voltage multiplier based on SS trafo - LONG

[ErikdeBest]

From some previous experiments I still have some transformers from SS amplifiers lying around which I want to use in tube amplifiers. Voltage multiplication comes to mind, and I looked through some threads here at Diyaudio and some Tubecad blogs and eventually built the multiplier as shown in the picture below employing 100uF/450V electrolytics and 1000V/1A diodes. It is a full wave multiplier as shown here.(One could argue that all savings from the transformers went into the electrolytics, but I got these 'surplus' for a very nice price...)

I did some initial testing with a 90V CT transformer and obtained the voltages as shown in the schematic. These voltages are about perfect for an amplifier as the most complex 'baby huey':
- 350V for plates of output tubes (OT) and driver tubes
- 210V for G2 of output tubes
- 70V to the drain of the mosfet source followers that drive the OT
- -70V for bias
- -210V to load the source followers

These are raw voltages, with lots of ripple and will drop under load. To eliminate the ripple I am thinking about source followers as in the 'engineers amplifier' from Pete Millet (P channel mosfets for the negative voltages), but also gyrators or regulators could be used.

Well, to my inexperient eyes it looks doable, but what do you think? I also used 450V capacitors throughout. I am sure that the capacitors between gnd and 70V can be of lower voltage, but I am not really sure about the voltage rating needed for the other capacitors. In the link to the 'full wave multiplier' they write 'Uniform component stress', so maybe all capacitors could be rated for 200VDC (when using a 90V CT transformer)?

Thanks for looking!

[Mooly]

Might be misunderstanding you, but if the 350 volts is to supply the output valve anodes, then the big problem is lack of current capability... that voltage will sag dramatically under load, causing severe modulation and sag of the others.

[leadbelly]

Originally Posted by ErikdeBest

I also used 450V capacitors throughout. I am sure that the capacitors between gnd and 70V can be of lower voltage, but I am not really sure about the voltage rating needed for the other capacitors. In the link to the 'full wave multiplier' they write 'Uniform component stress', so maybe all capacitors could be rated for 200VDC (when using a 90V CT transformer)?

Do you have a copy of Morgan Jones? He does a really good job of explaining how the voltage ratings of the caps and diodes must increase as you move up the ladder.

[Mooly]

The caps are in series... when the loading at different taps in the chain alters so does any imbalance in the chain,

http://www.techlib.com/files/voltmult.pdf

It's a variation of the "Cockroft Walton" multiplier.

[leadbelly]

Originally Posted by Mooly

The caps are in series... when the loading at different taps in the chain alters so does any imbalance in the chain,

Not sure I understand your point, but if you are suggesting that the fact that loading at different taps is relevant, I say it is not. The caps and diodes need to be rated for the max DC values they will face, which are no-load, and which varies as you move up the ladder. If you read the Morgan Jones book this is explained very clearly.

[gcwills]

You certainly can use voltage multipliers with a transformer with a low volt secondary normally used for SS amplifiers. I have posted an amp design here (6CM5 hi-fi amp) which uses a voltage quadrupler. It has surprisingly good regulation providing the transformer rating and cap values are adequate. The rule of thumb is that the transformer secondary current rating divided by 4 for a quadrupler is the max current able to be drawn from the supply. I attach a power supply schematic for reference

[ErikdeBest]

Many thanks for the replies!

Sag and modulation are indeed my main worries with these form of supplies, but I also read in an earlier thread (from gcwills) that regulation can still be pretty good, even with a quadrupler. Furthermore the amp will be operating in rich class A, so the current draw from the supply is constant.

I have a copy of Valve Amplifiers (signed by Mr Jones!). I remember the part on diodes rating, but couldn't remember he also deals with multipliers: I will have a look when I am back home.

It was indeed the thread on the 6CM5 amplifier that reactivated my interest in the voltage multipliers. I just built up the version above as the intermediate voltages come in quite handy, but if it doesn't work I will give the 6CM5 quadrupler a try!

Erik

[Brit01]

I'm looking to do the same, multiply 28V x 4 = 112v.
These amps are rated at 3 amps so plenty left if divided by 4. 730mA.

Your design has 2 x 33v transformers in series??

What would be the best design for 1 tranny to multiply it's voltage by 4? (planning to buy a lot of 5 of these for just 35 USD).

[gcwills]

Just to clarify - in my multiplier example, 2 X 30V AC secondaries in series produces 60V AC RMS. When this is applied to the quadrupler, the resultant DC output is 60V RMS X 1.4 (the peak value of the 60V RMS voltage) multiplied by 4 = approx 360V DC no load, and loaded to 200mA (with these transformers and capacitors) the supply produces approx 310V DC.

[Brit01]

thxs for the info.

so a 28v @3 amps:

28 x 1.4 = 39.2*4
= 156 volts @ 750mA
Perfect to run some 6080's I'm planning. Lets say a 10% sag also. Great
Could run 4 in parallel.

Just bought 4 of these trannies for 30 USD. Not a bad deal.