[Milsurplus] 1625 expert needed

[Thomas Adams]

I ran into something similar.

I was trying to build a period (1920s or 30s) Hartley oscillator transmitter, using a type 210. 
The tube I used was special... an original DeForest brand with a white ceramic base. It is
beautiful, and aesthetically PERFECT for the rig I was building.

The oscillator started and ran all right, but that big dome globe was filled with a yellow / purple
glow completely surrounding the plate.

Startled, I backed off of plate voltage on my bench supply; the glow died down somewhat,
and I let it run at reduced plate voltage while I investigated.

As I probed with the Simpson (I suspected a bias voltage problem for some reason), I noticed 
that the plate glow was gradually reducing. Cautiously, I inched up the plate voltage again to 
get more of the glow... same thing; in a few minutes the glow was reduced as the oscillator ran.

Repeat the plate voltage increase, gradually working to the intended Ep... and within 15 - 20 minutes, 
the oscillator was running nicely at 450 VDC with no signs of glow.

I came to the conclusion that the tube was indeed gassy, but running it with a substantial plate
current caused enough heat to reactivate the tube's getter, clearing the gas contamination.

That was about 10 years ago. I still have the same breadboard transmitter, with the same DeForest
210.  Occasionally I fire it up on 80 meters, and the tube still works nicely.


You might give the rig a try with a variable plate voltage supply. but the easiest method in your case
is to just swap out tubes; 1625s are cheap and common.

Mr. T., W9LBB






-----Original Message-----
From: sbjohnston at aol.com <sbjohnston at aol.com>
Sent: Jun 11, 2025 2:43 PM
To: milsurplus at mailman.qth.net <milsurplus at mailman.qth.net>
Subject: Re: [Milsurplus] 1625 expert needed

Light from what should be the vacuum space in a tube seems like it would have to be caused by gas.  Maybe something else is going on, but I do not know what it would be.
 
Gassy tubes usually do not perform well, but I suspect that can vary with the application and it may not make much difference in some circuits.
 
A related observation:
 
I have seen 807s and 1625s that show purple glow on the glass in a pattern that matches the inner structure of the tube.  In other words, when the tube was under power, stray electrons must have been spraying out of the cracks and crevices around the plate and when they hit the glass it caused a purple glow in the outline of the gaps in the structure.  I believe this is called fluorescence - a very cool effect.  
 
The first time I saw this I assumed it was a gassy tube, but upon closer inspection I could see it was only the glass itself that was lighting up - there was no glow at all in the vacuum space inside the tube or around the mount or metal elements inside. 
 
Steve WD8DAS   



 
 sbjohnston at aol.com   


http://www.wd8das.net/  

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