Load Line question

[Tonewood]

If I draw a load line, choose a bias point and see that a 3vp-p input gives about a 200 volt peak to peak out, does that assume a fully bypassed Rk? Would the p-p out be about 100v if Rk not bypassed?

Thanks

[J M Fahey]

Yes, load lines asume no extra resistance in series with the cathode.
For perfect measurement you use a PSU to set the cathode as negative as necessary relative to the grid.
And use different voltages (say 0V, -0.5 , -1 , -1.5 and so on) according to the line you are tracing.
Coming from a "good" PSU means its internal impedance approaches zero.
When actually building a guitar amp, you save $$$ by using a cathode biasing resistors.
Many old circuits (20's to 40's ) used a "Bias Battery". Google it.
So if you design with a cathode resistor, decouple it.
*If* you don't, it will work, of course, but gain will be less than calculated.

[Merlinb]

If Rk is unbypassed it affects the gain, but NOT the output swing, assuming Rk is small, which it usually is. In your case you could still get 200Vpp output with an unbypassed Rk, but it will take a larger input signal to do it. In stead of 3Vp-p it might require 6-8Vpp.

[Tonewood]

If Rk is unbypassed it affects the gain, but NOT the output swing, assuming Rk is small, which it usually is. In your case you could still get 200Vpp output with an unbypassed Rk, but it will take a larger input signal to do it. In stead of 3Vp-p it might require 6-8Vpp.

Checking my understanding-
1: Unbypassed, it should be able to take 6vp-p without distorting (much)?
2: If I have a fully bypassed input stage & it sounds great with a Strat(weaker pickup) but OD's with a high voltage humbucker or stomp boxes, switching the bypass cap out of the ckt will give the input stage more headroom?

J M Fahey & Merlin, Thanks again for your help!

[Dave H]

Checking my understanding-
1: Unbypassed, it should be able to take 6vp-p without distorting (much)?
2: If I have a fully bypassed input stage & it sounds great with a Strat(weaker pickup) but OD's with a high voltage humbucker or stomp boxes, switching the bypass cap out of the ckt will give the input stage more headroom?

1. Yes, unbypassed it will take 6vp-p before clipping but bypassed it will only take 3vp-p.
2. Switching out the bypass cap will do exactly what you want for the humbucker.

You have to be careful using the term 'headroom'. You could say the input has more 'headroom' when the cathode is unbypassed because it can now take 6vp-p vs only 3vp-p when bypassed but the output 'headroom' is unchanged because the output clips at 200v for both bypassed and unbypassed cathodes (as Merlin said). Removing the cathode bypass cap halves the gain so the tube can handle twice the input voltage before clipping at the output.

Dave H.