Screen voltage compared to Plates

[B Ingram]

Sorry to again be the dude that's pushing back...

... much of the fundamental “personality” of the amp can be distilled through more detailed design of the power section itself (assuming you get to turn it up enough to actually push the power section). Considering the type and “temperature” of bias (fixed, cathode, combination), where a PP amp will live between the extremes of Class B to Class A, and the B+ voltage regime for the power tubes will fundamentally influence the feel and tone of the amp, right? ...

I wouldn't say it like that. Rather, "the interaction of the power section and the power supply fundamentally influence the feel of the amp."

Tone is a tricky thing, and is often a byproduct of how other design-choices interact.

... It seems the power section often draws the short straw in design. ... we have become indoctrinated as “slaves to the knee” by vintage priorities. ...

I disagree on that one as well. Though perhaps the power section gets the short straw in design on amp forums.

When a designer is actually paid money to design an amp for production, the first thing to be sorted out is "power section design." It's essential to solving the problem of, "how hard will we push the speaker(s), to get how loud?" It also sets the requirements for what is needed from the phase-inverter/driver. And the choice of whether to add negative feedback around the power section further shapes, "How much drive-signal to the phase inverter is required?"

The power section design defines the demand placed on the power supply. The follow-on decision of how to design the power supply affects how the amp will feel when played loudly.


As for "slave to vintage priorities"... There are only so many output tube types available, and only so many power & output transformers. Manufacturers don't make (enough) profit making a completely-unique product each time, so there are a few standard choices available whose parameters were figured out in designs in (mostly) the 1930s. We can depart from the tried-and-true designs, but mostly we will just get "less" from the result. Like taking a modern car and modifying it to get "Less Gas Mileage" and "Slower Acceleration."

... manipulating screen voltage ... affects touch sensitivity, stiffness, squish, compression, and distortion characteristics of the power section ... Manipulating Eg2 for the chosen OT primary Z, we can ... influence our amp’s ... harmonic content by dictating the position of our design LL above, at, or below the knee. ...

I don't see the screen voltage doing any of these things directly.

Screen voltage will change plate current, which might create a need to change G1 bias voltage, which changes the "power sensitivity" of the output tube(s). That's a real thing, but sounds like "my preamp is now too wimpy" when tried in an actual amp.

Designs with high screen voltage can have high plate current peaks, which can pull momentary high current from the power supply. If the power supply is also under-built, that can create sag & compression. But only when the amp is played loudly.. So "screen volts" alone didn't impart stiffness/squish/compression.

• You could leave the screen voltage alone an just install a 10kΩ screen resistor between the filter cap & socket-pin. No change to the power supply or idle screen voltage, but you will definitely hear some sag when the amp is turned up.

And changing screen voltage is less-effective at changing whether the loadline is above/below the knee than just changing the output transformer (or better yet, doubling/halving the speaker load attached to the OT).