question about plate voltage to V1

[joesatch]

is there a difference in results if for example V1a plate is 150v from a 300v B+ into a 220k plate resistor vs 150v at the plate from a 350v B+ into an 330k plate resistor?  Plate voltage is the same but derived differently

[kagliostro]

Go here and play

https://www.ampbooks.com/mobile/vacuum-tubes/12AX7/

Ciao

Franco

[66Strat]

The RCA RC30 Receiving Tube Manual provides operating points for various plate voltages that would be a good starting point for experimentation.

See attached data and circuit symbol definitions for the 12AX7.

[joesatch]

i believe my question is being misinterpreted. not having an issue calculating plate voltages. i more want to know the difference between increasing the plate dropping resistor vs lowering the B+ influence on tone/feel with the plate voltage at the tube being equal.

[bmccowan]

Sorry, but I am going to do the same. Merlin Chapter 1 especially the parts about the plate resister & load. http://www.valvewizard.co.uk/gainstage.html
If I fully understood it, I'd try to explain it. But...

[shooter]

I'll do the quick n dirty;
as long as V1 stays in Class A (no distortion) the effect on the signal is minimal at best


IF, V1 moves into compression/clipping, then where the idle point (idle bias) is will have an effect, what effect and describing it is like describing the color blue, not gonna happen without lots of real-world examples - tinkering and tweaking.

[66Strat]

i believe my question is being misinterpreted. not having an issue calculating plate voltages. i more want to know the difference between increasing the plate dropping resistor vs lowering the B+ influence on tone/feel with the plate voltage at the tube being equal.

You're right. I don't understand what you're asking. What are you calling the "plate dropping resistor"? Also, there is more information provided in the 12AX7 operating points than just plate voltage. Columns Eo and V.G. indicate the voltage swing before clipping and the voltage gain of various operating points. Big Eo number and small V.G. number = clean and dynamic. Small Eo number and Big V.G. number = distorted and compressed.

[HotBluePlates]

is there a difference in results if for example V1a plate is 150v from a 300v B+ into a 220k plate resistor vs 150v at the plate from a 350v B+ into an 330k plate resistor?  ...

In Absolute Terms, yes there is a difference.

In Practical Terms, there is nearly no difference for V1 in a guitar amp.


Below are loadlines for the 2 conditions you want:  220kΩ plate load with a 300v supply idling at 150v plate, and 330kΩ with 350v supply idling at 150v plate.

The 330kΩ with the higher supply voltage accepts a little larger input signal, delivers a little more output signal before severe distortion, and exhibits a little more gain.

     -  However, rhe output signals realized depend on the input signal applied.
     -  Input will likely be closer to 100-200 millivolts rather than 1.3+ volts.
     -  Output for either is 14.8v peak to 15.3v peak, with very little (if any) difference in distortion.

     -  Gain, output, and input-headroom for both versions will be eroded by the Input Resistance of the next stage.
     -  The 330kΩ case will have its apparent advantage eroded more by that Input Resistance.

Net Result:  they're basically "the same" when looking at the V1 position.

[thetragichero]

here's my go-to when i'm trying to mess with preamp stages: https://hosenlander.nl/triodecalculator2/index.html

obviously don't change the numbers that are used for the tube model but have a play with the step 1 numbers and see what's different (as mentioned above, very little... 350v/330k ever-so-slightly more signal but not a difference any of us would likely hear)

[HotBluePlates]

...
     -  Output for either is 14.8v peak to 15.3v peak, with very little (if any) difference in distortion.
Net Result:  they're basically "the same" when looking at the V1 position.

"1dB" is "not much change" for sound.

     Soundfiles with -3dB and -1dB steps (start with your volume turned down until you know what the loudness is like)


Now look what I posted above, taken from the curves:  15.3v vs 14.8v ----> 20 log (15.3/14.8) = 20 x 0.01443 = 0.2886 dB

The average person probably cannot distinguish a <0.5dB step in loudness/gain.