Why does B+1 go to the OT?

[Ripthorn]

Hi, I am in the works of designing an amp that has switchable power sections between two different sets of tubes. The thing is they each have different supply voltages. I noticed that in a lot of designs, B+1 goes to the OT and I was wondering if I would need to switch the voltage going to the OT when I switch which power section is being used. Mostly, I am wondering what the reason is for sending voltage to the center of the OT.

[paulster]

Consider the OT as two individual primaries, one for each phase, and then it is comparable to a preamp gain stage with one half of the OT primary replacing the plate resistor.

The current flowing between the plate and cathode causes a voltage swing across the plate resistor (or, in this case, the OT primary) which is directly proportional to the current since the resistor is fixed.

Both halves of the transformer primary need to be out of phase so that the inverted signal ends up back in phase again, and it is this requirement that means the transformer can be simplified into a centre-tapped primary rather than two discrete primaries.

So, in answer to your question, if you want a different B+1 for different tubes then you'll want to switch it before the CT.

[Andy Le Blanc]

In push-pull trannies, both halves are inductively coupled, by the core.
so that a complete wave form is reflected upon the secondary as the current
swings from leg to leg. They are also out of phase so that power supply noise
and common mode signals are reduced resulting in less even order distortion.
If power were to be supplied not to the center then you would end up with two
disproportionate signals, of dis-similar phase, that could not be reflected to the
secondary by the core coupling, without severe distortion.

[Ripthorn]

Very good explanations, thank you very much. That now makes perfect sense. Is the value of B+ to the center tap a huge issue? I ask because I will have a power stage with tubes running at B+ ~70V and another with B+ ~110V or thereabouts. This is because the smaller tubes have a lower max voltage. Does that really matter in a case like this with the power to the center tap?

[paulster]

The value of B+ applied to the centre-tap of the OT is essentially your plate voltage. There'll be a small drop in the OT caused by the DC resistance of the OT but it shouldn't be significant, and so your plate voltage should be very close to the B+ applied.

This is the actual power supply to the power tubes and therefore needs to be within the specifications of the tube you are going to run.

A B+ of 110V will give a plate voltage almost certainly over 100V and your miniature power tube with a maximum plate voltage of 70V probably won't be very happy or last very long.

Conversely, a B+ of 70V will give you a plate voltage of, say, 65V which your other miniature tube should be quite happy with, but there will be tonal (and power output) variations to be explored in the region up to 110V.

Are you wanting to switch between two different types of tubes, arranged in push-pull pairs, or are you trying to combine different tube types at the same time?

[Ripthorn]

I want to be switching between push pull pairs. So I would put a switch after the phase inverter, routing the signal to one pair or the other. I will just need to get a switch for the voltage to the OT. Too bad they don't make 5PDT toggles. I need to two contacts for the phase inverter, one for the power to the OT and two contacts to switch the impedance ratio of the OT. I guess I can just use a DPDT for the OT secondary and a 3PDT for the phase inverter/B+ to OT.

[paulster]

If you want two different impedances on two different pairs you could always wire them so the plates are always connected to the relevant taps, the grids always connected, and then simply disconnect the cathodes from the pair you want to out of circuit.

Presumably this will be cathode biased, so if you switch on the ground side of the cathode resistors you could use a SPDT to change between tube pairs, by allowing only one cathode resistor to be tied to ground. Make it a DPDT and you can switch the B+ as well.

You'll still have the screens to take care of, but if they're derived independently from the B+ then you can handle these by default.