Better primary inductance with smaller transformers? I don't think so. Of course, a larger transformer (core) with the same number of turns will have more inductance. Maybe the cause of this misunderstanding is the fact that small transformers need more turns, and inductance goes up squared with the number of turns.
What I am missing in this "toroidal PT used as OPT discussion" is the importance of DC resistance (copper loss; insertion loss). I guess that with a dual secondary PT the two secondaries might have rather high DC resistance. This will to a large extent responsible for the amplifier output impedance (low damping factor).
2.13 ohm for each secondary.
That is over 1 ohm when parallelled.
Unacceptably high ...
That's acceptable. But what will be the primary inductance of the 120VA transformer? I just measured a 50VA toroid, wound for low core excitation of 1.2T. This means more primary turns, so good for inductance; that 50VA toroid has some 22H.
The larger transformers however will have substantially lower inductance, in most cases not suitable for tube amplifiers. I measure a low T 450VA toroid to have some 2,5H of primary inductance; unsuitable for any wideband OPT.
The problem is to find some compromise here, but in the end, IMO, it remains a compromise when compared to a dedicated OPT.
The larger transformers however will have substantially lower inductance, in most cases not suitable for tube amplifiers. I measure a low T 450VA toroid to have some 2,5H of primary inductance; unsuitable for any wideband OPT.
The problem is to find some compromise here, but in the end, IMO, it remains a compromise when compared to a dedicated OPT.
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There is little AC applied, typical for a normal L meter, though mine is a professional one. However, experience has learned that measuring at low excitation is the best approximation to how the transformer finally performs.
It is good practice to have enough primary inductance at low power levels; after all that's where by far most listening is done.
many years ago, there was this pp amp made using the 6336 tubes....that amp used toroids as OPT...should be on the net...i am trying to find it...
I am buying in the parts for a EL84 pp amplifier. One of the power transformers is 240v:12v so I might wire it up to a sig gen and see what it outputs at various frequencies and voltages. The sig gen only works up to 20VAC but it will give me an idea. If it works then great if not I will just have to buy a proper OPT.
240V:12V is 20:1 voltage ratio, and 400:1 impedance ratio. So loading with an 8 ohm speaker, your PP EL84 will see 3k2 anode-anode. Not good for PP EL84 which typically needs 8-10k. But for a guitar amp it might be sufficient; after all that's not HiFi.
toroids are intrinsically wide band... in general, it should work... but if your B+ is say 300 volts, then allowing 50 volts for tube saturation, then that is 250 volt peak or 175 volts rms, but your 240 volt toroidal is 120 volts split... so i doubt that that is suitable for an el84...
just my 2 cents...
Agree for HF. For LF, it depends. Inductance rules here; has nothing to do with being toroidal or otherwise.
Just measured several lower and higher power toroidal PT's for HF. It's miserable. None reaches 20 kHz without more or less attenuation with a 50 ohm (!) source impedance (signal generator).
Only dedicated toroidal OPT's have good HF bandwidth. PT's have just too much leakage inductance to make them good OPT's.
Only dedicated toroidal OPT's have good HF bandwidth. PT's have just too much leakage inductance to make them good OPT's.
