Drawing Load Lines for 4 valve (2 pair) push pull

[Glennjeff]

Well,
I've read about 6 semi-technical documents on drawing up design load lines for push pull PA's and they were all rather different.

My BIG question is :

When working with four valves do I use 1/4 OT impedance for Class B part of load line curve and 1/2 OT output impedance for Class A part of load line curve or do I have to put  a factor of 2 or 1/2 somewhere in the line art.

I actually measured one valve with my amp operating with sig gen, CRO and MM and decided the load line was a nice smooth curve which does appear to approximate 1/4 OT Z for class B part (towards saturation) and 1/2 OT Z for Class A part (towards cut-off ).

Any input would be much appreciated.

[Merlin]

Two options.
Either:
Take your characteristics graph and double the current scale so that it represents two tubes in parallel; then draw the load line normally (i.e. class-B slope = 1/4 of total impedance)
Or:
 Leave the characteristics graph alone but draw the load line with double the impedance figures you would normally have drawn  (i.e. class-B slope = 1/2 of total impedance). Remember, when two tubes are in parallel, each one 'thinks' it is driving twice the impedance.

[Glennjeff]

Thanks heaps Merlin,

I've been trying to visualize the two tubes into one impedance thing and it's given me insomnia.

Off to bed.

[PRR]

When one tube is pulling it feels the whole load.

When both tubes are working each feels half the load (double impedance).

It is rarely necessary to draw the full curve. Compute the class B situation. Would it do the job? (Neglecting the fact that hard B operation is very difficult.) Use that condition, but raise the idle current to some fraction of Pdiss. This may be set "by ear", but with a close eye on idle heat/temp. Tube cut-off is so gradual that this is "sure to work".

Absolute lowest THD does require plotting the gain through the A-AB transition, and perhaps adding small cathode resistors to limit peak Gm. But with the common tubes, real-low small-medium signal linearity usually requires HOT idle and low B+, so max power output is down from what the same tube-bucks "could" do.