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So that's why paraphase PIs don't sound horrible...
So that's why paraphase PIs don't sound horrible...
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Old 17th August 2018, 02:31 PM   #1
mashaffer is offline mashaffer  United States
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Default So that's why paraphase PIs don't sound horrible...

I was investigating a possible guitar amp technique of intentional throwing the PI out of balance to increase distortion at lower output levels and noticed that one of the main effects in simulation is a distortion spectrum that looks more SET like. An imbalance of about 2 or 3 dB looks very much like a classic SET.

Perhaps this is why many have reported paraphase amps sounding much better than expected. Not that we would want to intentionally throw things out of balance in Hi-Fi (although it might be an interesting mood control for small jazz combos ) but it may shed some light on why slight imperfections don't cause "in your face" nastiness.
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Old 17th August 2018, 05:59 PM   #2
6A3sUMMER is offline 6A3sUMMER  United States
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mashaffer,

I like your idea. But some people have the wrong idea about "balanced phase splitters" because a large number of them are Not balanced.

Making a phase splitter to be properly balanced is not always easy.

The best way for balance is with matched plate resistors, and with the hard connected cathodes connected to a Real (extremely high impedance) current source returned to a negative supply.

The splitters that have a pseudo current source (a resistor connected to the common cathodes) require special adjustment to the plate resistors, or it will not be balanced. And if you change the splitter tube, you need to re-balance by adjusting the plate resistors again.

The output tubes are not always balanced in the circuit voltages and current that they are run at (not just tested at the current and voltages that a tube tester uses to match). Any non-balance of the output stage will be 2nd order distortion. That distortion will Either add to, Or partially cancel any 2nd order distortion of the phase splitter.

Thee are some Hi Fi designs that do not have a very well balanced phase splitter, but are done as an approximation.

"Serial Phase Splitters" (driving the 2nd stage from a resistive divider off the 1st stage output), are extremely hard to match at all frequencies. But even more than that, the 1st stage produces 2nd order distortion, and the 2nd stage's 2nd order distortion cancels that. What you have is the 1st stage driving one output tube with 2nd order distortion, and the 2nd stage driving the other output tube with No 2nd order distortion. That is Not balanced. Understand?

The Dyna Stereo 70 exceeds the max rated filament to cathode voltage on the 7199 triode concertina phase splitter. After many hours of operation, the filament to cathode interface becomes very leaky. That causes the concertina to become un-balanced. it now has a larger signal on the plate, and a smaller signal on the cathode. Same problem of unbalanced signals to the output tubes. Oh, and by the way, it Sounds Good. Seems like you have re-discovered something.

Things are not always as simple as at first they seem.

Last edited by 6A3sUMMER; 17th August 2018 at 06:15 PM.
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Old 17th August 2018, 07:00 PM   #3
mashaffer is offline mashaffer  United States
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Things are not always as simple as at first they seem.
Undeniable truth.
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Old 17th August 2018, 09:17 PM   #4
jhstewart9 is online now jhstewart9  Canada
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Quote:
Originally Posted by mashaffer View Post
I was investigating a possible guitar amp technique of intentional throwing the PI out of balance to increase distortion at lower output levels and noticed that one of the main effects in simulation is a distortion spectrum that looks more SET like. An imbalance of about 2 or 3 dB looks very much like a classic SET.

Perhaps this is why many have reported paraphase amps sounding much better than expected. Not that we would want to intentionally throw things out of balance in Hi-Fi (although it might be an interesting mood control for small jazz combos ) but it may shed some light on why slight imperfections don't cause "in your face" nastiness.
Isn't that what one version of a guitar pedal is meant to do, insert distortion at low level. Then put it thru a linear amplifier to get what one would expect, lots of even order distortion. On that channel.

Like the 'firefly', from a few years ago, last stage one side driven.
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Old 17th August 2018, 10:22 PM   #5
Sodacose is offline Sodacose  United States
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I think the output stage of that Firefly is sometimes referred to as Self Inverting Push Pull (SIPP). Because of the shared cathode load and grounded grid on the "undriven" tube, it's acting like a differential amplifier.
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Old 18th August 2018, 12:03 AM   #6
6A3sUMMER is offline 6A3sUMMER  United States
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Well . . . that is an interesting circuit, but . . .
The firefly' "current source" of the self inverting output stage is a very poor one (440 Ohm resistor).
At low and medium signal levels, the output stage generates more 2nd harmonic than 3rd harmonic distortion (even if the signal from the driver has very low distortion).

If you put a Real current source instead of the 440 Ohm resistor, the output stage will generate more 3rd harmonic than 2nd harmonic distortion.

Of course the distortion of the earlier stages can cause more combinations than just the simple distortion of the output stage by itself.

Last edited by 6A3sUMMER; 18th August 2018 at 12:05 AM.
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Old 18th August 2018, 12:28 AM   #7
jhstewart9 is online now jhstewart9  Canada
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Here are some results of actual tests on some versions of the circuit. I managed to stuff the report into two parts to fit here.


Bottom line, lots of hype but no version works as advertised.
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Old 18th August 2018, 05:40 AM   #8
PRR is offline PRR  United States
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Quote:
Originally Posted by jhstewart9 View Post
Here are some results of actual tests on some versions of the circuit....
Here's a 1946 math-study leading to optimizations:
http://www.valvewizard.co.uk/SelfSplit.pdf

Linked from Merlin Blencowe's site.
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Old 18th August 2018, 04:03 PM   #9
20to20 is offline 20to20  United States
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Originally Posted by 6A3sUMMER View Post
The Dyna Stereo 70 exceeds the max rated filament to cathode voltage on the 7199 triode concertina phase splitter. After many hours of operation, the filament to cathode interface becomes very leaky.
This is incorrect and commonly quoted and repeated without checking. The 7199 cathode is allowed to be 200v above the heater voltage. It specifically states in the 1963 RCA manual that the heater may be 200v (-) with respect to the cathode. The typical cathode voltage on the PI is @100v. It must be individually checked because it varies tube to tube which is why the voltage is not specified in the ST-70 manual.
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Old 18th August 2018, 04:25 PM   #10
jhstewart9 is online now jhstewart9  Canada
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Quote:
Originally Posted by PRR View Post
Here's a 1946 math-study leading to optimizations:
http://www.valvewizard.co.uk/SelfSplit.pdf

Linked from Merlin Blencowe's site.
That is quite a good paper & very interesting. I believe the author has made two mistakes, but not in the math. First is assuming that small signal values of tube parameters will still work with very large signals. They don't.

When I first looked at this problem I used Electronic Workbench to simulate the cct. My first thought was to replace the tail resistor with a choke. That was a big improvement. Then I tried a FET in the tail, that was good too.

But both sidestepped the problem of large signals. The tube models used don't work for large signals. That became obvious in a test mule. It was impossible no matter what tail was used to pull the RHS cathode down far enough.

The other mistake in my opinion is not doing objective tests on the cct they claim to have built. Both kinds of mistakes are often made.
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File Type: jpg Merge Workshop ABC.jpg (312.7 KB, 128 views)
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