[AMRadio] UPDATE: Testing Transmitting Tubes

Geoff/W5OMR w5omr at w5omr.shacknet.nu
Wed Oct 20 05:40:05 EDT 2004

See response below.

> Wow! I have been inundated with stuff from higher minds than mine! Besides
> your excellent replies, I got a somewhat similar, yet different perspective
> from Patrick Turner from the R.A.T. group, (newsgroup: rec.audio.tubes) that
> I paste below. The guy wants me to calculate Mu, and plate resistance as
> well. After all, Gm = U / Ra.
> Patrick's reply:
> > What you are doing will broaden your ideas about tubes,
> > but why concentrate on the Gm?
> >
> > If I want to find out how to set up a tube I find out what
> > all three important parameters are, the Ra, U, and Gm.
> >
> > You can set up a tube with a choke load, and strap various values
> > of RL across the choke.
> >
> > Then after the bias is set apply a fixed signal input voltage to get about
> > half the maximum anode voltage swing before clipping, so thd is
> negligible.
> > Then measure the voltage gain, and do so for at least two different RLs,
> > say 5k, and say 10k.
> >
> > Voltage Gain, A, = U x RL / ( RL + Ra ), for all tubes.
> >
> > So since we know what A is, and RL is, we have two equations for the
> > two different loads, and from these we can work out
> > Ra, U and thus Gm, because for all tubes,
> > Gm = U / Ra.
> >
> > The most constant parameter for all tubes is the U,
> > because its determined mainly by electrode distances,
> > but Ra and Gm vary widely.
> >
> > The other way to measure Gm is to simply have no choke load,
> > or large value RL, and just have a 10 ohm R to the B+,
> > and measure the undistorted current change with grid voltage changes.
> > This don't tell you about Ra or U, which change dramatically
> > when G2 is connected to the anode.
> >
> > The first method of mine involves algebra.

Spreaking which...

 At Phoenix Sky Harbor airport today, an individual later discovered
 to be a public school teacher was arrested trying to board a flight
 while in possession of a ruler, a protractor, a setsquare, a slide
 rule, and a calculator. At a morning press conference, an FBI
 spokesperson said he believes the man is a member of the notorious al-
 Gebra movement. He is being charged by the FBI with carrying Weapons
 of Math Instruction. "Al-Gebra is a fearsome cult," the spokesman
 said. "They desire average solutions by means and extremes, and
 sometimes go off on tangents in a search of absolute value. They use
 secret code names like 'x' and 'y' and refer to themselves
 as 'unknowns', but we have determined they belong to a common
 denominator of the Axis of Medieval with coordinates in every
 country. As the Greek philanderer Isosceles used to say, 'There are 3
 sides to every triangle'."

 When asked to comment on the arrest, one of the arresting officers
 said, "If God had wanted us to have better Weapons of Math
 Instruction, He would have given us more fingers and toes."

 As the Bible says, "Let not many of you be teachers, for as such we
 shall endure a stricter security check."
 (or something like that.)

Smile :-)

73 = Best Regards,

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