pultec eq HUM *somewhere inside case*

[college101]

OK, guys...having a great build, BUT  I am getting the dreaded HUM at the 12ax7 tube in my Pultec. I re-checked my connections and everything is solid. Do I need to go the 100R route to float the heaters?

What should my next move be?

Replace the wiring on the heaters?

Everything else seems to be fine- I did cayacosta's lay out with the sowter transformers... Thanks Guys...

 

[gemini86]

EDIT:

Have you built the original eqp-1a? How do you know it's the 12ax7 that's humming?

 

[hop.sing]

Did you change the tube? I had a slight hum in one channel until I changed the tube for another one. I guess, the cathode-heater insulation is not very good with some tubes.

 

[college101]

Gemini- I built the Eqp1a that cayacosta built... Check Recproaudio.com to learn

I changed the 12ax7a I had installed to an older RCA 12ax7 I had just lying around...It still hums...If I pull the tube from the circuit(so there is no load) but leave the other tube and recitifer tube in circuit, the HUM goes away....

 

[college101]

Im actually starting to think its the Out put transformer that is producing the hum...re checked my wiring, and it is solid

re checked ground path, and bolts, and all are in the correct place

 

[pucho812]

time to do the chopstick test. get chopsticks or any orther non conductive stick and push wires around to see if it gets worse or better

 

[college101]

already did the orange wood test, and moved wires, nothing seems to make it change...

 

[lassoharp]

I changed the 12ax7a I had installed to an older RCA 12ax7 I had just lying around...It still hums...If I pull the tube from the circuit(so there is no load) but leave the other tube and recitifer tube in circuit, the HUM goes away....

ok

Im actually starting to think its the Out put transformer that is producing the hum...

Based on what you observed when pulling the ax7, this would suggest output and Au7 circuits are ok regarding hum.  It's suggesting the hum is originating at or before the 12Ax7.

Could you give us the approximate level and frequency of hum?  This can be helpful in ruling out a bad connection in the filterting section or loose ground.

 

[college101]

Well..the frequency is low...Only the low frequency knobs actually change the sound of the hum.....I can also hear a difference when the eq is in and out...

I have checked the main ground points in the circuit with my multimeter...Both test to ground and connection is good!

I re-solder'd the power supply caps and all of those connections...

Noise seems to be coming from the transformer(output)...when I connect the yellow wire to the middle to connected section, the hum almost cancels, but this is not how the lay out pics show it, nor does the schematic from Sowter make sense to do that...

My fuses has blown, and Im on hold till I can get another fuse...

 

[college101]

I found another thread about another guy having a similar problem...Ac ground problem...

I am going to try disconnecting Pin1 from both input and output XLR's...


My hum initially starts once the tubes warm up.. and dies once power is turned off to the unit...

 

[lassoharp]

Well..the frequency is low

Yes, we know it's low.  We need to know which frequencies and how loud for each or either.  If it's -20db @ 120Hz, that's a big difference than having -50db @60Hz.  It may be a combination of 60,120 and all octaves in between.  It's just a quick test to help narrow down places to look.

when I connect the yellow wire to the middle to connected section

I'm not sure why and where you are reconnecting the yellow wire.  For the Sowter 9530, the yellow wire is one side of the tertiary FB winding.  What is "connected section"?  The 1K CT resistor? 360r
resistor?


If the fuse is blowing you may have a shorting path developing somewhere in the power supply.

 

[college101]

Sorry Lasso, I dont think you understood what I meant..

How do I test what I am hearing in the unit?

 

[lassoharp]

Simplest way is to run the output into DAW recording software and use one of the onboard meters.  I use Logic which has a multimeter you can put across the main buss.  It can be scaled to read as low as -80db and also shows each frequency component and it's relative level.  It's not intended as precision testing software but it works fine for analyzing where noise is coming from and at what frequency.

 

[college101]

What about using an oscilliscope? I dont have a a DAW right now...Not getting the new one for a month or so...

 

[CJ]

if you have a fold out panel make sure the thing is grounded to the main chassis,

 

[college101]

ahh...CJ has spoken... I will re-connect the front panel and re-test today....with some new fuses of course...

 

[college101]

OK...guys..update...  Somehow, I got the wrong value fuse in the eq- I got a new fresh 1 amp fuse...Works great and has not blown


On to the more interesting news...I decided to try grounding the heaters with some 100 ohm resistors...
Once grounded, I applied power, and again, the dreaded HUM...

This time, I decided to turn my over head lamp off, and see the tubes lighting up....WOW> I start to hear a wierd noise...like the tube is sputtering!!  Like  a motor not getting enough air/fuel mixture...you get the point...

I look over at the 12AU7...and its flickering BLUE on off with the sputtering...***since Its a low frequency tone, I decide to adjust the Low Freq know, and the tone starts adjusting longer, and shorter based on the position of the freq selector switch..

I am geussing that its safe to assume, the 12AU7 is BAD and I need to replace it...

 

[CJ]

look for a crack in the glass,

or you might have the socket miswired,

weird, usuall the el-34's are the only tubes that glow all weird and blow up, 

 

[lassoharp]

I saw an EZ81 have it's plates meltdown and literally weld together during a PS short out.  No glow but it sounded like somebody cracking marbles with a shop vise.

 

[CJ]

if you over-fuse the marshall, you can get it to maybe melt down and catch fire, this way when the music stops, you get the stage show as a distraction,

fans will boo a blown fuse,

but they go ape when they see smoke and smell burnt resistors, get me?

keep it away from the backdrop curtains,