On Jan 1, 1:29 pm, K7ITM wrote:
On Dec 31 2008, 7:16 pm, ken scharf
wrote:



K7ITM wrote:
On Dec 30, 2:54 pm, ken scharf wrote:
Tio Pedro wrote:
"ken scharf" wrote in message
. ..
Even with it's 'hard vacuum' the 01 didn't have as good a 'getter' as
later tubes and would arc over with more than 130 volts or so. It was
used as a transmitter before the type 202 tube became widely available,
but was a QRP thing, less than a watt input.
Will it have enough output to drive a 245?
I'm sure it will drive a neutralized 245 to provide some gain.
You might not get enough drive for full power output from the 245.
(even if the 245 was run in class A, which requires NO driving power
you would still see SOME power gain).

Hmmmm...You think class A at RF requires no driving power??

Cheers,
Tom

Only voltage, no current.
Of course some power gets wasted since resistance isn't zero and some
heat is generated in the coils, etc. I guess there is some power factor
in the grid / cathode capacitance etc. The input impedance of the '45
in class A SHOULD BE infinite. It isn't, but it IS VERY HIGH. So
except for losses, no driving power, only voltage.

OK, to get an _accurate_ answer with respect to grid driving power,
you need to account for the effects of the electrons going from
cathode to plate as they pass by the grid. At high enough frequency,
this becomes significant. This is quite apart from losses in elements
external to the tube. One reference about this is Terman's "Radio
Engineers' Handbook, section 4, paragraph 9. But just consider that
it takes a certain amount of _energy_ to push those electrons around
and control them, even if they don't actually ever come in contact
with the grid.

Cheers,
Tom

Hey OM
I guess it's so ez to forget about the law. The law of conservation of
energy.
You can't create or destroy energy. Physics 101.

73 OM
n8zu

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