At 11:37 PM 8/2/97 -0700, you wrote:
>
>I own an Alpha 77 (not a D) and the bottom end of the Pi-L is 3Mhz.
>What's with 160 meters ? Did some 77s cover 160.. I thought the later
>"D" models did but not the 77 ?
>
>Peter Gerba
>pgerba@crl.com
You are correct, Peter. Only the Alpha 77DX and SX's have 160 meter
capability,
and it was an afterthought to say the least.
>
>Rich Measures <measures@vc.net>
>
>> ><SNIP>
>> >> adding a second tube is filter-C failure. When the second tube is
added,
>> >> the 120Hz ripple current in the hv filter C doubles, which in turn
>> >> quadruples the (P=I^2*R) dissipation in the filter capacitor. Mostly
>> >> during contesting, case seams sometimes split and the oil leaks out.
>> >> Peter W. Dahl reportedly has a retrofit upgrade.
>> >> -It adds up to a lot of dollars, grief, time, and less than a half
>> >> S-unit.
Actually, it was the original Peter Dahl 50 mfd @ 5kv replacement caps that
were failing. Peter got the manufacturer in Florida to redesign the units.
They ended up 35 mfd @ 4500 volts. I have several miles on mine, and they
do fine. The original Alpha stock filter cap was 25 mfd. It was very high
quality, but there was a demand from Alpha owners for better HV regulation,
and they thought another 10 mfd or so would help. My advice is save your
money! The Dahl unit is $398....when Alpha had parts, they charged $798 for
the 25 mfd @ 4000 volt stock cap. I see only 100-200 volt improvement in
regulation with the Dahl cap in my SX. 3900 volts at idle, and 3300 volts
at 1.8 amps. That is why most of the SX's today are running with outboard
supplies.
snip.....
>> >I just looked at a bone stock tank two tube Alpha conversion and it
>> >worked just fine.
It works great, but it is a matter of time until the plate blocking caps
crispy-critter, (running several amps above their rating on the high bands)
and the 1000 pf doorknob padding the load cap on 160 meters starts drifting
in value, causing the very expensive variable loading caps to arc.
Also the plate choke is a B&W 800 with turns removed.(about 80uh)160 meter
RF goes backwards through the choke and its inadequate bypass caps like a
freight train on 160 meters.
snip....
>> In serious contesting service? .......With the stock tune-C and 2-holer
>> conversion, the tank Q is under 6 on 160m. The 160m output is reportedly
>> substantively less than 4kW. Apparently, you believe that a tank circuit
>> which is engineered for 2000 ohms in, 50 ohms out, will also work
>> properly for 1000 ohms in, 50 ohms out. I don't.
snip....
Do not make the mistake of removing turns off the torroids to get better Q
on 160 and 80 meters. They will overheat in an instant. You need bigger
torroids or add another to the stack.
No Alpha 77D and some 77DX's are not convertible to two tubes because the
filament winding is only 10 amps. Newer DX's had a 20 amp winding. If money
is no problem, Peter Dahl has a "drop in" transformer replacement, or an
outboard unit that sits in its box behind the SX/DX.
I have just commented on the "tip of the iceberg" as far as Alpha design
blunders go; there is no problem, however, that cannot be fixed. IMO, the
77SX is the best table-top, one-piece 4kw amp ever made
(((73)))
Phil, K5PC
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