X-Y Plots of Output Tubes

[loudthud]

I installed 1 ohm cathode resistors in the output tubes in my 5F6A re-issue. I used my 1000x scope probe to look at the plate voltage (WARNING! This will fry most scope probes) and fed that to the horizontal input of the scope. This is the same graph you see in tube spec sheets where they plot plate curves for various grid voltages. I sketched these in the first photo and showed where zero is positioned on the screen. I also plotted the 30W plate dissipation line. The next photos show the bias point, a well behaved resistive load, and a not so well behaved resistive load. Apparently leakage inductance can cause some deviation from a straight line.

The second group of photos shows operation under typical playing conditions, not extreme overdrive. I think all controls were set to 12:00 o'clock.

I'd like to encourage somebody else to try this, but only if you have the proper equipment. If you don't have a high voltage probe, make one from 2watt resistors, several in series to get the voltage rating.

[trobbins]

They are fantastic plots - an artform!

The 5F6A probably has a relatively 'stiff' screen voltage (given the choke and cap, and not huge screen stoppers). Maybe that is associated with not identifying a curved 'down' end for the good resistive load plot - as the plot indicates it has hit saturation voltage and then travelled up a bit in current as grid is pushed into 'conduction' zone (the 5881 datasheet curves certainly show that character). If the screen was sagging under load, then perhaps the anode current would perhaps hook the other way a bit. If you get inclined, maybe repeating the resistive curve at a lower frequency may show up some screen sag.

The 'intensity' of the screen persistance indicates the time spent in that area - so the time spent at seeming high peak power dissipation levels is effectively average down - that power chord plot on the LHS is reminscent of switching losses in a switchmode. The 'wacky' one would have some high plate dissipation - what did you 'play' for that one?

Do you think the negative voltage excursion in the 'wacky' plot is clipped by the screen edge, or just not easy to see? The positive voltage peaks seems to get to about 1-1.1kV (ie. maybe up to 800V from B+), and it sort of looks like it goes negative from B+ by about the same amount - which would seem appropriate.