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Re: [Amps] Tube Class C operation no Drive power or grid current

To: "" <>, Amplifiers <>
Subject: Re: [Amps] Tube Class C operation no Drive power or grid current
From: "Fuqua, William L." <>
Date: Sun, 9 Oct 2022 19:44:52 +0000
List-post: <>
I am very familiar with operational classes of tubes.
This is the first time that I have seen class C1 in a spec sheet. And besides 
that it
has about the same efficiency as normal class C2. Some large power tubes that
have beam forming cathode structures that allow the control grid to be in the
electron shadows can operate C1, that is why I was very specific about it being 
in the medium power glass tube category.

Note, it is a special situation even with this tube. It has positive suppressor 
bias and could be driven into C2 if it had greater plate dissipation
as with the 4E27A .  It is like having a car with a high power engine with a 
very undersized radiator. You can't take advantage if the all the
available power without overheating. I'm wonder what the engineers were 

The surprise was efficiency, the tube is not driven to saturation as when you 
have large positive swing in
control grid voltage, thus grid current.

I built my first transmitter in the 9th grade, the 6146B was just introduced by 
It had a 6AG7 osc/buffer, 6L6 multiplier/driver and a pair of 6146B's.
  I sort of fashioned it after an older friend's that was similar but used a 
813 final.
It was rather costly since I used almost all newly purchased parts from a local 
Later I realized that I could have gotten stuff cheaper from other hams or 
hamfest or surplus sources.

I attended an electronics school across town for two summers. In discussions of 
tube detectors and later power amplifiers
my instructor emphasized the same point. "There are no grid leaks, grids do not 
leak. It is bias by grid rectification.".
Apparently, the term "grid leak" really annoyed him.

Bill wa4lav
From: <>
Sent: Sunday, October 9, 2022 3:44 AM
To: Fuqua, William L. <>; 
Subject: Re: [Amps] Tube Class C operation no Drive power or grid current

CAUTION: External Sender

You are not required to drive the tube into the grid current region in a 
Class-C amplifier.

The definition of Class C operation is a current flow angle of less than 180 
degrees, which
is accomplished by biasing the control grid beyond plate current cutoff. There 
might be a differentiation between
Class C1 and Class C2, similar to Class AB1 and AB2.

"Greatest efficiency is obtained when the input drive to the grid of a Class C 
amplifier is sufficiently high to allow grid current to flow during the more 
positive parts of the positive half-cycles. This is the method of working most 
commonly used and can be referred to as Class C2. If grid current does not 
flow, the mode of operation may be described as Class C1. References to RF 
power amplifiers normally state Class C, whereupon Class C2 operation can 
usually be assumed".

I believe that "Taschenbuch der Hochfrequenztechnik" goes into more detail 
about this.

As the tube is more efficiently used with grid current or Class C2 it is 
usually tacitly assumed that all Class C operations use grid current.

Regarding suppressor grid modulation, it is a quite simple and inexpensive way 
of obtaining AM without using an expensive and heavy Class B modulator. It had 
a brief popularity in mobile AM gear, especially ship's radio transmitters, 
before WW2 in Europe.

It shares its downsides with other forms of grid modulation which primarily are 
low carrier efficiency and being somewhat difficult to adjust.

In larger suppressor-grid modulated transmitters, an oscilloscope for proper 
modulation adjustment was often included.


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Datum : 2022-10-09 - 00:14 ()
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Ämne : Re: [Amps] Tube Class C operation no Drive power or grid current

Zero drive and grid current is only with 3kV plate voltage and as CLASS-C 
Oscillator or Amplifier. (CW, FM, PM)
Notice not the same for 2kV plate voltage but same efficiency.

Bill wa4lav
From: Fuqua, William L. <>
Sent: Saturday, October 8, 2022 6:09 PM
To: Peter Voelpel <>
Subject: Re: [Amps] Tube Class C operation no Drive power or grid current

I was discussing two separate things.
One was Class C CW Oscillator or Amplifier operation

The second was advantage and disadvantages of Suppressor grid modulation.

Bill wa4lav
From: Peter Voelpel <>
Sent: Saturday, October 8, 2022 2:54 PM
To: Fuqua, William L. <>; 'Amplifiers' <>
Subject: RE: [Amps] Tube Class C operation no Drive power or grid current

CAUTION: External Sender

The RCA data sheet shows the opposite:;;sdata=lC%2FkheYPtpdo5PRF%2FtkLMI2PBhChpi0J6OdmmzK%2FJD0%3D&amp;reserved=0

Peter, DJ7WW

-----Original Message-----
From: Amps [] On Behalf Of Fuqua, William
Sent: Samstag, 8. Oktober 2022 20:38
To: Amplifiers
Subject: [Amps] Tube Class C operation no Drive power or grid current

    Maybe I have not looked at the data sheets very closely , but I was
surprised to find a medium power glass Pentode that required zero grid
current, thus no drive power in class-C operation.  The 4E27, not the A

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