Pultec Clone build hum

[CJ]

ok this is going to sound stupid, but do it anyway, it don't take much time, leave the first jumper and get another clip lead and short out the secondary of the output transformer so we can box this trouble area inside the circuit,

 

[Rompstomp89]

ok this is going to sound stupid, but do it anyway, it don't take much time, leave the first jumper and get another clip lead and short out the secondary of the output transformer so we can box this trouble area inside the circuit,

So basically put the jumper across pins 2 and 3 on the output of the xlr jack? If so, that killed the buzz.

 

[CJ]

ok we checked the front porch and the back porch, let's check inside the house,

start in the kitchen,  pull the 12ax7a and see what happens,

 

[Rompstomp89]

ok we checked the front porch and the back porch, let's check inside the house,

start in the kitchen,  pull the 12ax7a and see what happens,

Pulled. Buzz get super loud. Pulled the au7 and buzz goes away

 

[CJ]

you have a voltmeter?  check AC volts on power supply caps, hook up clip before turning it on so you don't fry like bacon,

 

[Rompstomp89]

you have a voltmeter?  check AC volts on power supply caps, hook up clip before turning it on so you don't fry like bacon,

I do!

35.5 ACV  first stage 47uf
31                  Second 47uf
14                  Third stage 33uf
22.5              Last stage 33uf

seems like quite a lot...

 

[CJ]

that is very high, what kind of rectifiers , solid state or tube?

make sure your filter caps are grounded,

maybe bridge another cap across the existing cap to see if you have a bad cap,

 

[Rompstomp89]

that is very high, what kind of rectifiers , solid state or tube?

make sure your filter caps are grounded,

maybe bridge another cap across the existing cap to see if you have a bad cap,

Tube rectified (6x4)  I've tried swapping out recto tubes with no change.  I may try and experiment with adding some 1N4007s at the recto tube socket.

Ive re checked all ground connections.  Im going to recheck at star too. 


I will try the cap trick!

Thanks CJ!

 

[rouni]

HI! I m refreshing this thread because i ve had the same kind of problem with my last PTP eqp1. (The makeup gain is a pultec mb-1 circuit).
everything worked fine but with a super loud 50 htz hum, attenuating lows maked the hum louder!!! After 3 days of checking everything, ground loops etc... adding some metal shield inside and shieldind all the wires, changing the input and interstage transformers (without internal screen pin)... still a loud 50 htz...
Finaly there was this interstage 30k 1/1 schlumberger laying in my stuff... this one has an internal screen pin... ok let s go, not the perfect specs for an eqp1 but well... let s do it....
An then the magic happened! Noise gone! Noise totally disapear!! The quietest machine i ve ever built!
So the problem was just an unscreened interstage transformer! Hope this helps...

 

[Clbraddock]

HI! I m refreshing this thread because i ve had the same kind of problem with my last PTP eqp1. (The makeup gain is a pultec mb-1 circuit).
everything worked fine but with a super loud 50 htz hum, attenuating lows maked the hum louder!!! After 3 days of checking everything, ground loops etc... adding some metal shield inside and shieldind all the wires, changing the input and interstage transformers (without internal screen pin)... still a loud 50 htz...
Finaly there was this interstage 30k 1/1 schlumberger laying in my stuff... this one has an internal screen pin... ok let s go, not the perfect specs for an eqp1 but well... let s do it....
An then the magic happened! Noise gone! Noise totally disapear!! The quietest machine i ve ever built!
So the problem was just an unscreened interstage transformer! Hope this helps...

Oh wow, I am dealing with this EXACT problem. So glad you posted today (in an 8 year old thread). I am going to investigate what you discuss. I hope that fixes my problem too!

Edit: Do you remember what transformer you were using that had the problem?

 

[rouni]

Arf, the first insterstages i ve tested were Muirhead transformers... UK made from the 60' era.. i work a lot with vintage rare stuff like those... but i suspect any tranny without internal screen pin to generate problems in this position (interstage). But it s not a rule... it can be also a grounding issue, orientation and proximity of the audio transformers with the power transformer .