RadioBanter - View Single Post

July 18th 04, 12:10 AM

wrote:
....[snip]....

807's are becoming pricey these days. Still some cheap 1625's
available though. There must have been a google of them made
during WWII.

After reading an article about building an amplifier which used ten 1625's
with paralleled RF but with series filaments directly across the 120-volt
line, I've been saving them; hope to find the right round tuit someday....

--Myron.
Reminds me of an amp using a bunch of sweep tubes in parallel running
off the mains with a voltage quadrupler to get 600 volts.

With the 1625's there is a trick the old timers used to do.
Some brands of 1625's (I do not know which) had the beam deflection
electrodes ("grid" 3) brought out of the glass bulb with it's
own lead wire. (You might be able to see this upon close inspection
of the tube if the silver getter isn't in the way.)
The cathode and "grid" 3 were connected together
at the base pin. So what you did was to un glue and un solder
the base and then bring "grid" 3 out to it's own pin (there are
two un used pins on the 1625 base). This lets you run the 1625 in
grounded grid (cathode driven). Use 10 tubes, 9 in parallel as
the final, and the 10th as a driver stage. You now have a two
stage linear that will run about 500 watts out with less than
5 watts drive. The tube heaters could be run in series across
the mains, and use a voltage quadrupler to get 600 volts for the
plate supply. Use of an isolation transformer would be a very
good idea though!