Now you have done it. You have hit upon one of my favorite pet peeves. My
electonics instructor in high school, Hugh J. Phillips, always said â Grids
Donât Leak, It is bias by grid rectification!â. The resistance limit you
are referring to is due to what is called âcontact biasâ. In the old days
the standard all American five radio has one audio stage which was a triode
with a 100 Megohm resistor and no cathode resistor. This provided around a volt
or so of contact bias. There was no cathode resistor because the same tube
contained a diode which was the detector and they both shared the same cathode.
Due to this the cathode had to be grounded other wise the diode detector would
be reverse biased by several volts.
Grid rectification takes place when the grid is positive relative due the
cathode. The Grid coupling capacitor charges up to the peak value of the AC
voltage applied to the grid during this positive cycle. The capacitor then
discharges thru the âgrid leak resistorâ. The time constant of the grid
coupling capacitor and the grid resistor has to be much greater than that of
the period of one RF cycle.
Many power tube specifications do have a limit set on the grid resistor. You
will usually find it in the audio modulator specifications as a class AB1
amplifier. It is usually over 100Kohms. That means that the contact bias at
the specified value is negligible compared to the applied grid bias.
The bias due to the âgrid leak resistorâ only exist during times drive
is applied that is why class C amplifiers in many ham transmitters have
âClamp tubesâ on the screen grids. They drop the screen voltage to limit
plate current when drive is not applied because the grid bias is non-existent
with out RF drive (key up).
My other pet peeves:
mhz for Megahertz MHz. We only use mHz for moon bounce now days.
DB9 connectors. A bad habit started by the 70's computer jocks. The
subminiature D connectors have shell sizes (second letter) A thru D. The 9 pin
subminiature connector was an after thought.
The standard Subminiature D connectors were DA15, DB25, DC37, DD50 and DE9.
However after the problem became so wide spread that the sales people had to
rename the connectors to keep from losing business. So now you can find a DB9
connector in you catalog. I have a lot more pet peeves but expect that you
have heard enough for now.
âGrids Donât LEAKâ
From: Paul and Abbi Elliott <firstname.lastname@example.org>
Date: Mon, 29 Dec 2003 09:16:29 -0600
Subject: Re: [Amps] 3.5 kV 2A REGULATED Power Supply: Schematic ?
<If it is pure AB1, never driven into grid current, and if the capacitor
and circuitry have zero leakage
<then I don't see why not!
Won't work for the reason the resistor from the grid of a vacuum tube to
ground was called the "grid leak" back in the first days of tube type
receivers (yes, I was there). Some of the electrons ejected from the
filament will strike and become attached to the grid structure, and if not
" leaked" off, will make the grid negative enough to cut off the
tube. Look in the old RCA receiving tube maunal--you will find the
maxiimum value of the grid resistor listed for each tube type.
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