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Old 17th July 2005, 10:53 PM   #1
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Default Any pitfalls to tapping voltage doubler?

For an upcoming preamp, probably an Aikido, I was thinking that it would be elegant to use a voltage doubler (2.828 * VAC) for the B+, and then (excuse my poor terminology) tap the 1st ladder (1.414 * VAC) to use to elevate the heater supply. I would think that this would trim the parts count nicely and require a less $$$ xfrmr.

Are there any pitfalls to this? Would it work for both the non-floating and floating style doublers?
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Old 17th July 2005, 11:18 PM   #2
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LB,

A "full wave" doubler is fine for the B+ rail. Keep the draw on both halves of the PSU equal. Place a high resistance voltage divider across the entire rail to set the heater bias voltage.
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Old 18th July 2005, 04:11 AM   #3
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Both ideas are just fine. I don't know what you mean by ladder though, unless you are fudging around with a mere halfwave doubler- I wouldn't use this for anything but bias supply and AM detection.

Tim
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Old 18th July 2005, 05:46 AM   #4
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OK, I was confused, so I had to Google for an answer. I was looking at Morgan Jones' book, and what he calls "non-floating" is a half-wave doubler, and what he calls "floating" is a full-wave doubler. Thanks.
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Old 18th July 2005, 04:30 PM   #5
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Oh! I just remembered another question: can you make a voltage doubler with tube rectifiers?
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Old 18th July 2005, 04:53 PM   #6
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>>Oh! I just remembered another question: can you make a voltage doubler with tube rectifiers?<<

Yes, you can. The 117Z6 is designed for such service. However, I strongly advise that you use SS diodes. Each 1/2 of the doubled rail is half wave rectified. So, LARGE filter caps. are indicated in the doubler stack. Vacuum rectifiers arc over when combined with LARGE value filter caps. 600 PIV Silicon Carbide (SiC) Schottky diodes are as quiet as vacuum diodes. If you are concerned about cathode stripping, add a NTC thermistor to the PSU.
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Old 18th July 2005, 05:17 PM   #7
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Thanks Eli. You are quick draw on those responses.

Can I push my luck and try yet another question in this thread? It's sort of all on the same topic anyways....less $$$ tube supplies.

Can anybody point me to some good links on all-tube regulated supplies? Up until now I have been using SS regulators to avoid buying pricey chokes. I think I would like to attempt an all-tube amp or preamp, and a tube regulator sounds like pretty cool idea.
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Old 19th July 2005, 02:45 AM   #8
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Sure. Replace transistors with tubes, remove safeguards.

Click the image to open in full size.

I'd recommend 6W6 (well, something with more dissipation), or any number of triodes designed for the service (6AS7, etc.), for the pass tube, for any appreciable current.

Remember it's a total feedback circuit, so pick an output voltage, voltage reference, topology (there are a few arrangements for both pass, and, ugh...shunt) and work backwards from there. Ensure the tubes can pass the required current and voltage ranges for all voltage settings and current draws and you've got yourself a regulated power supply design.

Oh, and tube doublers work just the same- you need a floating heater supply or good heater-cathode insulation though. (Damper diodes are good for this.) For a full-wave doubler, imagine it as two half-wave rectifiers on opposite sides. As such you can also draw current from the center, same as using a FWB on a CT'd winding except the voltage output is higher.

The diodes need to be rated much hotter than the output current, since duty cycle for each half is much worse.

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Old 19th July 2005, 04:01 AM   #9
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> can you make a voltage doubler with tube rectifiers?

Sure. But look where your cathodes wind up. All different voltages. If you use naked-filament cathodes, you will need extra heater windings, well insulated. If you use heater-cathode rectifiers, you need very good heater-cathode insulation.

Also, voltage doublers either run tubes in series (multiplying plate-drop) or run the caps half-wave (increasing the peak/average current ratio and stress, with enormous resistance-droop).

That's why 99% of hollow-rectifier schemes use the center-tapped high-volt winding. Even though it needs more labor in the transformer, it use a single-cathode rectifier. Two plates on one cathode is a lot cheaper than many cathodes and heater windings.

There are a few exceptions. I have seen US market TV sets and radios running 250VDC from 110V voltage doublers. At 250V, heater-cathode insulation can survive, requiring just one heater supply. At TV set power, rectifier costs are huge any way you do it, and running the big H-sweep tube at higher voltage improves its efficiency enough to compensate some of the rectifier cost. OTOH, while there were volt-doubler radios and even (hot-chassis!) guitar amps, improved low-voltage output tubes soon made such drastic plans obsolete.

Taking some power from the middle of the full-wave doubler is trivial and often done. In fact when crystal diodes grew to the size that they could handle the stress, the FW doubler was a very common scheme. Especially for 6550 and 8417 output tubes where the G2 likes to sit about half the plate voltage. Done in the Bogen and Dynaco 8417 amps and in the monster 4x6550 servo-amps in the Ampex Quad VTR.

Regulating tube audio amps is, In My Humble Opinion, silly. (With a few specific exceptions such as vari-Mu compressors, and regulating the Screens of high-power amps.) Tube-regulating is extra silly. You have to make too much power, plus a little extra, and waste much of it. The extra power and heat and cost could do more good in other places. A tube regulator is another audio amplifier, just optimized differently. The extra iron needed to cover plate and heater loss alone would (in a rational world) buy a LOT of cool choke. Getting a low-low output impedance, or even high ripple rejection, is not at all easy. Just because the sand-heads regulate at the drop of a 7805 does not mean it is a good idea in the tube world. Good tube circuits will work fine with about any voltage they get, as long at it is clean (and chokes/caps do the job with little waste). But that's just my opinion.
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Old 19th July 2005, 07:25 AM   #10
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Quote:
Originally posted by PRR
Regulating tube audio amps is, In My Humble Opinion, silly.
<snip snip>
Ab
so
lutely.

Quote:
Just because the sand-heads regulate at the drop of a 7805 does not mean it is a good idea in the tube world. Good tube circuits will work fine with about any voltage they get, as long at it is clean (and chokes/caps do the job with little waste). But that's just my opinion.
Yep yep. I'll gladly show you how to make a tube reg, maybe even an amp schematic using one, but for the most part, they aren't necessary, especially with tubes, and I have little reason to recommend it. And as you know I make my opinion on these things just a little less humble than PRR here.

For example... something like a phono preamp. Just PSU noise. In the time and money it takes you to wire up a well-damped regulator for it, you could've had even just RC filtering so much better in rejection ratio. I have nothing against RC filtering for a preamp, it's not a very good way to run a power supply in general but as long as you're okay with it and it works out fine, sure, whatever. For anything with power in its title, go for LC, since Rs burn a whole lot more power, and it doesn't seem right hanging a power amp off a flimsy resistive filter anyway. Anything with varying power consumption (class A2 to a small extent, but more importantly, class AB and B PP) needs a solid power supply, but in particular, screen voltage, if any. Now, 6V6, 6L6, EL34 and so on were *designed* for simple circuits -- the screen voltage in typical use is 250, 300, sometimes 500 volts! This is very near if not the same as plate voltage, so you can get away with a cheap RC filter. A big drop like to 100V for a sweep tube (which is nice because it can take the 500V supply you started with, swing it 800 miliamperes and blast a hundred watts right off the bat!) is utterly impossible to accomplish with a resistor, particularly with screen current varying anywhere between 1 and 60 miliamperes! This range is also too much for a humble 0C3/0B2, good for only 30/25mA range respectively. (Two in parallel would work, but...good luck with that...) This is the perfect place to use a regulator, particularly a pass regulator since 70mA is a lot to burn at idle. The high voltage drop - 400V in this example - is suitable for pretty much any tube, given it can take the dissipation. The only problem is the low output voltage, a concern which can be fixed by referencing the error amplifier(s) to a negative supply (which you likely have in such an amp already), giving it more breathing room.

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