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In Reply to: RE: Broskie is correct on this one. posted by Stephen R on June 15, 2021 at 12:45:21
I am very fond of a floating paraphase inverter that is adjustable. Rather than making the "perfect" inverter with a "perfect balanced output".
The advantages of the adjustable floating paraphase inverter is that you can adjust symetry with output tubes that have either more or less different amplification factors.
I measured this when I built an Audio Note clone that had a floating parapahese inverter followed by a6SN7 before driving the output tubes.
when the inverter was adjusted for "perfect symmetry" the end result with "perfect (within 1%) matched 6SN7 followed by "perfect matched" output tubes was more than twice as large as when I simply adjusted the inverter for best output.
When using less than perfect matched 6SN7's or output tubes the difference became larger although a hefty dosis of negative feedback attempts to correct this. But then IMD started to shoot up (the result of GNFB).
The moral of this post? Start viewing a design in its totality rather than splitting a design up and ending up with a worse result.
AM
Follow Ups:
"The advantages of the adjustable floating paraphase inverter is that you can adjust symetry with output tubes that have either more or less different amplification factors."It's possible to adjust balance with any phase inverter, including the cathodyne. The LTP inverters in the amp I'm building now will have adjustable balance. The circuit is similar to the Citation V, although I'm using different tube types and component values. It's also worth nothing that imperfect balance here is largely compensated when NFB is used and the PI is included in the loop. Bandwidth is important for this purpose, and the LTP is excellent in that regard.
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Buy Chinese. Bury freedom.
Edits: 06/17/21
Looks good,, just be aware that different tubes and/or output transformer are highly likely to upset the feedback loop and the frequency compensation in the driver. (A lot of) testing will be require to ensure stability over different frequency ranges and output levels.
Personally I would be keen to add some resistors in series with the screens.
That's the schematic of the HK Citation V, just an example of the PI balance circuit in an LTP (and the same topology as the amp I'm building). I'm using HK transformers, but not from the Citation. Screens in each channel will be driven by a VR tube in series with B+, similar to the Leslie 122 amplifier. This will set the screens at 105V below B+, so just about any 6L6 variant can be used.
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Buy Chinese. Bury freedom.
For all the worry about balance, a lot of the stuff I have built of late runs a pentode LTP. Screen current is not going through the loads, but most certainly is passing through a cascode MOSFET current regulator at the cathodes. Phase-to-phase balance is not to be despised...LOL
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Two schematics from the RCA 26 manual, basically identical except for using a different output tube (and adjusting the B+ voltage).
To continue on my earler remark of having to adjust feedback loop and frequency compensation: Check the values of the feedback capacitor and the addition of frequency compensation in the phase splitter.
In the second schematic I like the way that the bias voltage is derived - an automatic adjustment when B+ comes on and there cannot be a run away because of a fault in the bias circuit.
This is what you have to figure out when not using the same components - especially transformers are troublesome, just any transformer which supposedly has the same impedance can result in total different behavior. This is due to interwinding capacitance and leakage inductance between primary and secondary. personally I would never ever consider trying to build a clone of the Williamson amplifier as it is only marginally stable and needed a very specially constructed OPT.
AM
"In the second schematic I like the way that the bias voltage is derived - an automatic adjustment when B+ comes on and there cannot be a run away because of a fault in the bias circuit."I do like the fact that bias tracks B+ regarding variations due to line voltage. However, this circuit brings the same liabilities to an AB1 amplifier as self bias. Namely, increased amplifier output and anode current causes bias voltage to increase, and that moves the amplifier closer to Class B. Small amplifiers operating near Class A may not suffer the effects, but large amplifiers biased in true AB1 will show crossover distortion at high output levels. This distortion will persist into quiet musical passages due to the time constant of the bias circuit, degrading fidelity. In addition, this circuit is essentially a voltage divider with the negative bias voltage being subtracted from available B+. That may not be a disadvantage, depending on the transformer used, but it has to be taken into account during initial design.
"I would never ever consider trying to build a clone of the Williamson amplifier as it is only marginally stable and needed a very specially constructed OPT."
The most significant issue with the Williamson IMO is the inclusion of too many stages and too many coupling caps. The Citation V topology is much cleaner, due in part to the simplification of the gain structure made possible by driving the outputs in pentode mode, rather than triode. Only two coupling caps are necessary, and there is significant headroom in every stage ahead of the outputs.
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Buy Chinese. Bury freedom.
Edits: 06/18/21
And many rate the floating paraphrase for it's sound. From those that like the single ended distortion characteristics but have been persuaded by going PP. WE124 amplifier being a case of this.Anecdotal of course. No science involved in this post.
Edits: 06/16/21
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