Power tubes as rectos??? Going mad again :D

[Satamax]

Hi everybody!

Well, at it again. How much curent and voltage could i expect using power tubes as rectifiers. I think i recall either Enzo or Dai replying there was a problem with cathode to heater limitations, but what if i connect cathode and heaters as per directly heated tubes? Ok, i gonna go and bang my head against another wall elsewhere

[Rob Mercure]

Hey Max,

I'd expect the same maximum amount of current that the tubes could pass when used as an amplifier - after all the plate dissipation doesn't change. As to the other question I suspect that what you connect to "what" affects the answer. That is, are you going to connect the grids to the cathode or the plate or some combination.

In my collection of 1940-50s electronics engineering magazines there is circuit for a variable, but unregulated, power supply using BPTs as both the rectifiers and variable power devices. Plates to the AC supply, cathodes to the filters, g2 connected to the plates (I believe) and g1 to the wiper of a potentiometer, one outside lug of the pot grounded and the other lug connected to a lower voltage negative source. The circuits used either a 6H6 or selenium rectifiers connected to a 6.3 VAC winding wired as a voltage doubler. And I'm pretty sure that the power tubes used were wired to their own heater supply.

If you use a dedicated heater winding you can bias close to B+ and not worry about the H-to-K rating. R.G. has pointed out that most internal tranny insulation is good for 600 VDC but if you're worried about this you could either see how good your power tubes heaters worked on 5 VAC or use a bunch of 5V6s or 5AQ5s in parallel or use one of the "horrible" output tubes which have quite high H-to-K voltage ratings.

Rob

PS: That is "horizontal" output tubes - TV techs are irreverent.

[Satamax]

Thanks a lot Rob!

Talking about television tubes and regulators

http://members.aol.com/sbench/reg1.html

Well, does a regulator induces sag??? No i would say. Otherwise i'd try a self biassed regulator like dis! Wait a min, same problmem again! Cathode to heater limitation! Tho, i could use something like this only on the preamp

I don't even know why i try to complicate my life!

[Satamax]

Well, checking some tubes on duncanamps.

Seems that the best common tubes are 6550 and KT90, suposed to cope with 300VDC above heaters. At respectively 42 and 50 watts dissipation. May be a bit more on the cathode since it gathers G2's electrons as well!

509 only does 150V, 6L6 200 and el 34 100. KT88 150 too.

But for this aplication, there's a special tube! meant for this, 6C33B-C

http://www.tubebuilder.com/images/tu...c33c-bdata.txt

300V at 60 watts. THought i can't remember if it's per plate, or per tube, since there's two plates in the tube.

[dai h.]

must've been Enzo. Not power tubes, but I recall in some cases small signal pentodes were used as rectifiers. The expensive Sony tube mic, plus supposedly some piece of Western Electric equipment (some thread from rec.audio.tubes quite a while back where Ned Carlson ex.Triode Electronics guy mentioned something like that IIRC). Dunno if this is germaine, but there was something about using power tubes as 6550s as a "pass tube for a regulated power supply" in my Rosenblitz(sp??) book too if memory serves.

[Satamax]

Well, i've found some "reference" to some Western electric circuit doing something like this.

http://www.audioasylum.com/scripts/t.pl?f=set&m=19588

[R.G.]

All vacuum tubes are at heart vacuum rectifiers. The things other than the heater, cathode and plate are only there to make them back away from full conduction. They will all, every one of them, work as rectifiers.

Rob is correct - the rating as a rectifier is the same as the rating as a tube in terms of current, voltage and power. However, for signal tubes they don't TELL you the correct max current rating or voltage rating. For signal (including power) tubes they tell you the design-center power supply, not how much voltage it will withstand, and they tell you the max average signal current, not the max pulse current, which is what rectifiers do.

It is also possible - this is a big gap in my knowledge - that the cathode coatings and manufacture is different in rectifiers because rectifiers have big peak currents and signal tubes don't. The peak currents in a tube can have deleterious effects on the emitting surface of the cathode if they are too high.

Frankly, it doesn't make any sense to waste a power tube this way when you can almost perfectly fake a vacuum rectifier with silicon diodes and other parts added to simulate the vacuum rectifier's imperfections.

On the question of heater to cathode voltage, most tube data sheets give it as "300V". That is not where they break, it's a guaranteed safe working voltage. They do work higher. How much higher? No one knows, and it probably varies per tube. Use a floating cathode winding, even if you have to put in a small 6.3V/ several amp transformer just to get it and then you don't have to worry about the voltage limitation.

On the question of regulators and sag.
Regulators are one-sided amplifiers which are provided a DC signal to "amplifiy". They have exactly the same characteristics as a normal audio amplifier of the same construction, in that their accuracy is a function of the forward gain and feedback, and they don't need an output transformer. A power regulator does whatever it is told to. If you want precisely controlled sag, feed a regulator a sagging reference voltage. I believe this is the subject of one of Randall Smith's patents if I remember correctly. Indefensible in court, IMHO.

Again, IMHO, it is wasteful to use vacuum tubes as series pass regulator elements. A big tube like a 6L6, 6550, etc. will have perhaps a 1000V, 400ma rating in this use and will burn out semi-regularly. A power MOSFET which can do the same job if attached to a properly designed heat sink costs under $3.00 and will last forever if the design is done well.

There are some things that vacuum does irreplaceably well, notably audio amplification. There are some things that tubes simply don't do all that well. Power supply regulation is one of these second items. I would bet a substantial amount of money that no one can do better than chance guessing in a properly designed double blind test at even telling apart equipment with vacuum versus solid state regulated power. It's always possible that there is someone, somewhere that could do it, but I'd be rich on the majority of the bets I won.

[Enzo]

I don't recall ever discussing this myself.

One day I was thinking about the cathode injected output stage like some old PVs such as the Heritage and a bunch of the Music Man amps. Since the grids were just tied to steady voltages, I wondered if one could get some sort of amplification out of some rectifier tubes, since the cathode transistors took the place of control grids. I don't remember writing about it here, but I might have. COuld you be thinking of that?

( I didn't think about it much, even at the time. Not amplification so much as just a pass element.)

[R.G.]

I don't recall ever discussing this myself.

One day I was thinking about the cathode injected output stage like some old PVs such as the Heritage and a bunch of the Music Man amps. Since the grids were just tied to steady voltages, I wondered if one could get some sort of amplification out of some rectifier tubes, since the cathode transistors took the place of control grids. I don't remember writing about it here, but I might have. COuld you be thinking of that?

( I didn't think about it much, even at the time. Not amplification so much as just a pass element.)

That's the cascode connection. What is happening is that the output tube is running in common grid mode, and the input is fed from the cathode. Effectively, the input is a current into the low-impedance cathode, and the plate is forced to follow the same current. However, the element controlling the current in the source only has to deal with the voltage from grid cutoff to grid conduction. The plate has to follow that current, even with a much bigger plate voltage supply.

The common grid connection is used a lot at RF because the AC-grounded grid eliminates the multiplication of the grid-plate capacitance by voltage gain, as well as shielding the cathode input from plate voltage effects.

It doesn't work with rectifiers, as it's still the changing Vgk that changes the tube current. A rectifier with no grid is just a rectifier.

[luis]

There was a now extinct spanish builder using a rectifer(EL-34 no more no less) as rectifier(early 70īs design),you can see in the schematic below.It was very courious.I have one at home to restore.

[Enzo]

That looks to me more like a voltage regulator, there is a diode bridge rectifier on the power transformer secondary feeding it. The EL34 is acting as the pass element, and the output voltage is controlled at its grid by the smaller parts.