pultec eq HUM *somewhere inside case*

[college101]

Haa... Well when I first put the tube in, one of the pins looked a little off...Never really thought anything of it till yesterday when I found out it was causing the problem...

Marshall's on Fire? what a shame...

 

[CJ]

good find, did the hum go away?

early marshalls were the best, partridge transformers,

partridge came up with a buch of cool formulas for transformers, check it out if you can find it,

 

[college101]

I am getting my replacement tube tommorow durring the day via Fedex..once I get home, I will check...BUT I am geussing that is what was wrong all along...Im also going to remove the float set up for the heaters...

I will look up patridge...Ive actually been wanting to buy some transformer/inductor books to start getting into em...
I wanna build a Sovtek AMP I had about 10 years ago but my favorite amp is a jcm 800...Those sound pretty mean

 

[college101]

Well...New 12au7 tube is working great, but the sound still remains...ZOSO on this one...

Still cant figure it out...Whats wierd is...As I adjust the low freq select knob, the sound adjust's as the knob is turned...

I am going to remove the 100 ohm resistors, and try re wiring the heaters, but Im honestly NOT convinced that is the problem..

advice is welcome guys...

 

[CJ]

short out the input to the 12au7 , clip a grond lead to the grids,

see if the hun is coming from the 12ax7 or 12au7 circuit blocks,

try to localize the trouble, then you can fix it,

 

[lassoharp]

.As I adjust the low freq select knob, the sound adjust's as the knob is turned...

You may be picking up power transformer radiation in the passive circuits, or maybe a ground connection in that section.  It sounds like part of the hum is getting into some portion of the EQ section.  Your input doesn't have anything connected to it when testing, correct?

You can try temporarily disconnecting the EQ section from the interstage transformer and see what happens.  That should help in differentiating between amp problems or EQ section problems.

 

[college101]

CJ..so I should ground 2 and 7 of both ax7 and au7( one at tube at a time) to see if this stops the hum...

Lassoharp...I do not have anything connected to the input when this noise is on...Today...I replaced the 12au7 tube, turned the eq on, and once the tubes warmed up, the sound started again..

I will try and disconnect the interstage and see if that is the cause...

I have the layout exactly the same as cayacosta's pultec layout...

 

[college101]

CJ..you were right...the NOISE is originating from the grids on the 12u7...once I grounded them...it was quiet...

Where should I go from here? amp section connected to the grids?

 

[college101]

So basically, something needs an additional ground...after looking at the original schematic..  My 1k off the output transformer that goes to ground, is apart of a daisy chained ground..maybe it needs to be comepletely seperate ground?

 

[CJ]

ok, so we have isolated it to the 12ax7,

this is because if the 12au7 were making noise, the hum would still be there when you short the grids,

now which circuit are you using, the original pultec with the weird output transformer or the new one without the feedback winding?

does the input transformer have a shield terminal on it?

you can try grounding the grids to the 12ax7 also,  2 and 7 again,

if the hum stops, then we have it isolated to the input circuit,

 

[college101]

I grounded the grids in the 12ax7, and the hum was still there...The hum only stops when I ground the grids of the 12au7

I am using the same transformers as the cayacosta layout from Sowter...

The input transformer does not have a shield on it, neither does the output tranny

 

[soapfoot]

Since your experiment with shorting the grids of the output tube stopped the hum, we can say (at least) that it's not magnetic coupling between the power and output transformers. I would bet you have a ground loop somewhere.

Here's what I did to get my recent build quiet, and hopefully this will help. I had four separate ground points as follows. Now it's among the quietest pieces of gear in the whole studio.

Ground XLRs together and to case right where XLR connectors enter case.

AC mains safety ground is grounded to the chassis right where the power cord enters (or where the IEC connector is).

Filter caps are daisy chained and grounded to one point which is also the star ground for the PT high voltage center tap, the ground in the heater circuit, and 12AU7 cathode resistor grounds.

Another star ground point for T1, T2, and EQ grounds together (star) near the interstage transformer.  Bypass switch grounds here as well.  Bonus points if T1 and T2 are physically close together and near this star point so that their cases are effectively part of this star.

Make sure everything grounds to one and only one point.

Heaters are powered off the 6.3V winding with an artificial center tap using 150 ohm resistors. This is DC biased as per Pultec schematic, and the 62k resistor bypassed with .3 goes to the power supply star ground described above. Twisted pairs are run in twisted pairs around the perimeter of the chassis, away from any low-level signal-carrying components. 

All other AC wires (mains, high voltage pre-rectifier, etc) are also run in twisted pairs away from low-level audio.

Power transformer is oriented so the 'null' of the hum radiation points away from audio transformers and EQ inductor, and also as physically far from them as the chassis allows.

Following these basic principles, I believe you should have a quiet pultec.  Double-check the rectifier tube, sometimes these can go noisy as well. 

 

[college101]

Cj..whats my next move  9530x output transformer from sowter

Im using the original circuit from pulse

 

[CJ]

check the wiring to the pots,

 

[hop.sing]

I grounded the grids in the 12ax7, and the hum was still there

I would say you can savely assume, that the hum originates somewhere around the 12ax7, which is a blessing, because it is not in the EQnetwork or the input/interstage transformer.
Maybe its somewhere around the feedbacknetwork? Ground the two 360 ohm resistors on the transformerside. If the hum is still there you can rule out the tertiary transformerwinding and its connection to ground.
Good luck

 

[college101]

will do guys...will do...

 

[college101]

If I ground the 360 ohm resistor on the left side...quiet, but I can hear some high frequency oscillation...

If I grounf the 360 ohm on the right, then I hear a very quiet low freq sputter as I heard before...

CJ.. I checked my pots wiring, and I am sure it is correct...

 

[college101]

I got it!! It feels great to build sometihng with so much wire and finally get it to work...
It is dead quiet!


Pultec Eqp-1a* Completed!!

CJ.. thanks for making me realize what I did wrong... as well as Hop.sing...

 

[lassoharp]

I got it!!

So, what was it?  It would be helpful to others who may have the same problem as you in the future to know the details of what you found.

 

[hop.sing]

Great, what was the problem?