OK, here's a totally crazy OT question . . . which I presume won't work, but I'll ask anyway.
Someone on another forum recently posted a link to an old thread here about cheap output transformers. I had seen this before but it's been years. Anyway, in one post someone asked about using a PP OT in a SE amp.
One response was "In a PP application, the tube bias current flows through the primary winding from the center tap to the ends, which cancels out any magnetic flux in the core due to this DC current. In an SE application, this DC current flows from one end of the primary to the other, which will saturate the core resulting in poor performance. Manufacturers get around this by introducing an air gap in the core."
This made me wonder . . . if current flowing in only one direction through the primary of a SE OT is what creates the core saturation problem, would it possible to use a PP OT for SE with the B+ connected "in reverse"?
So, B+ to the ends of the primary winding and have the center of the winding connected to the plate of the tube. Since the current would not all be flowing in the same direction, would this cancel the magnetic flux in the core and make the transformer usable for SE operation without the need for an air gap?
Someone on another forum recently posted a link to an old thread here about cheap output transformers. I had seen this before but it's been years. Anyway, in one post someone asked about using a PP OT in a SE amp.
One response was "In a PP application, the tube bias current flows through the primary winding from the center tap to the ends, which cancels out any magnetic flux in the core due to this DC current. In an SE application, this DC current flows from one end of the primary to the other, which will saturate the core resulting in poor performance. Manufacturers get around this by introducing an air gap in the core."
This made me wonder . . . if current flowing in only one direction through the primary of a SE OT is what creates the core saturation problem, would it possible to use a PP OT for SE with the B+ connected "in reverse"?
So, B+ to the ends of the primary winding and have the center of the winding connected to the plate of the tube. Since the current would not all be flowing in the same direction, would this cancel the magnetic flux in the core and make the transformer usable for SE operation without the need for an air gap?
There would be no net flux in the core, but there would be no secondary signal output,
since the ends of the primary must have opposite phase signals.
since the ends of the primary must have opposite phase signals.
It would not only cancel the magnetic flux due to the standing current from the output tube, but also the magnetic flux from the signal, as Rayma says.
You could feed a constant DC current that equals the plate current from a high impedance CCS into the other end instead. OTOH, this reduces the poor SE efficiency even more.
Best regards!
Best regards!
Yes, you can also do that in a regular push-pull output stage, by disconnecting the grid drive to one of the output tubes.,
but still having proper DC grid bias voltage.
but still having proper DC grid bias voltage.
Dave Slagle tried it -more than a decade ago-, ... but this solution did not spread.
https://forum.intactaudio.com/viewforum.php?f=4&sid=510a311c8d38b11f8c6475a0139e669e
Try to find the schematic of a Klimo Beltaine there is a good ( not perfect) solution about this issue
Walter
No wonder. You'll have more effort than with a PP, this just to decrease efficiency? I'd clearly prefer a PP design, not only for it's better efficiency, but also for it's better CMRR, which makes the PSU simpler.
Best regards!
Hi
I agree about pp; specially with pentode-tetrode configuration ( is not easy ) , then with triode tubes.
Also a study with s.e. can be interesting and some solutions are to be investigate.
Walter
The only scheme that looked practical for SE DC cancellation was called "anti-triode". It used a P-P OT with a Triode on one side and a Mosfet on the other side with cathode and source electrodes connected above a CCS tail. The Mosfet gate (at DC voltage) was adjusted to equally share current with the triode at idle (class A). The high gm of the Mosfet kept the tail voltage near constant (to avoid 3rd H generation of a typical LTP ), and the CCS tail made sure the Mosfet conducted a perfect complementary current to the Triode. Hence the "anti-triode" name. This gave DC balance and SE power output was doubled.
Those who tried the scheme said it did not sound the same as conventional SE using a gapped inductor OT. My conclusion is that the gapped OT distortion (2nd and 3rd harmonic) is essential to SET sound.
One can remove OT distortion using just 3 resistors using an old patented scheme, US patent # 4614914, using negative resistance to null out the OT primary winding resistance (using positive current Fdbk). Also written up in RDH4 pgs 354, 355. So there never was any need for SE gapped OTs to supposedly remove zero crossing magnetic distortion. Quite the opposite, you need the gapped OT to generate essential distortion.
Those who tried the scheme said it did not sound the same as conventional SE using a gapped inductor OT. My conclusion is that the gapped OT distortion (2nd and 3rd harmonic) is essential to SET sound.
One can remove OT distortion using just 3 resistors using an old patented scheme, US patent # 4614914, using negative resistance to null out the OT primary winding resistance (using positive current Fdbk). Also written up in RDH4 pgs 354, 355. So there never was any need for SE gapped OTs to supposedly remove zero crossing magnetic distortion. Quite the opposite, you need the gapped OT to generate essential distortion.
Last edited:
Has anyone ever quantified the distortion profiles of the various amplifier (typologies) worth discussing? I have to say I'm lost; there's clearly a no distortion quadrant, there's amounts of 2nd and 3rd via SE, then there's the "kind of" 2nd and 3rd via P-P, then there's 2nd in phase, 2nd out of phase. Seems to be like crafting wine, beer or cooking, striking certain sonic flavor balances that "hears good"...
As yet said, I'm not really a SE addict. Anyway, I'm thinking about a SS current mirror to set the conterbalance current exactly to the plate current. Would that be feasible, at least with a triode SE, where's no screen current?
Best regards!
Best regards!
Very interesting - Broskie suggested a "single ended circlotron" on his blog years back. Essentially it's a triode with mosfet+current source (IIRC) or triode acting to balance. The same concept just having a speaker rather than an output transformer.
You can balanced out the DC using a LPF and then simply have the SE drive the AC.
I use P channel mosfets with LPF to balance the DC on my circlotron. I've not measured the difference as it was below the multimeter. Just started the amp up - measures a mean of about 1.8mV DC with the noise (unregulated RCRC filter input).
You can balanced out the DC using a LPF and then simply have the SE drive the AC.
I use P channel mosfets with LPF to balance the DC on my circlotron. I've not measured the difference as it was below the multimeter. Just started the amp up - measures a mean of about 1.8mV DC with the noise (unregulated RCRC filter input).
Last edited:
The "Anti-Triode" scheme may have simply been a little -too- perfect. The low inductance of a typical gapped SE OT develops more magnetizing current than a high inductance P-P OT. Xfmr magnetizing current is a good source of 3rd harmonic.
So one might try a pentode of similar gm to the triode (so a pentode in place of the earlier Mosfet) to get some 3rd harmonic from typical LTP tale voltage variation for similar gm devices. The pentode gets a fixed grid voltage, adjusted to match the idle current of the triode side for DC balance.
Both devices still set over a common CCS tail, so that complementary AC currents from the devices to the P-P OT ends double the power output. (versus just a fixed current from the pentode) The complementary operation is also needed to get the 3rd H from tail voltage variation.
To get even higher 3rd harmonic, one could try a lower turns power toroid for the P-P OT, but that may not be necessary.
Getting higher 2nd harmonic is easy by choice of the triode, but again may not be necessary. Most top end SET tubes are selected for lower 2nd H.
However, the usual gapped SE OT, with pulsing DC operation. has variation of its inductance during a cycle. This would likely produce some further modulation of 2nd H and 3rd H with some phase shift. A spectral analysis of a typical SET could be useful to determine if further "imperfections" are needed.
So one might try a pentode of similar gm to the triode (so a pentode in place of the earlier Mosfet) to get some 3rd harmonic from typical LTP tale voltage variation for similar gm devices. The pentode gets a fixed grid voltage, adjusted to match the idle current of the triode side for DC balance.
Both devices still set over a common CCS tail, so that complementary AC currents from the devices to the P-P OT ends double the power output. (versus just a fixed current from the pentode) The complementary operation is also needed to get the 3rd H from tail voltage variation.
To get even higher 3rd harmonic, one could try a lower turns power toroid for the P-P OT, but that may not be necessary.
Getting higher 2nd harmonic is easy by choice of the triode, but again may not be necessary. Most top end SET tubes are selected for lower 2nd H.
However, the usual gapped SE OT, with pulsing DC operation. has variation of its inductance during a cycle. This would likely produce some further modulation of 2nd H and 3rd H with some phase shift. A spectral analysis of a typical SET could be useful to determine if further "imperfections" are needed.
Last edited: