From: Garey Barrell 
Subject: [drakelist] Re: Claas C operation of 6jb6 finals for CW


Garey Barrell  made an utterence to the drakelist gang
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Pierre -

Even with your situation, I would strongly recommend against trying to 
implement Class C operation of the final stage. 

In addition to the bias level, you will also need considerably MORE 
drive than is available from the 12BY7 driver stage.  Remember, your 
drive has to overcome that much higher bias level since the tube has to 
be turned ON for at least a small portion of the cycle.  This would 
probably require, at the very least, a voltage step-up transformation 
from the 12BY7 to the 6JB6 grids.  The 12BY7 would probably not be able 
to provide enough driving  _power_, particularly on the higher bands, to 
drive the 6JB6's in Class C.

Keep in mind.  Class AB1 linear stages require NO driving power (since 
the grid is never driven positive), only voltage into a very high 
impedance grid circuit.   Class C stages require considerable driving 
power, since you need to generated a MUCH higher RF voltage (it has to 
exceed the "several times cutoff bias voltage" over at least part of the 
cycle) into a much lower impedance grid circuit, and the grid is 
actually driven into the positive grid current area.

To answer your direct question, bias "switching" is generally achieved 
by having some "fixed" protective bias to protect the finals against 
loss of drive, and then develop the "operating" bias across a
grid resistor.

Sorry, I still say it's a bad idea, especially for negligible (at best!) 
"potential" benefits.

73, Garey - K4OAH
Atlanta

Drake C-Line Service Manual
http://hr99.home.mindspring.com/R-4C_Servicez/>



F3WT wrote:

>Thanks a lot , gentelmen , for your replies - very  much appreciated .
>
>As all my antennas ( wire - based) aren't  fed barefoot but  with various
>high -Q  couplers (silvered coils,  symetrical into Lecher feeder always
>tuning to SWR=1), this should  add to the plate PI output network filtering
>by cleaning  out any further  the spectral overtones generated by the
>C-class operation. Shouldn' t it? (  Can in most instances tune the couplers
>while in reception by noise peaking only , suggesting Q is probably rather
>high- at least in low bands.)
>
>>From what you suggested, a  seperate biais generation will probably  be
>necessary . Now, using a ramp-up from AB2 to C class op biais when key is
>down and reverse when up again all in a few millisec or so should reduce
>clicks , right?
>
>I read abt the many warnings  as you, Garey , signaled to ensure that  if
>biais is  not present, plate voltage has not to be either!! hi! This could
>be ensured with a relais safety  circuit arrangement. Any comment and
>reference of past realisations  or mods on this as well?
>
> thank you,
>73.
>Pierre.
>F3WT
>
>
>le 13/02/05 17:20, Ron Wagner à wagner{at}dma.org a écrit :
>
>  
>
>>The bias voltage is used in other places on the rig besides just the
>>finals.  That said, have a look at the "drake mods"
article written by
>>VE3EFJ  http://www.dproducts.be/drake_museum/Drakemods.htm  select the PDF
>>version and look for bias. The 5 or so hit will be the CW discussion.
>>
>>He discusses backing down the bias provided that you are only running CW.
>>Keep in mind that the 6JB6s are a small linear amp, and adjusting then out
>>of linear will effect you CW note.  Might get the FCC looking your way, or
>>at least a few fellow ham unhappy with your key clicks.
>>
>>73,
>>Ron
>>
>>On Sun, 13 Feb 2005, F3WT wrote:
>>
>>    
>>
>>>I run only CW and would like to bias for class C operation 
those finals as
>>>I did once for my home - made CW Xmter which has a couple of  807's.
>>>      
>>>
>
>Vy 73
>de Pierre.
>F3WT
>- 
>
>
>
>
>  
>
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On Behalf of Garey Barrell 
Submissions:        drakelist{at}www.zerobeat.net

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