Garey Barrell <[EMAIL PROTECTED]> made an utterence to the drakelist gang ---------------------------------------------------------------------- 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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