Tio Pedro wrote:
"ken scharf" wrote in message If you
want to use tubes from the same era, then I'd pick a '27 for the
oscillator. The 27 is probably the first tube in a chain of evolution
that led up to the 1626 which was used for the same purpose in the ARC-5
transmitters. (though the 1625 is a heck of a lot easier to drive than a
'45!).


I agree Ken. A lot of the early 30s rigs used a tetrode driving a 45.
I'll have to dig deeper. (Looking for retirement projects!)
I saw a neat design using a pair of 30s to drive a P-P pair of 33s in
one battery TX QST ran in 32 or so.

Pete


The 27 is an indirectly heated triode.
Except for the Kellog tube, it was the first one ever made.
(not talking prototypes though).

The 24A is a tetrode. Than might make a nice xtal oscillator to drive
the '45.

I've noticed a lot of the early designs from the late 20s
and early 30s used cathode bias (resistors to B- off the
directly heated filaments) on triode RF power
amplifiers. Were they adding a small amount of
bias to make them easier to drive? Or, for what reason?

One other thing, I don't remember seeing parasitic
suppressors on early rigs; did the need become
evident when TV became popular in the late
40s? I know those early TXs could take off in
the nether regions

Pete

Tio Pedro wrote:
I've noticed a lot of the early designs from the late 20s
and early 30s used cathode bias (resistors to B- off the
directly heated filaments) on triode RF power
amplifiers. Were they adding a small amount of
bias to make them easier to drive? Or, for what reason?

One other thing, I don't remember seeing parasitic
suppressors on early rigs; did the need become
evident when TV became popular in the late
40s? I know those early TXs could take off in
the nether regions

Pete


Cathode bias resistors on rf power amps were a safety measure.
If the tube lost drive with no bias it could draw enough plate current
to MELT the plate, especially if run with a high voltage near (or OVER!)
the maximum ratings. Of course, using a C- supply would serve the
same purpose. Many rigs actually used batteries. Since the grid
current flowed in the reverse direction from the battery, a C battery
would actually be RECHARGED in normal use, so they tended to last a long
time.

Parasitic suppressors were not used in the early days since no one was
on the vhf frequencies there wasn't anybody to interfere with!

Actually, parasitic oscillation might show up in other ways making the
amplifier hard to load, and if detected this way the builder would take
steps to stabilize the circuit.

"ken scharf" wrote in message
...

Cathode bias resistors on rf power amps were a safety measure.
If the tube lost drive with no bias it could draw enough plate current to
MELT the plate, especially if run with a high voltage near (or OVER!) the
maximum ratings. Of course, using a C- supply would serve the
same purpose. Many rigs actually used batteries. Since the grid current
flowed in the reverse direction from the battery, a C battery would
actually be RECHARGED in normal use, so they tended to last a long time.

Parasitic suppressors were not used in the early days since no one was on
the vhf frequencies there wasn't anybody to interfere with!

Actually, parasitic oscillation might show up in other ways making the
amplifier hard to load, and if detected this way the builder would take
steps to stabilize the circuit.


The reason I asked this that Bill (Exray) ran
into some problems with his early TX project.
The old neon lamp trick showed
the presence of VHF parasitics, and adding suppressors
improved a few of the problems he was having.

We were discussing the use of cathode bias resistors, and
I couldn't quite grasp the reason for using them, but
tube protection makes sense.

Pete