>Features include smooth vernier
>loading and tune controls...big heavy bandswitch with the WARC bands tapped,
>and the appropriate meters. The unit is heavy and well constructed. The
>variables caps are wide spaced. ... ...
However, a tune-C that has a wider spacing than the open contacts on the
bandswitch is, in my un"expert" opinion, an open invitation to toasted
bandswitch contacts. During a 'glitch', by far the safest thing to arc
is the tune-C. Or, in other words, you can't file down evaporated band
switch. Heath learned this lesson the hard way when they supposedly
upgraded the SB-221 by installing a wider spaced tune-C.
> ... ... A lot has not been said about these QRO
>amps here on the reflector...and I hope they do not become a target for the
>critics and the naysayers.
Critics/naysayers/"experts" who technoblather turn themselves into large
targets, Lee.
How to evaluate QRO's amplifier design:
1. Is the grid bias V adjustable?
2. Is the amplifier neutralized at operating frequencies above about
10MHz?
3. Is the grid terminating R connected as directly as is practicable to
the grid?
4. Is the screen V for the 4cx800s obtained through a R that is
connected to the anode supply, and is the screen voltage regulated by a
string shunt of c. 20v, 5w zeners?
5. Does the amplifier use high VHF-Q/high VHF-Rp VHF parasitic
suppressors?
6. Is the heater voltage easily measureable and adjustable,
sans-soldering?
notes for 1 - 5:
1. The bias V needs to be adjustable so that the operator can adjust
the amplifier for virtually zero grid current at a variety of common
drive levels.
2. Eimac recommends neutralization for virtually all tetrodes. I
learned this lesson the hard way, by first making the mistake of
listening to a number of amplifier "experts".
3. This is a VHF stability issue. (note: the cold end of this resistor
needs to be triple-bypassed with capacitors that exhibit capacitive
reactance at the low end of HF, at the high end of HF, and at the anode
circuit's VHF resonance). IMO, a satisfactory test for capacitor
resonance can be performed by clamping the DUT in a copper-jawed vice and
dipping the DUT with a dipmeter. IMO, it's important to keep in mind
that above it's self-resonance, a capacitor looks like a choke.
4. 4cx800s have tender screens with a proclivity for potentially-fatal
current reversal. The safest kind of screen protection for these guys is
automatic current-limiting, with loss of anode-V protection, both of
which the described circuit provides. The zener-string shunt regulator
handles reverse screen current with ease. (another Eimac idea) A rotary
switch for changing the zener tap would be nice for adjusting the ZSAC
after setting the grid bias V for zero grid current.
5. Higher VHF-Rp in the VHF suppressor means higher VHF
voltage-amplifiction.
... < http://www.vcnet.com/measures/Rp_comp.html>
Rich---
R. L. Measures, 805-386-3734, AG6K
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