Hi Ron,I experianced nearly the same symptoms after modifying a Dentron
2000L (8877) With A Dahl replacement transformer upping the plate voltage
from the standard 2500 to 3800vdc. I replaced the plate tune cap with a
Jennings UCS-300/10KV vacuum variable as well as replacing the plate choke
with the 1.5 amp choke from RFParts (W8JI design)...All is fine here now and
the amp performs flawlessly on all bands 160-10..I guess you could nearly
classify this amp as home brew now...I re-mounted the tube vertically,added
a small blower of proper cfm rating,tuned input board the origional never
had,and a W4ZT adjustable bias board...A lot to squeeze into this relatively
small box...It`s now this poor mans Alpha 77dx..GL Ron,73, Jim, K7RDX..
----- Original Message -----
From: <w2cqm@juno.com>
To: <amps@contesting.com>
Sent: Monday, December 10, 2007 3:47 PM
Subject: [Amps] Drake L4B/8877 Retrofit Problem
Perhaps some of the Amps Reflector experts out there can help with an
amplifier construction project problem! Recently completed the fourth in a
series of similarly constructed Drake L4B/8877 retrofits. All have
upgraded, Hipersil, step start, higher voltage supplies. All the projects
utilized basically the OEM Drake bandswitch tank circuit components and to
this point have been well worth the effort making 2200w output.
Unfortunately, the last one is giving me grief. On the 40/80m band, I'm
unable to load beyond the 600w output range (10w of drive) without
experiencing RF "fireworks" in the plate tune capacitor. This plate cap was
replaced with a wide spaced, heavy duty unit because the original Drake
cap initially exhibited the same characteristics; and I thought the
replacement would solve the problem. Didn't help! I'm able to Variac
control the plate voltage and the problem appears not to be voltage
initiated. Oddly, on 20m, the amp (4+KV plate voltage) develops 1800w
output (Bird Meter/dummy load) without a whimper with only 30w of drive.
To address the problem, I tried a variety of parasitic choke combinations
from no choke, "U" shaped, and the typical coil/resistor combination (all
with 50ohm non-inductive resistors). The latter style did help only
slightly until a higher level of output was tried. I substituted a single
1000pFd blocking capacitor for a pair of parallel 1000pFd units with
somewhat better results. However, tuning remains critical on the lower
bands and output remains about the 600 watt level with accompanying
flashing. RF "fireworks" manifest itself within the confines of plate
tune capacitor. The flashing is either plate to plate or plate to ground
and usually never in the same location. In this installation, I used the
Drake HV plate choke since my custom wound 1" porcelain units have
essentially the same configuration and electrical characteristics.
Apparently the stable/unstable boundary line is very critical and really
hasn't changed much with any of the component changes. Incidentally, higher
output can be achieved on the lower bands but only with fast tune/load
capacitor combinations and lots of unsettling flashes and noise. Not a
comfortable scenario especially with the price of a replacement tube.
Meshing of the capacitor plates are all normal for the three bands tested. I
have several spare tubes but did not make any substitutions as yet. When I
regain some motivation, I plan to pad the plate tune cap with small
increments of capacitance but the outcome is uncertain. I've already
prepared a posting for a 0-300 pFd vacuum variable. That may or may not help
but at this point I'm at a loss. Incidentally the positioning of all the
components are close clustered so the typical caveats have been addressed.
After reading my abbreviated tale of tribulation, is there anyone out there
who may recognize where I may focus next? Any help would be appreciated.
I sense I'm close to getting the old gal up and running. For the Drake
purists, I exercised extreme care to keep the amplifier OEM so it appears
to be a factory stock unit. Regards, Ron W2CQM/3
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