From: Jim Thomson
Sent: Saturday, January 30, 2010 1:00 AM
To: amps@contesting.com
Subject: Fw: Good amp to buy /amp notes
From: Jim Thomson
Sent: Friday, January 29, 2010 8:36 PM
To: amps@contesting.com
Subject: Good amp to buy /amp notes
You may be surprised to know that the AL-82 uses the same capacitor vendor
with the same plate spacings as the SB-220. It uses the same bandswitch
construction as several Dentrons. Generic Modutec meters.
Carl
KM1H
## I bought a few new Modutec meter's years ago. They are pure junk imo.
As for the pair of 813's... I got 1100 w out of several amps I built yrs ago
with pairs
of 813's. I had loads of spares, and never used any of the spares. The 813
[or any
other triode strapped tetrode, like a 4-1000, 4-400 etc] is an imd generator.
## I don't know why the ARRL has this aversion to pulse tuning a GG triode
amp. I
changed the CW dot length on my mk-v's.. from stock of.. '10'.. down to 4-6.
This
reduces the duty cycle [ while sending 60 wpm dots] from 50%.. down to just
20-35%.
Then just use a pep meter, like the bird /cd.... or the array solutions
wattmeter, or scope.
## then when done, hit it with a 3 x sec dead cxr... just long enough to take
some steady
state grid /plate current meter readings.
## for a good hb amp... the 3CX-3000A7 is superb. The 225 watt CCS grid
can be used as
a dummy load for ur 200 w xcvr ! IMD is superb. 50 watts of drive = 1.5 kw
out 100 w of
drive = 2.5 kw out, etc. Since the grid is bonded to the cold chassis, actual
CCS grid diss is a lot
more than 225 W.
## On brand new /rebuilt 3CX-3000A's , this is the procedure I use for
max tube life.
Thoriated tungsten Fil and blower ONLY are turned on. I use a 750 VA Sola
constant v xfmr, that
in turns feeds a constant, regulated 240 vac to the variac. Variac is set
for 7.5 vac [ measured at the
socket, via 100-200 uh chokes in each fil metering lead, + 200 ohm 3 watt mof
, also in each metering lead
plus .01 uf bypass caps. ] Tube is run for a full 48 hrs [ hr meter on
fil xfmr primary] with NO B+ and
NO drive applied. The getter is heat activated by the fil. After 48 hrs,
all residual gas is purged.
## next, B+ and drive applied.. and tube put into normal operation. AFTER
200 hrs have passed on the
fil hr meter.... fil V is REDUCED in small increments... until the pep meter
JUST drops a few watts [while pulse tuning]
[knee effect] . Then fil V is increased by .1 V At this point fil V is
running at 15% below nominal 7.5 vac or 6.375 vac
As the tube ages, fil V is increased to maintain full pep out. I USED to use
a separate fil step start set up.. with adjustable
0-15 sec agastat /P+B timer. These days, I use the variac itself to do the
controlled ramp up of fil V. [ then zero pri/sec surge
like a step start does] I also use the variac to ramp down the fil V..
when done for the night. A simple cam on the variac shaft + microswitch /relay
ensures that IF the variac was left cranked up the night b4.... that the fil V
won't be slammed on. The variac HAS to
be reduced to almost zero, to engage the micro switch, which in turn activates
a relay, which in turn, applies
regulated 240 vac to the fil xfmr. Now, you can increase the variac, slowly,
to desired voltage [ as measured on
a fluke 87 or HP DVM]
## run any thoriated tungsten tube in the above manner, and the tube will last
forever. Blower over run
is also used, and blower runs for 5-10 mins after the fil has been ramped
down to zero.
## I USED to have rock solid 239.9 vac in winter.. and rock solid 247.2vac
in summer.. for 10 yrs.
Now it's all over the map. As I'm typing this, the laptop screen just got
brighter, and so did room lights.
Now vary's from 225v to 243 v
## the SOLA cured all of these problems.. and will easily regulate to well
under 1% . IF you do use typ step start for
fils, it's a trick to get both the pri and sec surges the same value. Even
then, they are both too high imo. The small variac
ramped up slowly, results in zero current surge, and no thermal shock. I also
ramp down slowly, when done for the night.
Jim VE7RF
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