Like many pieces of collectible vintage electronic gear, the 51J receivers are now over half-a-century old.
News Dead links corrected.Rebuilding the Collins 51J Series Receivers.
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.Ī 6BF5 works just fine and Collins had a reason to use them! Copyright hb9aik - vk2ani Base Design Copyright by edg3. 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 VDC. 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. This yields better performance and stability than e. 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. 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. The higher line voltages in the US today the 51S-1 was designed for VAC, not V or so have certainly also something to do with the overheating of the tube - but that's not the whole story. Just keep this in mind when looking at e.
The 6BF5 was designed to produce audio power with low plate voltage in TV sets and for this purpose has a powerful heater. 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. My goal was to implement the original design choice, just to get rid of some problems. 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.