At 04:57 PM 12/22/97 -0700, Yellowstone Photo - Barri wrote:
>I've built a few amplifiers over the years, but only one using Zener
>diode bias. This is the most recent one: 8877 for six meters.
>
>I built it "cookbook" style, following a couple of different plans
>published in CQ. I used a 14 volt Zener. The amp appears to work well,
>and according to reports sounds good.
>
>My only problem, if it's a problem, is that the efficiency appears to be
>low: 55%,
55% is not bad, not dazzling but not bad IMO if it were below 52% I'd
start looking around for places in your tank circuit where power is lost.
The parasitic suppressor if not properly designed can dissipate power.
Unless you have a bad tube the power available at the anode lug is 60% for
AB1 operation. The tank circuit itself is the most likely culpritt. LEAD
LENGTHES MUST BE KEPT AS SHORT AS POSSIBLE. This especially so at VHF.
Long leads will increase tank coil inductance, and thus require higher
tuning capacitance to obtain a match. This inturn will increase tank
circuit Q. The higher the Q the more lossy the circuit. As has been stated
in this group a Q of 12 is SORT of a standard. A Q of 15 is the maximum
that I consider acceptable. A Q of 20 is excessive. Remember: Conservation
of Energy.
>if I can believe the meters.
if you have reason to doubt your meters check them
>Would a different value of Zener have any effect on efficiency?
Yep sure would. The higher the zener bias voltage the closer you push the
amp to Class C. Resulting in lower idling plate current, higher efficiency,
higher drive requirements and more distortion. Only let on air signal
reports be your guide when the operator giving you the report furnishes
proof that his ears have been brought into the shop for calibration in the
last 3 monthes.
Your idling plate current at 100 mils sounds about right. But don't hold me
to it. Check the tube's spec sheet or the ARRL handbook.
All in all Barrie considering your at VHF I would say you have a job well
done. Can you do better? Probably, but I don't think it would be worth the
effort to ring another 40 watts out of it.
That's my 2 cents
Bob K1TA
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