The Magician 807 !

[IZ0MFI]

In a full index  plate_screen modulation of a 807 C class final rf stage, after the partition of two audio signals (plate and screen), it's very important to select the correct value of grid resistor related to anode supply according some application notes of this tube .
The resistor will be RF only bypassed because during modulation, an audio swing voltage of 6-7 Volt is readable with o scope so is possible to reach over 100% of upward modulation when only 92-93% was the max value obtainable (in mine line-up .
This resistor allows to obtain around -90 v dc with 4 ma of grid current so in my transmitter, i have selected a value of 11200 ohm for two 807 at 700 vdc  .
Why this resistor audio un_bypassed is so important for modulation
I thinking at a form of grid modulation contribute
73' Marco
IZ0MFI

[WBear2GCR]

Marco,

I think you are discussing a "screen modulated" 807 transmitter?

It might be good if you could post a schematic.

I am pretty sure that screen modulated output stages are limited to <100% modulation.

Others may have some better information.


                   _-_-

[IZ0MFI]

Hello WBear,
Sorry but is a true Plate and S.Grid modulated push pull TX, this  :
http://amfone.net/Amforum/index.php?topic=33194.0
The schematics is very close to the attachement but drived from a very stable e.c.o. 




 

Marco,

I think you are discussing a "screen modulated" 807 transmitter?

It might be good if you could post a schematic.

I am pretty sure that screen modulated output stages are limited to <100% modulation.

Others may have some better information.


                   _-_-

[Opcom]

The resistor is unbypassed for audio so that the variations in grid current causes modulation of the grid bias. It's a grid leak set up for audio and DC, unaffected by RF drive.

If I understand, It makes the stage work more consistently when operating voltages and RF levels are not exactly right. Seems like something the military would do.

[IZ0MFI]

Yes a form of automodulation like a sort of current pulling effect during modulation process of the tube or the tetrode only?
What the military only would do?
73' Marco

 

The resistor is unbypassed for audio so that the variations in grid current causes modulation of the grid bias. It's a grid leak set up for audio and DC, unaffected by RF drive.

If I understand, It makes the stage work more consistently when operating voltages and RF levels are not exactly right. Seems like something the military would do.

[IN3IEX]

Marco made a very interesting observation indeed.
In my TX I use only plate modulation, the screen grid of the 4/65 is connected to 250V by a 10H reactor for automodulation. It could have been connected to 600V with a large resistor and still have "automodulation" . With low plate voltage the screen grid current increases and because of the resistor (or reactor) the screen voltage decreases, thus automodulating the screen grid. It works.
Now it seems that a similar phenomenon happens to the control grid. Could it be useful to understand and improve it?
Like the plate/screen_grid interaction there must be the screen_grid/control_grid interaction, which may appear as soon as there is control grid current, like in class C with a large grid resistor.
We have to think about that, and wait for some more results from Marco.

[IZ0MFI]

Thank you Giorgio,
It's very hard job obtain an improvement if the modulation envelope is this  .
73' Marco

Marco made a very interesting observation indeed.
In my TX I use only plate modulation, the screen grid of the 4/65 is connected to 250V by a 10H reactor for automodulation. It could have been connected to 600V with a large resistor and still have "automodulation" . With low plate voltage the screen grid current increases and because of the resistor (or reactor) the screen voltage decreases, thus automodulating the screen grid. It works.
Now it seems that a similar phenomenon happens to the control grid. Could it be useful to understand and improve it?
Like the plate/screen_grid interaction there must be the screen_grid/control_grid interaction, which may appear as soon as there is control grid current, like in class C with a large grid resistor.
We have to think about that, and wait for some more results from Marco.