Voltage Doubler Power Supplies

[AlexHempel]

You're right that 450 seems to be a hard voltage point to hit with OTS iron. For closer to 400, I was looking at this: https://www.hammfg.com/files/parts/pdf/169G.pdf?t=1697661946

Sort of ridiculous but hey, fits the bill. 115v primary so with modern line voltage the 133 NLV secondary should yield over 400.

 

[gadget73]

Just depends what you need. My big Bogens work that way, 700 on the plates and 350 on the screens. Things with more typical voltages would likely be too low though.

 

[AlexHempel]

Some of the Hammond guitar transformers have pretty low voltage/high current secondaries, but with a doubler voltage is more like 6550/EL509 range. They are perfect with a bridge though.

 

[AlexHempel]

@gadget73 Yeah, I bought a ton of 6CD6s and have been working on a design for them. Lots of sweep tubes want the screen at even less than half the B+ (at least according to the datasheet).

 

[GordonW]

The voltage drop seen in practice is also a function of the difference in no load/full load current draw. Many push-pull tube amps draw only a little more current at full output, because the tubes idle close to Class A. This can create the appearance of substantially better power supply regulation. Actually, there's no reason any of the PS configurations can't regulate equally well, it's just a matter of component values and quality.

Jack

The one I mentioned with 4.5v drop between idle and full power, was at 51 watts output at 20Hz, and close to 65 watts at 1KHz, from a pair of KT88s on 3.25K primary PP transformers, with 460V B+ and idling at only 65ma per tube. That's VERY MUCH into AB. As in peak current at full conduction, is twice the combined idle current for the pair (close to 275ma, as opposed to 130ma).

Regards,
Gordon.

 

[GordonW]

Another handy bonus from voltage doublers when dealing with pentodes, you have a convenient point for the screen supply, tapped out of the middle of the supply. Its going to be 1/2 the B+ which may not be the proper voltage but if that happens to work out its a simple way of powering them without also having regulation issues or needing anything even remotely complex. Probably going to need a choke though since its a half wave rectifier at that point which means a fair bit of ripple. Not super hard to filter that though, screens don't need a huge amount of power so the choke doesn't need to be a massive thing to get a decently quiet screen supply.

That, with something like an 807, could be just the trick. 550 to 600v plate, 275 to 300V screens is right in their ballpark.

The same would apply, with many sweep tubes.

The "bottom" cap on the doubler needs to be a lot bigger than the "top" cap, if you're doing this.

Regards,
Gordon.

 

[gadget73]

I claim no credit for this design, I can just tell you it works quite well.