>I have two questions regarding tuning of vacuum tube linears.
>
>I have a few questions about tuning a grounded-grid amplifier running in
>class AB2.
>
>The manufacturer of my ampl supplies representative values of grid
>current, plate current, and power out as an approximate indicator of
>normal output. I find that I can frequently obtain the suggested P.out
>and Ip with substantially less grid current (sometimes 50% less).
> Is this an indication of improper operation, or perhaps worse linearity? If
so, why?
>
Grid current varies from tube to tube, so grid current is pretty much
moot in AB2 g-g service. Grid current and screen current is important
in Class AB1 amplifiers. The grid bias V should be adjusted for zero
grid current with max drive applied. Screen-current is adjusted as per
the manufacturer's recommendation with the Load-C and Tune-C. No RF
output meter is needed.
- Class AB2 g-g is a whole nuther ballgame. Provided that no more than
the rated drive is applied, when tuned for max relative RF output with
max drive, grid current will be ok. The reason for this is that, when
mistuned, too many electrons are being diverted to the grid (because of
too-light loading), not as many electrons reach the anode and tank, so RF
output drops.
However, with most ham radio amplifiers, the anode supply sags too much
in AØ tuneup, so a tuning pulser or a CW keyer sending 50wpm dits
produces a more accurate tuneup for SSB as well as CW.
>Second question. If I want to run my amplifier at reduced power (say 50%
>for RTTY), regardless of tuning, it seems like the efficiency of the
>amplifier is less then when operating at full power.
Amplifiers need to be tuned up at 100% of the drive that will actually be
applied.
> By this I mean that
>the P.input - P.out yields a *relatively* larger plate dissipation number
>at lower input powers than at higher input power. I'm interested to
>know why, and the practical application of this is to run RTTY within a
>safe anode dissipation budget.
>
- good idea. What kind of amplifier are we talking about?
cheers, Ed
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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