[Test-Equipment] TV-2

[Richard Knoppow]

----- Original Message ----- 
From: "Pete Lancashire" <xyzzypdx at gmail.com>
To: "Discussion of Electronic Test Equipment" 
<test-equipment at mailman.qth.net>
Sent: Friday, July 16, 2010 10:10 AM
Subject: Re: [Test-Equipment] TV-2


>I submit I have been fooled .. but having a tube that has 
>its
> characteristics accuratly known
> as a verification I guess is like having a known resistor, 
> cap etc as
> a check for a bridge.
>
> -pete
>
    If you _really_ want to know what a tube is doing you 
can set up a "first principles" arrangement and measure its 
characteristics. You will need a couple of power supplies, 
one for the plate and screen grid, one for filament, and 
another for bias, the latter can be batteries. All but the 
filament should be adjustable over enough range to make tube 
curves. In addition you will need a couple of voltmeters. 
Current can be measured using a voltmeter and series 
resistors. One can do this with a single voltmeter but its a 
PITA. The tube handbook will give you some idea of what the 
tube should be doing. To measure "dynamic" characteristics 
you need the above plus an oscillator which is adjustable 
for both frequency and output level and an accurate AC 
meter, preferably two. The set ups for measurements can be 
found in many places, I think the older editions of the RCA 
tube handbooks included it. They also have a very good text 
section on how tubes work and how to design tube circuits.
      In addition to the tube you will need an appropriate 
socket. The tube socket and terminals for the supplies can 
be mounted on a wood board, the most versitile arrangement 
is simply to run short leads from each socket terminal to a 
terminal like a Fahnstock clip or binding post. An 
oscilloscope is useful but not necessary.
     This arrangement will allow you to measure all static 
(steady state DC) characteristics, generate tube curves and, 
with the oscillator, measure AC characteristics such as 
distortion. You can determine true Gm, amplification factor, 
and impedances this way. Its somewhat time consuming but not 
difficult. I had to do this in school for one of my 
electronic labs.
     Since there is published data on virtually all tubes 
its possible to know the "typical" characteristics and from 
those find tubes that are worn out. One can also set up to 
measure leakage shorts and gas.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com