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.
## I have 3 x L4B's... hooked up nose to tail... and all tuned up on
different bands. The stock L4B
will easily handle 2 kw on bypass. A rotary switch ensures only one amp at a
time is in the 'operate'
mode. [ key line form yaesu to rotary switch, then 3 x key lines from rotary
switch to each amp, each amp
has it's op /stby switch set to OP. ] The modified L4B uses vac relay's for
input, output, and bias, same deal
handles 2 kw on bypass mode]
## 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.
## I dispensed with the normal electronic grid protection scheme. Instead,
a simple 3agc fuse holder on rear apron,
with a super fast fuse installed, is simply wired between chassis and NEG
terminal of grid meter. If excess grid current drawn,
grid fuse blows open.. and now NO path for dc grid current. Amp shuts down
asap. Zero power output on wattmeter's.
If you can't draw dc grid current, amp can't be driven ! Also, the INPUT
swr [ between xcvr and amp] will rise to infinity]
Xcvr will shut down. [ mine does anyway, since I use high swr protection on
both the input and output of the amp... -7 vdc
applied to xcvr alc buss, also key line is opened off, all TX inhibit lead
is disabled, the result is xcvr is putting out zero watts].
## 3agc grid fuse is simply replaced.. and you are good to go again. This
scheme works fb on the L4B amps as well.
The super fast 3agc fuse just has to be sized correctly. Some of the above
may or may not be applicable to typ table top
amps. One advantage of the HB amps is, you are free to size any component how
you want, meter it how you want, cool it
how u want, etc. You can upgrade any portion of it too. I use ceramic vac
cap for both tune /load. And vac relays for input,
output, and also bias.
## The tuned input consist of 2 x 4 x section broadcast variables [16-540 pf
per section, all sections in parallel] Both caps
are padded on 160m. Coil is 17 turns of 6 ga CU.. on a 1.5" ID. Coil tapped
on each band. A separate 2 wafer, 6 pos small
ceramic switch, changes coil taps and also padding on 160m. These days I
use a 3 x wafer switch, and 3rd wafer used to
switch the vac relay used for the plate choke relay. Plate choke is just 180
uh + 50 uh Zero resonance's from 1.8 to 30 mhz.
## I used 6:1 Jackson bros calibrated ball drives for the tuned input.
Input swr is dead flat across all band's. Input tuning
is silky smooth, and pre-set work every time. Pre-sets for vac tune and
load loads work superb. Typ vac cap requires
18-26 turns for min -max C. Each revolution can be split into 100 x
increments.. or 1/100th of a turn. This puts pre-sets
dead on. Total time to change bands, including the tuned input is 45 seconds
[ 160m... directly to 15m]
sri for the diatribe.
Jim VE7RF
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