[Amps] Alpha 77D anode choke swap?
kd6vxi at gmail.com
Sun Apr 29 10:08:42 EDT 2018
On Tuesday, April 24, 2018, Rob Atkinson <ranchorobbo at gmail.com> wrote:
> ## You are not after XL. You are after Z..and there is a big difference.
> No pure resistive component to the imaginary number. Anything else?
> ## The 50 uh plate choke on the hb YC-156 amp is wound with 20 gauge
> It can handle more current. That doesn't reduce RF voltage to the
> power supply.
I don't believe current capability was the point here. It was the
inductance value. He was including the wire size as a gauge and to give a
complete idea of what the choke was.
> ## Bypassed at the base with a 4700 pf @ 10 kv disc ceramic capacitor.
> Also a
> 1500 pf
> @ 15 KV HEC doorknob cap. The 4700 pf cap does a better bypassing
> job on
> the lower freqs.
> The 1500 pf doorknob does a better bypassing job on the higher freqs.
> If they are paralleled you have 6200 pF. I don't have time to do the
> math but I would still want more inductance. That much capacitance
> might be okay for a linear RF amplifier though.
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.
> I find your comment about the disc caps being "better," curious. With
> ceramic capacitors, "better" relative to frequency usually means lower
> frequency current rating, and higher current rating means lower or no
> value drift. Disc ceramics don't usually do well on lower
> frequencies, especially 160 m. It makes much more sense, if you need
> 6200 pF to get that job done with one or more doorknobs rated for a
> few amps at 1000 kc.
> ## HV filter in the 6700 vdc supply consist of 4 x 47 uf @ 4.5 kv oil
> filled caps in series parallel.
> That's all you need. Get rid of the electrolytics. Too much filter
> capacitance and stored energy. Oil much more reliable.
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.
You have to remember. Some of us aren't doing 1.5kw pep. Some of us live
where that isn't the legal limit.
But, if your doing 15kw pep, you better have your imd down by 10 db more
than the yoyo running an ameritron, or you'll instantly be run off the
band. You won't do that with 30 uF of capacitance. Just //wont// happen.
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