pultec eq HUM *somewhere inside case*

[college101]

I ve had it on for a few hours now, and its running great...burn in for 24 hours...then were gonna listen to it

 

[orangechili]

Ok, reviving this old thread because I have a very similar low level hum issue in a eqp-1a build. PTP Cayacosta layout.  Grounding scheme exactly the same as the rec pro audio layout. Here's what i've tried so far.

-hum goes away if i ground grids on 12ax7(v1) pin 2 or 7
-chopstick test can't identify a wire or solder point that causes hum
-triple checked wiring around pots/both boards
-experimented with different xlr grounding pin 1 to chassis, etc to no avail


All voltages in the unit are spot on the schematic and the unit functions great except for the little low level hum issue.  i'm pretty sure its around the input section because the shorting the 12ax7 makes the hum go away...  Any ideas on how to proceed? I hate to start changing around the grounding scheme in the layout especially since it seems proven good over the years.... I'm curious  to know what caused 'college 101's' hum issue that he corrected?

 

[orangechili]

Also tried many different tubes as well so no issues there

 

[lassoharp]

i'm pretty sure its around the input section because the shorting the 12ax7 makes the hum go away

What that is suggesting is that the hum is originating upstream in the passive section, or possibly a bad tube so tube swap first.  Second, I would disconnect the leads from the interstage primary and see if the hum is still there.

Just to rule out things step by step,

1)Has the unit always had the hum since you completed it?  Or was it fine then just appeared later?

Hum in the passive section may be due to radiation pickup from the power transformer.

 

[orangechili]

Thanks! Will try disconnecting the interstage primaries, already tried tube swaps.

The hum is concentrated in two areas, 60 hz and around 150hz. There's a good amount of hum though in the whole low end area, just those are the highest levels. It goes up in those areas to -60db.. The hum has been there since i completed the unit.

Also tried a ground lift and no change in hum

The PT is a Triad R-7A mounted on back panel under the IEC socket

Everything else works awesome on the unit except this pesky hum, usual tricks not working so far

 

[orangechili]

Ok, disconnected the primaries on the interstage tx and the hum is still the same. The only thing so far that makes the hum go away is shorting the grids(2 and 7) on the v1 12ax7 tube.

 

[lassoharp]

Is it possible to rotate the interstage while observing hum level?

 

[ruffrecords]

When you disconnected primaries of interstage the hum went away so it sounds to me like magnetic interference into the interstage transformer itself.  To verify this check what happens if you disconnect interstage secondaries?

Cheers

Ian

 

[orangechili]

The hum did not go away from disconnecting the primaries of the interstage unfortunately.

I took the chassis screws off the interstage tx and moves it around gently and the hum decreases a tiny bit 2-3db to like -58db instead of -60db.

The hum goes up a tiny bit when I increase the lf atten and boost knob as well with the eq in. hum is there with eq in or out

 

[lassoharp]

It sounds like it could be picking up radiation from the power transformer.    You can also try taking a piece of metal and moving it between the interstage and the PT and observe hum.  Move it around in as many ways and positions as you can.  Sometimes it can be most effective in a position you might not expect.  Start by using the rotated position on the interstage that gave you the biggest hum reduction, then add the metal and go from there. 

Are you using the Sowter interstage?  Mounted inside or outside chassis?

 

[orangechili]

Given the distance between the PT and the interstage tx (complete opposite ends of chassis) i'm not sure that could be the issue. I tried moving some metal around each transformer and random positions in the chassis and no difference

 

[orangechili]

I'm not using a sower for the interstage tx, just a Edcor wsm 2.4k/10k mounted on the inside of the chassis all the way on the left side. I'm using a carnhill with the territory wiring for the output tx.

 

[lassoharp]

I've seen hum pickup at up to 13-14 in away with a shielded transformer, though with the PT on the outside of the case it should help.  Having hum at -60db with a transformer within that range is still a good possibility.  The Edcors have virtually no shielding.  Do you have any other suitable transformers to sub?  Preferably potted ones.

Also, how long are the leads coming from the secondary to the 12ax7 grids?  If they are more than a few inches you could try using shielded coaxial if all else fails.


Only other things left in that part of the circuit are the center tap of the IS secondary and the zobel if you're using one.  Check grounding on the CT.  Check connection on zobel.

 

[ruffrecords]

How long is the cabling to/from the interstage transformer? Is it screened?

Cheers

Ian

 

[orangechili]

Zobel connected right and interstage CT secondary to ground is correct.I don't have anymore iron in spec to test unfortunately

The wire lengths coming off the interstage tx is about 5 inches. The 12ax7  grid wires are a little longer at 7 inches. The eq out wire to the amp in wire is pretty long. I'm also using a rotary switch instead of a dpdt toggle switch for bypass. I might try switching that out to see if that helps somehow. I believe that wiring is correct but running out of options of things to try.

 

[orangechili]

I may try shielded wire on the grids and see if that helps. Thanks for the suggestions!

 

[orangechili]

Shielded wires on grids(grounded on one side) doesn't work. still the same hum... I tried replacing the bypass switch just on a whim and that didn't work either

 

[orangechili]

Ok, i've made a little progress on this puppy.  Between moving the interstage and the output transformers i can get the hum down to -80db from -60db. Both of these are unshielded so i believe that makes a difference as well...The OT was on the back pretty close to the PT so moving that makes a difference. The inductor is unshielded as well but i notice no difference  when moving that around. Moving the input tx around has no effect either.

I'm still getting a healthy increase in hum in the 150hz-250hz area while increasing the low atten and a minor jump in hum when increasing the low boost control. This makes me wonder if there's an issue with using the Edcor WSM 2.4k/10k  as an interstage. I have double checked my pots/board wiring again just to be sure....

Any thoughts?

 

[orangechili]

With the OT relocated I'm seeing the hum level going bonkers and all variating like crazy while moving the interstage around. I think that is the culprit. Any recommendations for a good shielded interstage tranny thats in good spec? I live in the US so sower is a little expensive after shipping, etc...

 

[ruffrecords]

With the OT relocated I'm seeing the hum level going bonkers and all variating like crazy while moving the interstage around. I think that is the culprit. Any recommendations for a good shielded interstage tranny thats in good spec? I live in the US so sower is a little expensive after shipping, etc...

Look at Cinemag and Jensen.

Cheers

Ian