Virtual center tap OT?

[Satamax]

Hi everybody.
4
Long time no see.

We, i was wondering if, on a small OT, for a single 6v6 for example, i could create a virtual center tap

Like this



To use it as a phase inverter, to couple the 6v6 and a PP output stage?

Thanks a lot guys.

Max.

[Helmholtz]

The 100k resistors will (asymmetrically) drop voltage during grid conduction and reduce drive capability down to that of a normal PI.
Might work satisfactorily with lower value resistors (1k?) though.

[J M Fahey]

We need to know what´s driving it , and what load will it drive in due time so ppostb the full schematic, at least a provisional one.

[Satamax]

Thanks a lot for your replies guys.

JM, this is a theoretical question.

But the circuit could look like a marshall plexi preamp, feeding a gain stage and a 6V6 tweed champ like output, which would serve as a phase splitter, and run a pair of big bottles, 6L6 EL34, or 6550. Something along those lines. I have played amps with long tail pairs enough. It's time to try other phase inverters.

Thanks again.

Max.

[eschertron]

Thanks a lot for your replies guys.

JM, this is a theoretical question.

But the circuit could look like a marshall plexi preamp, feeding a gain stage and a 6V6 tweed champ like output, which would serve as a phase splitter, and run a pair of big bottles, 6L6 EL34, or 6550. Something along those lines. I have played amps with long tail pairs enough. It's time to try other phase inverters.

Thanks again.

Max.

What voltage swing do you expect to get from the OT?

[J M Fahey]

Chiming in to ask the same.
I fear a regular speaker out OT will not do, "speaker voltage" is usually way lower than what you need to drive power grids.

As in a 6L6 needs some 52V peak at least (same value as negative bias and that while still in Class AB1) , so some 36V RMS .
And you need twice as much end to end because the resistive tap divides that in two, so 72 V RMS ... some 650W RMS ....

So yes, in theory yes, but not in practi9ce.

Just thinking aloud: if you want to make a powerful transformer driven Class AB2 amplifier and don´t want to spend a fortune on a custom wound driver transformer by one of the famous guys, you *might* pull secondary from a standard SE OT , since primary is fine,and rewind a many more turns thinner wire secondary, with the needed voltage ratio.

And since you are at it, you might even wind a real center tap

Of course if primary and secondary are interleaved, that complicates matters a lot, because you´ll have to at least partially unwind and rewind some primary.

[g1]

I have played amps with long tail pairs enough. It's time to try other phase inverters.

If you're interested in uncommon types, have a look at this one (early Garnet L190D). It uses what is referred to as a 'center-tapped choke'.
Maybe JM would comment on the ease or difficulty of winding such a thing. (or whether a OT primary could be used like this)

[J M Fahey]

Well, this one is being driven from a tube plate loaded with 47k, so we can imagine the choke/primary winding must be even higher impedance at Guitar frequencies ... that implies A LOT of turns of very fine wire.

Physically possible but tons of turns of very fine wire

That said, I would love to Class AB2 drive a few power tube grids ... from a transformer coupled TDA2030 or similar.

[SoulFetish]

I fail to see how a DC ground reference for the 0V crossover would provide a balanced output at all? you would need one half of the winding to be wound out of phase.
unless hes driving the secondary from a single ended stage..., but then why bother with the centertap at all? Then you would have to find a turns ratio that works, and I don't see how a OT makes sense for this.

[Dave H]

I fail to see how a DC ground reference for the 0V crossover would provide a balanced output at all?

Imagine the winding has a grounded centre tap (like an OT primary). The voltages at the opposite ends of the winding seesaw about this real centre tap. It's just the same for the artificial centre tap.

[Satamax]

Thanks a lot to all of you guys.

JM has answered, in a simple manner, that i can understand.

Not enough voltage swing!

Thanks a lot.

[eschertron]

Not enough voltage swing!

Not for big bottles, but maybe a pair of EL84s? I'm curious now about trying something like that. My question is:
Do I need to load the OT with something closer to a normal speaker load to protect the windings? Herr Helmholtz has suggested 1k instead of the 100k, maybe even smaller (100R) would be desired? This is now a practical question rather than purely theoretical.

[mhuss]

Another practical example (using a dual secondary) is the Fender PS series. See (PS300 Schematic).

[Helmholtz]

Not for big bottles, but maybe a pair of EL84s? I'm curious now about trying something like that. My question is:
Do I need to load the OT with something closer to a normal speaker load to protect the windings? Herr Helmholtz has suggested 1k instead of the 100k, maybe even smaller (100R) would be desired? This is now a practical question rather than purely theoretical.

My first reply above was just a principal consideration regarding asymmetrical loading of a virtual CT. As long as there is no grid current the 100k resistors may work.

The main problem with using an OT as driver transformer is actually available drive voltage, as Juan explained. A 8k/8 Ohm has a voltage step-down ration of 1/32. So if the driver tube allows for a positive voltage swing of 250V at the primary, the max. secondary peak voltage is 7.8V. Minimum necessary drive voltage peak is given by the bias voltage, meaning 12..15V for EL84s. This is without grid current (class AB1).
Transformer secondary should be terminated by a resistance reflecting to the desired primary tube load.

For class AB2 the driver circuit requires a thorough single ended power stage design.