[Amps] Alpha 77D anode choke swap?
jim.thom at telus.net
Mon Apr 30 17:10:37 EDT 2018
Date: Sun, 29 Apr 2018 07:08:42 -0700
From: Shane Youhouse <kd6vxi at gmail.com>
To: Amps <amps at contesting.com>
Subject: Re: [Amps] Alpha 77D anode choke swap?
<You should do the math.
<A 6200 pf cap would suck (in almost all cases, unless your having Draloric
<build you custom caps) at 10 meters.
<A high value and low value capacitor in parallel give differing values of
<reactance at different frequencies. They will also have different resonance
<This is ham radio amplifier 101, in almost all handbooks and amplifier
<tutorials since the 60s.
## all u have to do is look at the various HEC graphs and charts.
We are not talking about huge amounts of RF current at the bottom
of the plate choke to begin with.... IF the plate choke is designed
correctly. For these multiband 80-15 and 80-10m and 160-15
and 160-10m amps, its an easy matter to just install 2-3 X
4700 pf @ 15 kv ceramite disc caps... PLUS a 200-1500 HEC
doorknob for the upper bands...then be done with it.
## If you want a chuckle, check out the hb plate choke W8JI used
on his... SW AM linears he built... on his website in 2 x places.
14 gauge magnet wire, 110 uh, on a 1 inch solid teflon rod, really long,
on his https://www.w8ji.com/shortwave%20broadcast%20amplifier.htm
HB 3x15 triode. He used a single huge HT-59 for the bypass cap
at base of the huge plate choke. Then a huge mica for the plate block cap.
It’s a 3-22 mhz, linear amp for AM, 8 kw cxr, 32 kw pep out.
## Plate block caps are a bit more critical application, they have to
be carefully chosen to handle RF current, on the upper HF bands.
<The entire rabbit hole of capacitance can ultimately be traced to a phone
<call I had with Jim almost a decade ago. Both in the bias line and the
<B+. I had made a comment that we where using gargantuan piles of C on the
<11 meter amps I was involved with then. Think pairs of tetrodes with
<handles. The IMD stopped bouncing around with a large value of filter C.
<It dropped even more with large values of bias C.
<Of course, this makes utter sense. A rock solid choke input filter has
<lower imd out of the subsequent amp than a lightly filtered C in out
<supply. We use Rock solid supplies for the screen, suppressor and control
<grids. But when it comes to a triode, let's just throw a string of diodes
<in, no filter, and it's just as good. Then wonder why the imd is in the
<I made the comments. Jim told me I was full of feces. Then he bought a
<couple cases of capacitors and found out conventional wisdom was wrong and
<that you can have large amounts of C, and the crappy problems on C input
<filter supplies is either eliminated or lowered completely.
## Not quite. I installed the 20 x 2000 uf @ 450 vdc caps into the
box back in 1991. Thats 27 years ago ! Then it just continued from there.
Plenty of surplus lytics available these days. I use 100 K @ 3watt MOF, 1% tolerance
EQ resistors from mouser. I bought 3 x boxes of em, 100 per box.... and all 300 of em
measured exactly 99,980 ohms. Zero % tolerance between em. Vishay brand...
from mouser. No need to play match up, they are dead on..all the same value.
A single 6A10 is wired across each lytic, rvs connected. In the event of a FWB
failure..or FWD failure, say one leg shorted...... you dont want raw AC being pumped
into the lytics on half cycles. In this event, the safety diodes across the lytics
conduct instead...fuse blows, end of event.
## Amps that run ten times more power require ten times more energy storage, its that simple. Then
toss in the requirement for low to no ripple..and the requirement for better dynamic regulation on ssb,
cw, the no brainer solution is bigger C...that and a bigger plate xfmr.
## I believe it was me who cooked up the idea of using a huge lytic across the string
of bias diodes. I was testing series strings of 6A10 diodes in the shop, on the bench,
and noticed the aprx .715 vdc across em... (with 200 Ma flowing)... increased by 12 %...
when current increased to 2.0 A . I also noticed the 6A10 ran pretty warm with 2 A CCS..
and the 1N5408 ran pretty warm with 1 A CCS. I also noticed the 1st and last diode in the
string of 50 x diodes ran cool. I attributed that to the test lead clips on the input of diode #1
and the output of diode #50.
## Then it occurred to me since the V drop increased as current increased, good, lets wire a
big 47,000 uf lytic across the entire string. Presto, problem solved. Bias V does not budge,
rock solid. A simple 20 position tap switch was used on buddys 3x6 amp. 1st 10 x diodes
left in the circuit at all times. The rotary switch just tapped every 2nd or 3rd diode after
that. For ultra fine bias adjust, a spst toggle can be wired to either 1st or last diode. A 2nd
spst toggle can be used to shunt out the 1st 10 diodes if you want class A operation. Ok,
now you can tweak the bias.... on the fly if you want to, and run anything from class A-AB-C
and anything inbetween. Bomb proof, you can blow it up. I also found you can parallel 6A10s
for more current..and the current will split 50-50 every time.... or 1/3 1/3 1/3 if 3 used.
## If a surplus 20 position rotary switch is not available, a more common 12 position switch can be used,
or 2 of em, with the last position of switch #1... feeding the input of switch #2. Then 23 x positions can be
obtained if required.
Later... Jim VE7RF
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