Yes, that works as well with the grid lifted, but it´s much better to have
it tight RF grounded and supply positive voltage to the cathode, which is
done on that circuitry
The positive bias voltage is coming from the emitter of pass transistor
2N6124, connects through the 1 ohm resistor to the 10k resistor together
with -HV.
The collector of that transistor is connected to ground and therefore closes
the circuit between grid and cathode.
With that the grid is effectively negative biased with the voltage defined
by the 5k "bias set" pot and the TL431 shunt regulator.
73
Peter
-----Original Message-----
From: Alex [mailto:alexeban@gmail.com]
Sent: Donnerstag, 10. Juli 2008 06:31
To: 'Peter Voelpel'; amps@contesting.com
Subject: RE: [Amps] A Grounded Grid biasing question
...noooo, it doesn't: the source of the FET is connected to the -HV point
through the 0.1Ω resistor! This is where it goes when the FET turns on.
The FET is just replaces the usual Operate/ Standby switch or VOX relay; it
is not a current source!
What appears to be the bias source is actually the reference voltage for the
comparators on the left side of the schematic, actually part of the
protection circuit.
What I meant by negative was that you can lift the grid from ground for DC
and apply THERE a negative voltage to set the idle current to a safe value.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Peter Voelpel
Sent: Wednesday, July 09, 2008 9:40 PM
To: amps@contesting.com
Subject: Re: [Amps] A Grounded Grid biasing question
A negative voltage source will not work for biasing at the cathode, it will
do the opposite.
I see the necessary (preadjusted) positive voltage reaches the cathode when
the IRF540 switches through.
73
Peter
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Alex
Sent: Mittwoch, 9. Juli 2008 20:09
To: wc6w_amps@yahoo.com; amps@contesting.com
Subject: Re: [Amps] A Grounded Grid biasing question
...you misunderstood the schematic: the bias generator is supplying bias
just to the 2 opamps in order to set the trip reference.
The tube is controlled by the IRF 540 as a two state switch only, moving the
tube from STDBY to operate.
For setting the bias you need either a negative voltage source, like we did
with the dual 6146, or else connect a zenner in series with the cathode AND
the FET. Don't try to read in the schematic something that isn't there:
it'll only confuse you.
Get a copy of Will Orr's book: you have plenty of examples on how to.
BTW concerning the silver strip: guys, drop the nonsense! There is not
anything better than good old plain copper strip cut from flashing copper.
It will tarnish a bit in 20 years' time, so what?
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Radio WC6W
Sent: Wednesday, July 09, 2008 7:45 PM
To: amps@contesting.com
Subject: [Amps] A Grounded Grid biasing question
Has anyone ever seen a design wherein an 8877, or similar triode, operated
in grounded grid has the bias generator placed in the grid return line,
rather than in the typical cathode (grid + cathode current) location?
This implies that the HV return must be allowed to float at least as
positive as the bias value. The grid remains chassis grounded as usual. And
the advantage of this arrangement is that the bias generator need only pass
the grid current which is typically 1/10 of the cathode current.
A proposed circuit may be seen at:
http://wc6w.50webs.com/wc6wamps/index.html?fr40.html
73 & Good morning,
Marv WC6W
http://wc6w.50webs.com/
*
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