P2P Pultec 50hz noise help

[Clbraddock]

I just finished building a P2P pultec and overall am pretty happy with it, but would really like to reduce a noise bump I am getting at about 60hz (see pic).

Things I've tried and observed:

1. Checking that the casing of the DPDT and all pots and switches have continuity to ground.
2. With eq in, boosting the bass boost increases the 50-60hz bump a few db.
3. With eq in, increasing the bass attention all the way increases the 50-60hz bump more like 10db
4. With all eq controls zeroed out there is no different between the eq filters in or out.
5. Swapping each tube made no difference.
6. When sending a test signal, turning the unit off results in the 50-60hz bump dying within about a second even though the test signal continues for more like 8-9 seconds (The upper harmonic distortion keeps climbing with power off until the signal dies)
7. Placing a steel sheet adjacent to the power transformer and grounding it against the cap cans makes no difference. Same for mu-metal

What else should I try? I would like to get the noise down a bit, because I had planned to use a pair of these on the mix bus mid-side. To use them in their proper gain staging though, I would probably be applying 10-15 db of gain AFTER the pultecs to get up to reasonable LUFS. The noise floor is okay where it is, but boosting it 10-15 db it is going to be noticeable. Is there anything I can do, or is this just the way real pultecs are?

 

[ruffrecords]

On your graph, the signal looks to be at about -30dB and the hum around -90dB. You don't say what the absolute signal level is that the -30dB represents but if it is 0dBu then you have less than 1mV of hum which is a good start for a design using AC heaters. The fact that the hum disappears when you turn off before the signal dies away does imply it is a magnetic coupling of some sort.

Heater lead dress and overall layout can make a difference. Do you have a hum dinger fitted or do you just connect one side of the AC heater supply to analogue 0V?

Cheers

Ian

 

[Clbraddock]

Thanks Ian. Test signal is -20dbfs at 1khz. I think that should be about 0 dbvu (interface is an Apollo set to +24 db mode). If -20dbfs is 0 dbvu, doesn’t that mean that the -90 dbfs noise is really more like - 70 dbvu? I found an old pultec manual that quotes the noise spec as -90 db +/- 10.


The heaters are wired as per the vintage layout. No humdinger, but the heater tap on the power transformer has a center tap. The center tap connects to ground through the cathode bias resistor of the output tube (12AU7).

I was under the impression that humdingers were for non-center tapped heaters. Would a humdinger still apply to the heater wiring I described above?

 

[Clbraddock]

As far as lead dress of the heaters they are twisted tight and tucked in the corner crease, except between the input 12AX7 and output 12AU7. There there tubes are so close together there isn’t room to do any sort of twist. I wired it like shown here (heaters in light blue):

 

[ruffrecords]

OK, so with the centre tap there is no opportunity for a hum dinger., but you have elevated heaters which will also reduce hum.

The -90dB noise figure is, I believe, relative to +10dBm output so in reality it is about -80dBu (which is about what you would expect from a well constructed modest gain tube device.

Can you post a picture of the inside wiring/construction?

Cheers

Ian

 

[Clbraddock]

Thanks for your help. I really appreciate it. It's hard to get good photos with the front plate still on. I could take it off, but would have to unscrew all of the pots, switches, etc. If that would be more help let me know. Thanks again!

 

[Clbraddock]

In the 2nd to last pic (IMG_0093) I'm wondering if I should pull the wires away from the back panel to get them further from the tube heaters. There is a tube heater approximately where the red line is Ive drawn below:

 

[Clbraddock]

Also, here is the full wiring layout I more or less worked off:

 

[Ike Zimbel]

I just finished building a P2P pultec and overall am pretty happy with it, but would really like to reduce a noise bump I am getting at about 60hz (see pic).

Things I've tried and observed:

1. Checking that the casing of the DPDT and all pots and switches have continuity to ground.
2. With eq in, boosting the bass boost increases the 50-60hz bump a few db.
3. With eq in, increasing the bass attention all the way increases the 50-60hz bump more like 10db
4. With all eq controls zeroed out there is no different between the eq filters in or out.
5. Swapping each tube made no difference.
6. When sending a test signal, turning the unit off results in the 50-60hz bump dying within about a second even though the test signal continues for more like 8-9 seconds (The upper harmonic distortion keeps climbing with power off until the signal dies)
7. Placing a steel sheet adjacent to the power transformer and grounding it against the cap cans makes no difference. Same for mu-metal

What else should I try? I would like to get the noise down a bit, because I had planned to use a pair of these on the mix bus mid-side. To use them in their proper gain staging though, I would probably be applying 10-15 db of gain AFTER the pultecs to get up to reasonable LUFS. The noise floor is okay where it is, but boosting it 10-15 db it is going to be noticeable. Is there anything I can do, or is this just the way real pultecs are?

WRT #1, have you checked that all of the places where you have tabs going through to the chassis have actual continuity to ground? The paint on the rear chassis looks a) great, and b) like it might have good insulating properties. You might want to grind off some paint at those points (like your main chassis ground screw in particular). Taking off some paint where you have metal-to-metal contact between the chassis panels (back to sides, sides to front panel etc.) might also help. Star toothed washers can also help get through the paint.