UTC A-43 & 2uF output caps for G-Pultec

[bigidibo]

Hello there!

I have laying around a couple of UTC A-43s, wondering if they would be a good fit for a G-Pultec?

And have a couple of 2uF paper oil caps. What will change with lower capacitance? Will I lose some lowend?

Thanks for any tips.

 

[user 130681]

Firstly, we need to know the impedances and load factors to determine the frequency response curve. Secondly, Paper in Oil capacitors are notoriously faulty due to electrical leakage over time.
There is no advantage over a quality electrolytic ... you are feeding a transformer, so you have a low quality device to start with as opposed to direct coupling.

 

[bigidibo]

Ops, sorry. The Primary is 600 Ohm and has two secondaries, 600 Ohm each.

I see, thanks. Where would you suggest experimenting with the color of the unit, apart transformers and tube? where are the most critical caps that influence the signal?

Cheers

 

[user 130681]

Ops, sorry. The Primary is 600 Ohm and has two secondaries, 600 Ohm each.

I see, thanks. Where would you suggest experimenting with the color of the unit, apart transformers and tube? where are the most critical caps that influence the signal?

Cheers

1/2πrc is the formula to work out the 3db point for frequency response.
We cannot comment further as no schematic.

 

[bigidibo]

I'm using G-pultec makeup, but a bit different filter.

https://www.gyraf.dk/gy_pd/pultec/gy_pd_sch.gif

 

[beatnik]

The A-43 is a 1:1:1 splitting transformer (one input -> two outputs), the Gyraf gain stage calls for a 2:1 transformer, so I don't think it would work properly.

You could try to redesign the gain stage to use a 1:1 output transformer, maybe with parallel triodes in cathode follower configuration, that could potentially bring the output impedance low enough for driving a 1:1 transformer.

 

[bigidibo]

I see, thanks for the info. Will keep digging something for the input.

Wondering about heater voltage... I have a transformer with a 6.3V sec output. Do I need a higher voltage to downregulate it to 6.3 or 6.3V will that work?
Or it is possible to bypass regulation and wire straight to the heater?

Thanks

 

[beatnik]

Actually it might be fine for the input since that's 1:1 - try it and see

Re heaters, first of all you should try ac heaters properly twisted and routed. Only if that doesn't work, move on to dc heaters.
Pultec eqs work with line level input so you are not dealing with small audio signals like on a mic preamp of phono preamp.
Original Pultecs had ac heaters and nobody is complaining, it doesn't seem to affect the value of vintage units !!

if you use unregulated dc you will be fine just dropping the voltage to 6.3V with resistors. Check out valvewizard website there is a good explanation of this method

If you used a regulator you need one with a low dropout voltage.
6.3V ac with a bridge rectifier would lead approximately 8.8V dc after rectification, so you need a regulator that drops no more than about 2.5V
The LM1084 might be a good candidate.

Pay attention to the current rating of the transformer as with dc heaters it will draw almost 2x the current