Re-posting an old music-electronic forum discussion from awhile back…
As promised, here are my attempt at drawing the composite graphs for the Ampeg SVT 6550A. Sorry the graphs aren’t annotated, if you have questions, please let me know. In any case, here is a brief description of what you are looking at:
Note: The following results assume zero power supply sag, zero voltage drop due to screen resistors, ideal OPT with purely resistive load. The actual circuit performance may be quite different.
– Class A and Class B load lines (light blue and dark blue), note the Class B line is largely covered by the Class AB1 load line;
– Class AB1 load lines (turquoise), you can see it extends to the saturation voltage of the other side (Min Ep);
– Point A – quiescent point at 700V, 16mA (nearly Class B bias!)
– Point B – Max Ia, Min Ep1, tube at saturation;
– Point B’ – the other tube at cut off;
– Point C – tube at saturation, corresponds to Point B;
– Point C’ – the other tube at cut off, corresponds to Point B’.
Ampeg SVT with 5K load (only one pair shown, there are three pairs in the actual amp):
Ampeg SVT with 10k:
From the above graphs, we can see that for the 5k load, Epmax = 1400-73.3 = 1326.7V and for the 10k load, Epmax = 1400-27.8 = 1372.2V, so not that much of a difference really. But the screen grid dissipation is a different story as I shown in my earlier post.
Originally Posted by bob p
Also found this at diyaudio. If you follow his formula, then the 6550A ought to be able to handle 660*2.5=1650V before arc-over occurs. Assuming the amp is working fine, then simply doubling the output load should not cause a catastrophic failure, unless the switch is done on the fly without any precaution, then it would not surprise me that something bad could happen. Talk about a large d(i)/d(t)!
Since I have never even work on an Ampeg, my take on the situation may very well be far from reality… so experienced designers and techs please feel free to set me straight.