Date: Mon, 13 Feb 2017 13:57:40 -0600
From: "Jeff AC0C" <keepwalking188@ac0c.com>
To: "MU 4CX250B" <4cx250b@miamioh.edu>, "Paul Christensen"
<w9ac@arrl.net>
Cc: "Amps Amps" <amps@contesting.com>
Subject: Re: [Amps] Alpha 77Dx two hole
Generically speaking, I don't think the 2nd tube changes the tank tune
settings - assuming the same total plate current and the HV is unchanged.
What does change is the stray anode capacitance (Cout) which roughly doubles
in a 2-tube arrangement. The increase Cout shows up across the tune cap
which is why you can run against the min stops. Of course with that 2nd
tube, the temptation to push more total Ip results in a lower tank input Z
and that does change the tune settings.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
## The 77DX was touted as having a 4.4 kva CCS plate xfmr inside....at
least per the
original qst review. IF you try and double the plate current, the plate load Z
will drop
in half. IF taps are in the same place, the tank Q will drop in half, like
from 12 down to 6.
## To configure correctly for 2 tubes, you require half as much uh for each
band, and double
the pf for both the tune and load cap. K0DD converted a lot of 77dx to
77sx. If an external
B+ supply used, the torroids in the 77 will go up like a roman candle. I
think they used a
B+W 800 plate choke, which has to be swapped to a 802 to work on 160m.
There was a
bunch of other issues on the 77sx....esp getting it to function right on 160m
band.
The peak v across the load caps was higher too. I think the
77dx used a paralleled pair of air variable load caps ganged at the shafts.
The only ones that
could be converted were ones that had the bigger fil xfmr..to handle a second
tube. I forget what they
did with the cooling situation. Oem blower would be marginal with a 2 holer.
K0DD did an extensive
write up a few yrs back on the 77sx conversions, on the yahoo amp reflector.
Its really not worth
the effort to install the 2nd tube. If its already there, better to just
operate it at 1.5 to 2.0 kw out..and
call it a day. Converting back to a single tube is not complicated. As noted
the metering shunts were
all changed. If I remember correctly, you had to mentally double all the
readings.
## Then u also have twice as much tube C..which is bad news on 10m..but easily
fixed with some uh b4 the
main PI-L tank. I think the B+W chokes would also resonate on 30, 17,12 m
bands. 8877 are not cheap to replace.
I would seriously consider modifying back to a one tube amp.
## I think they used a 2:1 un un to get the oem tuned inputs to work with
the 2 x tube cathodes. 8877 is aprx 50 ohms,
and only 25 ohms with 2 of em. The un un was a cheap and simple method to
re-use the oem tuned inputs.
## If u want to run more power, imo, a pair of 8877s is the wrong way to go
about it, not cost effective, esp with a pair
of throwaway tubes. A 3CX-3000A7 would be far better suited for that
application.
Jim VE7RF
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