Pultec eq1-pa repair hints

[Zander]

Hi,

I have build two pultec eq’s ptp with rainton’s chassis and layout.
They worked very well and measured surprisingly quiet for tube eq’s.
Now one of the two is faulty. It works well and after a minute or two it shuts down. There a loud static and everything is distorted. When the eq is bypassed.
I have switched the 6x4 with no luck.
The voltage rails are within spec when turned on until a few minutes after the 345v drops to 338v,  290v to 274 and 250v to 263v. I’m a bit scared to switch the other tubes because I bought really decent ones...

Edit: the voltages rails in the other, working, unit stay within spec.

 

[Zander]

I put the tubes from the faulty unit and I get the same voltage drop within a few minutes when I put in the 12ax7a tube. Could it be that? The voltage are 265v where it should be 250v and 275v where it should be 290v.

 

[Cardinen]

Yes it seem a tube issue, try with a new one and see what happen.

I suggest also to check the PSU (is it from the original pultec schematic, without regulators ? )

 

[Zander]

Yes. Its the original. With the 6x4. Psu seems fine. I only get lower voltages when i put the 12ax7a in. Voltage also drops between 220ohm resistor and 6x4.

 

[Tubetec]

Hi Zander
From what you have said if changing the tubes from the broken unit to the working one the fault reappears ,you most certainly have a bad tube in there .
Its just a guess on my part but do you happen to be using a JJtesla Ecc83 or 12ax7 when you find the fault , Ive had quite a few bad tubes brand new from JJ ,including extremely microphonic preamp tubes with very unbalanced currents in each section ,tubes that oscillated for no apparent reason , and one set of E34L's which were suposed to be a 'high grade long life' version ,this garbage very nearly killed the mains transformer on my priceless vintage Hiwatt 100 ,due to some kind of grid current issue causing the bias voltage to drop away over time .

All the best and I hope you get it sorted soon .

 

[Zander]

Hi,

Thanks.
No it’s rca branded. I was really happy with them. They sound very good. I think I will order the same one and a spare or so.
Good to know. I also have an hiwatt which I’m getting new tubes for.