>This question is likely too simple, but I'll ask
>anyway: What is the "fool-proof" way to tune an
>amplifier?
>
>My amplifier is a Command HF-2500 and was used
>this weekend for the SARTG RTTY contest (and it
>was the first time it was ever tuned up by me on
>a band desides 20m).
>
>Somehow in tuning up on 80m with my antenna all
>preset etc, I found a combination of settings in
>which power out seemed to peak (a local peak).
>
>Upon increasing drive and going to operation, I
>noticed a lot of grid current. It took little time
>to discover that the problem was mistuning, but I
>must admit that sometimes tuning things up I feel
>like a monkey twisting dials.
>
>Can you help in providing a "fool-proof" solution
>to tuning (one that works in the middle of the night
>in the middle of a contest etc)?
>
>I like to take the very best of care of my equipment
>and hate to see excessive grid current even momentarily
>(2x3cx800's).
Class AB2 Cathode-Driven Tuneup?
A pair of 3CX800A7s can be dangerously overdriven by many transceivers. If I
were driving a pair with my TS-440S, I would add enough cathode feedback R so
that they could not be driven to more than the rated 600mA each. Reportedly,
this requires adding 35 to 40 (low-L) ohms in series with each cathode. An
added benefit of independent cathode feedback resistors is that they tend to
equalize currents between unmatched tubes.
Linear amplifiers work best at the volts/current ratio their tank is designed
for. A common mistake during tuneup is to try and 'take it easy' by tuning up
at reduced anode current. In order to produce an accurate minimal-stress tune,
enough drive needs to be applied to produce full anode current, either
continuous or pulsed. Tuning up at reduced drive is sure to produce a mistune
when full drive is applied.
Theory:
Electrons from the cathode pretty much wind up in two places, the antenna or
the grid. Too many electrons flowing to the grid, due to light loading,
subtracts from the flow of electrons flowing to the antenna---in addition to
increasing distortion. OTOH, too heavy loading, reduces the anode voltage
swing---which reduces the flow of electrons to the antenna as well as to the
grid. Thus, to accurately tune a grounded-grid linear amplifier apply enough
drive to produce the rated anode current, and alternately tune/load for maximum
relative power output.
- If you have an amplifier with less than a 2500w-capable power supply, or if
you want to reduce stress and strain during tuneup, use a tuning pulser. A
schematic diagram is available on my Web site.
http://www.vcnet.com/measures
Rich---
R. L. Measures, 805-386-3734, AG6K
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|