[Amps] Alpha 8410 Pics

[Jim Thomson]

Date: Sat, 5 Mar 2011 12:02:35 -0500
From: "Carl" <km1h at jeremy.mv.com>
Subject: Re: [Amps] Alpha 8410 Pics

Very nice and its easy to spot the lineage right back to the vintage 274 
model.

Is that an Ameritron plate choke?

##  No it's not. It's Alpha's  version of the infamous Ameritron choke.
Alpha  ripped off the idea  from Ameritron.  The older Alpha choke as used in the
87-A  was a 2 x piece affair..with a cap and  a bandswitch position used at the junction point
of the 2 x chokes.....to chassis.   To really ensure the cap is  dc grnded on the cold end, Alpha
used 3 x 1 meg @ 1/2 watt carbon resistors..wired in series.. between  cold end of cap and chassis.
Both chokes used in series on the low bands... and  cold end of cap is RF grnded  to chassis  on the higher
bands.   I used the same scheme on the hb amps... except a relay was used instead of a spare bandswitch contact. 
Without those resistor's from cold end of cap to chassis at all times.... all hell breaks loose  for  some reason. 
The cap hanging off the junction point should DC block the B+.... regardless of whether the cold end of the cap is
resistance grnded or not.   In desperation we added 3 x 1meg resistor's... and all was well.  That cap  hanging off the 
junction point , once chassis  grnded, now becomes the plate bypass choke  for the smaller choke..for the higher bands. 
Smaller choke is closest to the anode.   Of course, there is still another bypass cap at the base of the bigger  choke, which 
is hardwired to chassis  at all times.  




They have made a few changes over the years and its nice to see them trying 
to address the weak link bandswitch that also goes back to the 274. Doubling 
up on the high current wafers, and flat strap everywhere instead of braid, 
shows that they read the various amplifier forums.

###  Gee, I wonder where they ripped off BOTH of those idea's from?  Braid
is a dead loss imo,  and those bandswitch  current ratings are for 60 hz/dc... and
NOT RF freqs.   Paralleling 2 x wafers  for more RF current is easy..but it has to be done 
right. All contacts are strapped in parallel  between the 2 x wafers alright..BUT the taps
to the tank coil taps  are  done on wafer #1  [ closest to the coils].... meanwhile the
OUTPUT of the assy [ COM]  has to be on wafer #2.    Done that way ensures the 
RF current will divide equally between the 2 x wafers.   IF you use the COM on wafer #1
instead, 95%  of the current will flow through wafer #1..and only 5%  on wafer #2. 

##  same rules apply with 3 x wafers  used in parallel.   Coil taps connect to wafer #1
and output of assy  [COM]  is done on wafer #3.  

##  Alpha used my strap and parallel wafer idea....and I used their resistor fix for the 2 x piece
plate choke..so I guess that means we are now even.   Those Emtron amps  run a  LOT cooler
when the braid is replaced with strap.  

Later... Jim   VE7RF






 Even using more nylon 
washers in order to minimize the arc path to the support hardware is a good 
move but for the money they charge you would think that Electroswitch would 
supply Teflon.

What is with that switch section where they use 2 ceramic spacers seperated 
with a pile of washers? Some sort of torque absorber? Id expect that from 
Dentron.

On the 76PA I recently replaced the bandswitch in (as well as other makes 
and models that blow them at the same place) I covered the steel mounting 
screw threads with liquid polystyrene ( Q Dope) and then coated the external 
ceramic spacers, wafer and nylon washers junctions with more.

Its good to hear that the EBC is working, its something to take a look at 
for some HB projects using big tetrodes running lots of idle current.

Carl
KM1H