Specifications

An important point of 2X loading performance with pentode and UL modes is not the actual power drop-off, as that is principally

a function Ohm's Law: The source generator can only develop so much output voltage, so if the load resistance attached to it is

doubled in value, then the power developed must fall off. This is true with any amplifier. The notations of the reduced power were

simply to show what it fell to.

The takeaway points are that with both pentode and UL operation, power was maintained as the load was reduced to 1/2 rated

load, but with pentode mode, the distortion rose notably, and particularly so with the 2X load condition, where as with UL

operation (as with triode), the rise in distortion with either 1/2 load or 2X loading was only modest under either of these

conditions. That the UL connection can basically perform as well as triodes do in this regard, yet do so while delivering pentode

levels of power output, is the real beauty of UL capability.

SUMMARY

While UL and pentode operation produce basically the same power response across the audio bandwidth and with respect to

loading conditions, UL operation ALSO shows the same INsensitivity to distortion with varying load impedance levels that triode

operation does -- while watt for watt, producing the lowest overall distortion of all three output stage configurations. This is the

beauty of UL operation, combining the best that the other two modes have to offer in one package.

In the ST-70 then, UL operation of the output stage with feedback would appear to be the best compromise, considering the

power, distortion, frequency response, and damping factor targeted for the unit. Triode operation can certainly be a viable

alternate configuration, with the advantage of that mode typically producing lower order distortion products, but at a cost to HF

response and power. Pentode operation of the ST-70 would not be recommended with EL34 tubes, although may prove

acceptable with 6L6 class tubes. This was not investigated for this project.

The Driver Board

Of all the criticism leveled at the ST-70, the driver board has surely absorbed the lion's share of it. Critics point to its quality of

parts, topology, lack of adjustments, the tubes it uses, board quality, and so on -- which is really unfair, because when this unit

was introduced, its design was considered state of the art in most of these areas. Today however, parts quality is far superior,

alternate topologies can offer greater consistency of performance, AC and DC balance controls are more important due to

production of lower quality output tubes, other more common and/or in production tubes are easier/more economical to source

for driver duty, and board quality has come light years from where it was in 1959. So there is room for improvement today to be

sure. But based purely on a model of performance CAPABILITY, the original driver board is still a very hard design to beat.

Getting beyond concern for its quality of execution and features then, what design based PERFORMANCE issues are

associated with the original driver board? Well, there are basically only two.

LF OSCILLATION:

Unlike the MK II & MK III designs that predated it, the ST-70 uses a resistor connected between the B+ source and cathode

terminal of the pentode input stage, to provide a measure of fixed bias for this stage to help maximize its gain. Along with this

Fig. 4: 10 kHz square wave in triode mode.

Fig. 5: 10 kHz square wave in pentode mode.

Fig. 4 10KHz Triode Mode

Fig. 5 10KHz Pentode Mode

Note: