes, and this one has worked for you before, but I'd change that. I'd put the PI ground to it's negative filter cap terminal, and ground the pots/input jack bus elsewhere. Also, because the pots aren't insulated from the chassis, each pot/bus wire section is making a ground loop. Might not be killing you, but then again, it might.

The 22uF and 8uF wasn't in the circuit - thanks for asking.
I'd already changed the PI ground to it's filter, before I added a second bias cap.

[dai h.]

Dai you hit the bullseye.
I clipped another filter cap across the bias circuit (now there's one before and one after the dropping resistor) and the hum decreased.
It's now less than half as loud as before.

good to hear there is some improvement

Also, because the pots aren't insulated from the chassis, each pot/bus wire section is making a ground loop. Might not be killing you, but then again, it might.

I only looked at the diagram(not the photos), which seemed okay (at that point) as drawn (ground bus connecting pots not connected to chassis through backs of pots) however, it appears as (defaced/Mike) points out that the grounds there are connecting to chassis there and that appears to upset the grounding scheme (makes a loop where returns can come back through two paths).

Also, (Mike) even if the pots are connected together (such as on a typical older style grounding scheme on Fenders, Marshalls, etc. which use the chassis) I don't think it's a loop. I see it more as one connection (the potentials are brought closer together by connecting the backs of the pots). I think it also makes sense in case a pot becomes loose (what would happen if there wasn't a wire connecting the backs?).

[dai h.]

looked over the diagrams a bit more, and so far as I can see, if this was my amp I would connect some of the grounds a bit differently. (First take off the connections to the chassis bolt point where the 82 ohms 2Ws, bias tap wire and bias circuit ground are connected together.) I'd have the bias tap return (the yellow wire in the diagram which I'm assuming is one of the two wires from the bias winding) go to the bias circuit board where the 100uF/100V + and 10k are. (The noisiest parts of the bias circuit return ground and the B+ grounds are separated but they still connect to each other--as all grounds should be. And, AC ground for the grid load/bias return Rs referenced to the power tube cathodes.) Then from that point to where the 1 ohm 2W (power tube cathodes). That point is where the bias circuit ground ties in to the rest of the ground line. The ground for the two 82ohm 2Ws (which I'm assuming is the false filament center tap), I would connect to the ground of the first cap at the output of the B+ bridge rectifier. Can't guarantee success, but since the extra bias circuit filter worked (implies some problem there) hopefully it's the right direction.