[AMPS] Perfect Amplifier

[Phil Clements]

At 12:06 PM 9/1/97 +0100, you wrote:
snip.....
>Looking at Rich's points:
>
>>IMO, it would be feasable to produce an amplifier that had:
>>1.  2mS - 3mS switching.
>>2.  Rheostat-adjustable filament-potential.
>>3.  Filament voltmeter function on the multi-meter.  
>>4.  Low VHF-Rp parasitic suppresson.   
>>5.  Step-start.  
>>6.  Tank circuit with appropriate RF current rated components.  
>>7.  Good input swr with any transceiver.  
>>8.  Glitch protection.  

IMO, number one on the list with a ceramic tube amp is grid-trip protection.


>#1.  2mS - 3mS switching. 
>
>Some of the smaller vacuum relays are quite OK at 1Kw plus - carrying,
>NOT hot switching. However, they are abt $75 in the US. A few years ago,
>they were available here at $7.50 (yep seven dollars fifty cents!)
snip...

Not too bad in the Colonies any more...$25-$35 @ flea markets and
re.radio.swap.
A good point was made by Rich. Make sure you hi pot test these "bargains" to
see if a vacuum still exists, and the contacts have not been crispy-crittered.
The DC rating of a vacuum relay is much much less than the RF contact rating.
The DC will cause a big bang and burn up the coil. 4000 volts @ 1 amp will
do this readily. A ham that used to work at Jennings said they had a slogan
when shipping deadlines were getting tight on vacuum relays...."Glip it and
ship it"....referring to using glyptal to patch a flaw. I am sure Kilovac
and the others had similar policies.

>Question: has anyone been really successful with PIN diodes?
>
>I've got 10 off 1000volt 2 microsecond PIN diodes on insulated stud
>mounts, rated a 1A DC, and 10 of the same wire ended. One day, I'll see
>what can be done. Interesting that W2VJN's company is avoiding PINs
>because George is dubious about the reliability under amateur
>conditions.

I have heard of great success in QRO mods by WB3EEU, who substitutes
Macom 4P709-487 diodes for the smaller ones used by Ameritron and the now
defunct Design Electronics. This makes a 3KW or 5KW (with blower) unit out
of a marginal 1500 watt box. I called Ameritron a few years back when a QST
article challenged them to use the Macom diodes, but they had no plans to do
so at that time. Too bad; this limits most QRO amp builders to vacuum relays
which are 5 times slower than PIN's on a good day.


>
>#2.  Rheostat-adjustable filament-potential.

There is a plentiful supply of 2.25 amp variacs on the flea market circuit
now in like-new shape for around $10 each. I use them in all my projects.

snip...........there, the rheostat is better. I also argue
>that step start on filaments is no bad thing either.

When using the variac, I add a 10-15 ohm resistor between the variac output
and the filament transformer. It stays in the circuit all the time and does a
nice job of inrush protection with no moving parts. A $1.00 part protecting
a very expensive tube(s) just makes good sense whether Eimac thinks so or not.
>
>#3.  Filament voltmeter function on the multi-meter.
>
>Amen to that. It's not a difficult or costly thing to do, either.

Of course this is a necessity with adjustable filament circuitry.

>#4.  Low VHF-Rp parasitic suppresson.   
>
>I guess we all have our own opinions on this, and its implentation, but
>the basic principle (as I understand it, the parasitic suppressor should
>have a low Q at VHF) is agreed.

Eimac is rewriting "Care and Feeding".....the section on this subject should
answer many questions about designs for the newer tubes.

>
>#5.  Step-start.  
>
>An alternative that can be very cheap after a good flea market, is a
>motor  (or even hand?) driven Variac. Otherwise, I would say that step
>start is a sine quae non. For production, it isn't that expensive,
>either.

The use of variacs with capacitor input power supplies is not good engineering
practice. It can reduce a nice, stiff 240 volt line to mush in many cases.

>
>#6.  Tank circuit with appropriate RF current rated components.
>
>This surely comes down to deciding what the application is. If you want
>RTTY at 1500 watts for half an hour or more, then it's very different to
>non compressed  speech with one minute messages every hour. If you want
>a contest amplifier, then you're better off with the RTTY approach.

IMO, this is the most flagrant violation of good engineering practice in
today's "store bought" amps. Tank circuits that were used in 500 watt amps
15 years ago are now passed off as 1500 watt units. 1/4" tubing on 10-20
meters, tapering off to at least #10 wire for 40-160 meters should be a bare
minimum. A 100% duty cycle 1500 watt amp just cannot be tucked under one arm
and walked away with, and it is time the newcomers were educated about this.

>
>#7.  Good input swr with any transceiver.
>
>I must admit I don't understand this. If the input SWR is good
>(<1.5:1?), then why should the driving transceiver come into it? Why
>should the input impedance be dependent upon the driving transceiver? Or
>have I missed something?

What a lot of folks do not realize is that the new broad-band solid state
transceivers do not have a 50 ohm output across all nine bands. The power
ratings are "into a 50 ohm load". A variable tuned input with controls
brought out to the front panel would be convenient and produce a cleaner
output.

>
>#8.  Glitch protection.  
>
>I assume that by this, Rich means the 'spike' of energy of much higher
>level that the transceiver can produce before its own ALC grabs the
>output.
snip....... 

What Rich is talking about is "big bang" HV arcs that blow plate IC meters
across the room and do other terrible things. Solid state circuitry is now
available at a reasonable price that will limit anode current, shut down the
AC mains, and begin discharging the filter cap. at a very fast speed.

snip.......
>
>From all this, I think it's easy to see that there are two approaches
>for most of these points. As I haven't yet seen anything available which
>will allow me to get an amplifier which meets my additional criteria for
>the perfect amplifier, which is a cost of less than 10 cents per watt
>output, home brew is the answer. Besides, I like building things! It's
>the only chance I get to do real radio engineering these days - work is
>meetings and typing on a computer!

AMEN, Peter...do not look in a ham store or catalog for any answers! I like
to "roll my own" also...apparently our standards are a bit higher than the
current state of the art!
(((73)))
Phil, K5PC

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