[Collins] 75S-3B audio output tube
Dr. Gerald N. Johnson
geraldj at storm.weather.net
Fri Dec 12 21:40:05 EST 2008
On Fri, 2008-12-12 at 16:31 -0500, Jaime Gomez wrote:
> The output tube (6BF5) in my 75S-3B shorted out, I need to replace it.
> I have been checking some sources but cannot find it. I saw that it is
> similar to the 6AQ5, base connections are the same and the 6AQ5 can
> use higher plate and screen voltages. Can I replace the original 6BF5
> with a 6AQ5? Any suggestions?
> Thank you, 73,
>
> Jim, HK1N
> Barranquilla, Colombia
> Tel: +57 312 6642680
>
I know more than one S-line has been used with a 6AQ5 in the output tube
socket.
>From my tube manual (at least the first one I've consulted), while the
6AQ5 can stand higher plate and screen voltages, it also draws more
current and with the right output transformer puts out more audio power.
Be sure to check the bias on the grid pin, it should be -11 volts
according to my 75S-3B manual, it can be down because of a leaky
coupling capacitor (not a common problem with a ceramic, but possible)
or a bad electrolytic in the bias side of the power supply or a resistor
that has changed value from age, humidity, and tube heat.
The 6BF5 certainly does run hot for producing only 1.9 watts of audio.
I will check more tube manuals with more than one line of data for each
tube and try to see what other problems there might be. My first manual
says the grid voltage on the 6AQ5 with 250 volts on the screen is going
to need to be -12.5 volt, a little more than the 75S-3B will provide,
but at the lower voltage, it may actually need less bias to draw enough
plate current to produce full audio output.
I'm sure a search on S-line receivers or 75S and 6AQ5 will turn up many
discussions on the topic, and a few web pages. I might try that later
too.
--
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer
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