Any pitfalls to tapping voltage doubler?

[leadbelly]

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?

 

[Eli Duttman]

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.

 

[Sch3mat1c]

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

 

[leadbelly]

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.

 

[leadbelly]

Oh! I just remembered another question: can you make a voltage doubler with tube rectifiers?

 

[Eli Duttman]

>>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.

 

[leadbelly]

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.

 

[Sch3mat1c]

Sure. Replace transistors with tubes, remove safeguards.

[IMGDEAD]http://webpages.charter.net/dawill/tmoranwms/Circuits/Regulated_PS.gif[/IMGDEAD]

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.

Tim

 

[PRR]

> 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.

 

[Sch3mat1c]

Regulating tube audio amps is, In My Humble Opinion, silly.
<snip snip>

Ab
so
lutely.

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.

Tim

 

[leadbelly]

Wow! Thanks for all the great info. I am thinking that I will try CRC filtering in my next preamp to avoid SS.

I thought of one last question: can a rectifier bridge be made out of tube rectifiers? I have seen the 2 SS rectifiers plus 1 tube technique, I was wondering if there was an all-tube solution? The reason I ask is that a 230VCT isolation transformer is much less pricey than even a Hammond, and it takes a bridge to get 300+ B+ out of that.

 

[Eli Duttman]

Check the 6JU8 out. It can be used as a low current all tube bridge. It works at 120 VRMS; you'll have to check the data sheet for higher voltages.

A big disadvantage of an all tube bridge is the extra forward voltage drop. A hybrid bridge is FINE, as the vacuum rectifier blocks SS diode switching noise. So, a pair of UF4007s and a 7Y4 are an EXCELLENT low cost solution.

 

[leadbelly]

6JU8 seems to fall short, 300V PIPV, but thanks for the tip! I could see it being useful for B+ <200 service.

Wow, what a great thread for me! My next preamp will use a iso xfrmr, hybrid bridge, CRC filter, and cost a ton less $$$ than if made with tube iron. Woo-hoo!

 

[leadbelly]

Oh, I just thought of another question: are there any techniques to rectify low voltage DC for heaters without SS? I know AC is just fine for heaters, but the next preamp will be a phono stage and I am worried about hum.

 

[Sch3mat1c]

I thought of one last question: can a rectifier bridge be made out of tube rectifiers?

No, not really. I mean, you can, sure, but it sucks donkey hind quarters.

Tim

 

[PRR]

> techniques to rectify low voltage DC for heaters without SS?

Tungar rectifiers. I have not seen one since the early 1950s. They had huge voltage drop: we fed 60VAC-120VAC to get a charge on 6V car batteries.

Copper-oxide.... oh, that's solid-state.

How do you feel about rotating machinery? Run a wall-power motor, spin a DC generator. You think your silicon diodes have switching hash........

You can get hum as low as you need even with AC heat. We all did that in the days before selenium. But the trick is forgotten and everybody is too lazy to rediscover the painstaking details.

If you must obsess: a sealed lead battery for an emergency light will run one tube (the first stage which is the most critical) for more hours than you can sit without a bathroom break. A 6A battery charger will fast-charge it part-full in the time it takes to microwave a burrito too hot to eat. With a little care you will never run low on clean DC.

60Hz is not the only AC around. Build a 30KHz radio transmitter, use it to feed the heaters. This was actually the way we fed the incandescent lamps in optical film playback, before solid diodes and huge capacitors were around.

 

[Sch3mat1c]

> techniques to rectify low voltage DC for heaters without SS?

How do you feel about rotating machinery? Run a wall-power motor, spin a DC generator. You think your silicon diodes have switching hash........

Better than that, and slightly more innovative: synchronous motor with contacts at exactly 0 and 180 degrees. Instant switching, perfect zero crossing - can outdo any squalid state piece! Mind shoot-through if you time it too close though, and forget about instant-on. (As the rotor spins up, it probably puts a monstrous stress on the first filter cap as voltage goes positive and negative until it locks phase... 5-10 sec delay relay would be a good idea...)

Tim

 

[leadbelly]

Tungar rectifiers. I have not seen one since the early 1950s. They had huge voltage drop: we fed 60VAC-120VAC to get a charge on 6V car batteries.

It can't be that bad. I was looking through RDH4 and they showed 6H6's used to generate -9V bias from a 6.3VAC secondary.

You can get hum as low as you need even with AC heat. We all did that in the days before selenium. But the trick is forgotten and everybody is too lazy to rediscover the painstaking details.

Do tell. I was thinking that a twisted pair inside a grounded braid would be the way to go. Is that the right track?

 

[Sch3mat1c]

It can't be that bad. I was looking through RDH4 and they showed 6H6's used to generate -9V bias from a 6.3VAC secondary.

Oh, sure it can. 6H6/6AL5 is high perveance and you don't need much load on a bias supply. Tungars on the other hand had truely massive emission, I'd love to have a few.

Say, weren't they mercury vapor? They'd only need 30-50VAC then. I'd hate to see a high vacuum tube taking on ten amperes...

Do tell. I was thinking that a twisted pair inside a grounded braid would be the way to go. Is that the right track?

I don't know what he's getting at, but the usual tricks are biasing heater, hum balance (look at any 40s-60s stereo to see how the hum balance pot is wired) and in extreme cases, DC power. Ohhh, I bet I know what he's getting at- only works with class A power amps though.

Tim

 

[PRR]

> 6H6's used to generate -9V bias

At what current?

A "bias supply" might be 0.000,1 Amps. Preamp tube heaters need 0.3 Amps. Makes a difference.

> weren't they mercury vapor?

There are mercury vapor tubes, a whole zoo full of them. They were the workhorses before hockey-puck SCRs and such. They are fussy about temperature; in commercial and military use they needed pre-heaters or they would burst at power-up. In low-volt work like typical preamp tube heaters, you would need 3-electrode "ignitrons" with a high-voltage pulse into the starting electrode to initiate the arc at the beginning of each half-cycle. Short the load, and they happily pull all the current the utility company can make. These were really too fussy for incidental work, like garages charging batteries. Tungars are 1,000 Watt lamps with plates inside. True thermionic. Huge.

For 30 years, very low hum was done in broadcast, recording, and hi-fi simply by twisting the leads, routing them close to a steel chassis, aWAY from grid and plate circuits. Good amplifiers look obsessively "neat" inside: such obsession was the key to low-hum operation.

Refinements include center-tap or vari-tap ground. Biasing up to a positive voltage only helps if the heater insulation is leaking: in an ideal world, you'd replace the tube. In mass production, it was often cheaper to run a DC bias than to sort-out the few-percent leaky heaters.

Printed Circuit Boards are not at all good for AC-heated audio amps (impossible to twist the heater leads).

Shielding the heater leads was almost never done. If the leads are twisted, the external field cancels. And distance (layout) is a better shield than a conductive wrap.

Specifically: you can put the hum below the thermal noise of the tube.