>In message <19970818022013.AAH142@[205.231.11.55]>, Rich Measures
><measures@vc.net> writes
>>
>>Class AB2 Cathode-Driven Tuneup
>
>> 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.
>
>Now I know how to *properly* tune up my pair of 3-500Z's ;-)
>
>So, for a different type of amp using a pair of 4CX800's (a ETO 91b) any
>useful tips out there on tuning?
>
Grid-driven linear amplfiers require a different tuning method, Andrew.
First off, for acceptable linearity, the grid bias V needs to be set high
enough so that virtually no grid current flows when maximum signal is
applied. Zero signal anode current [ZSAC] is then set by adjusting
screen potential. (note: although ZSAC can be slightly adjusted by
increasing the grid bias V, under no circumstances should the bias V be
set lower so that grid current flows during a maximum signal condition.
- Instead of using the relative output meter for adjusting the tune and
load adjustments,, one uses the screen current meter to tune and load a
grid-driven linear amplifier. With max. signal/full drive appled, the
tune and load controls are adjusted until the manufacturer's specified
screen current is indicated when the tank is in resonance.
- Lighter loading (more C-tune) produces more screen current. Heavier
loading produces less screen current. Resonance is indicated when the
screen current peaks.
- For more information on adjustment of grid-driven linear amplifiers,
see *Amplifiers* on my Web site:
http://www.vcnet.com/measures
- - A caveat: If a double-peak or other vagary is observed on the
tuning indicator as the tune C is rotated though resonance, the amplifier
may not be unonditionally VHF-stable. This applies to both
cathode-driven and grid-driven amplifiers. Another indicator of marginal
VHF stability is intermittent arcing. On page 14 in the Trio-Kenwood
TL-922 owner's manual, the term 'normal arcing' is used. IMO, arcing is
a normal indicator of VHF instability. There is nothing 'normal' about
arcing. Some photographs of arced bandswitches are available on my Web
site.
Rich---
R. L. Measures, 805-386-3734, AG6K
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