Princeton Reverb - Reverb Noise

[Bombacaototal]

Yes, hoffman amps stuff is outstanding for their known working designs, built by tons of people, (myself included).

As seen on this, the reverb grounds are separate even from all other parts of the grounding. You may want to try grounding reverb stuff quite a ways away from both power sections and input.

Other than that, I'm at somewhat of a loss myself.

~Phil

Thanks for the reply Phil. Currently I have the cathodes of the tonestack jumpered together and a lead going to the pre-amp ground and separately I have the reverb cathodes (Driver + Recovery) jumpered together and grounded with another lead at the pre-amp ground. The rcas are grounded separately as per Hoffman.

To make sure I understood your suggestion correctly I should create another separate ground for the reverb (cathode of the reverb driver and recovery). Should I include the pots and supply on that new ground too?

[pompeiisneaks]

You may want to try grounding the pots and cathodes of the tubes related to reverb with the reverb pan RCA connections.

That way all parts of that signal process are in the same location and not able to be influnced by higher current areas.

~Phil

[Bombacaototal]

What a battle.... I worked maybe 20 hours on the amp from Friday night until yesterday and unfortunately did not manage to fix, although the ground loop improved ever so slightly.

The 4th pan that arrived and was by far the worst of them as far as noise.

I used a number of alligator clips to move some of the grounds around and try to debug the issue. With the tonestack disconnected I also disconnected the footswitch (because it is grounded to the chassis and I wanted to eliminate the most grounds I could), and I also removed the 3.3M dry path resistor to only leave the reverb circuit and PI on.

I experimented with the FX Loop ground, and it hums more if isolated via washers and grounded to the pre-amp then grounded on its own spot. The PI cathode ground did not make much difference between grounding it on the FX Loop or the Pre-amp ground.

Now to the reverb circuit, I added a new supply cap for the reverb transformer so I could play around with its ground too. With both RCAs (send/return) grounded to the chassis, the least amount of hum I managed to get was creating a separate ground for the reverb driver cathode, reverb recovery cathode, reverb supply and reverb send or dwell pot (but there was still hum) and in combination with the Reverb return pot and the mixer cathode grounded at the pre-amp ground straight (not the buss bar). The worst hum was grounding the reverb transformer supply with the screens or having the reverb recovery and mixer cathode grounded on the new reverb ground too.

I then went into the mission of isolating the RCAs which are under the board, so I had to gut the amp and remove the board to do that...Oh my...It improved a tiny little bit but with the return (red) isolated and grounded to the pre-amp and with the send (blue) grounded at the chassis. With both isolated it had more hum. I tried moving both and either between the pre-amp and the new reverb ground. I also tried the PI experiment again under this last configuration but did not make much difference.

The only thing I haven't tried is making all the power section and power tubes share the same ground like Hoffman. Worth a try?

Otherwise I guess I will have to live with it as there is nothing more I think I can do.

When I hooked everything back up, for one triode I did by mistake a cathode into the tube plate and the plate into the tube cathode. Did I fry the AX7? Now I have very little signal coming through and a bit of a distorted sound. I need to debug this today. Probably a cold solder somewhere.

[Bombacaototal]

The Ax7 was indeed fried, very boomy and no high end. The mid switch was no longer connecting well to the chassis and was causing a lot of weirdness on the tonestack. Everything back in place and working.

Going full Hoffman grounding tonight as the last attempt, otherwise I might have to live with this hum

[Bombacaototal]

Yesterday I worked the pre-amp back to Hoffman style, with all cathodes from Tonestack and Reverb Circuit, plus the Buss Bar and the isolated inpup jack going to a single pre amp ground near the input jack. I also have the supply capacitors for the tonestack and reverb driver and reverb recovery + mixer there.

I will work on the power section moving the Power Transformer CT (Red/Yellow), bias supply and large supply capacitors (B+) to a single point together with the bias pot and the power tubes ground.

My question is what to do with the PI cathode and PI Supply grounds and what to do with the screens supply. Do I move all of them to the power amp ground lug too?

I am also going through this to see if I can get any further ideas
https://ampgarage.com/forum/viewtopic.php?t=27991

[sluckey]

My question is what to do with the PI cathode and PI Supply grounds and what to do with the screens supply. Do I move all of them to the power amp ground lug too?

I do.

[Bombacaototal]

My question is what to do with the PI cathode and PI Supply grounds and what to do with the screens supply. Do I move all of them to the power amp ground lug too?

I do.

Great, I will attempt and report back.

For the single power amp ground point do you use the power transformer bolt, or is it fine having this ground righ in between the 2 power tubes?

[pompeiisneaks]

I have often heard working amp techs say that using a transformer bolt for any important connection is a recipe for disaster. The transformer undergoes the most flex/movement throughout the lifetime of the amp and they've found many amps that were blown pretty badly or the like because the transformer flexing the chassis caused the nut to loosen and the ground/earth/etc comes loose. It's often much smarter to put your own grounding nut in it's own hole int he same general vicinity so it isn't prone to the flexing caused by the transformer.

~Phil

[Bombacaototal]

I have often heard working amp techs say that using a transformer bolt for any important connection is a recipe for disaster. The transformer undergoes the most flex/movement throughout the lifetime of the amp and they've found many amps that were blown pretty badly or the like because the transformer flexing the chassis caused the nut to loosen and the ground/earth/etc comes loose. It's often much smarter to put your own grounding nut in it's own hole int he same general vicinity so it isn't prone to the flexing caused by the transformer.

~Phil

Thanks Phil, I will try grounding everything in the Power Tubes ground point which is between the 2x 6V6s.

[Bombacaototal]

The only things I am not moving to the power tubes ground are the heaters 2x 100R and the mains cable ground (as I think by regulation this has to have a standalone ground).

Sluckey, since you have build some Hoffman style amps do you see any issues in leaving the 100R on its separate ground?

[sluckey]

I will try grounding everything in the Power Tubes ground point which is between the 2x 6V6s.

Not saying this is a bad idea but I don't like it. Ideally, the power ground would have the HT center tap connected directly to the reservoir cap negative lead using a dedicated lug that is bolted to chassis. This may not be possible, especially if you are fond of using cap cans. I like cans. My power ground will be a lug/s bolted to chassis very near the cap cans.

Sluckey, since you have build some Hoffman style amps do you see any issues in leaving the 100R on its separate ground?

I've never built any of Hoffman's amps. All my layout designs are my own. When I use an artificial heater center tap, the 100Ω resistors will be mounted on my board. The ground end of the resistors will be connected to the on-board power ground. If the PT has a real heater CT wire it will be connected to chassis at the same point as the HT center tap.

My grounding scheme is not a hard/fast rule. There are many successful variations. And since I like to repurpose transformers from old Hammonds, I often have to adapt my grounding schemes a bit. The only rule I strictly follow is connect all power grounds to chassis near the PT and reservoir cap. Connect all preamp grounds to chassis near the input jack.

[Bombacaototal]

I will try grounding everything in the Power Tubes ground point which is between the 2x 6V6s.

Not saying this is a bad idea but I don't like it. Ideally, the power ground would have the HT center tap connected directly to the reservoir cap negative lead using a dedicated lug that is bolted to chassis. This may not be possible, especially if you are fond of using cap cans. I like cans. My power ground will be a lug/s bolted to chassis very near the cap cans.

Thanks for the reply Sluckey.

As far as the power section I actually have it now similar to how you are suggesting,
1) a dedicated lug bolted to the chassis with the HT center tap, a lead coming from the negative side of the reservoir cap and a lead coming from the bias supply cap, very similar to Dumble ODS grounding.

2) I have another dedicated ground lug bolted to the chassis with the power tubes grounds, the bias pot and the PI cathode.

3) One for the 100R and

4) one for the mains.

Hoffman seems to have all of these in a single point, which is what I am planning to try

[Bombacaototal]

I tried putting all the power section on the same ground (except for the 100R) as per Hoffman and it made little to no difference.

So I put every back as before...

Any other ideas or should I call it the day

[sluckey]

Worth a try... Use shielded cable between the reverb pot and the board. If you have some shielded twisted pair (STP) use that. If not, use two shielded cables. Ground shield at the pot ground.

[Bombacaototal]

Worth a try... Use shielded cable between the reverb pot and the board. If you have some shielded twisted pair (STP) use that. If not, use two shielded cables. Ground shield at the pot ground.

Thanks Sluckey, I am already using coax cables (RG316/U) from the board to the pot and from the pot to the tube socket of the mixer tube. I am using normal hook up from the board to the socket (PIN 6, 7, 8 )