Pultec EQP-1A Voltages

Tom Thumb · Jun 19, 2005

I've built a Pultec EQP-1A clone as per the layouts etc. using Sowter transformers. The valves aren't anything spectacular. 6X4 is Brimare.

After a bit of debugging, it seems to be doing something useful though I don't have anything to compare it to directly.

However, the voltages don't quite match the ones on the schematic (contained on the second last page of the manual, DWG E-72, 658-2).

The values I get are detailed below. I notice that the 12AX7 voltages aren't balanced (i.e. pins 1 & 6). Any ideas?

All measured +DC wrt star point ground.

Diagram Measured
6X4 pin 7 355 320
CT Heater 60 54.5
1K/1W left 345 312
1K/ 1W right 325 293
12AX7 pin 1 140 136.5
12AX7 pin 6 140 127
12AX7 pin 3 1.3 1.04
12AX7 pin 8 1.3 1.05
Centre 1Meg 31 26.8
12AU7 pin 3 40 36
12AU7 pin 8 40 36
O/P Transfo 290 261

cayocosta · Jun 19, 2005

Looks like all your voltages are about 10% low. Could be mains AC is low (~107 VAC in US), or possibly the 6x4 is weak. (I have a 5Y3GT that is very weak.) You will have to search for a matched 12AX7 - the two triodes can vary.

Tom Thumb · Jun 19, 2005

I'm running it in the UK, though the mains voltage could still be low I guess.

With the 12AU7 and 12AX7 not fitted, the 6X4 is outputting 359V dc, though that's no load, so probably doesn't count...?

I doubt my 12AX7 is matched, but it's certainly something I can consider when this one pops, or I decide it's worth it.

I know it's a bit on the vague side, but do you think the voltage discrepancies will have a 'bad' effect?

cayocosta · Jun 19, 2005

I wouldn't worry about it.

But I would try some other 6X4's though - they're only $5 in the US for new old stock RCA.

Used (but tested good) RCA or equivalent 12AX7's are relatively inexpensive also, and might yield a closer match if you can pick up a few. (Assuming each pair of plate and feedback resistors are matched.)