[SixClub] Question on Amplifier

[K8RI on six]

On 2/15/2011 8:03 PM, Frosty wrote:

Arggg! I spent all this time composing this and just found the Internet 
has gone down!

Sorry about the dissertation<:-))
> I would like to take a survey of the 6 met
> r and vhf operators on the following:
>
> WHAT WOULD YOU LIKE IN A 6 METER AMPLIFIER?
Rugged, reliable, stable, clean AND economical with a short warm-up!
> Power out:
> 1.500 watts
> 2.1000 watts
> 3.1500 watts
None of the above!   I want something that will run 1500 out while 
contesting and digital 24X7 which means it has to have enough overhead 
(surplus capability) in both the tube and power supply to run close to 
double  that. IOW to run 1500 reliably without over heating, or 
shortening the life of the tube it needs to be capable of close to 3kW.
A tube to run 1500 out must have a *minimum* of 900 watts dissipation 
(at 60% eficent) which covers most of the common power tubes for ham 
amps, but even a kw of dissipation is very near the limit.

The power supply at that same 60% must be capable of providing 1500/0.6 
or 2500 watts CCS, not intermittently.
I'd much rather see a bunch of "Kalifornia Kilowatts" running 
conservatively than many of the old amps rated for 1KW DC/2KWPEP input 
(bout 1200 PEP out) and a lot of today's smaller amps being pushed for 
all they are worth.  Many of those old amps didn't have the best IM 
products, but then again neither did our transmitters back in those 
days.  Many of today's smaller amps get pushed for far more than they 
are worth.  Actually a few of them are rated by the manufacturer for 
more than they are worth.  At-any-rate, I dislike seeing, or hearing, 
marginal amps being pushed.

Particularly on 2-meters, RF exposure limits can become a problem. With 
stacked 12's at 130 feet I'm limited to a continuous 380 watts because 
of the distance of my nearest neighbor who's house is about 175' to the 
South of me. Of course figuring duty cycle for SSB I can run very close 
to the legal limit.  FM? Not so good. Digital, unless short bursts, not 
good at all.

But by default I have to vote for #2.
> Drive:
> 1. 25 watts
> 2. 100 watts
There are problems with many of today's rigs running reduced power. IE, 
a 100 watt rig running 25 out.  *Some* and I have to emphasize that only 
some have a leading edge pulse that will hit the amp with over drive. 
These rigs need a pad between the exciter and amp so the 100 watt rig 
running 100 out will drive the amp with 25.  Often you hear an easy to 
drive amp sounding like...crap because of the way it's being 
driven/operated.  People blame the amps for the keying pops and splatter 
rather than the rigs.

BUT if some one can afford a good, rugged, reliable, clean amp they 
should be able to afford a good, clean, rugged exciter.

So by default I vote for #2
> Futures:
> 1. built in P/S
REAL legal limit amps with build in Power supplies are real hernia 
builders, tipping the scales at or close to a 100# (or more).  I have 
one for HF.
Here, I'd have to vote for an external PS unless some one comes up with 
a good reliable switcher.

Questions 2 and 3. are we talking just for 6 meters.
> 2. one input
I'd vote for #2 with a pass though capable of the legal limit 24 X 7.
IOW you can have an HF amp feeding though a 6-meter amp or the 6 meter 
amp feeding through the HF amp.
> 3. one output
Possibly 2 inputs, but definitely 2 or more outputs. I have both tuners 
and dummy loads.
My system is set up so I can (on HF) set the tuner using an antenna 
analyzer, tune the amp into the dummy load, and then switch the amp 
through the tuner to the antenna without ever putting more than a tiny 
signal into the antenna instead of tuning up on the air.  We've all head 
the station or stations trying to squeeze the very last watt out of 
their amps.  Most of those amps are not meant for that kind of 
service/abuse.  On VHF and more so on UHF this an get a bit complicated 
or messy.
> 4.simple metering
> 5. Solid state/ Transistor.
There is a lot to be said for either approach besides nostalgia.  A 
*good* SS metering system is almost analog as it's  not just one segment 
and then the next. The next segment slowly gets brighter as it gets more 
signal.

So again by default, I'd vote for #5

> What about 2 meters and 440?
Definitely 2 meters be it a dual 6 and 2 style amp with a pair of 
3CPX800s or a pair of Chinese FU728F tubes. Unfortunately the FU728's 
are not rated for 2 meters, BUT I think derated they'd still work in the 
classic 6 and 2 amp.  For 6 and 2 I much prefer separate amps. Be 
warned, a pair of these hummers can run 5KW PEP at full power.

440?  Not much reason. Here in Lower Michigan the power limit is 50 
watts. Although...you can get special permission for EME but the 
antennas must stay pointed 20 degrees or more above the horizon.

You didn't mention "auto tune" but on 6 (or two) I hardly find that a 
necessity. It does create a sweet spot in the hearts of those who have 
used them on HF though.

Now as to the real world. On HF I currently run an Emtron DX-2sp which 
will do a nominal 2KW/2.5KW PEP out and lets the FU728F loaf along at 
1500 out.

BUT I've started an amp project. Initially it will use separate RF decks 
from the power supply(s). Eventually it may consist of self contained 
consoles but with remote control heads with the HF amp(s) eventually 
being  auto tune.

The original question eliminates solid state, but you can now build a 
relatively rugged solid state KW for a very reasonable price.  That next 
500 watts gets expensive and requires a lot more protective circuitry.
The same for instant on!  Thoriated Tungsten filaments are nearly 
instant on, but are likely to require 2 or 3 times the power of an 
indirectly heated filament.

The question of Triode Vs Tetrode did not come up but can be essential. 
  There are more Triodes available than Tetrodes.  Most Triodes are 
simple, easy to drive, and will work well through 6 - meters and some do 
well on 2-meters. They often require a tuned input for a clean signal 
while the Tetrodes can use a passive input and require very little drive 
with LOTS of gain.

If home brewing there are a number of Triodes and Tetrodes available as 
pulls although most of them are a bit more hefty than most of us are 
used to.

My amp project uses 4CX3000A7s.  These tubes will work well right up 
through 2 meters. They can be cooled with a muffin fan at the legal 
limit in SSB and CW although I'd go for a bit more air for contesting 
and digital.
My goal, which will probably never be finished, is for an amp on HF, 
6-meters, and 2-meters with each built around the 4CX3000A7.  I like the 
3CX3000A7 but it will not go to 2-meters.

There are many tubes still available as pulls that are in good working 
order.  They are available tested from several sources.  I'm hesitant to 
purchase tubes off e-bay, but have done so without getting burned so far.

If you are going to brew your own there are many older tubes that should 
not be written off either.
Unfortunately many articles on amplifiers use tubes constructed of that 
rare element "unobtanium" such as the 4CX1600B for 6-meters in the ARRL 
handbook.  It's a great tube, better than the 8877 as far as I'm 
concerned. Unfortunately IF you find one and build around it, you may 
not be able to find a spare.

Amps like the MLA2500 and others using the tubes from the 8873 
(conduction cooled), 8874 (axial cooled), and 8875 (transverse cooled) 
are becoming available at reasonable prices due to the very high prices 
of the tubes. finding the 8873 and 8875 is almost impossible.  However 
the GI7B tube can be modified to fit some of these and as it uses a 
cooler than can be removed it's not that difficult to build another 
cooler that will work for transverse cooling making another great amp to 
convert to six.  8877's and 3CPX800A7s are available as pulls as are 
3CXa nd 4CX3000s.  the most popular tube at present for home building is 
the YC156 as it does not require a socket, has reasonable filament 
demands other than it has a very long warm up time, and can be found for 
$250 to $350 tested. The only drawback other than the long warm up time 
is this tube is BIG at roughly 9" tall, 5" in diameter, and a weight of 
just over 9#. OTOH it is basically a 4CPX5000A7

If you don't mind BIG the Glass bottle (GU81M) will work on six at 
reduced ratings and the GS35B will make a whale of an amp as well.

OTOH it's difficult to build an amp that works well and looks good for 
less than the price of a new one on the market...UNLESS you have 
connections, are good at scrounging, and very good at long range planning.

73

Roger (K8RI)
> Charles F. Frost
> Frosty K5LBU
> frosty1 at pdq.net
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