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Re: [Amps] power limiting in 8410 amps?

Subject: Re: [Amps] power limiting in 8410 amps?
From: GWP <>
Date: Fri, 2 Nov 2018 14:34:09 -0600
List-post: <>
There is no intentional OUTPUT power limiting circuit in the Alpha 8410

The 8410 firmware WILL put the amp in standby if the DRIVE power exceeds
75w for too long, to protect the input circuit 50 ohm resistor, but short
peaks do not (intentionally) trigger the protection.

There are resistors between the tubes cathodes (tied together) and ground.
These do limit the gain of the amplifier, and are necessary to prevent the
amp "taking off" and going into uncontrolled oscillations.

On early 8410 production amps there were three 22 ohm resistors in
parallel, giving approx 7 ohms to chassis, this can be measured at the test
points TP1 to chassis and TP4 to chassis, the test points are located at
the right-rear edge of the top power supply board (the Mains Board).

Later 8410 amps and all repairs have 4 of the same 22 ohm resistors
installed in parallel, this increases the gain slightly and also adds
wattage to keep them from being damaged by severe overdrive.  The 4 in
parallel typically measure about 5.7 ohms from each of the two test points
to chassis.

If the resistors measure more than 7 ohms it will be difficult to drive the
8410 to 1500 watts output.  Driving the amp harder and harder can damage
the 50 ohm input resistor.  The cathode resistors can be replaced, they are
inside the tubedeck, between the tube sockets.  They can be accessed by
removing the plate under the tubes, and then removing the Tubedeck Board.
If any 8410 still has the 3 original resistors we always replace them with
4 new ones if an amp is repaired at Alpha for any reason.

Glenn AE0Q
Alpha RF Systems

On Fri, Nov 2, 2018 at 11:40 AM Steve Bookout <> wrote:

> Hello all,
> Over the last week, or so, I seem to remember 'glossing over' messages
> and seeing some comment, by someone, mentioning that 8410 Alpha amps
> have some circuitry that tends to limit their output. This being, done,
> I guess, to get FCC approval.
> I ask because, I have 4 of these amps in my M/M contest station and I've
> always noticed that the higher the power out, the more input it takes to
> get there.   They tune up real nice and easy to about 1200 to 1300
> watts, with about 40 watts of drive, but to get to 1500 seemed to be a
> challenge.  Seemed like the gain would fall off, maybe taking twice as
> much power to get the last 200 watts.
> Can anyone shed some light on this?
> Details would be great, as I would like to maybe remove such limiting
> circuitry.  It just seems like I'm pushing the amp in a bad way to get
> the 1500 watts out.  I know there is no real difference in the effect of
> the power out, but I would rather get 1500 watts out @ 45 watts of
> drive, than 65-70 watts of drive.
> 73 de Steve, NR4M

Glenn and V-NATCH Katie,
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