[Collins] 75S-3B audio output tube

[Garey Barrell]

Jim -

This question has been batted around for a LONG time, MANY times!!  :-)

Here is a good description from January 2000

+++++++++++++++

A follow up on the use of a 6AQ5(A) in place of the 6BF5 in the 75S-3B.

I discovered as I was doing some tube swapping that the cathode resistor was open
and ended up having to fix that along the way.  Another good reason to get that
darned hot 6BF5 out of the receiver!  What I finally did was to put a 3-lug
terminal strip on the screw for the tube socket that is farthest away from the
back of the receiver.  I've used it to anchor cathode resistors and the additional
330K (I used 350K which I had) resistor from grid to ground as well as a cathode
bypass capacitor.  The way I've put them in it's very simple to change the
configuration back to the original electrical configuration with the use of the
soldering iron at only one point.

I "split" the 47 ohm recommended cathode resistor and put in two 22 ohm (I
selected high valued ones as I had no 27 ohm resistor) in series so that the
entire amount of resistance can be bypassed or only one of the two resistors if
one wishes more negative feedback to improve fidelity.  I used a 22 mfd bypass
capacitor which I also had available and chose to bypass the entire cathode
resistor to get as much gain as possible.

I made these DC measurements to ground with the 6AQ5A in the circuit and audio
gain at minimum.

pin 1,7    -6 V (control grid)
pin 2        1 V  (cathode)
pin 5    183 V  (plate)
pin 6    144 V  (screen)

If you look at the characteristic curves for the tube, 7 volts of grid bias and
imagine a load line with the 190 volt B+ available it looks reasonable.

I used an inexpensive Leader audio generator set to 750 Hz and the signal applied
through a .01 mfd capacitor to the high end of the AF gain control pot.  I made
signal measurements with a Tektronix oscilloscope.  The waveform measurements are
purely subjective and done by eye!

---------------------
With the 6BF5 in the original Collins circuit and a 4 ohm resistive load in place
of a speaker I measured these APPROXIMATE AC values:

Output across 4 ohms backed off from when top and bottom of the waveform appears
to distort/flatten 2.5 W.
Voltage gain from the audio pot SLIDER to voltage across the 4 ohm load about 12X.

-------------------
With the 6AQ5 in the circuit as described above with the cathode completely
bypassed:

Output across 4 ohms backed off from when top and bottom of the waveform appears
to distort/flatten .5 W.
Voltage gain from the audio pot SLIDER to voltage across the 4 ohm load about 7X.

===========
REAL WORLD
===========

The 6AQ5 in the original circuit sounded fine to me even with no circuit changes
but who said it's HI-FI?  There appeared to be plenty of audio to listen to but I
didn't listen for long.

Right now I'm listening to the receiver upside down on the bench with a cheap
Radio Shack small "mobile" speaker in a plastic housing attached.  I have the
audio gain control at approximately 8 O'clock and listening to SSB signals on 20
meters that peak on the meter at about S7 (I have no decent 20 meter antenna).   I
have adequate audio when 10 feet away and though my "good ear" is in the direction
of the speaker as I type here and I definitely have some hearing problems, it
sounds plenty loud to me.

I think the 6AQ5 will work fine and if not, it'll be easy to return the circuit to
the 6BF5 configuration.

Ian, K6SDE

+++++++++++++++++++++++++++++

Here is one source in the US for the 6BF5

<http://www.esrcvacuumtubes.com/vacuumtubes_tubelist_tubes_6A3-6CZ5.html>

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>



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
>