Ok D
I ran a AM screen modulated 807 long ago , with a resistive divider
for the screen voltage , mod transformer [ output transformer , with
speaker winding as the input from small amp] to tune open cct
[switch] the earth of the divider ,screen volt = max , tune for max RF
out, shallow dip , then switch -on- dropped the screen to 50% volts ,
gave good upward mod , had a 8 ? uF cap at the tapping to take audio to
ground ..
73-G.
--------------------------------------------------
From: "Dimitrios Tsifakis" <[email protected]>
Sent: Wednesday, December 19, 2012 12:47 AM
To: <[email protected]>
Subject: Re: LF: Better picture of the 807 based 500kHz TX
That is not the final schematic, this is one I drew as a starting
point and there have been many changes (including a cathode resistor
to provide some of the bias and a resistor with a few turns on it on
the plate for avoiding parasitic oscillations). It's not quite
class-c, it's somewhere between class-c and class-b by the way. The VR
valves were not used when I configured this for AM, but I am only
using CW now, so I thought the regulated screen voltage is not a
problem... Need to revisit that I guess...
73, D.
2012/12/19 Graham <[email protected]>:
lol well that config is for linear service , you should have the
screen fed via something like 6.8K from the plate supply , with
a
0.1 uF decoupling cap , no regulation for class C as the screen
voltage
will vary as the anode amps / secondary emission changes [ negative
resistance type of thing] , you need a 100 ohm 2 watt resistor ,
shorted with a over wound 8 turn 10 mm id coil directly to the
anode
as acp , would normally have 50/100 ohms in series with the
cathode
as auto bias , should the drive fail, or IA = inf very quickly
...
starts off with orange anodes by the wire supports , then yellow ,
then sparks inside , then white anodes , then glass shatters /
melts
= game over , with cathode keying , looking at a big gap cw key
!
Happy days : )
G..
--------------------------------------------------
From: "Dimitrios Tsifakis" <[email protected]>
Sent: Tuesday, December 18, 2012 11:46 PM
To: <[email protected]>
Subject: Re: LF: Better picture of the 807 based 500kHz TX
2012/12/19 Graham <[email protected]>:
Why the stabilised screen supply ?
G..
It's embarassing. I like the glow of VR tubes! :-)
73, Dimitris VK1SV
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