Higher plate resistor values

[Rhodesplyr]

As I've worked on more amps, I've noticed that it wasn't uncommon for designers of the late 1940s and 1950s (Gibson, Premier) to use fairly large value plate resistor values for 6SC7/6SL7/12AX7 gain stages, from 330k to 470k. In later amps, you almost always see 100k or maybe 220k.

In listening to these earlier amps, they are definitely colored rather than clean, but colored in a way that I tend to like. My question is: What circuit effect am I listening to? Is it the output impedance of the stage interacting with the next stage?

As one example, on the Gibson GA30 schematic, you see 100k plate resistors on a 6SC7, but in an actual 1953 GA30 I just worked on, the plate resistors were 470k.

[Jazz P Bass]

The plate resister typically goes hand in hand with the cathode resistor to set the gain stage.
What are the cathode resistors on the 470K plate circuits?

[Rhodesplyr]

The plate resister typically goes hand in hand with the cathode resistor to set the gain stage.
What are the cathode resistors on the 470K plate circuits?

It varies.

The Premier 120 uses a contact-bias (aka grid leak) input stage with 330k on the plate and a grounded cathode. The paraphase PI uses a 2.2k cathode resistor on each AX7 section, with 470k on each plate.

The Gibson GA30 has a 6SC7 paraphase PI with a 1.5k shared (as all 6SC7s) cathode resistor and 470k on each plate.

[g1]

Maybe look at where the plate voltage sits in relation to it's B+ node. You may be liking a tube biased closer or further from cutoff, which will affect whether it clips more on the top or bottom at higher signal levels.
As Jazz mentioned, it will depend on the cathode resistor, but off the top of my head, I think stages with higher value plate voltages are usually biased more toward cutoff.

[J M Fahey]

Agree and add: higher than normal plate resistors are a way to get a little more gain from a stage, but that coupled to next stage (or stray) capacitance rolls off highs.

Remember old amps are usually "smooth" or "mellow" or "dark".

This is one reason why.