> 1.) The 240VAC tap seems out because either way you
> will be hitting the amp with a 1.64% to 3.23% over
> voltage.
> 2.) I am thinking that the 245VAC tap would be the
> choice to make; the line voltage might be a bit soft
> at times of high grid demand (0.4% under-voltage) but
> that is awful close, 1.20% over-voltage of the 248VAC
> potential during those late light hours on the low
> bands shouldn't hit the amplifier to hard.
> 3.) The 250VAC tap also seems like a poor choice as
> either way the amp with a 2.46% to 0.80%
> under-voltage.
Jim,
I picked the transformer windings in the AL80 series of
amplifiers (and all Ameritron amps) other than the original
AL80 below S/N 300.
Here are the facts:
People are WAY too overboard about filament voltage. The
reason is they like to apply what is important to a 24 hour
365 day a year commercial operation to Ham radio.
With a BC station the tube is normally operated at a small
fraction of rated dissipation, anode and screen voltages,
dissipation, and emission currents. The filament and tube is
on and hot, and it almost never cycles. Since there is no
real thermal stress and the tube is loafing along at a
fraction of rated emission and dissipation, emission life
can be an important parameter.
With Ham radio, people complain about noise. They complain
about price. They complain about size. They also turn the
gear off and on (sometimes several times a day). The tube is
run at or beyond CCS limits, the airflow is at or near the
minimum value, and everything goes through big thermal
cycles. Many tubes are also not made as well as they once
were so far as seal quality and pumping down of the tubes.
The materials inside tubes are sometimes not as good.
Because of that, Ham radio applications are MUCH different.
You will virtually NEVER find a low emission tube in amateur
service compared to other failures like overdissipation,
bent or warped elements from handling or thermal cycling, or
tube manufacturing defects like seal leakage or outgassing
of materials inside the tube.
Like the tank Q of 12, some people just get way too dramatic
about making something that is down in the noise floor be
some *exact* value, and they often don't even understand why
they are using a certain number. For example, did you know
the voltage of transformers drops as they heat? How many
people caution you to set the operating voltage with a fully
warm transformer or filament choke?
The real truth is if the voltage on your 3-500Z is between 5
or 5.25 volts, or even if it is a little high, it won't make
a bit of difference in the life of the tube if you lower it
to some magical number. You won't see 10,000 operating hours
in amateur service, and you almost certainly won't have a
tube that fails because of low emission unless you run the
thing at 6 volts. If you keep the voltage somewhere near 5
volts it will be like 99% of all the other tubes that fail
and dies from something you really can't control.
The main reason you have taps on that transformer is to set
the HV. The bandswitch and tuning cap, because of cost
necessity in the Ham market, don't have a lot of headroom.
The most damaging thing is to let the HV get too high, so
the taps allow you to keep the HV at a reasonable value not
exceeding a safe margin.
The filament, if you watch the HV and follow guidelines,
will be right in the ballpark. This is why the manual tells
you to set the tap to the next HIGHER voltage UP from the
maximum voltage you expect to see.
You can measure the filament voltage and do whatever you
like (it's your amp), but the REAL common cause of problems
is letting the HV become higher than normal and the way you
prevent that is to follow the manual and set the line taps
on the transformer to the closest setting HIGHER than the
maximum expected voltage.
73 Tom
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