I love seeing how some creative guys can come up with such interesting little experiments:
http://www.youtube.com/watch?v=80Io7zLamaM

When designing for "starvation" mode I prefer to use tubes which don't require forward-bias of G1. The subminiature tubes work well for this. I have a case of 5840Ws that are outstanding low-signal amps at low voltage.

He uses a 70v line matching transformer for output..
I'm not familiar with that.
What is the impedance ratio? .. or turns ratio?

I tried playing around a little with this setup tonight for fun.... using an 8 ohm speaker and a regular (6.2k) PP output-transformer for 6AQ5 and a 6AQ5 tube.
I used a 50k pot to set bias...
I only got some tiny bit of sound when there was +500mv grid bias .. and tube draws 2.5ma.
More positive grid bias above 500mv did no good.
The output hardly worked at all... with the SG tied to the transformer's CT... but I did get a tiny output with SG tied to plate.

Is a 6 B Q5, that he used, all that different from the 6 A q5 that I used tonight?
.... or was my output puny because of the OT ratio?

Looks like a solution searching for a problem, but interesting. Uncharted waters for the most part.

Starved plate amps can work, but not even close to being linear. Auto radio tubes (space charge) from the 50's worked fine on 12 volt plate voltage. Plenty of information on the web about these, and not too hard to come by if you want some to play with.

Using 70.7 volt transformers with multiple taps for an output works sort of. The taps are intended to provide different levels of impedence mismatch to vary volume at that particular speaker. So you can manipulate them to reflect a differrent (than 500 ohms nominal) primary impedence. That is what I have been taught by others. I tried it once with a 6GF7 (develop a simple cheap guitar amp with stuff nobody wants,) and it was OK. Put out a couple of watts before delightfully distorting. Lost interest and didn't save circuit.

He uses a 70v line matching transformer for output..
I'm not familiar with that.
What is the impedance ratio? .. or turns ratio?

With these the magic number is 5000. The 1 watt tap has an impedance of 70.7 *70.7
The 10 watt tap, on larger ones, has an impedance of 1/10th of that or 500 ohms.

As output transformers, the lack of an air gap causes trouble.

There is also 25, and 100 volt versions of these types of transformers.

I think they chose 70 volt to keep the wiring under a 'low voltage' classification in
the code.

The 100 volt version was common on European PA stuff in the 60's.

You should look at the 12K5 space-charge power amplifier tube. 40 mA plate current, 75mA grid 1 current, (grid2 is the control grid) It develops 40 mW with an 800 ohm load, a usable volume, with 12.6 volts.
There was also the 12J8 which had two detector diodes but only developed 20 mW into a 2700 ohm load to drive the output transistor.
Don

Interesting 9 volt amp. Back in the late 50s they made special car radio tubes for 12 volts on the filaments and plate, I don't recall an output tube but IIRC most or all had one or two big Ge Power transistors that initially were subject to runaway failure.
Hank

"He uses a 70v line matching transformer for output..
I'm not familiar with that.
What is the impedance ratio? .. or turns ratio?"

Off topic LV amplifiers, but on normal PA amplifiers the 70 volt output tap is equivalent to about 250 ohms output impedance and the 100 volts tap to 500 ohms, o/p impedance. Both are used to drive a distribution line on which remote speaker with matching transformers are placed (each tapped for relative volume levels to balance the sound.)
Cheers,
Roger

Yes, there were a lot of special purpose tubes that worked great on very low plate voltage. Goodle "Space Charge Tubes, " for more. Early transistors weren't as reliable as tubes for some applications. And yes, one or more early Germainum tranistors to amplify the result to the speaker. So the vibrator, transofrmer, and rectifier disappeard instantly from the radios. Dropped the cost of making a car radio at least in half. Yes, 99% of the time the transistor failed, but many of them went on until surrendering to the crusher. Come to think of it, I can't remember any of my cars, or parent's cars save one that ever required service. And then it was, I think the '62 Impala, needing a $1.49 power transistor.

Many tubes can work for various purposes at very small voltage potentials talthough this is now a soloution looking for a problem. Interesting though.