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Old 4th December 2010, 01:27 AM   #1
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Default 809's in Class B P-P w/feedback

Greetings,
How horribly sounding will 809's in class B push-pull be with feedback or should I be asking how hard would it be to implement feedback in a class B push-pull amp.?

Thanks,
Ray
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Old 4th December 2010, 02:43 AM   #2
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In true class B P-P operation (40-60% efficiency), crossover distortion will be your worst enemy. You'll need a very tightly coupled output transformer of the best quality you can get. Plate current virtually stops flowing during the negative half cycle. Because of this a very stable power supply with good regulation is required as the plate current rises rapidly to a large value on the positive half cycle.

Crossover distortion can be corrected to a large extent by applying positive bias to the cathodes of the output tubes with a small resistor in series to ground. The exact amount of this positive voltage will need to be determined by experiment since 809s are low bias tubes to begin with. Perhaps a volt or two will be enough. Further reduction of crossover distortion will be helped by lots of inverse feedback in the usual manner. But the real challenge will be the output transformer. Sound quality will largely depend on it's construction. But since there are no class B high quality audio amplifiers in use that I can think of, don't expect to produce a good one. Even the McIntosh's like the MI-200 aren't true class B. They're mostly AB2
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Last edited by HollowState; 4th December 2010 at 02:56 AM.
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Old 4th December 2010, 03:56 AM   #3
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Originally Posted by rsumperl View Post
Greetings,
How horribly sounding will 809's in class B push-pull be with feedback or should I be asking how hard would it be to implement feedback in a class B push-pull amp.?
Pretty awful, I'd say. You have to remember that when they're talking about Class B audio, they're talking AM plate modulators, where fidelity isn't a consideration. With Class B, X-over distortion is inevitable, and it truly sounds horrible. It's not a consideration with RF amps, since these have LC tuners and/or BPFs to remove whatever harmonic distortion that produces. For PA amps and modulators, you don't care if the sound quality isn't what it should be anyway.

If you put 'em into Class AB2 instead, and use source follower grid drivers, then they'll probably sound pretty good, especially if you include NFB to take care of the otherwise higher than normal r(p).

Then, it's just a question of finding a good loadline, and experimenting.

You can probably get Jack at Electra Print to make you the OPTs. (Edcor also does designs, I believe).
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Old 4th December 2010, 05:07 AM   #4
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If you do build it I'd love to see the measurement results. People will drop $20k on a class B theater amp without blinking...
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Old 4th December 2010, 05:52 AM   #5
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What was called Class B (-9V G1, 20 mA/tube idle, 3.7W dissipation on first grids) when 809 was designed now is called class AB2, I believe.
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Old 4th December 2010, 04:15 PM   #6
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Originally Posted by Wavebourn View Post
What was called Class B (-9V G1, 20 mA/tube idle, 3.7W dissipation on first grids) when 809 was designed now is called class AB2, I believe.
Yes! The 809 data sheet (has anyone else looked at it today?) shows "Class B" as having 40 mA zero signal plate current; that's 20mA per tube. This is today's definition of class AB. The 3W grid power is at max signal. Grid power is very small at idle.

And it's a great example of the old assumption that class B amps are driven into grid current. Notice thay don't call it "Class B2" but there is grid current.

Today we would call it class AB2 with small overlap.

Crossover distortion:

The turn-off glitch occurs whenever one side of a switching push-pull amp turns off. Even in AB, where there is overlap, there is still a turn-off glitch. The glitch does seem to be smaller if one side turns off after the other has turned on, but it's still there and is an effect of OPT leakage inductance.

Build it!

I would not worry about the definition of class B, built the 809 amp with whatever idle current and positive grid drive makes sense for your chosen load line, and tweak it from there, just like any other amp. The only thing these tubes need vs. a "regular audio triode" is provision for high plate resistance and grid current drive (MOSFETs are our friends). Treat it like a pentode and apply local feedback. It can probably sound amazing and be really pretty with the bright filament and all...

Cheers,

Michael

Last edited by Michael Koster; 4th December 2010 at 04:20 PM.
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Old 4th December 2010, 05:20 PM   #7
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Quote:
How horribly sounding will 809's in class B push-pull be with feedback or should I be asking how hard would it be to implement feedback in a class B push-pull amp.?
What output power are you aiming at ?

This tube has potential for 145 W out as PP-connected.
Quite sufficient level for most purposes.

This tube can be used ofcourse at lower supply voltages and power levels but allways it needs to be driven with power i.e. grid current.
So you need a real driver stage.

I have experimented with relatively similar triode 811A.
A 6N6P double triode as parallel connected cathode follower works fine as a driver stage with it.
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Old 4th December 2010, 05:52 PM   #8
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Thanks for the info Michael. Would you have any schematics of circuits I could try?

Thanks,
Ray

Quote:
Originally Posted by Michael Koster View Post
Yes! The 809 data sheet (has anyone else looked at it today?) shows "Class B" as having 40 mA zero signal plate current; that's 20mA per tube. This is today's definition of class AB. The 3W grid power is at max signal. Grid power is very small at idle.

And it's a great example of the old assumption that class B amps are driven into grid current. Notice thay don't call it "Class B2" but there is grid current.

Today we would call it class AB2 with small overlap.

Crossover distortion:

The turn-off glitch occurs whenever one side of a switching push-pull amp turns off. Even in AB, where there is overlap, there is still a turn-off glitch. The glitch does seem to be smaller if one side turns off after the other has turned on, but it's still there and is an effect of OPT leakage inductance.

Build it!

I would not worry about the definition of class B, built the 809 amp with whatever idle current and positive grid drive makes sense for your chosen load line, and tweak it from there, just like any other amp. The only thing these tubes need vs. a "regular audio triode" is provision for high plate resistance and grid current drive (MOSFETs are our friends). Treat it like a pentode and apply local feedback. It can probably sound amazing and be really pretty with the bright filament and all...

Cheers,

Michael
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Old 4th December 2010, 07:16 PM   #9
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Re: Michael
"The turn-off glitch occurs whenever one side of a switching push-pull amp turns off. Even in AB, where there is overlap, there is still a turn-off glitch. The glitch does seem to be smaller if one side turns off after the other has turned on, but it's still there and is an effect of OPT leakage inductance."

I think there may be a couple of ways to fix that leakage glitch. (other than Mac OT or Circlotron or Twin Coupled schemes) Seeing as how the leakage L causes the plate voltage to swing up wildly positive after the tube cuts off. If one added "nominal gain" (0 dB NFdbk, ie, Closed Loop gain set to match the tube's normal gain into the load) Schade plate feedback resistor(s), the tube would be held on just enough to absorb the glitch (ie, correct tube output voltage maintained). Of course a regular Schade NFdbk should work too.

Then there is "magic" Schading, which uses current sensing Schades and a differential driver to subtract them. This forces the tubes to make a smooth current handover. No abrupt cutoff is possible then. The crossover region is widened from what the bias and intrinsic tube curves overlap would normally obtain.
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Last edited by smoking-amp; 4th December 2010 at 07:23 PM.
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Old 4th December 2010, 07:20 PM   #10
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Then there is "magic" Schading, which uses current sensing Schades and a differential driver to subtract them. This forces the tubes to make a smooth current handover. No abrupt cutoff is possible then.
Do you mean local FB by current around each tube to minimize glitches, then global FB by voltage around both of them to minimize output resistance?
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