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Old 8th March 2007, 07:44 AM   #1
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Default class AB2 who's using it for hifi?

Despite it's home in MI apps with higher peak currents it actually makes the tube output stage behave better on overload. I'm using diodes across the neg biassing resistor with Cath on signal. Anyone else tried it ?

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Old 8th March 2007, 09:06 AM   #2
Yvesm is online now Yvesm  France
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Bah !

The grid already forms a diode with the cathode !
There is no definitive advantage to add another one in parallel.

Going in AB2 may be the best or the worst, it depends more on the plate load than anything else.

However, this voids the use of any link cap in the grid wich disturbs the bias being quickly charged by the grid current and slowly discharged throught grid leak resistors.

This means also that neg bias source MUST have a low impedance/resistance.

For exemple:
http://www.dissident-audio.com/PP_6L6/Page.html

This amp goes in AB2 on signal peaks when the load impedance value is low.
Of course, peaks could still be somewhat clipped, but DC coupled cathode follower driver insures that the bias is not altered and that there is no recovery delay.

Yves.
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Old 8th March 2007, 10:57 AM   #3
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With source or cathode followers direct coupled t the output stage grids and the proper loading and tube choice, it's a fine technique. I've used it with 6L6-types very successfully.
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Old 8th March 2007, 12:03 PM   #4
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Quote:
Originally posted by Yvesm
Bah !



This amp goes in AB2 on signal peaks when the load impedance value is low.

Yves.
Yes .....I'm using a conventional low Z driver Zout app 2K. Recovery delay performance is good and circuit can take +15dB extra global nfb without going into oscillation. Square wave at 10Khz is very square with no overshoot/ringing into dummy L.

A couple of 6550B's in fixed bias and 4.5Ka-a (may seem a bit high) but have a couple of surplus trannies on hand. 20W is attainable from B+ 350V; Iquies = 90mA. THD @ 20W 1Khz = 0.2%

Putting a diode + 12K res in series across each grid leak(68K) definitely improves HF & Mid R response & thd just before clipping. There may be better methods but this one is the simplest to poke about with. This is true AB2 i.e forcing positive drive and the anode current doubles (dangerously to tube self destruct), one must select series resistor of diode to limit this). Avoiding s/c the output terminals !!
There is a sinister side to AB2.....sound quality close to clipping is pronounced harsher than class AB1 or A.....However since nearly all the op amps in the music industry operate in class B, I cannot see no reason to follow the same trend ?

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Old 8th March 2007, 12:13 PM   #5
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Most of that harshness goes away if you drive the grid with (gasp) a low impedance source follower. That was probably the biggest improvement I noticed when I went from a CF driver to solid state.
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Old 8th March 2007, 01:09 PM   #6
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Thanks for tip.....shall try even lower drive Z......Silicon diodes do have a reputation for snappiness and adding to harmonics....even though I'm using soft recovery ther'e still too fast for audio apps. A tube doube diode is very well worth a try.
The rest of the AB2 performance hangs on the power supply.

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Old 8th March 2007, 04:35 PM   #7
Yvesm is online now Yvesm  France
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Quote:
Originally posted by richwalters

. . .
The rest of the AB2 performance hangs on the power supply.

richj
Yes ! Specialy when using triodes.
With penthode/tetrodes just regulate the screens supply.

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Old 8th March 2007, 05:17 PM   #8
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I strongly suspect that AB2 conditions would make softer sounding tubes "sit up right" and sharpen up sound definition. In another thread (6550 tubes) the arguement persists (often) against the same vendor for an identical tube.
In another way, a duff slack tube would immediately be noticed in AB2. I'm going to AB2 to lighten-up my Peerless system whatever make of tube.

I've been around with tubes for 40 years......would others agree that true AB2 could be considered as " equivalent current dumping " ?

richj
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Old 8th March 2007, 05:21 PM   #9
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Quote:
Originally posted by richwalters


I've been around with tubes for 40 years......would others agree that true AB2 could be considered as " equivalent current dumping " ?

richj
Yes, very similar.
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