Output Tube Load Resistance, Tube Current and Voltage

[tubeswell]

Sorry for these dumb questions

1) What's happens in an output tube (in terms of bias) if supply voltage is constant and load resistance (through the OT) increases? (I guess the PT dissipates more heat, but does the tube get hotter (and draw more current, and alter the bias point) as well?)

2) In a push-pull OT, is rated load resistance calculated on the basis of the load on each side of the primary winding? (Seems like it should be, to my reptilian brain, because each side of the primary is hooked up to one tube)

[Rob Mercure]

If the load resistance increases the flow of current though the tube diminishes. If the tube is fixed bias the bias supply remains the same unless the the load is so reduced that the overall ouptput voltages of the power transformer increase. If there is cathode bias the lowered current flow will cause the voltage drop across the cathode resistor to decrease effectively descreasing the bias voltage and causing the tube to draw increased but with increased current there will be a greater voltage drop across the load. current. Of course this a cyclic phenomena here as the increased load resistance will again cut resistor current which will decrease the bias which will allow the tube to draw more current until some point of equilibrium is established.

The load impedance provided by a push pull transformer is usually given as "plate-to-plate."

[tubeswell]

If the load resistance increases the flow of current though the tube diminishes. If the tube is fixed bias the bias supply remains the same unless the the load is so reduced that the overall ouptput voltages of the power transformer increase. If there is cathode bias the lowered current flow will cause the voltage drop across the cathode resistor to decrease effectively descreasing the bias voltage and causing the tube to draw increased but with increased current there will be a greater voltage drop across the load. current. Of course this a cyclic phenomena here as the increased load resistance will again cut resistor current which will decrease the bias which will allow the tube to draw more current until some point of equilibrium is established.

The load impedance provided by a push pull transformer is usually given as "plate-to-plate."

Thanks Rob

So increased load resistance = less power (all other things being equal)?, but in cathode bias too much increased load resistance will tend to overbias the tube, whereas with fixed bias it wil tend to underbias the tube? Did I understand you right? (Forgive me it is 4:30am over here and I can't sleep for thinking about this)

Also, with the plate to plate measurement of load resistance, does therefore altering speaker impedance not have a strictly linear relationship to load resistance? (i.e. if I plugged 16R into 8R tranny tap on push pull OT, and if load resistance is measured plate-to-plate, I should therefore not expect a doubling of load resistance? Only I'm pretty sure the OT on my recent 5G9 build is wound to give a '5k primary' (but I could be wrong about this), and I want to know whether using the 16R speaker on the 8R secondary tap will result in 10K or something less?, Or am I confused about the way tube datasheets describe different load resistance ratings for push-pull vs S/E?

[Enzo]

Transformers are like a set of gears. In gears, the input and output speed relationships are determined by a ratio, but there is no inherent speed of a gear. The output transformer has no inherent impedance, if reflects the secondary load impedance back to the primary through its ratios. SO a 5k primary transformer only offers 5k to the tubes if the rated load is on the secondary.

[tubeswell]

Transformers are like a set of gears. In gears, the input and output speed relationships are determined by a ratio, but there is no inherent speed of a gear. The output transformer has no inherent impedance, if reflects the secondary load impedance back to the primary through its ratios. SO a 5k primary transformer only offers 5k to the tubes if the rated load is on the secondary.

Okay, thanks Enzo. So it is a straightforward thing then. Throwing the 16R speaker into the 8R tap will double the load resistance. That's what I initially thought, but then I think I was having difficulty reconciling this with how a push/pull OT load resistance ratio should be referenced at each end of the primary, as opposed to between one end of the primary and the CT. Is a push pull OT not wound so that the secondary is 'referenced' to one side of the primary at a time? (if you get what I mean?)