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[AMPS] Alpha 77's

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Subject: [AMPS] Alpha 77's
From: (Ron Hooper)
Date: Sat, 09 Aug 1997 02:49:50 +0100
Phil Clements wrote:
> At 09:26 AM 8/7/97 -0700, 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.
> 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!
> (((73)))
> Phil, K5PC

Hi all,
  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. 
  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
  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
  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 thier 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

Ron W4WA

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