On 4/21/2021 2:18 PM, John Larkin wrote:
On Wed, 21 Apr 2021 14:02:41 -0400, bitrex wrote:

On 4/21/2021 1:53 PM, John Robertson wrote:
On 2021/04/21 10:48 a.m., bitrex wrote:
On 4/21/2021 1:28 PM, John Robertson wrote:
In my field of arcade repairs we run into older jukeboxes - one
particular model which uses the 2D21 tube and was from about 1954.

I have the spec sheet he

https://www.flippers.com/pdfs/2D21_Thyratron_tube.pdf

It would be fun to make a solid state replacement for it as nothing
seems to exist. These tubes are still available from several sources,
but they get wonky with age.

Being able to simply wire in a TRIAC or SCR (with appropriate bits of
electronic glue) to remove this tube from the suspect list when
servicing this classic Seeburg V200 jukebox would simplify future
servicing. I always like to make things easier for the next tech to
work on machines that passed through my shop...

I should point out that Seeburg only used these 2D21s (three 2D21s
used) on their very first control center for their Tormat (200 x
magnetic cores) memory and dropped it like a hot potato. There were
numerous service bulletins culminating in the factory providing a low
cost replacement for the control centers that used 2D21s (three of
these) and the replacement used a single 2050 tube and was a very
reliable design. Not everyone took advantage of the program and all
these replacement were used up long, long ago.

John :-#)#


What happens to them as they age? is that with respect to the unused
NOS types?

I was under the impression that small-signal gas thyratrons were very
reliable in service. They're much less temperature sensitive in
service than solid state devices, at least. If you blast a relaxation
oscillator made with a 2D21 with cold spray it barely drifts.

The 2D21 tubes become erratic, sometimes they respond, sometimes they
are slow (the solenoid grumbles as it sluggishly engages), and sometimes
they just ignore the control signal. Yet these tubes test OK on my
mutual conductance tube checker (of course).

John :-#)#


The tube tester can test for conductance from thermionic emission but
IDK if it can test for the conductance when the tube is operating in the
ionization-cascade mode.

A tube tester may not apply enough voltage or current capability to
fire the gas, so would miss a problem with ionization.

What's the Gm of a thyratron anyhow?

Thyratron anode conductance is grid current-controlled when in the
ionization mode, you can read off the parameters from fig 2 of the 2D21
datasheet. For grid 1 voltages between about -1 and -5 volts and Ib
between 25 and 200 mA it's vaguely linear at about 50mA per 1 mA.

One could build a simple thyratron tester.