Pultec Clone build hum

Ive been pulling my hair out for a few weeks trying to troubleshoot some power supply hum in my Pultec Build.  the hum starts when the tubes warm up and starts off loud and harsh (around 0db) and then fades to about -65db and just sits there. I had an idea thinking it was the main filter caps but replaced them and reflowed solder joints with no change.  The hum fills the lower spectrum from 60-150hz. 

    Ive searched for hours on here and found some ideas.  I tried shorting out the grids of the ax7 to see what happened to the hum.  I found that shorting pin 2 got rid of the hum and just left a normal ground buzz then I tried pin 7 and the hum actually got louder and squealed a little. Ive changed tubes, checked solder joints, moved grounds around according to the layout (off of recproadio) I've grounded out the eq, input and interstage with no hum change. I checked resistance across my heaters and the most I could find was 0.4 ohms.  I was thinking it could be a layout thing but looking at the original pultecs and my power transformer doesn't seem any closer to noise sensitive parts. 

Im using Edcor input (all the way on the far side of the chassis from the PT right by the input jack)
Sowter output (outside of chassis by the output jack right next to the input jack)
Cinemag Interstage (mounted on back panel inside chassis between in and out jacks)
Heyboer PT 290-0-290 (center tap directly to star main ground.  The heater center tap has been moved from the position in the layout to directly to star ground with no change)
LAYOUT:
http://www.recproaudio.com/diy_pro_audio/diy_files/eqp_1a/eqp-1a_layout_final.jpg


any insight would be amazing on this! the EQ sounds great other than this HUMMMMMMM.

thank you.

On that layout I do not see anywhere that one side of the heater is connected to ground. If the heaters are just floating you will get lots of hum.

Cheers

Ian

ruffrecords said:

On that layout I do not see anywhere that one side of the heater is connected to ground. If the heaters are just floating you will get lots of hum.

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The heaters are Center tapped. Are you suggesting that pin 9 should go to ground as well?
Thanks

Rob

Rompstomp89 said:

The heaters are Center tapped. Are you suggesting that pin 9 should go to ground as well?
Thanks

Rob

Click to expand...

No, but the centre tap should be. Is it?

Cheers

Ian

ruffrecords said:

No, but the centre tap should be. Is it?

Cheers

Ian

Click to expand...

Yeah,  they are center tapped.  In the layout I believe it goes through a 65k and a .3 uf cap in paralell to ground. I had it that way but changed it to see if the hum would change.  Now the center tap is going directly to ground. The hum did not change.

When you flip off the power, does the hum go away?
You should still hear audio through it for a few seconds as you turn it off.

dmp said:

When you flip off the power, does the hum go away?
You should still hear audio through it for a few seconds as you turn it off.

Click to expand...

When i flip power off hum is gone.  There is a big pop however and then a trail with some subtle white noise that fades away.

Would it be possible to move the power transformer away from the case by extending the wires?
It is possible the circuit is picking up hum from the power transformer, which would be ruled out by testing it moved away.

dmp said:

Would it be possible to move the power transformer away from the case by extending the wires?
It is possible the circuit is picking up hum from the power transformer, which would be ruled out by testing it moved away.

Click to expand...

I can't move it a whole lot because leads are short after i installed it.  I do however have another one because i was planning on building two units. However, if it was the PT wouldn't the hum start as soon as the unit is flipped on? it doesn't start humming until the tubes are warmed up?

Just some thoughts

Thanks DMP

Tubes need to be working before you hear the hum usually.
It depends on where it is being picked up.
Testing with the second PT is fine - just disconnect the current one.
Not sure that is the problem, but it is a good test to rule out a common culprit.

dmp said:

Tubes need to be working before you hear the hum usually.
It depends on where it is being picked up.
Testing with the second PT is fine - just disconnect the current one.
Not sure that is the problem, but it is a good test to rule out a common culprit.

Click to expand...

I will try it.  I was thinking its about time ha  I attached a link to the mp3 of the problem.  At the beginning I turn the unit on listen carefully you can hear the hum start loud then go softer.  I raise the gain up to hear the hum towards the end right after that i turn the unit off and there is a big pop so watch your ears and headphones/speakers! it happens at 18sec

https://www.dropbox.com/s/1f31a4q9lblenr7/Pultec%20HUM%20ref..mp3?dl=0

dmp said:

Tubes need to be working before you hear the hum usually.
It depends on where it is being picked up.
Testing with the second PT is fine - just disconnect the current one.
Not sure that is the problem, but it is a good test to rule out a common culprit.

Click to expand...

Reporting back:  Wired up the other (but same) PT with no change.  Moved it around and the low end hum did get lower by rotating it 90 degrees from the way i had it installed which is some good evidence! So end bells down is better in my case. however, the buzz is still there.

For the buzz I would try disconnecting the heater center tap (just tape it off) and run two 100 ohm power resistors to ground from each heater lead.
You may be able to keep the power transformer oriented the way you had it in the case, but rotate the input/output transformer or inductor. Those are the three most likely components to be picking up hum from the PT.  You want them to be off axis and 90 deg rotated with respect to the PT core.

dmp said:

For the buzz I would try disconnecting the heater center tap (just tape it off) and run two 100 ohm power resistors to ground from each heater lead.
You may be able to keep the power transformer oriented the way you had it in the case, but rotate the input/output transformer or inductor. Those are the three most likely components to be picking up hum from the PT.  You want them to be off axis and 90 deg rotated with respect to the PT core.

Click to expand...

The 100ohm trick defiantly helps. buzz is still present.  I moved the input tranny around and that just brings back the low freq hum.  Is there a way to lower the upper octaves of the buzz?  can i put a cap across the two heater legs as well or one to ground? any other tricks? thanks so much for the help!

If it is the heaters that are still causing the hum, you could elevate the heaters (instead of putting the 100 ohm resistors to ground, put them to an elevated stable DC voltage). I don't think that needs to be tried yet. It seems like you have a few things going on.  Do not put caps across the heater resistors. They are supposed to be AC.

What is the status of the hum with the 100 ohms on the heaters, and the PT a foot away? Also make sure you don't have anything else really close that can induce hum like a CFL bulb or computer monitor.
Once you have it quiet & working in a 'baseline' position, bring the PT closer and try to set the transformers and inductor in an orientation that does not hum. It may be impossible, however, and you'll have to go back to the drawing board on arranging the physical layout of the transformers. . 

dmp said:

If it is the heaters that are still causing the hum, you could elevate the heaters (instead of putting the 100 ohm resistors to ground, put them to an elevated stable DC voltage). I don't think that needs to be tried yet. It seems like you have a few things going on.  Do not put caps across the heater resistors. They are supposed to be AC.

What is the status of the hum with the 100 ohms on the heaters, and the PT a foot away? Also make sure you don't have anything else really close that can induce hum like a CFL bulb or computer monitor.
Once you have it quiet & working in a 'baseline' position, bring the PT closer and try to set the transformers and inductor in an orientation that does not hum. It may be impossible, however, and you'll have to go back to the drawing board on arranging the physical layout of the transformers. .

Click to expand...

I can't move the other tranny that far.  I'm still using its orig stock leads that are only about 12" long.  Ive lifted it above the unit as high as it could go (about 7-8") without buzz change.  its about -70 Db with the 100 ohm trick.  The layout i went off of actually had elevated heaters with the heater CT going to a 62k in parallel with a .3 uf cap.  I went back to that method and in stead of a .3 I did a 10uf, it seemed to help a hair.  Maybe that means its not really the heaters that are causing the buzz? what about a cap on the rectifier.  Ive seen  a matchless spitfire layout having the AC mains going into a 1n4007 on each leg and then a cap across them going into the rectifier.  Is that for reducing hum? I'm kind of just rattling off ideas now haha refer to this link to see what I'm talking about: http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/08/SpitFireCeriatone.jpg

what about a cap on the rectifier.  Ive seen  a matchless spitfire layout having the AC mains going into a 1n4007 on each leg and then a cap across them going into the rectifier.  Is that for reducing hum? I'm kind of just rattling off ideas now haha

Click to expand...

Don't think so... sounds like you have some coupling though the air and maybe the grounds.

dmp said:

Don't think so... sounds like you have some coupling though the air and maybe the grounds.

Click to expand...

super weird, so i had another PT laying around and installed that one.  Its a little under voltage (like 20V on the mains) and got no change in buzz.  I decided to check voltages around the unit and when i got to the plates to the OT i put the meter on it and the whole circuit freaked out. the plates on the 12au7 jumped up a 100V on both sides and just sat there! the buzz got ridiculously loud so i shut down the unit and powered it back on and it was back to normal buzz.  Tried it again and back came the loud buzz and voltage.  I had a chopstick on hand and poked around a little but could not manipulate anything to change the loud buzz.  Ive already double and triple check solder joints, tubes and components for right values.  it defiantly seems like a connection to me.  or maybe even a cap or something.  Im so lost in all of this.

tube circuit? walk in the park,

first off, split the problem in half,  take a jumper clip alligator thingy and clip one end to the innerstage primary hot and the other end to the innerstage primary cold (gnd) and report back immediately.
no xlr cord connected to input.

CJ said:

tube circuit? walk in the park,

first off, split the problem in half,  take a jumper clip alligator thingy and clip one end to the innerstage primary hot and the other end to the innerstage primary cold (gnd) and report back immediately.
no xlr cord connected to input.

Click to expand...

Hey CJ,
Thanks for chiming in.  I connected like you said and the buzz stayed the same.