[Collins] 6BF5 audio

[Ulrich N Fierz]

Hi all,
this issue keeps coming up regularly in S-Line radios and 51S-1 and most of
the important issues were adressed by Jerry and others. High line voltage
(i.e. more than 115VAC) is another cause.

The tube runs hot yes, but very often too hot indeed. Some time ago I went
into this systematically and came out with comments and a solution for the
51S-1. This approach is however applicable to all those 6BF5 circuits. What
it amounts to is to check bias and set it according to the book, replace the
coupling cap and reduce screen voltage by connecting a resistor to ground.
That takes care of excessive plate dissipation (also due to higher plate
voltage) and maintains the proper audio characteristics. The original text
ran as follows:

Hi 51S-1 owners
almost everything has been said but for the curious
there is some additional food for thoughts: not only
is the 6BF5 operated close to or outside the published
limits (higher AC voltage may be an additional cause)
but the bias voltages on the circuit diagram do not
compute when you take the resistor ratios... They will
again be different when measured as an overheated 6BF5
might draw grid current.
My solution was to leave the unbypassed 39ohm cathode
(AF and DC feedback) resistor in place and pull the
screen grid voltage down by connecting a 27kohm (2W)
resistor to gnd (there is a convenient tie point on
the turret nearer the p.s. circuitry). This yields
better performance and stability than increasing the
screen grid resistor only.
I also replaced the original screen grid resistor with
a higher wattage type and the control grid coupling
capacitor (which might or might not be leaky).
With a good 6BF5 there is no grid current and the bias
voltages measured should track what is supposed to be
there according to the dividing ratios (starting with
approx -37VDC).
Now the 6BF5 is operated well within its limits -
still getting hot of course...
As I spent some time analyzing this matter armed by
tube manuals, 6BF5s (which still is not an expensive
tube) and various pieces of test equipment I thought
this might be helpful.

I hope this helps, my receiver has been ok ever since and not blown any more
6BF5s.

Best regards to all
Ulrich Fierz  hb9aik - vk6bqd
Ulrich N Fierz
Telecommunication Engineering Services
Dunantstrasse 9
CH-8044 Zurich
Switzerland