negative voltage doubler for ccs

[Jaap]

I want to use a Pimm CCS under the cathodes of a el84PP amp. I only have a free 5 volt tap on my transformer. So I thought about a voltage doubler for the negative supply under the CCS.

I suppose that i cannot connect the bottom of the CCS to earth.

I do not know how to make that negative voltage doubler.

Anyone ? Thanks !

[Eli Duttman]

Jaap,

The "full wave" doubler circuit is differential. You can treat either end as "hot".

Connect the cathode of a Schottky diode and the anode of a 2nd Schottky diode to an end of the 5 VAC winding. Connect the free cathode to the + terminal of the doubler cap. stack. Connect the free anode to the - end of the doubler stack. Connect the free end of the 5 VAC winding to the center of the doubler stack.

Use BIG caps. in the doubler stack. Follow the doubler stack with a choke and more capacitance. Available DC current will be approx. 1/4 the AC RMS value. The rail voltage will be slightly above 12.5 V.

[leadbelly]

Just for my own understanding, Gary Pimm told me the CCS typically has 30V drop across it. Does that mean your cathode will be biased up to about +20V?

[Jaap]

thanks for the useful information about the doubler.

I didnot realize that the voltage drop could be that big around the Pimm-ccs.
I saw ccs with -15 volt under them, so I figured I could try with my 5 volt transformertap.

Perhaps i have to use a different ccs under the el84 (?)

[Shoog]

In this role why not consider a simple LM317 regulator. Its a far less demanding position for a CCS than an anode load and I understand these regulators work well in this position.

Shoog

[Dave Cigna]

You could probably get away with a cascode mosfet ccs there.

But, a negative supply is needed only if you want to keep the grids at ground - at the input stage for example. Are your el84's cap coupled? If so you can bias the grids positive (derived from B+) and forget about the negative supply entirely.

-- Dave

[2wo]

Try This...John
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