[Amps] Fw: Good amp to buy /amp notes

[Jim Thomson]

From: Jim Thomson 
Sent: Saturday, January 30, 2010 1:00 AM
To: amps at contesting.com 
Subject: Fw: Good amp to buy /amp notes





From: Jim Thomson 
Sent: Friday, January 29, 2010 8:36 PM
To: amps at 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