Clay,
Sounds like a cool idea, and I always thought it should work well
since there is no mod iron.
I would go for a lower output using lower voltage though, with different
tubes.
4500 volts is a lot to expect a filament transformer to handle, and if it
arcs over, it could be into the 110 ac side!
The 450th is a big triode, so I dont think you can run it single ended
pie net output, you would need to do 2 250th tubes in push pull or
something,
but the 450th would do the cathode modulator quite well.
It would be interesting to look at what available tubes would work
with this type of circuit.
There are some nice modern tubes to play with, 3cx800's and other
big triodes, or, if it could work, tetrodes.
Not sure how cathode modulating a tetrode would work out.
If you could cathode modulate a pie net output tetrode, that would open
up a lot of possibilities.
Maybe find one that runs a lower voltage but is good for some power...
Brett
N2DTS
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Clay W7CE
Sent: Thursday, September 30, 2004 12:52 PM
To: Discussion of AM Radio
Subject: Re: [AMRadio] Home brew AM
One option that isn't discussed often because it is considered inefficient
at high power is series cathode modulation. This is accomplished by putting
the modulator tube between the RF amplifier tube's cathode and ground. The
only apparent inefficiency is that you run the RF amp at one fourth of it's
Class C CW power rating. I'm currently considering building such a rig
using a 450TH for the RF amp and another 450TH for the modulator. At 4500V,
the 450TH is good for about 1500W out on CW, and with the modulator tube
added will drop to about 375W of carrier. On modulation peaks it will hit
the tubes CW power level of 1500W. The tube will be doing fairly light duty
at this power level since it can handle 1500W CCS output at 4500V, and
should last a very long time as a result. Based on the price and weight of
450TH's vs. large mod transformers, I think it's a good trade off. In
addition, the fidelity and audio response will be limited primarily by the
audio driver, not the mod transformer. On the negative side, I have to run
the plates at 4500V vs. about 2500V for plate modulation. The filament
transformer for the RF amp will also need to be hi-pot tested to 4500-5000V
(anyone willing to part with a pair of 7.5V, 12A filament transformers that
will work for this?).
Any thoughts and advice on this approach will be appreciated.
Clay W7CE
- Original Message -
From: Brett gazdzinski [EMAIL PROTECTED]
To: 'Discussion of AM Radio' amradio@mailman.qth.net
Sent: Thursday, September 30, 2004 9:03 AM
Subject: RE: [AMRadio] Home brew AM
That would be hard, but not impossible.
You would need to read up on mod transformer construction,
find something to use as a core, and wind loads of wire on it,
keeping in mind the very high voltages that it would have to withstand.
You can buy mod transformers, peter dahl makes new ones, ebay, etc.
Old surplus is best, old military rigs, broadcast rigs, old ham
rigs...
I am using mod transformers that were built by RCA in the 1940's
for some sort of ship board radio for the Dutch navy I have been told.
Without any mod iron, you would have to go to some sort of
screen modulation with large tubes.
Brett
N2DTS
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of GGLL
Sent: Wednesday, September 29, 2004 7:55 PM
To: Discussion of AM Radio
Subject: Re: [AMRadio] Home brew AM
Hello people, I agree with the nice look and smell of glass AM. Few years
ago, I made my CW/AM transmitter, with an 807 plate/screen modulated, 50
watts
CW, 40 watts AM output. Started with a triode/pentode oscillator/doubler
VFO,
then converted to solid state oscillator/buffer, followed by a tube driver
(6BQ5) and finally the 807 to a pi tank output.
Making my own plate modulated transmitter (200..300 watts out) is a buzzing
idea in my mind since a couple of months. But here the most difficult item
to
get is the modulation transformer, does someone know a method to build it by
myself?.
Many thanks in advance
Best regards
Guillermo - LU8EYW.
Brett gazdzinski escribió:
Steve,
What do you need, everything?
Here's a partial list of what you need:
3 Chassis 17x3x14,
3 panels 19x10.25?,
3 sets of side supports,
Power transformer to give 1500 volts at 500 plus ma,
Choke at 500 plus ma, two is better, swinging for the modulator,
Rectifiers (I like the k2aw? bricks),
Modulation transformer,
Bleeder resistors,
Big relay and time delay relay for step start if you go that route,
4 Tube sockets,
Filament transformers,
Killowatt plug in tank coils and its socket and swinging link,
Push pull grid coils for the bands wanted,
Push pull grid tuning cap (150 to 200 pf per section, low voltage)
Modulator grid drive transformer 8 to 10,000 ohms roughly at 20 or more
watts,
2 Neutralizing caps,
4
