PT5 PR. TRANSFORMER FOR FOREPLAY II

[coca]

Good day folks! I'm thinking that the PT-5 power transformer would be a good choice for a FOREPLAY II linestage, that would allow the use of four rectifier diodes in the power supply. There are only 10 volts difference in the secondarys of the FOREPLAY II transformer and the PT-5. However, I would like clarification on the statement in the bottlehead catalogue, and I quote, "DESIGNED TO RUN INTO A VOLTAGE DOUBLER"

Thanks in advance.

Bernie.

[Caucasian Blackplate]

I think another important question is if you are prepared to DIY a regulated heater supply?

PJ will have to chime in about the use of the doubler vs. FWB.  I would imagine a FWB would work, but I would also mention that the PT-1 for a Foreplay II circuit is IMO a better choice and much more compact.

[Paul Joppa]

The thing about "designed for voltage doubler" has to do with shielding the external electrical field. With a full-wave voltage doubler, one terminal of the winding is at AC ground (through the first capacitors) and I put that terminal at the outer end of the high voltage winding so that the outer winding layer acts as a shield. There will be a much larger electric field around the transformer with a fullwave bridge, and small signal circuits with high impedances are likely to pick up some hum from capacitive coupling. This is a lesson we learned from the PT-1 in Seduction, and is the reason for the copper shield around that power transformer. (There is also a magnetic shield tape around the PT-1).

You could use the Eros PC board and drop some voltage by using the Eros shunt regulator. If it were me, I'd jigger the Foreplay a bit to run at the higher voltage (225v) with more current, but you could instead run the shunt reg down to 150v (or 190v, which many Foreplay IIs actually ran at.) The Eros board has a voltage regulator for the heaters. You could also just use a 150v gas regulator tube, as in the stock FP-III.