Using a standard pentode-tetrode as output "space-charge tube" any ideas-experiences?

[bembel]

Hi All,
Had anyone experimented with using a standart (or especially TVline) pentode-tetrode as power stage output "space-charge tube" ?
- With a B+ < 35V
- G1(with substential grid current ) & A tied to B+
- signal driving G2

Thx for your ideas.

[SY]

Close. I tie the control and suppressor grids to the cathode, then drive the screen. The output tubes then look much like zero-bias triodes, but with a very low mu. Works very well with most high-perveance sweep tubes.

[bembel]

what are your voltages & currents please?

[SY]

In my big amp, I'm using 800V of B+, 3mA idle current, 1250 ohms plate to plate. 6LF6 output tubes.

[bembel]

mmmmhmmm I was more thinking about a circuit in wich
G1>10mA @ 25V B+ (why not put a diode to block AC ?)
OT Z<1k tied to anode (for a single tube SE or why not Z=300 headphones?)
small bypassed cathode resistor
I was thincking of a few 5763 i have and I got also a 6DR6 wich tolerate well G1 current (at least a bit)
but my masterword here is low voltage (maybe also it can be interesting to raise filament voltage to achieve better efficiency despite tube life (as do real space-charge tubes))
I hope my english is clear enough.

[kevinkr]

Seems unlikely to work in a power stage due to the very low voltages you propose using, even high perveance sweep tubes need more voltage than this. I would hazard a guess that the minimum you need for a low power application such as a head phone amp would be somewhat over 100V.

[Tom Bavis]

Take a look at the 12K5 data sheet for an example of this class of operation. 6W of filament power, 1W G1 dissipation for 35 mW output. I'll send a couple of used 12K5s if you want 'em...

http://www.mif.pg.gda.pl/homepages/f...137/1/12K5.pdf

[kevinkr]

I hadn't considered the possibility of using real space charge tubes due to the difficulty in procuring same, but it looks like a totem pole output using a couple of pairs of these might work.. I wonder about distortion though..

[Miles Prower]

^^^^^

It SUX: Pentode Connections See second to last paragraph.

[bembel]

thank you for your answers,
Tom pointed at it, I was looking for a "alternative" to real space charge tubes like the 12k5 that I can't find here in France, My old tube databook list also europeans EF97 & EF98 but I didn't manage to find them either.
I've just found a source of ECC86 (wich is a real space charge tube (from 6V to 30V B+) but not intended for output stages. very funny to play with.
My Idea was to get the same 10-40 mW , from a rugged TV line tube G2 driven, with also 10mA G1 (tied to B+), & maybe 20-100% increased heater voltage. (I got several bunches of those TV tubes so tube life span is not a problem).
Thx Tom for your 12K5 sending proposal, but for now, It will surely be costly to send few of them, I have better to take some more weeks to find space charge substitutes. Anyway if I can't find any, I wil be interested, It's very nice from you, and I'll be glad to send you some tubes you're looking for too ! (any needs ?)

the whole idea is to make distortion (we're not talking HiFi) guitar effect-amp(mW) FOOLPROOFED, as guitarists have often crazy ideas.

Kevin's idea to parallel tubes in a totem pole may help also, but i'm not so familiar with totem poles, could you give me a few datas for the first prototyping approach.(let's say with a typical TV line sweep tube).

Thx again Bembel