Collins 51S-1 Tube Receiver

 

6BF5 Audio Output Tube: Very Hot!

The "Hot-6BF5-Question" keeps popping up - as do projects for replacements - and as it is many years ago I posted the results, it is described here. My goal was to implement the original design choice, just to get rid of some problems. As I spent some time analyzing this matter armed by tube manuals, several 6BF5s by different manufacturers (it is still not an expensive tube) and various pieces of commercial test equipment, I thought this information might be helpful and save others time better spent.

The 6BF5 was designed to produce audio power with low plate voltage in TV sets and for this purpose has a powerful heater. Just keep this in mind when looking at e.g. 6AQ5s and the overall series/parallel heater balance in your radio.

The higher line voltages in the US today (the 51S-1 was designed for 115VAC, not 125V or so) have certainly also something to do with the overheating of the tube - but that's not the whole story. There is some additional food for thoughts: not only is the 6BF5 in this radio operated too close to - or even outside - the published limits but curiously the bias voltages marked on the circuit diagram do not compute when you take the actual resistor ratios... They will again be different when measured, as tolerances, aging and grid current by an overheated 6BF5 will change the values further.

My solution was to leave the unbypassed 39 ohm cathode (AF and DC feedback) resistor in place and reducing plate current by pulling 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 e.g. 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... As the tube was designed for TV receivers and usually had to work very long hours - perhaps more than your restored 51S-1 ever will - it should last a while. A 6BF5 works just fine and Collins had a reason to use them!