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Re: [Amps] Current draw fan Alpha 77

Subject: Re: [Amps] Current draw fan Alpha 77
From: Martin Sole <>
Date: Wed, 25 Oct 2017 07:21:58 +0300
List-post: <>
Hi Gary,

If you take a look at my 77D pictures here:

In the 77D gallery about the 6th picture. You can see the thermostat switch mounted to the side of the exhaust plenum. The two orange wires exit the RF compartment through 1,000pF feed through capacitors and are then wired to short out the blower rheostat. You can see the rheostat mounted on the rear panel just below the top cover interlock switch. As you can see it is set almost at one end so the blower is running only slightly faster than its slowest speed. Even on 50Hz this seems enough for the single 8877. With the blower at full chat I can imagine its deafening, quite the howling banshee I'm sure.

I will be home again shortly and plan to pull the blower and do the bearings, assuming they are standard and readily available. I also want to arrange a way to monitor filament voltage and I will probe the exhaust temperature to see what values I have. Perhaps someone here could suggest reasonable exhaust temperatures. Your 150F (66C) at maximum seems quite low.

73, Martin, HS0ZED

On 24/10/2017 20:45, Gary Smith wrote:
Hi Martin,

They made several of the 77 line and I
should have been more specific, this is a
two tube variety. I'm not sure if it came
originally with two or a 2nd was added
along the way. It's an amp with a lot of
overhead available to me as I leave it in
the CW/Low Volt mode and it loafs along at
legal limits regardless of what contest
I'm in.

The blower is just plain loud and it is
set at full speed by the setting on the
adjustable wire wound resistor, it came
that way and I haven't lowered the speed.
It's running right now and moving curtains
10 feet away & that doesn't make sense if
I don't need the CFM. I don't see a
thermostat or probe located anywhere
(except for the Sunbeam oven thermometer
probe I have in the back to keep track of
the exhaust temp), it ranges from 91F at
idle to between 130-150F, tops.

I just bought a thermostat & probe with a
10 amp relay which I'm going to utilize.
I'll have to figure out which way will
allow me to have the fan run at a slow
speed & run a connection to the other end
of the adjustable enamel wire-wound
resistor, to the relay. When it reaches
the target temp (I'm guessing 110F should
be a good point to engage full speed),
it'll run the fan at maximum till it cools
down and defaults back to slow.

Noisy bearings are a bugger and I don't
hear that kind of sound, it's just a loud
blower running at maximum. I rarely
transmit any more, I listen more so a
slower speed & less noise just makes
sense. I did just recently post my
cleaning the squirrel cage when I first
bought it, and how that made a marked
difference in the turbulence & sound. That
was 6-7 years ago and I'm going to do it
again while I have the top off, just to be

I remember the cage had a very small
amount of surface rust which I removed,
that roughness added to the turbulence. If
I see any has returned maybe it would help
if I made it spray painted the cage with
enamel to increase the laminar flow & make
it a snap to clean again, easy to do and
wouldn't hurt anything.

I'll let you know how the thermostat works

Thanks for the reply & 73,


Hello Gary,

My 77D(x) has a rheostat in the blower supply. I'm not sure if that
was the same for all. Since mine is an early model it also has a
thermostat switch mounted on the exhaust plenum that shorts out this
resistor. I believe that was later deleted. I have the blower set
quiet slow with the rheostat but the air out the back never seems to
get too hot and the thermostat has never switched to high speed but
then I don't tend to use it for high duty cycle modes.

My blower is quite noisy, definitely worn bearings and I plan to
replace them in the next few weeks. If that proves successful maybe I
can run the blower a bit harder without needing ear defenders.

Whilst some might argue the original 77 transformer was a bit weedy
and the tank circuit somewhat undersized in some situations (2 tubes
on 160m) the blower by comparison seems almost excessive in the 77,
almost able to achieve that desired state of being able to lift the
tube from its socket :) This on a 50Hz supply where a lot of blowers
are distinctly under sized.

73, Martin, HS0ZED

On 23/10/2017 18:35, Gary Smith wrote:
Not sure why my message didn't send
properly but I'd sent:

Got it. Thanks for the direct replies.

I am finally getting around to adding a
thermostat to the amp to allow slower fan
rotation at idle and full rotation when
transmitting. The blower on this thing is
a beast.


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