Yes, my inherited homebrew 811 amp uses something similar. As I
believe you and I discussed before, the 811 circuit is a copy of the
Collins 30L1 bias circuit.
Any dowside risk to gutting the grid cap/choke circuit in the L-7 and
replacing them it Cu straps between the grid terminals of the socket
and chassis? Will this affect the termination impedance that the tuned
input sees significantly?
Making this change to the 811 amp will be a bit more work as it requires
implemention of cathode bias to replace the grid leak resistors that are
in parallel with the grid bypass capacitors (the L-7 operates in zero bias
mode). Of course, I am in less of a hurry to modify the 811 amp because
it has other problems that I need to fix before its fit to go back on the
air:)
Thanks,
Mike, W4EF.....................
----- Original Message -----
From: "Tom Rauch" <w8ji@akorn.net>
To: <amps@contesting.com>
Sent: Wednesday, September 12, 2001 7:22 AM
Subject: [AMPS] Henry 2kd classic
>
> > Hmmm, if there are no good rules of thumb and no peak current/energy
> > ratings for anode supply components, how does one go about sizing
> > the glitch resistor?
>
> I'd just make it as big as I reasonably could, depending on plate
> current. There really isn't any reason to dissipate more than a
> couple
> percent of the output power in the resistor from I^2 R losses, and we
> don't want to noticeably reduce HV regulation.
>
> > BTW, speaking of grid components, how come Drake grounds the grids
> > on the L-7 thru a parallel combination of RF chokes and capacitors?
>
> That is a mistake that was promoted by Bill Orr (now SK). Bill
> talked Heathkit into doing it also, as well as other companies. The
> original idea came from the 30S1 where it worked quite well. The
> idea
> is you form a capacitive voltage divider between the cathode and
> ground and tap the grid up on that divider to make the amplifier less
> sensitive to drive power.
>
> It works perfectly in the 30S1 because it is a grounded-grid tetrode
> that draws no grid current. The screen shields the input and output,
> and since the divider is lightly loaded by a very high grid impedance
> the division is the same from band-to-band.
>
> The moment the 30S1 draws grid current, the impedance between
> cathode and grid drops like a rock and the grid is "tied" to the
> cathode. While this produces distortion when grid current appears,
> it
> does prevent hurting the tube from excessive grid current. It also
> increases driving power requirements. So it was an excellent idea in
> the 30S1 because it was an AB1 TETrode!
>
> The results are different in a triode, because there is no shield
> between the anode and cathode other than the control grid. Also,
> since
> the tubes generally draw considerable peak grid current the grid
> voltage waddles all over the place on SSB. This causes increased
> IMD,
> and the gain varies from band-to-band.
>
> This is a good example of what happens when people try to apply a
> good
> idea in one application to other systems without understanding how
> they work. Every system is different, and should be evaluated on its
> own merits.
>
> That system actually moves the amplifier towards LESS stability
> and poorer IMD if it is a GG triode!
>
>
> 73, Tom W8JI
> W8JI@contesting.com
>
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