[Top] [All Lists]

Re: [Amps] AL-80B transformer tap

To: "James M. Daly" <>, <>
Subject: Re: [Amps] AL-80B transformer tap
From: "Tom W8JI" <>
Date: Mon, 23 Jul 2007 09:53:25 -0400
List-post: <>
> 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.


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

Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>