[ARC5] 10 meter BC-454 - yet more

[Bill Fuqua]

   Indeed screen dropping resistors had been used for years even when I 
first started working with radios in
around 1958. But I had not seen any reference to using the screen dropping 
resistor as a means of
making a sharp cut off pentode function as a remote cut off pentode. Even 
my in my electronics
classes in high school.
    These are the only two data sheets I have found actually having some 
data that reflect this.
Maybe there are more but don't have time to search.
     The point being that the screen dropping resistor is not only being 
used to determine the screen
voltage but it is acting in a dynamic way to alter the characteristic 
curves. And curves reflecting
this are published in the data sheet.
    Look at the 6SK7, no hit that the screen dropping resistor can extend 
the cutoff voltage of the
tube.
    I am no newcomer to radio.

73
Bill wa4lav


At 12:00 PM 2/6/2013 -0500, Geoff wrote:
>There is nothing new about that as almost all RF and IF amps used at least 
>one screen dropper of 22-56K going back to the 30's.  As AVC circuits 
>advanced it was used on all stagesa nd right to the end in the 70's.
>
>The 6BA6 can trace its lineage back thru the 6SK7, 6K7, 6D6, and 78, plus 
>the 44 and 35 tetrodes.
>
>Carl
>
>
>
>>  Just looked at GE's data sheet on the 6BA6 and guess what.
>>They show an optional 33k screen resistor instead of fixed voltage to 
>>extend the cut-off voltage even more negative.
>>It is a remote cut off already, this suggest that this technique may be 
>>used for later stages to
>>reduce clipping and allow larger plate voltage swing.
>>73
>>Bill wa4lav
>>
>>
>>At 09:49 AM 2/5/2013 -0500, Geoff wrote:
>>>There is a difference between remote cutoff and extended cutoff that 
>>>some manufacturers referred to as semi remote. In a simple radio without 
>>>multiple AGC loops it was prone to easily overload.
>>>
>>>The 6AC7 had an almost zero use as a gain controlled RF amp in any 
>>>commercial products that I am aware of and was often used as an AVC, S 
>>>Meter, panadaptor IF amp, or similar application.
>>>
>>>Another problem as an RF amp is that the grid circuit loaded the input 
>>>coils, broadened the bandpass, and reduced image rejection in low IF 
>>>sets. Not a problem on the high IF ARC-5's with images.
>>>
>>>
>>>
>>>
>>>----- Original Message ----- From: "Ian Wilson" <ianmwilson73 at gmail.com>
>>>To: "ARC-5 List" <arc5 at mailman.qth.net>
>>>Sent: Tuesday, February 05, 2013 1:41 AM
>>>Subject: Re: [ARC5] 10 meter BC-454 - yet more
>>>
>>>
>>>>The 6AC7, according to the data sheet, is either sharp cutoff or remote
>>>>cutoff depending on how you control the screen grid:
>>>>
>>>>*When a fixed screen voltage is used, as shown in condition 1, a
>>>>sharp-cutoff*
>>>>*characteristic is obtained.*
>>>>*
>>>>*
>>>>*When a screen resistor is used, as shown in condition 2, an extended 
>>>>cutoff
>>>>*
>>>>*characteristic is obtained which may be utilized in applications where 
>>>>the*
>>>>*gain is controlled by variation of the grid bias.*
>>>>
>>>>(GE data sheet).
>>>>Condition 1: 150v fixed screen voltage.
>>>>Condition 2: 60k screen resistor to +300v.
>>>>
>>>>73, ian K3IMW
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