From: Garey Barrell 
Subject: [drakelist] =?windows-1252?Q?Re=3A_new_Re=3A_Claas_C_operation_o?=
 =?windows-1252?Q?f_6jb6_finals_for_CW__=80__old_Re=3A_?=
 =?windows-1252?Q?=5Bdrakelist=5D_R4C_xtal_bank_bad?=


Garey Barrell  made an utterence to the drakelist gang
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Pierre -

This has been suggested before.  There is a single reason to DO it, and 
several to NOT do it.

The reason to DO it (start optimistically, right?) is that it increases 
the efficiency of the final stage because of the higher efficiency of 
Class C operation over AB1, PLUS it reduces the total efficiency of the 
stage by eliminating the "between elements" dissipation of the
"idling" 
bias.  Great!

Reasons NOT to do it.

1.  The bias supply as designed can't provide a negative enough bias to 
reach Class C.  Not enough voltage.  Partial "self-bias" could be used, 
but...

2.  The bias supply voltage is used elsewhere in the transmitter, so you 
would have to separate the final grid bias from that circuitry.

3.  Class C operation of an amplifier tends to have very sharp "on and 
off" transitions, leading to "clicks", worsening any existing clicks 
from driver stages AND generating some of it's own.  Old-time Class C 
stages usually had some sort of "shaping" applied to driver stages to 
give a "soft" keying characteristic that was "sharpened
up" by the final.

4.  These same sharp transitions also result in a higher level of 
spurious and harmonic generation by the final stage, tolerated under the 
old regulations for "purity of emissions", later filtered by additional 
circuitry to keep them in accordance with tighter specifications.

So.   Overall, I wouldn't recommend it.   The increase in output power 
isn't all that great, (perhaps 10% "real world" increase,) and the 
reduction in dissipation / heat can be more than compensated by a small fan.

73, Garey - K4OAH
Atlanta

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



F3WT wrote:

>Hi Garey:
>
>I find this exchanges very very interesting also for  a TR4!
>
> Now I wonder abt the following:
> 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.
>Ever tried this on a TR4/T4Xany?
>I presume the bias just  to be set to further  below -60V so that at least
>iddle Anode current become zero.
>But  more precisely  what is the exact class C polarization ( bias) of the
>6JB6 finals?
>
>TU!
>
>Vy 73
> Pierre.
>F3WT
>- 
>  
>
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