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[Amps] Which Amp

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Subject: [Amps] Which Amp
From: (Steve Katz)
Date: Mon, 26 Aug 2002 15:59:21 -0700
Re "foolproof" amplifiers, in this particular case I'd recommend an old
Alpha 78.  Haven't been made in years, but therefore very reasonably priced.
They're no-tune and cover 160-10 meters.  Replacement tubes are very
expensive, but with small modifications the amp can be converted to more
commonly available tubes including 3CX800A7s (using two, instead of three
8874's), and if a used one with very good condx tubes can be found, it may
well last another ten years or more before the tubes require replacement.
The tubes in the 78 loafed at only about 750W dissipation total under normal
operation, and they're rated for 1200W total.  I had the original set of
8874's in one purchased in 1980 or so and sold in 1998, eighteen years later
-- still running full output, no signs of wear.   The amp makes >1kW PEP on
each band in the "no tune" (bandpass) mode, and is difficult to overdrive
and damage, if the input padding circuit is set to the "high drive" tap
(which takes about 150W drive power for normal operation).


"If everything seems under control, you're just not going fast enough." -
Mario Andretti

> -----Original Message-----
> From: 2 []
> Sent: Monday, August 26, 2002 3:40 PM
> To:   Nathan, Kevin; '';
> Subject:      Re: [Amps] Which Amp
> >Hello All,
> >
> >In the past, I have gotten some good advice from this list so I hope
> those
> >of you on here can do it again.  <GRIN>  I am totally blind so please
> keep
> >that in mind during the request that follows some brief information.  I
> do
> >have very accurate means of measuring power and for achieving the correct
> >dip and load on my amp to tune it up so these are not considerations.
> >However, I do not have a means of reading meters on the amp itself.
> >
> >Last December, after briefly trying a Kenwood TL-922 which had some
> >problems, I reviewed a mess of information on the internet and finally
> >purchased an Ameritron AL-811H.  I felt this would provide me with the
> power
> >level I wanted to use and seemed to be the best buy for the money.  Now,
> 8
> >months later, I have a serious problem.  The other night, while I was
> >operating on 20 meters, the amp gave a mighty bang followed by almost a
> >buzzing sound.  I turned off the power immediately and when I tried to
> turn
> >on the main power switch later, I found both fuses were gone as well.  I
> >pulled the top cover to have my wife check for anything obviously
> blackened
> >or burned such as a filter cap in the power supply but there is nothing
> like
> >that and no smoked smell.  However, two of the four 811's are rainbow
> >colored inside.  The technical staff at Ameritron tells me that from the
> >description, these tubes are shorted and probably took out the grid
> >resistors 
> A grid-filament shorted tube does not blow grid the resistor because 0V 
> from grid to cathode makes 0mA of grid current.  A grid-filament short 
> causes the tube to draw some idling current on standby because there is 
> 0v bias instead of the usual c. 25v cutoff-bias.  
> > gris cdoesfor them and the protection diode on the power supply board.  
>  It takes over 30 amps to short a 1 amp diode.  This can not happen from 
> a grid-filament short. 
> >They
> >are not wanting to cover repairs because they say I have over driven
> these
> >tubes and this is not a warranty item.  
>  20W of drive per 811 cathode will normally drive them to the full rating 
> of 175mA per tube.  How much drive did you apply?
> >He kept emphasizing over and over
> >again that you must monitor the grid current on the 811's very carefully
> >which, of course, I cannot do.
> >
> He was ''talking through his hat'' because a blown grid resistor is one 
> symptom of an intermittent parasite.  Tuning a g-g amp correctly is done 
> by tuning for max out with max drive.  When the output is tuned and 
> loaded for max, output, most of the electrons are flowing to the antenna, 
> and grid current is ok.   Another symptom of a parasitic oscillation is 
> greatly changed resistance of a parasitic suppressor resistor.   To 
> measure such, one side of the resistor must be unsoldered from the 
> suppressor's coil.  
> -   A friend of mine in Simi Valley tested the AL-811 prototype for a 
> magazine column he writes.  After about 1/2 hour of use, the amplifier 
> banged and the filaments went dark.  He brought the kaput amp over to my 
> garage for a pow-wow.  The 811s sustained broken filaments during the 
> bang.  My guess is that a sudden current and the resultant Electro 
> Magnetic Force pushed the filament wires sideways, causing them to 
> shatter and fall in pieces to the bottoms of the envelopes.  I sniffed 
> around the AL-811 with a dipmeter.  I found a sharp dip c. 95MHz at the 
> DC bloclking cap.   This suggested that the parasitic suppressors have a 
> high VHF-Q.  Decreasing the VHF-Q could have been realized by either 
> using resistance-wire instead of copper-wire or by adding inductance to 
> the suppressor coils.  The trade-off with adding inductance is that the 
> dissipation in the suppressor resistor increases exponentially on 10 
> meters as a result.    However, if a suppressor resistor burns out, the 
> VHF-Q skyrockets and a parasitic is likely.  [see "Calculating Power 
> Dissipation in Parasitic Suppressor Resistors". *QST*, March 1989.]
> >I have someone locally who will look at the amp and fix any blown
> >components.  But now I need to make a decision.  If the grid current in
> >these tubes is such a major thing, 
> Grid current becomes a major thing only during intermittent VHF parasites 
> or when some schmuck tunes the amp for max grid-current instead of max R. 
> F.  out. 
> >I want to sell this amp and find
> >something that is more bullet proof for me as a blind operator.  So, here
> is
> >my question.  What is the most forgiving amplifier out there that would
> give
> >me 600 watts to 1Kw output power, not be so critical about grid current
> and
> >cover 160 through ten meters?  Price is also a definite factor here since
> I
> >will most likely take a beating on this amp when I sell it.  I am
> >particularly interested in amps running a single 3-500Z or similar but
> >really need some advice here.
>  3-500Z amplifiers are not totally immune to parasites between 90 and 150 
> megahertz.  Virtually any amplifier on the market could be made less 
> squirrelly if the owner is willing to make do with c. 2% less power out 
> on 10 meters --  which is done by lowering the Q of the VHF suppressors.  
> If you are interested, I will send you a freebee suppressor retrofit kit 
> in the interest of HF amplifier science.
> >I thank you for your feedback and assistance.  I am not signed up for
> amps
> >at home yet so please address your replies with a cc to my home address
> of
> >  73
> >Kevin, K7RX
> >
> >Kevin Nathan, Independent Living Coordinator
> >Dept. of Services for the Blind 
> >3411 S. Alaska
> >Seattle, Wa 98118
> >Voice:  (206) 721-6450
> >Cell:  (206) 604-4767
> >Toll Free:  (800) 552-7103
> >Fax:  (206) 721-6403
> >
> >_______________________________________________
> >Amps mailing list
> >
> >
> >
> -  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
> end
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