Bob,
I gotta say, the writer of that letter sounded like he was surely mad about
something. When an amp is built cheap, it's cheap and no matter how much paint
and flashing lights you add to the outside, it's still cheap on the inside. It
sounded to me the designer pretty much admitted to it. However, there's some
folks that don't have the money for an Alpha on the other hand and get by fine
using the 811, etc. What I noticed though is the author never mentioned the
internal layout of the amp as that has as much to do with an amp as every part
in it. Long lead lengths cause stray capacitances which can lead to strange
behavoir I'll call it. It's funny too that they'll argue there's no such thing
as this problem but still use supressors to stop it. That's a big can of worms
I don't want to reopen here as it's been quiet for a looooong time. Some amps
are more prone to this behavior too not only because of layout but because of
the type of tube used. These tubes have high gain and h
igh interelectrode capacitances. Eimac is fully aware of this and have wrote
numerous times about it in their literature and letters to some hams. I think
I'll stop there and let others comment more as that's about all I know to say.
Best,
Will
*********** REPLY SEPARATOR ***********
On 9/30/05 at 7:32 PM Bob Maser wrote:
>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@hawaii.rr.com>
>To: <amps@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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