At 02:49 AM 8/9/97 +0100, you wrote:
>>
>> >Hi Phil;
>> >
>> >On the 77 grid trip.. I don't know about the "all". There seem to be many
>> >flavors of the 77. From my manual (model 77, no D) "Alpha 77's grid
>> >overcurrent relay system will not permit the amplifier to be grossly
>> >overdriven.." I've tripped the relay; it's in there.
>> Phil wrote:
>> Thanks, Peter, I stand corrected on the grid-trip on the 77. I have only
>> seen 1 or 2 at flea markets, and never a manual. Too bad they did not
>> continue the circuit in the 77DX/SX models.
>>
>> All this arguing about grid dissipation is moot. Grid-trip circuits work on
>> current, not watts. The important thing is that a single 8877 grid be
limited
>> to 70-90 ma at all times, and as Tom said, be a very fast acting circuit.
>> This will protect the tube in case of loss of load or light loading.
>>
>> I never cease to continue my education here on the reflector....thanks!
>> Ron wrote:
>I guess some of my other posting was not clear. The 800 ma choke, is
>in the tank circuit not the power supply section of the amp. My B+ drops
>to something like 3600 volts under load, 2880 watts input, at 800 ma, is
>about 1800 watts out, with one tube. I got to believe the choke is under
>rated for 2 tubes.
Not to worry about the current on the choke. This was an AM rating, I have
felt of the B$W 800 after several hours of heavy DX'ing and it is not even
warm. The little VHF choke connected to the bottom of the B&W 800 is made
of much smaller wire than the 800 is. Also, the same value choke is used in
the cathode circuit. I didn't like this...I changed to Ohmite Z-14's there.
> Ron wrote:
> Now the transformer in my unit is rated at 2.4 kva about the same as
>the one in my commander. At 2800 watts input it is going to get hot. I
>might not burn it up but is still a long way from being a 2 tube
>transformer. The guys that operate 40 wpm cw or RTTY will certainly know
>what I am talking about here. I have worked with many different
>transformers and for the hell of can't figure how they got even a 4 kva
>to physically fit in any of the existing 77's.
> If your going to run 4kw out you will have to run about 6500 input,
>right. How long will even the SX's run at that level in a CCS duty
>cycle?
That's what I was talking about. There were several transformers used in these
models. Some of the early DX's didn't even have a 20 amp filament winding.
The transformer in my factory SX is BIG....just barely fits in its hole,
weighs 48.5 pounds. You have to lower it in place with a rope looped around
it. It, and the Peter Dahl replacement unit are 3350 kva units...1.2 amps
CCS. Folks that had smaller transformers just took them out when they went
to two tubes, and use outboard box made by Dahl, which contains a 2 amp
CCS transformer.
> One other small thing I forgot to mention. I believe it was Rich that
>said the Plate Choke bypass Cap's in the 77's are too small, he was
>right. The value needs increasing, at least in the D model for the lower
>frequencies.
I told you that also. 160m apparently was an after-thought. They didn't
want to loose the continuous 3-30 mhz coverage for their commercial
customers by putting in a larger plate choke, an didn't want to spring
for a two choke system as is done now by Alpha, Henry, and Creative Labs.
> I havnt got into the xmitting caps yet, but all this talk about the
>door knobs are interesting. The caps are not much for handling current
>in the tank circuit. If their temperture starts to rise the tank circuit
>tuning changes. The place to see this effect happening is the grid
>current changing while transmitting a carrier. I had a argument with the
>folks at Command Tech about this a few years back. Their solution was to
>put a negetive and positive coefficient door knob in series, so each
>could drift away and still be about the same amout of capacitance. I
>thought that really sucked and was not a good pratice, even if it did
>work.
>
Truly, that was curing a symptom, not the disease. Some later DX/SX's
only had 500 pf 858 doorknobs padding the load caps on 160. My SX came
with a 1000 pf, which gives you more lee-way with those funny 160m
antennas we use. I replaced the 1000 pf with two 500's in parallel. (did
not require any hole drilling) This combo runs cool enough now and does
not drift. I have had no problems with the 80 meter C1 padding doorknob
stack as yet, but the plate blocking caps are a disaster just waiting to
happen. There is plenty of room up there to put in the larger doorknobs
and cure the problem. I always end all my 77DX/SX bashing by stating that
these amps are still the finest table-top 9 band amps ever built, IMO.
(((73)))
Phil, K5PC
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