[Alejandro Lieber[_3_]]

flipper wrote:
On Sun, 14 Oct 2012 00:35:12 -0700 (PDT), patrick-turner
wrote:

On Saturday, 13 October 2012 16:12:47 UTC+11, Alex Pogossov wrote:
"Alejandro Lieber" wrote in message ... Alex Pogossov wrote: "Alejandro Lieber" wrote in message ... Very interesting. Do you know if it has been done in practice ? Will the final valve still work in class C all the modulation cycle ? I also remember seeing a rig using 2 6W6 (or 6Y6) in parallel in the cathode of the RF output valve. Cathode modulation has more advantages. 1. It is a sum of control grid modulation and partly screen modulation. As the cathode voltage rises at the troughs of modulation, voltage difference between the screen and cathode reduces. Thus the "pull" of the screen grid reduces which cooperates with the effect of the cutting of by the control grid. Thus the cathode voltage excursions are smaller than control grid excursions if it were purely grid modulation. 2.
Modulation is not affected by variation of excitation voltage. In a common grid modulation, if the excitation voltage say is modulated by 50Hz hum, output will also be modulated. With the proposed cathode current modulation output will not be affected. Cathode voltage might be modulated by hum, tracking it like in a cathode detector, but since the current is stable, the anode RF component will be almost unchanged (ignoring cut-off angle effects). But the greatest advantage is no need for any adjustments of bias, etc. The only thing to be set is the cathode current. It is done by adjusting bias on the gate (base) of the modulator series transistor. Very interesting. Where can I look for a practical circuit ? Nowhere. The whole fun is to create this circuit and experiment. Patrick has done similar thing (cathode current modulation) in his low distortion test AM modulator. You have a chance to try in a higher wattage range. I did it man
y
years ago with a smaller tube, similar to 6BW6, for a 10m band transmitter. Regards, Alex -- Alejandro Lieber LU1FCR Rosario Argentina Real-Time F2-Layer Critical Frequency Map: http://1fcr.com.ar --- news://freenews.netfront.net/ - complaints: ---

One small tube which might be good for G3 AM modulation could be the small mini 7 pin 6DT6, because it was common for an FM demodulator using quadrature detection, but fairly simple, and 6DT6 was made to have good G3 gm character.
Once the RF is AM modulated, it can just be amplified by a following RF output tube, maybe 1 x 807 is good.

Patrick Turner.

The 6DT6 is a "dual control pentode," g3 being the 'second' control,
and is the current 'tube of choice' for what has become the defacto
'standard' BCB AM transmitter on www.antiqueradios.com (Homebrew
Radios), because it is a 'dollar days' tube. That, and other
variations, trace back to Norm's 6888 transmitter. The 6888 and it's
loctal equivalent the 7ak7 are the most 'powerful' of the lot. The
7ak7 is also a 'dollar days' tube but being loctal seems to nix it's
popularity for some reason.

Prior to the 6DT6 the 6GY6 was the 'tube of choice' because it's
cheaper but allegedly produces almost the same output level as the
6888 did.

The 6GX6, 6HZ6, and 6AS6 are also dual control pentodes and the
compactron 6BV11 has two dual control pentodes in the one bottle.

The 'standard' configuration is a solid state brick oscillator
exciting G1 with audio, after a triode preamp, to G3. Norm's original
used an input transformer and the higher cathode bias needed was
enough to power the brick under the 6888.

Since there are only 2 readily available brick oscillators in the BCB
there are also tunable versions.

The current 'tunable' 6DT6 schematic is here

http://amradio.freeiz.com/transmitter/6bk76dt6.jpg

I think this is the 'original' 6888 thread (although in a previous
thread Norm is 'working on it')
http://antiqueradios.com/forums/viewtopic.php?t=63310

And the follow up when it was built
http://antiqueradios.com/forums/viewtopic.php?t=63526

Of course, there are dual control (when sharp cutoff) heptodes too.
Here's one using a 6AW8 as local osc, feeding the 6CS6, and RF final.
http://home.comcast.net/~suptjud/6CS...ransmitter.JPG

This is the 'standard' antiqueradios version, which is the same thing
as the dual control pentodes but using the 12CS6.
http://amradio.freeiz.com/12av612cs6rev2.jpg

And then there are 'twin pentodes', like the 6LE8, 6HS8, 6GS8,6BU8,
with common g1 and k that, when g3 and plate are tied together are
dual control pentodes.

Lots of g3 modulation.

Probably the most ingenious is this one
http://www.radiomuseum.org/forum/hi_...ansmitter.html
He subtracts screen from plate to keep modulation wholly on the
'linear' portion of the G3 curve.

Another interesting transmitter is Bob Weaver's screen modulation with
carrier control and while that's not g3 modulation I mention it
because it's sort of a counterpoise to the above 'hi-fi' idea since
compression *is* distortion. But why go to all that linearization if
you're going to compress in the preamp?
http://www.antiqueradios.com/forums/...?f=12&t=170605
He also built one using a 6V6.


I remember seeing some 40 years ago a 1 watt one tube AM short wave
transmitter using suppressor modulation using a single 6BH6.

--
Alejandro Lieber LU1FCR
Rosario Argentina

Real-Time F2-Layer Critical Frequency Map:
http://1fcr.com.ar

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