On Jan 17, 2007, at 2:52 PM, hbmandel wrote:
[snip]
Seems to be alot of hams going to the GU84B tubes for their Big
amps over the 8877 or even the old 4-1000.
[snip]
I agree. Here's the breakdown:
The 8877 tube is hellishly expensive and
has a filament that is extremely sensitive
to shortened life because of a few hundredths
of an overvoltage. Additionally, its 25 watt
Grid Dissipation rating makes it vulnerable
if not sutably protected, to damage from
mistuning and arcs.
Judging by the gold-evaporation patterns I've seen on the grid during autopsies, the 8877's grid is damaged by intermittent UHF oscillations wherein the top layer of the gold-plating boils off and forms gold meltballs.
The 4-1000A being a Tetrode, is happiest being
Grid-Driven, but few amateurs have the inclination
to design and experiment with driving and neutralization
techniques, so many of these tubes end up Grounded Grid
at a miserable efficiency.
AB1 grid-driven is slightly less efficient than g-g cathode-driven because the anode's headroom is reduced by the screen potential.
Additionally, the 4-1000A, with a 6KV at 1 Amp
800mA
anode requirement needs a power supply to suit, and
while transformers and diodes are plentiful, adequate
filter caps are not,
Not the case with new-design electrolytics.
and those that are available are
hellishly expensive once the array is gathered to
afford low ripple at maximum amperage.
A 9kV, 25uF, low-ESR filter for an 8170 or 8171 currently costs c. $190 using Panasonic electrolytics.
Hal Mandel
W4HBM
...
R. L. Measures, AG6K, 805.386.3734
r@..., rlm@..., www.somis.org
