[Amps] 3.5 kV 2A REGULATED Power Supply: Schematic ?

[Bill L. Fuqua]

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 “ Grids Don’t Leak, It is bias by grid rectification!”.  The resistance limit you are referring to is due to what is called “contact bias”. 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 “grid leak resistor”. 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 “grid leak resistor” only exist during times drive is applied that is why class C amplifiers in many ham transmitters have “Clamp tubes” 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.

“Grids Don’t LEAK”   
 

73
Bill wa4lav

-----Original Message-----
From: Paul and Abbi Elliott <paab at gte.net>
To: amps at .contesting.com
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.

72

Paul W5DM 

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