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Is Western Electric selling a Push-Pull amplifier (91E) as a Singlended ?

Balle Clorin

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Take a look at this.. Makes me wonder if not the buyers are mislead by the selling arguments...at a quite high price.
Do they understand what they are getting? Not a Single ended 300B triode, but a push-pull amplifier consisting of a Mosfet and 300B triode in combination..
Would they buy it then?


WE:
  • The Western Electric Type 91E is a Class A, single-ended (SE), integrated amplifier for high-fidelity audio reproduction.
A WE quote from 2018 posted on AudioAsylum... "In addition to announcing the availability of the 300B, I'd like to share some of our adventures in product development. Continuing our 80th anniversary celebration of the 300B, we are unveiling a new single-ended amplifier called the 91E, as an homage to its famous predecessor the 91A, first introduced in 1936. The 91E embodies a new proprietary Class A2 parallel feed current source topology (patent pending), combined with toroidal output transformers, and microprocessor controlled automatic bias. It will achieve in excess of twenty watts per channel-a never before realized level of performance for the 300B in a single-ended circuit."
1660116185085.png


And the evaluation of the patented circuit here by TubeCad.
Patent here

From Tube CAD:"It certainly looks unconventional, but is its operation as unconventional as its appearance? First, let's add some signal relationships throughout the circuit.

Steered%20Current%20Source%20for%20%27Single-Ended%27%20Class-A%20Amplifier%20with%20Signals.png

The triode receives the input signal at its grid. As the signal swings positively, the triode increases its current conduction and its plate voltage is pulled down, which then pulls down N-channel MOSFET Q2's source voltage, which in turn provokes a decrease in the voltage drop across its drain resistor, making P-channel MOSFET Q1 draw less current. When the input signal swings negatively, the opposite occurs; the triode draws less current and MOSFET Q1 draws more current, pulling the triode's plate voltage up. The capacitor coupled output transformer relays this amplified signal to the loudspeaker. In short, push-pull operation, not single-ended functioning, as MOSFET Q1 actively varies its current flow match the triode in anti-phase, just like every other push-pull output stage.

Parafeed%20Topology%20Example%201.png

The patent claims that this a para-feed arrangement, but really is just a push-pull totem-pole arrangement of dissimilar output devices with a coupling capacitor attaching to the output transformer's primary, which allows us to use a non-air-gapped output transformer. In fact, we could just as easily use a bipolar power supply and DC coupled the output transformer primary at one end and ground the other end."
 
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sq225917

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Maybe mention your thoughts to the patent office?
 
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DonH56

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Does not fit my definition of push-pull outputs after a two-second look. Only one FET is actually driving the output; push-pull implies active devices "pushing" and "pulling" the output. I also wonder if the signal shown on the drain of the lower FET is really that large...
 

Blumlein 88

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Just looks like they are using the mosfet to bias the tube to me.

Not dissimilar to how Nelson Pass made his Aleph 0 single ended mosfet amp. One mosfet supplied current to bias the circuit and the other was the only one modulating the output signal.
 
OP
Balle Clorin

Balle Clorin

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If you read Tubecads evaluation it is hard do regard this as anything but push pull. other words seems like semantics invented by Western Electric to hide the facts. I will trust Tubecad more than WE here,
 

fpitas

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Some gray area there, but I'd think the bigger show-stopper is it's full of transistors.
 

DonH56

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I trust my textbooks and various standard definitions of amplifier topologies. The FETs bias the 300B, providing essentially an active current-source load for the tube to increase gain and perhaps lower distortion. It may also increase tube life through dynamic bias; someone like @SIY would know. I have not played with tubes for years.
 

fpitas

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I trust my textbooks and various standard definitions of amplifier topologies. The FETs bias the 300B, providing essentially an active current-source load for the tube to increase gain and perhaps lower distortion. It may also increase tube life through dynamic bias; someone like @SIY would know. I have not played with tubes for years.
Gotta agree. That high-impedance current source at the anode won't affect the bizarre loadline of the transformer. I guess a gapped transformer might be even worse. I mean, euphonic.
 

SIY

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I trust my textbooks and various standard definitions of amplifier topologies. The FETs bias the 300B, providing essentially an active current-source load for the tube to increase gain and perhaps lower distortion. It may also increase tube life through dynamic bias...
I can't see why it would.

The current source, of course, is shunted by the load so won't be terribly helpful for distortion reduction. John did his usual good job of analysis.

Maybe mention your thoughts to the patent office?

In the US, the PTO examiners will not accept any input from anyone other than the inventor and his agents. This caused me some grief in the past...
 

Tom C

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Images were taken from the Western Electric website.
Here is the schematic of the original 91A and 91B:

B2BDB5D7-D11D-44B0-92BC-58FCF94CBD00.jpeg

This is a single ended amp. Simple enough to see, there are four tubes total in this design. One is a rectifier for the power supply. The rest are for the audio signal, one for each stage of amplification. The input stage is a times one pentode, the grid of which is connected to the input signal by T1. The output (plate) of V1 is capacitor coupled to the grid of V2, the intermediate stage. The output of V2 is capacitor coupled to V3, which is the output stage, a triode. The output of V3 is connected directly to the output transformer, T2.

Here is the schematic for the 92B, a model from the same era. Rather than being a single ended design, it is push-pull:
A00022EF-8B49-4063-8ADF-C2E1DD72A5AB.png

You can see that there are two triodes for the output stage, V4 and V5. The arrangement requires an additional transformer, labeled T2, between the intermediate stage, V3, and the push-pull pair used at the output stage. The secondary of T2 is center tapped, and coupled directly to the grids of V4 and V5, which are the output pair. The plates of V4 and V5 are coupled directly to the primary of T3, the output transformer. Note that T3’s primary is also center tapped.

Mating a vacuum tube with a solid state device in push-pull, and doing it to good effect may take some uncommon engineering prowess. And to what end? My formal training and work experience is as an electronics technician, not an engineer, so take my comments for what you will. But looking at the schematic for the 91E provided by the OP, I just don’t see a push-pull design. It’s single ended. But, you already knew that from comments by people much better qualified than I.
 
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Balle Clorin

Balle Clorin

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What WE made in the 1930s does not matter here,...

Don't just look at the schematic ,read the text. The current though the mosfet may look like a constant current source, but is is in fact variable current in anti-phase with the 300B tube current. That means it does exactly the same as the "top" tube in a push and pull pair of tubes or transistors.=Push-pull amplifier.
I am not an expert either , but TubeCad's analysis and conclusion makes sense to me.
Maybe I could simulate it in Spice if I get enough info,
A closer investigation of the current and voltage swing of the lower 300B may bring some clarity. If the 300B swings less than 20watts it cannot be a single ended design that delivers 20 watts to a load

Here is the voltage and current swing required to get 19.6watts from a single 300. It has to operate far into A2 positive grid operation and have 270mA at some positive grid bias., The curves I use her are close to the original WE, I used the EML tubes myself and teste the whole characterisic with a curve tracer.A very goood 300B on paper even better than WE 300b.

I do not think this is how WE do it,the tubes would not last long even WE tubes

PS . To go into A2 as this would require a driver device with far more current capability than a ECC81, I guess some transistors are feeding the 300B also.
Nothing wrong with a hybrid design , just do not lie about it. The amplifier sounds fine and is very nice , I have auditioned it.

1660320942259.png
 
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Tom C

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Well, I will admit I don’t have the patience to pick through the text. But the input signal is connected to the input terminal of only one active device in the circuit, and that’s the vacuum tube. The Mosfets simply are not wired to operate in push-pull.
 

DonH56

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I can't see why it would.

The current source, of course, is shunted by the load so won't be terribly helpful for distortion reduction. John did his usual good job of analysis.
Thanks, I wasn't sure and didn't try to analyze it. Was thinking modulating the current source might help. Of course, when I have done something like that in the past (in the RF world), it could improve slew and reduce average power (maybe), but usually hurt distortion, so there's that... But, that was on an IC, no tubes around.

So you consider it a push-pull circuit? I did not read the article, just questioned the conclusion that it was a push-pull output rather than single-ended. Don't really care what WE Marketing has to say about it... :)
 

SIY

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Thanks, I wasn't sure and didn't try to analyze it. Was thinking modulating the current source might help. Of course, when I have done something like that in the past (in the RF world), it could improve slew and reduce average power (maybe), but usually hurt distortion, so there's that... But, that was on an IC, no tubes around.

So you consider it a push-pull circuit? I did not read the article, just questioned the conclusion that it was a push-pull output rather than single-ended. Don't really care what WE Marketing has to say about it... :)
It looks push pull to me, but a nonsymmetrical push pull with a bit of current-dumping tossed into the mix. John's rather scornful analysis is pretty dead on.
 

Blumlein 88

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I just looked at the diagram. Having read the article, yes it is a hybrid push-pull. Pretty sneaky of them.

Of course I've wondered why someone didn't make Mosfet amps that were transformer coupled like a triode since I think some of their sound is due to the transformer. I also prefer the sound of push-pull triodes to SE triodes. But then again, I don't use either anymore.
 

VMAT4

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If you read Tubecads evaluation it is hard do regard this as anything but push pull. other words seems like semantics invented by Western Electric to hide the facts. I will trust Tubecad more than WE here,
Maybe you should get an Electrical Engineering degree so you could reason this out for yourself as opposed to trusting Tubecad over WE.
But I also wouldn't trust any tube amp to be high fidelity in this day and age.
 

fpitas

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I just looked at the diagram. Having read the article, yes it is a hybrid push-pull. Pretty sneaky of them.

Of course I've wondered why someone didn't make Mosfet amps that were transformer coupled like a triode since I think some of their sound is due to the transformer. I also prefer the sound of push-pull triodes to SE triodes. But then again, I don't use either anymore.
Some of the First Watt amps by Nelson Pass use MOSFETs and transformers.
 

fpitas

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Maybe you should get an Electrical Engineering degree so you could reason this out for yourself as opposed to trusting Tubecad over WE.
But I also wouldn't trust any tube amp to be high fidelity in this day and age.
Making it push-pull will only help cancel even harmonics. We certainly can't have that.
 

fpitas

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Well, I will admit I don’t have the patience to pick through the text. But the input signal is connected to the input terminal of only one active device in the circuit, and that’s the vacuum tube. The Mosfets simply are not wired to operate in push-pull.
I kind of think of it as a mu-follower, or SRPP idea.
 

DonH56

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It looks push pull to me, but a nonsymmetrical push pull with a bit of current-dumping tossed into the mix. John's rather scornful analysis is pretty dead on.
OK, looked again, and skimmed the Tube CAD Journal article. I had not heard of TCJ, but had heard of John Broskie, though forgot he lived just a couple of hours (more like three now, traffic) north of me. Don't know him at all, just by reputation. Wish TCJ had been around back when I was actually trying to design tube circuits. Mostly preamps, though; aside from some conventional designs, my only unconventional (direct-coupled, OTL) tube amp worked fairly well -- until it failed spectacularly.

My excuse is that I've been peering at some feed-forward stuff just for fun, using dynamic current dumping to enhance the output slew rate, but all I can say is "brain fart" when looking again at the schematic. Duh. Keeps me humble, hopefully. I would've expected them to just split the input signal off to the top device instead of mirroring it around (sort-of) and needing the N-FET, and also imagine the toroidal transformer hits the latest audiophile buzz-words better than a typical EI gapped transformer with the B+ routed through it conventionally'ish. This thing looks like it was invented mostly to give the appearance (but not the fact) of a single-ended tube design, with a patent thrown in for good measure because most of us probably wouldn't do it that way (for good reason). Must be large degeneration Rs to keep the thing marginally stable without bias drifting all over...

But, it's pretty... :cool:

Edit: TCJ's front page had a link to "DVC" that I had to click to see what that meant... My first subwoofer was a servo design using a dual voice coil woofer from an Infinity IRS, using the second coil for feedback. The actual speaker used an asymmetric crossover, Arny's design I think, pretty much as John described. Sharp cookies, them guys!
 
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