Pultec EQP1A power transformer, which one ?
« on: September 03, 2008, 08:16:38 PM »
I've read thru many pages of Pultec EQP1A thread and the only off the shelf power transformers I've seen are the following.

Original :
Chicago PCC 55 ( electroststic screen )
(540 VCT) 270-0-270VCT - 55MA, 6.3VCT- 2A, 5.0 - 2A

other recommended specs :
(500VCT) 250-0-250VCT - 100MA, 6.3VCT- 2A, 5.0 - 2A

Is there one I have missed that better suits this project


Sowter 9527 :  500VCT - 60MA, 6.3VCT - 1.5A, 6.3-5.0V - 1A  $200USD
capable to power two units , has eletrostatic screen

Hammond 260C (500VCT) 250-0-250 - 75MA / 6.3VCT -2MA / 5.0V - 2MA
no end bells, universal primary,

New Hammond 270CAX : 500VCT - 100MA / 6.3V -2A ( no ct ) / 5.0V-2A $60 requires a fake center tap with two 100-250 ohm resistors on either leg of the heater filament wires to ground. Or floating arrangement.

Old Hammond 270CAX : 500VCT - 70MA/ 6.3VCT- 2.5A/ 5.0V - 2A  $47
reported as lacking enough current in the high voltage secondary.

Hammond 271X/371X (560 VCT) : 280-0-280 - 60MA / 6.3VCT - 2 MA / 5.0V -2A

Hammond 370BX 55VA, (550VCT) 275-0-275 @ DC ma 50, 5VCT @ 2A , 6.3VCT @ 2A.  $59.99.
This looks like another good choice. universal primary.

Allied 6K88VG : 500VCT - 40MA / 6.3VCT - 2.0A  $21
 Requires seperate transformer for 5VAC  pilot lamp ( on/off light) or use 110VAC bayonet lamp off the primary for pilot on/off lamp. the 500VCT secondary  is rated for powering only one channel only at 40MA. Shielded (grey wire)

Allied 6K56VG : (540VCT)270-0-270 -120MA/ 6.3VCT -3.5MA/5.0V - 3.0MA $41
This seems to be a great option since the Chicago PCC 55 original power trasnformer has a 540 VCT also, only .055MA. or less than half the current of the 6K56VG. 5V lamp filament included

Edcor XPWR106 $44
(550VCT) 275-0-275, 125ma / 6.3v - 4A (no ct)
no pilot lamp filament , 6.3V heaters must be floated

are there any others like  Antek , Heyboer that offer a off the shelf power transformer that is reasonable in price and has the minumum requirements to power a EQP1A circuit. The main issue I have read seems to be voltage deficiencies in the circuit some have reported with some power transformers lacking correct specs.
curiousaudioparts at  gmail  dot  com

Pultec EQP1A power transformer, which one ?
« Reply #1 on: September 03, 2008, 10:19:43 PM »
I'm planning on this for my stereo pultec.  it's $30


-A bit large in amps, but more is more ;)

-You can fudge the RC Resistor to get the right voltage or use PSU designer.
But with PSU designer you need to know more specs than what is listed.

Make sure to set the PCB up for full wave center tap for this xfmr and the other 500V transformers.

Pultec EQP1A power transformer, which one ?
« Reply #2 on: September 04, 2008, 11:12:22 AM »
This is good for two channels , but I'm looking for a transformer for one channel. definitely a cheaper way to go. 6lbs. of Iron for this one.

So two secondary's  460VCT - .9MA and 12.6VCT ?

Will adjusting the RC like you say bring the 460V legs closer to 250V

What about the voltage drop at the 6X4 rectifier tube ? Silicon Diode is more efficient , but I plan on using the rectifier tube.

Electrostatic screen , not needed due to toroid design ?

curiousaudioparts at  gmail  dot  com

Pultec EQP1A power transformer, which one ?
« Reply #3 on: September 04, 2008, 12:11:18 PM »
The center tap goes to ground.  put the bottom diode on the bottom side of the 230-0-230 leg of the xfmr.

Using Full Wave Bridge, an estimate is 230*1.414 -2 to get your volts after rectification.  Then we need to find the load for HT to adjust the dropping resistor of the CRC section.

You may want to go with Gyraf's plan with back to back xfmrs since you are just doing one channel.

From my calculation with PSU designer, Tube rectifiers will drop 10V from that calculation above.  Maybe someone else can chime in since I am not very familiar with tube rect.

I would get PSU designer and play with it.  it's a great learning tool.  You can get in the ballpark for the CRC, but you will need exact resistances for  better estimates.

feeding a 15mA load I got 4.3K for the CRC resistor, but the load is a guess.  Can someone tell us the load of the Gyraf pultec?

Pultec EQP1A power transformer, which one ?
« Reply #4 on: September 04, 2008, 12:54:43 PM »
Bill - yes I agree , all those details will be important in adding the correct transformer , full wave rectification and tuned CRC filtering. I'm gathering by your explanation the pultec draws an estimated 15-20 milliamps of current between 230-260VAC, yet 40 MA should be plenty for one channel. Looks like the Allied 6K88V transformer is the close to what I'm looking for, but the if the heater filament (6.3V) did not have the center tap like the Hammond 270CAX then the conections are defaulted as floating. Could some one explain how a floating heater arrangement is done. I'm assuming its a direct connection from the two filament wires in a series arrangement to each of the EQP1A tubes (3 total) ?

with the antek transformer, I'm thinking a layout like the original. any other transformer would look the same. except for the center tap missing below for the tube heaters. this is the part that confuses me.

Is the center tap really important for the heaters or would the floating method be good enough ?

Obviously , I would consider a regulated circuit ( not shown ) for the tube heaters.


curiousaudioparts at  gmail  dot  com


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