EL83 OT impedance question

[Helmholtz]

I know. But I've noticed that it's common practice among guitar amp builders and even manufacturers to use the same primary load for both (one tube se and two tubes push/pull). I was openly wondering why and thought the higher minds might some insight

I just checked with the help of some tube literature. And I found that I was not the first to invent the modified simple formula. And yes, the modified formula can be applied to Class A PP amps as well. As it gives the optimum load impedance per tube, the Zaa is twice this value (only Class A).

The load impedance in Class B PP per tube often is about half the Class A value. But as in Class B operation the Zaa is 4 times the single tube load, the resulting Zaa may be close or the same as in Class A.

(The formula does not apply to single ended triode power amps.)

[tubeswell]

I just checked with the help of some tube literature. And I found that I was not the first to invent the modified simple formula. And yes, the modified formula can be applied to Class A PP amps as well. As it gives the optimum load impedance per tube, the Zaa is twice this value (only Class A).

The load impedance in Class B PP per tube often is about half the Class A value. But as in Class B operation the Zaa is 4 times the single tube load, the resulting Zaa may be close or the same as in Class A.

(The formula does not apply to single ended triode power amps.)

The reason that you use the plate-to-plate 'equivalent' for calculating push-pull Zo-o, is because in the Class-A part of the signal cycle, both halves each see the plate-to-CT load, therefore the turns ratio for each half needs to be half of what an OT Pr would be in an SE amp (for the same type of tube), because there are two push-pull halves of the OT primary in parallel - both conducting (albeit in anti-phase to each other) into the single secondary winding. Therefore, the total number of primary plate-to-plate turns is 'the same' as for an SE OT primary (albeit that each half of the PP primary is wound in opposite turn-directions, with each having half the number of plate-to-plate primary turns).

When the B-load-line kicks in, half of the PP OT primary is not conducting, therefore the Pr:Sec turns ratio of the other ('on') half is now half of what it would be when both halves are conducting. As each half of the OT primary (on its own) has half the number of turns of the total primary, the Pr:Sec turns ratio of the resulting 'on' half, is now delivering 1/4 of the total possible plate-to-plate load (that you have when both sides are conducting).

The B load line is of course steeper and there’s less propensity for screen overdissipation when this kicks in as long as sufficient negative bias is maintained (but fitting Rg2 is still prudent)

[Helmholtz]

I know. But I've noticed that it's common practice among guitar amp builders and even manufacturers to use the same primary load for both (one tube se and two tubes push/pull). I was openly wondering why and thought the higher minds might some insight

Here are some real numbers for EL 84s from tube literature/ manufacturer's recommendations:

SE plate load: 5k
PP Class A plate-to plate load: 10k (each tube "sees" 5k)
PP Class B or AB plate-to plate load : 8k (in Class B each tube sees 2k)

All values for Vp=Vg2=250V.

No good idea to use the same OT for SE and PP even if plate load fits. OTs cannot be optimized for both types.