On 9/24/2010 3:38 AM, W2XJ wrote:
> This discussion pertains only to poorly designed AMPS. There should be no
> voltage sag and the amps should be ccs at 1.5 kw out (in the US). If digital
> modes are contemplated, 5 to 10 KW amps in class A mode are necessary.
In that case I can't think of any to which it doesn't apply. Even home
brew amps with massive power supplies have some drop although not nearly
as much as most commercial amps.
Much depends on how much overhead you have. IF you want an amp that will
run cool at the legal limit all day long be it SSB or CW contesting, or
some form of digital you have very little choice as the amps that can
easily do that are unlikely to pass FCC approval. Very few US made amps,
or those imported that get the FCC's blessings have this overhead and
end up running at or near being maxed out which stresses most of the
components in the amp. Of course there are the typical 1200 watt out
amps in abundance that were designed for IAS (Intermittent Amateur
Service) with most having insufficient pate dissipation, cathode
emission, power supply capability, or any combination up to and
including all. Then the owner pushes it, trying to get 1500 out. This
results in over driving, current saturation, non linear operation, and
excessive plate dissipation leading to shorter tube life along with
other failures.
Plate dissipation is not a hard set figure for most tubes, but emission
is. The 8877 "is capable" of 2400 to 2500 watts out, IF and I emphasize
the IF it has enough air on it, AND the plate supply is beffy enough to
handle 2500/.60% = 4.1 KVA for continuous operation. This is making the
assumption that the system is really 60% efficient and does not take
into account losses in the transformer, or the ability to keep the
transformer cool. Now even keeping it to the legal limit we are now
looking at roughly 2.5KVA, again neglecting losses and being generous
about efficiency. At to plate dissipation, getting the legal limit out
key down only requires 900 watts plate dissipation at 60% efficiency.
However that 8877 is a relatively small tube that takes enough air flow
at a given back pressure to start getting noisy.
OTOH that extra plate dissipation is only available IF you have the
proper air flow. Most US made amps do not. They were/are designed for
the legal limit and even then there are many who complain about the
noise. Put the airflow up to give the 8877 1500 watts of dissipation and
it will be LOUD.
Now if you are willing to, or like building you can go to any one of the
larger tubes like the YC156, or the 4CX3000A7. Keep them even close to
the legal limit and they will be almost silent. OTOH don't forget to
put enough air on the seals as these take a wee bit more filament power
with the nearly instant on 4CX3000 taking something like 365 Watts while
the YC156 IIRC is 225. These are pretty hefty compared to the 100 watts
of the 8877. OTOH compared to the size of the 8877 both of these tubes
are BIG and heavy at between 8 to over 9" tall and 5.5 to nearly 10#.
They are also readily available as pulls for a fraction of the price of
a new 8877.
There are a couple alternatives for cooling with less noise as well.
Remote the fan and use a hose to get the air to the amp, or remote the
amp. Then there is "water cooling". Most of today's external anode tubes
can be modified to use water cooling which is, or can be virtually
silent and it doesn't take a lot of flow to handle the legal limit.
OTOH don't forget the tube seals still need air on them.
73
Roger (K8RI)
<snip>
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