Date: Sat, 5 Mar 2011 12:02:35 -0500
From: "Carl" <km1h@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
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