[Top] [All Lists]

Re: [Amps] Current draw fan Alpha 77

Subject: Re: [Amps] Current draw fan Alpha 77
From: Martin Sole <>
Date: Wed, 25 Oct 2017 13:31:27 +0300
List-post: <>
Hi Gary,

Definitely a later model, you have a pcb for the RF meter circuit whereas mine is just open wired around the socket and that sure is some gunk on the blower.

If it were me I would first slide that wiper on the blower resistor to almost the other end, to set up a default low speed for the blower during rx periods then I think you could do like Jim said and wire a relay, coil from 12v to the PTT line and with some suitably rated contacts just short out the resistor or parallel in a much smaller value. That would cause the blower speed to go up and down with keying.

A dry contact thermostat could be used in the same way but would just vary the blower speed based on temperature. I don't think you would need a supply voltage for that.

On 25/10/2017 09:01, Gary Smith wrote:
Hi Martin,

Thanks for the link & the info. There are
some differences, our Plenum's are
different and this one doesn't have the
temperature switch in it. I will open it
tomorrow and see what I can do, taking
current photos. I need to find a 12V DC
source inside the amp for the thermostat
or I'll have to come up with something
else, I have some ideas that will do.

Here are photos from when I brought the
amp home; the first photo shows the
Wirewound Resistor as it sat and still
does. Very high RPMs & noise.

This one is dark but it shows a view of
the plenum and it also provides a view of
the dirty squirrel cage I was saying
needed cleaning and when cleaned, the
noise went remarkably down. All that junk
caused turbulence you could hear at higher

This last one is a close-up of the cage &

More later, getting late here.

Thank you!



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.


Amps mailing list
Amps mailing list

Amps mailing list
Amps mailing list

Amps mailing list

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>