[Collins] 51S-1 and 6BF5

[Ulrich N Fierz]

Hi all
  the 6BF5 question keeps popping up and as it is more than 6 (?) years ago I worked on it and posted it to the CCA list I'll do it again here. The higher line voltages these days have certainly also something to do with the overheating of the tube - but that's not the whole story.

Here my edited original comment:
  
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 too close to or outside the published limits in the 51S-1 (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 (often) draw grid current.
  
My solution was to leave the unbypassed 39 ohm 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 in the grid bias circuit (starting with
approx -37VDC).
  
Now the 6BF5 is operated well within its limits - still getting hot of course... However the tube was designed for TV receivers and there usually had to work long hours, perhaps more then your restored 51S-1 ever will.
  
As I spent some time analyzing this matter armed by tube manuals, several 6BF5s by different manufacturers (which still is not an expensive tube) and various pieces of commercial test equipment I thought this might information be helpful and save others time better spent. 

Thank you for the bandwidth and again I hope this proves helpful to the operators of e.g. 51S-1 receivers.

  73 and keep them working - a 6BF5 works just fine and Collins had a reason to use them!
  Ulrich  hb9aik - vk6bqd
   
  
 


Ulrich N Fierz
Telecommunication Engineering Services
Zurichbergstrasse 203
CH-8044 Zurich
Switzerland
<hb9aik at yahoo.com>