[Amps] Ameritron AL-811H-- Again

[Bob Maser]

Bottom line, Ameritron is junk.  Its big plus is that they are cheap. 
Aren't they owned by MFJ?

Bob  W6TR
----- Original Message ----- 
From: "Jim Reid" <reidj021 at hawaii.rr.com>
To: <amps at contesting.com>
Sent: Friday, September 30, 2005 5:33 PM
Subject: Re: [Amps] Ameritron AL-811H-- Again


> Here is the reply I rcv'd this morning from Tom Raush, W8JI,
> who designed the AL-811H for Ameritron.
>
> Some personal stuff at the opening, is deleted, and he did give
> permision for me to pass this info along to those of interest::
>
> "Hi Jim,
>
> ...snip...
>
> 1.) The common failure in 811 and 572 amps is tubes. The
> common mode of failure is high vaccuum or gas arcs. Although
> most likely under periods of high peak anode voltage
> (operation), flashovers can occur under ANY condition when
> HV is present. Sometimes the arc itself breaks down the gas
> and getters the tube, sometimes a tube is not recoverable.
>
> 2.) The resistor in the grid are to improve balance between
> tubes. The capacitors keep the grid at chassis potential for
> RF, and this is to SHIELD the cathode from the anode. The
> grid is the ONLY isolation for feedthrough capacitance. The
> values were chosen to minimize unwanted coupling through the
> tube and to equalize the drive to tubes over a normal range
> of replacement tubes. I wouldn't let a backyard mechanic
> without a network analyzer suggest new values based on
> emotional opinion.
>
> 3.) You can add as much nichrome as you like and you will
> have the same tube problems. The problem is not parasitics.
> The original suppressors are perfectly [adequate]..
>
> 4.) The meters are already protected. 100% of [such] meter
> failures are caused by the negative rail of the
> electrolytics rising above ground when the HV dumps to
> chassis ground through the tube anode to grid flashover. The
> current flows back through the grid shunt to the negative
> rail, and that is what blows out the grid shunts and meters.
> If anyone took the time to trace the current path they would
> see a single negative rail clamp diode protects the metering
> system.
>
> 5.) The "bang" occurs because things outside the tubes move
> and arc when you dump over a thousand volts to ground
> through five or ten ohms of power supply ESR. If you double
> that resistance by adding a 10 ohm series resistor, you
> reduce the surge current to half the value. It will still
> damage things. 75 amps of HV current isn't much better than
> the 150 amperes. If you really want to do glitch current
> suppression you should add at least 25 ohms using a HV surge
> rated resistor.
>
> Anyone is welcome to do whatever they want, but the fact is
> if the tubes are good there isn't a problem. If one or more
> is bad you'll have a problem even with dozens of circuit
> mods. If you have problems with an 811 or 572 amp it will
> almost always just be a tube or tubes. That's because tubes
> are cheaply manufactured and inadequately tested and
> conditioned before being sold. 3-500Z's are getting that way
> also.
>
> Tubes are costly to build correctly, that's why Eimac dumped
> glass tubes years ago, and why the people who bought the
> glass tube line weren't sucessful. We have to live with what
> is being manufactured and sold.
>
> For example, the Chinese manufacturers I've seen don't even
> highpot the tubes for voltage breakdown. In the mid 1990's
> they were testing production 572B's at 200 watts output with
> 1700 volts on the anode!! This is fodder for people with an
> agenda because it means there will be significant numbers of
> field failures related to flashovers.
>
> If you want to increase tube life in the 811 series, plug in
> 572B's. They are tested about like they should be testing
> 811A tubes. They have high reserve dissipation, so you are
> not likely to "cook" the anodes by overdissipation. The
> stated intention when designing the 811 amp was to build a
> very cheap amplifier. The cheapest amplifier possible for a
> given output power.
>
> There could have been $100 dollars of additional circuitry
> like a 25 ohm 25 watt HV fault resistor, a series 50 ohm 50
> watt cathode feedback resistor, and even a bigger tank coil
> and more expensive meters. Even with all that the tubes
> would by far remain the weakest link in the system. There is
> little point in installing rear wings on the back of Hondas
> with weak 4 cylinder engines that go 100 mph maximum, but
> some people want to do it. Myself, I prefer to get the BIG
> problems first. Change the tubes. Chinese 572B's are about
> as good as old RCA 811A's were at the same power.
>
> 73 Tom"
>
> Very interesting information.
>
> 73,  Jim W6KPI/KH6
>
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