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6EM7 Floating Paraphase (I know humor me please)
6EM7 Floating Paraphase (I know humor me please)
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Old 8th February 2018, 12:18 AM   #11
mashaffer is offline mashaffer  United States
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Amedeus is it just the R from the junction of R4 and R5 to ground that I am missing? I admit that I am getting a little confused with the Crowhurst circuit due to the fixed bias.
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Old 8th February 2018, 08:43 AM   #12
AmadeusMozart is offline AmadeusMozart  New Zealand
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Originally Posted by mashaffer View Post
Amedeus is it just the R from the junction of R4 and R5 to ground that I am missing? I admit that I am getting a little confused with the Crowhurst circuit due to the fixed bias.
No, it is the shared cathode. A true floating paraphase inverter can have two totally different tubes which cannot be the case if you have a shared cathode. Be aware that a cathode that is not bypassed has negative feedback, the 12AX7 drops from approx 40x amplification to approx 14.2x according to my calculation. The drawback is that when you do not bypass the cathode-heater leakage can give hum so you have to use a tube with little leakage. On another note: the 6SN7 is about the most linear double triode that you can find, I like that tube a lot.

edit: I do not like fixed bias. That is fine if you want maximum power output but you run the risk of shortened tube life and higher distortion, especially if the tubes are not well balanced. I cannot find the article anymore (but am certain about it since I queried the Audio Note setup) but for the least distortion Mullard recommended to bias the output PP a bit more towards class B than the normal AB recommended and to use individual cathode resistors.

Last edited by AmadeusMozart; 8th February 2018 at 08:55 AM.
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Old 8th February 2018, 12:45 PM   #13
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by AmadeusMozart View Post
edit: I do not like fixed bias. That is fine if you want maximum power output but you run the risk of shortened tube life and higher distortion, especially if the tubes are not well balanced.


This surely doesn't hold true universally! Fixed bias usually means small cathode currents and about two thirds of the plate dissipation rating at idle, whereas cathode bias means both full cathode current and full plate dissipation - at idle!
Best regards!
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Old 8th February 2018, 02:28 PM   #14
mashaffer is offline mashaffer  United States
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Quote:
Originally Posted by AmadeusMozart View Post
No, it is the shared cathode. A true floating paraphase inverter can have two totally different tubes which cannot be the case if you have a shared cathode. Be aware that a cathode that is not bypassed has negative feedback, the 12AX7 drops from approx 40x amplification to approx 14.2x according to my calculation. The drawback is that when you do not bypass the cathode-heater leakage can give hum so you have to use a tube with little leakage. On another note: the 6SN7 is about the most linear double triode that you can find, I like that tube a lot.

edit: I do not like fixed bias. That is fine if you want maximum power output but you run the risk of shortened tube life and higher distortion, especially if the tubes are not well balanced. I cannot find the article anymore (but am certain about it since I queried the Audio Note setup) but for the least distortion Mullard recommended to bias the output PP a bit more towards class B than the normal AB recommended and to use individual cathode resistors.
Thank you that clarifies it well.

In this case being 6EM7 based the VAS/PI tubes will be the same type by definition. In simulation a 1V peak input swings over 20V peak which in theory should be enough as the output tube bias point is likely to be around -20V and I thought the NFB would be good for linearity. I will investigate the other approach and see what happens.

For the heaters I was planning either elevated heaters or DC depending on what I have on hand (still checking my inventory).

The output stage is to be biased class A the PS for the output stage is limited to about 130V-140V so I was considering mixed bias. I have to spend more quality time with the curves and see.

Later on I will probably play with disparate tube types as well since I find the whole thing quite fascinating (yes I am a bit quirky but it is a hobby because it is supposed to be fun right? ).

The whole thing makes me wonder about something really quirky like a self split output stage using two different tube types.

6EM7 Floating Paraphase (I know humor me please)-6em72curves-jpg
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Old 8th February 2018, 02:33 PM   #15
mashaffer is offline mashaffer  United States
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Originally Posted by kodabmx View Post
On small power amplifiers I usually use this acrosound circuit.
For higher power amps I use split phase (cathodyne), and I've never used an LTP.

Click the image to open in full size.
Thank you for joining in. Interesting circuit. What is the reason for different bias points for the two halves of the AX7?
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Old 8th February 2018, 02:53 PM   #16
mashaffer is offline mashaffer  United States
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Is this more proper?

6EM7 Floating Paraphase (I know humor me please)-6em7_paraphaseschem_sepcath-png
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Old 8th February 2018, 03:15 PM   #17
audiowize is online now audiowize  United States
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Quote:
Originally Posted by Kay Pirinha View Post
This surely doesn't hold true universally! Fixed bias usually means small cathode currents and about two thirds of the plate dissipation rating at idle, whereas cathode bias means both full cathode current and full plate dissipation - at idle!
This isn't necessarily true, but I will agree that if you are designing a class B push-pull amp, you can have very low PD at idle and you'll need fixed bias by design.

It's not impossible to design a cathode biased output stage that runs at 2/3 PD.
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Old 8th February 2018, 04:17 PM   #18
Ketje is offline Ketje  Belgium
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Originally Posted by mashaffer View Post
Is this more proper?
With R1 and R8 to ground, R9 is useless, even contra-productive (makes the open loop gain of the x(-1) stage smaller).
And one cathode resistor without decoupling capacitor.The two triodes function with opposite phase, current in the cathode resistor doesn't change.If it does change the circuit is out balance.In that case, since the circuit is a (not very long)LTP, there is some correction.
Mona
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Old 8th February 2018, 06:43 PM   #19
Kay Pirinha is offline Kay Pirinha  Germany
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Originally Posted by Ketje View Post
With R1 and R8 to ground, R9 is useless, even contra-productive (makes the open loop gain of the x(-1) stage smaller).
This resistor, in line with the voltage divider between both anodes, appears to be essential with the original Floating Paraphase idea. Their value shold be much smaller w.r.t. the following stage's grid leak resistors, though.
Best regards!
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Old 8th February 2018, 07:14 PM   #20
audiowize is online now audiowize  United States
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What is the purpose of R5?
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