PPP primary impedance load Q

[Le Basseur]

Let' say we have a PPP output stage w/ fixed bias(6L6s,EL34s,whatever).
OPT's primary impedance for 4 tubes in PPP is around 4 kOhm for a quartet of 6L6s,reflected in one and only 8 Ohms secondary.
Question:
Assuming that the filaments are continuously working on all four output tubes BUT I lift a pair of output tube's cathodes from the ground (of course,one from each side,like "reducing output power in a Twin"...,not otherwise),
what happens in terms of primary impedance?
Does the OPT "sense" all four tubes (as long as the cathodes are hot) or just two,the ones being completely in the circuit?
I landed today in an argue regarding this subject and my position was that no matter the filaments are hot or cold,if the cathodes are decoupled from the ground,the electrons' emission doesn't follow the phisical path,from - to +,therefore the only tubes that the OPT "sees" are the ones fully in charge.
(BTW,some Acoustic tube amps have this feature via a switch that lifts a pair's output tubes' cathodes from ground,halving the output power).
Am I wrong?
Please comment!
TIA!
Regards,

[Jag]

I've always assumed it's as you say but, I've also always wondered.

[Old Tele man]

...basically, the inter-electrode capacitances are always there, but without current flow, the inter-electrode impedances will go to infinity (open circuit) for the non-operating tubes.

[stokes]

I think you mean lift a pair of output tubes cathodes,not anodes from ground.To answer the question"what happens in terms of primary impedance",the effect will be the OT would want to see a 16ohm load on the secondary.In a case of a Twin it would be 4x6l6's into a 4ohm load,lifting or pulling two tubes you would want an 8ohm load.

[Le Basseur]

Stokes,
Typo!(corrected in the meantime)
Yes,I'm aware about the 2-vs.-4 output tubes and how this reflects in the primary-vs.-secondary loads.Every couple of months somebody posts here a question regarding this issue.
I asked about the relationship 2-or-4 tubes vs. strictly primary impedance.
Thanks!

[hasserl]

I have tested this with Carvin amps that have a switch to lift the cathodes of two of the four power tubes. That is all the switch does, the filaments continue to operate, the anodes remain connected to the OT primamry, everything stays the same except the cathode to ground circuit, for one power tube on each side, is lifted, opening the circuit and turning the tubes "Off".

In half power mode (with the cathodes lifted) the power production is greatest when setting the impedance switch to half the actual load. i.e. with an 8 ohm dummy load the amp puts out the best power using the 4 ohm tap. Setting the impedance switch to 8 ohms causes a loss of power, and setting it to 16 ohms causes a greater los pf power.

4 ohms - 51.5 watts
8 ohms - 30.5 watts
16 ohms - 15.1 watts

When hooking the amp up to a speaker cabinet and playing thru it the difference in sound between the three position impedance switch is clear, with the fullest and strongest sound coming with the impedance switch set to 4, setting it to 8 ohms causes the sound to thin out with less bottom end response. Setting it to 16 ohms reduces the bottom end response fuller and causes the tone to sound squashed, with less depth and clarity. There is also a corresponding drop in volume.

[stokes]

I'm not sure what you are looking for as to effect on the primary impedance,but the OT only knows turns ratio,the primary impedance and secondary are a product of the power tubes on the primary and the load on the secondary.For instance if you are running four power tubes into an OT with a 4k primary into an 8ohm load and switch only the speaker load to say 16ohms,your reflected primary will change accordingly,it would seem like an 8k primary.So if you pull two tubes and keep the same secondary load,your reflected primary impedance would change accordingly.