In on 10 Nov 2009 09:45:09 -0500, in
rec.radio.amateur.boatanchors, 'Scott Dorsey' wrote:

Dave J. wrote:
If it measures characteristics of which I have only the faintest working
knowledge then by my standards it's complex enough.

What makes it complex is the fact that tubes all have different pinouts.
There are three or four controls on there that set voltages and loading for
the tube, and the rest of the controls just are for selecting the pinout.

If the 'curve tracers' you mention are the sort of tool I imagine from the
name then in those (pre d - a / computer) days they must have been
beautiful designs.

The curve tracer is basically a device like what you have, except that it
makes one parameter adjustable and displays a plot on a CRT of the plate
current with respect to that parameter. Some of them make two parameters
adjustable and display a family of curves.

Anything you can do with a curve tracer you can also do by hand with a
transconductance tester like you have, and a sheet of graph paper, and a
lot of labour making individual measurements and plotting the curve.
The transconductance tester is normally used for simple go/no go tests, and
for matching tubes based on their plate current at a single point in the
curve.


Yes, that sounds close to what I was picturing. Perhaps a LF oscillator
providing input(s) and a synchronised scope as a readout?

These days I imagine you'd produce a reasonable semblance by knocking up a
two way interface between a sound card and some breadboarded testkit?
Although preferably with a decent linear opto-isolator somewhere in
between

A PC makes quite a handy substitute for an audio 'scope. I'm slowly
meandering toward digging up the right 'back to basics' video capture card
to give me something that'll work at lowish RF. (maybe a few MHz) My
lifelong love of the tinkering perpetually exceeds my budget by a couple
of orders of magnitude.. ;-{

[..]

Hadn't thought of a guitar shop, though I'm surprised your average guitar
amp's all that fussy WRT valve specification? I'd have imagined more the
sort of pass/fail testing I'd cobble together as a psu/pot/avo birds-nest
on my workbench.

The guitar shops usually aren't too worried about that, but everyone would
rather have nice test equipment than poor test equipment.

Bit expensive for a pass/fail piece of kit I'd have thought. Although, I
s'pose it'd be one of those investments that won't drop in value too
quickly .


Thanks and apols for time-warped reply

Dave J.