Does anyone have a copy of the schematic for this unit on Youtube?
Looks to be well thought out.
http://www.youtube.com/watch?v=Q6ZS83F0-uI
Thanks
Hank

Two power supplies, a 50W socket, 4 meters and a Variac. Who needs a schematic for anything that simple?

Transformers, rectifier diodes, some filter caps and bleeder resistors are the basics. Circuits are on line, in ham handbooks, etc. Tube manuals have the curves and you can copy them and then plot your results on the same graph.

Get it started and post in the Homebrew Forum on here and plenty of us will walk you thru it.

Carl

The Gm measurement (top meter) adds some complexity but you can get Gm just as accurately, if more slowly, by static grid shift.

I don't have a problem with the basic design and can do it but I wanted to see what the Youtube guy did. I have an old Hunter Bandit linear carcass in storage that someone stripped the RF parts from. I was thinking of using it for the basis. I'd install a Variac, more meters, sockets and fil xfmrs with a variable bias supply. I've got quite a few assorted high power Tx tubes that I acquired over the years to build but only did one. Too much work and it's much cheaper to buy a non working linear and fix it. I'm interested in the safest, simplest. easiest with versitlity and a good Gm circuit (I'd like the subject one).
Hank

I can't tell what the YouTube guy did to read Gm but I presume he fed the grid with an AC signal and read off AC current in the plate. Grid isn't too much of a problem: you'd use a transformer with the secondary in series with the grid. Monitoring the plate signal is trickier. A series 10 ohm or even 100 ohm resistor will develop the AC voltage without adding too much impedance but you have to watch the plate current. I would personally transformer-couple that AC signal out, and drive an amplified meter. All of the good tube testers use an amplifier there, so you can get the series resistor low enough. But I think the YouTube design is just an AC panel voltmeter.

Then there's the question of signal frequency. It can't be 60Hz for directly-heated tubes or it would be swamped by hum. Either you run the filament from DC or you raise the signal to 1kHz or something (AVO used 18kHz for instance).

Good points Alan. Hopefully someone who has done it with much success will fess up. Gm is an area where I really don't want to do trial and error, I'd rather have push button and read Gm directly.

I dont see where a direct gm reading is necessary. If you can vary bias and HV then a plot against the published curves will tell you if a tube has excellent, good or poor emission. With many TX tubes the gm is listed at a voltage and current that you may not wish to reach plus there is a notation that it is only approximate.

Ive used the same tester to both regetter (when possible) and characterize triodes and multi grid tubes up to 4-1000A's for decades and have not been dissapointed with what they actually do in an audio or RF circuit.

Carl