Crazy tube idea?

[R.G.]

No problem. Just use a separate little power transformer for the heater power for those top tubes..

That's the conventional answer, and the one that's used for powering the heaters of rectifier tubes.

However, there are also problems with that. The heater windings have to be waved around at the same speed that the cathodes move. They represent a loading on the cathode and plate that they're tied to by internal tube capacitance. For this hypothetical use, the cathode will be moving from a low point of perhaps 50-70V to a bit less than B+, so it's going to go up and down by about 300V or more. Even small capacitances charged and discharged by 300V in the upper octave of audio might be a problem.

For low frequencies this isn't a problem because the capacitive coupling to the cathode isn't all that big so the capacitive current to fill it is small. But at high frequencies, the loading needed to change the voltage on a separate transformer winding needs at least looked and and thought about, at least to the point where some reasonable estimate of how negligible the loading is.

It may be that it's not a problem. But good design practice demands that one think about it. Every electronic component is a capacitor, inductor, resistor, semiconductor and antenna all at the same time. Whether that's a problem or not depends on the circuit and what it does.

It may also be no problem at all, and we can tell that ahead of time. That's why God gave us math.

[diagrammatiks]

Maximum plate voltage is the measured plate voltage with respect to it's cathode, not to ground.

-g

aha. you gain at least 100 volts maybe that way.

[Diablo]

Thanks to all for posting your thoughts. It's certainly way beyond my knowledge to try to implement this concept. But if any of you want to be pioneers, you have my encouragement.....

[Alan0354]

Also the problem of the standoff voltage between the cathode and the heater. They are not supposed to have hundreds of volts difference. You have to look at the spec to confirm.
Even if it is safe, how are you going to drive the top tube? The whole tube is going to float on the plate of the bottom tube and is swinging. You almost has to use an isolation transformer to drive the top tube. Why do you even want to do things like this? Plenty of tubes can take 500V!!!!

[J M Fahey]

Early transistors in series were driven by separate secondaries in driver transformers.
Technically, it can be done with tubes too (taking care, besides, of providing floating heater supplies), but it would be klunky, very expensive, poor performing, and would be definitely worse, on all counts , than using proper tubes.
All of that for tiny weak 6AQ5's.
If I had a stash of them (say, 100 or 200) definitely would not waste them and design (and sell) a bedroom 1 or 2 W amp, so fashionable now.
But if you have a couple only ... not worth the effort.

[R.G.]

Early transistors in series were driven by separate secondaries in driver transformers.

Class G and H (the terminology on these are confusing and often mixed up) do this today, and some early transistor amps simply connected a resistor string between the power supply and the output, with the base of the (usually darlington for easy drive) series devices tied to the resistor string. The resistor divider drive the outermost pairs, and the innermost pairs were driven by the audio signal from the predrivers or voltage gain stage. The resistor string drive automagically divided the voltage stress between the devices... as long as it was proper compensated for AC conditions as well as DC. But that's another story.

[Austin]

I am planning to run power tubes in cacode for a test, anybody ever tried this? Here is a guy that seems to have done it with success: https://sites.google.com/site/powercascode/home/power-cascode

[tubeswell]

I am planning to run power tubes in cacode for a test, anybody ever tried this? Here is a guy that seems to have done it with success: https://sites.google.com/site/powercascode/home/power-cascode

Doesn't say what he/she did about the heaters

[MWJB]

"All this strikes me as an exercise in Yugojetting. If you bolted a jet engine to the roof of a Yugo, it would probably go pretty fast. But you could buy a better car for the price of a used Pratt & Whitney."

Ha ha...isn't it supposed to go airborne and collide with a multi-storey tenament block? :-) Is that the Glaswegian equivalent of the "Arizona Chevy Impala JATO" myth? "Yugojetting" love it, if it's not in the OCD next year there'll be stern letter of dismay....

[silverfox]

Isn't a circuit like this susceptible to generating a high voltage spike that could blow caps?? I don't know so I'm asking,

Silverfox.

[jazbo8]

I was wondering if you could run a pair of power tubes in series. My idea is if you have some type of tubes with a relative low max plate current rating of say 250 volts, could you run two in series and apply 500 volts safely?.....like capacitors in series?

Not sure if you are still interested in the idea, but it can be done, if fact, it works very well for Class A as shown by Mr. Turner here and here.

[Enzo]

Interesting for sure. But note all the extra tubes and complexity just for the sake of the circuit. TWO KT88s to get 18 watts and an EL34 just to drive THEM.


But look at the top of each schematic, it says the amps have not been built or tested.

[km6xz]

There you go spoiling someone's dream again......reality can be such a bad sport sometimes :>)

[jazbo8]

Interesting for sure. But note all the extra tubes and complexity just for the sake of the circuit. TWO KT88s to get 18 watts and an EL34 just to drive THEM.


But look at the top of each schematic, it says the amps have not been built or tested.

They were designed for hi-fi use and Turner is a very good engineer/builder, so even if the amps were not built or tested, IMO, they would work as designed with some minor mods, and/or different part values... In any case, I do not see any glaring error that would prevent them from working, do you? Perhaps something to try when there is nothing else to do...

[Leo_Gnardo]

Perhaps something to try when there is nothing else to do...

HA! When there's nothing else to do at Enzo's, you'll find him and Wenzo having a snowball fight with the devil, because hell will have frozen over. Same at my workshop and - maybe yours too. Oh I'd like to build some of my ideas, and others, but here I got so many repairs to do - and chores besides - there's never any time to experiment with circuits old or new.