V2 Plate resistor values

[caglej]

What have you guys discovered with the plate resistor values for V2? I just wanted to find out before I soldered them in. What would be the difference if I used 220k and 180k, versus say 180k and 120k?

[Bob-I]

What have you guys discovered with the plate resistor values for V2? I just wanted to find out before I soldered them in. What would be the difference if I used 220k and 180k, versus say 180k and 120k?

The importance is the ratio between the plate and cathode as well as the absolute value. I used 220k/3.3k and 150k/2.2k and it's really starting to sound great now that I've tweaked the tone stack.

[caglej]

Thanks Bob........those are the values I planned on, so if you're getting good results.....I'll just keep keep them as is.

[Funkalicousgroove]

Very simply, the plate and cathode resistors affect the bias of the tube, so, it really depends on what you want it to do for you as to which values you choose. Smaller Plate load resistor= higher voltage, and a wider possible dynamic range, larger resistor= less headroom and browner sound. The cathode resistor will then set the bias of the tube, 1/2 of a 12AX7 has a 1W Max static dissipation, and are generally biased 50-80% of that. I would suggest that you experiment and then measure your results, write down the plate voltage and cathode voltage difference and keep a chart. If you like what you come up with leave it, if you don't like it try something more traditional and note the difference. Here's the tough part though, every tube is different, so they'll all sound different and act differently with the same RP/RK values.

[Robert]

Very interesting. Thank you for the explanation Funk. It is so much more useful when you know the "why" behind the values in these circuits.

[llemtt]

Plate resistor is part of the "load" seen by the tube, which is actually the plate resistor in parallel with the impedance of the next stage.

Changing the load of the tube changes many things but mainly gain and available swing (headroom).

If you look at plate resistor from a different viewpoint you can consider it part of the "source impedance" of the tube stage so changing it affects gain and swing but also frequency response (think coupling caps, shunt caps, interelectrode capacitance,...).

The cathode resistor sets the operating point of the tube stage, i.e. the amount of current that flows at idle. Tube characteristic are different with different operating points!

Take some good tube amp books (lot of material online! I suggest Aiken site also and Kevin O'Connor books), tube plate curves and begin a learning process, at the end you will be a master of each little aspect of a tube stage.

Short answer: differences between 180/2.7 vs 220/3.3 and 120/1.8 vs 150/2.2 are very subtle, if you can build two preamps and footswitch between them while you play, then choose the one you like the most...

Teo