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Old 13th November 2008, 03:03 AM   #11
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Default Re: Re: Re: Re: 6L6GC AB2 Amp

Quote:
Originally posted by N1ESE
The '2' means that the grids ARE driven positive and grid current will flow. An output stage in which there is grid current such as AB2 or B must be transformer coupled instead of R-C coupled. If R-C coupling IS used the grid behaves like the plate of a diode and rectifies some of the audio signal which makes the grids more negative. Instead of class AB2 you would have a kind of moving class AB1 and a lot more distortion than you had counted on. If you were to attempt class B with R-C coupling, the extra grid bias could actually bias the tubes beyond cutoff and lead to heavy distortion. The rise in average, DC, plate current may be anywhere from 50 % to 200 %.
I'm not sure that's it; I think HollowState is talking about something else, since we are discussing DC coupling via source followers, not transformer coupling or RC coupling.

I am SO interested because I am thinking about an A2 6L6 SET myself.
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Old 13th November 2008, 03:20 AM   #12
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Victor, are you sure you aren't confusing AB2 with a unity-coupled output stage?

AB2 could have high quiescent currents if you set it up that way. In fact, it could be only slightly different than A2.

Quote:
An output stage in which there is grid current such as AB2 or B must be transformer coupled instead of R-C coupled
...or DC coupled as from a follower stage

Quote:
Is there something else about AB2 mode that results in worse crossover distortion than plain old AB?
No.
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Old 13th November 2008, 03:22 AM   #13
SY is offline SY  United States
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Quote:
Also, a question on the post above. I thought AB2 was when the tube's grid is driven positive. I had read that when this happens, the grid impedance suddenly drops, and unless the driving stage is designed with adequate capabilities, gird current starvation can result. Is there something else about AB2 mode that results in worse crossover distortion than plain old AB?
This is correct- a very low impedance driver is needed because of the drastic change in grid impedance at the drive signal approaches then exceeds zero volts between cathode and grid. In the old days, a transformer or cathode follower was used. Nowadays, even us old die-hard tube guys have to admit that a source follower does better. I've been using them in AB2 drive without seeing anything like crossover distortion. And likewise, I've used them in screen drive, which is even more demanding of the driver.

The advantage of a follower over a transformer, besides bandwidth and lower source impedance and distortion is that it's not a stepdown. This reduces the distortion of the driving stage. It's a win-win.
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Old 13th November 2008, 03:58 AM   #14
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Quote:
The '2' means that the grids ARE driven positive and grid current will flow. An output stage in which there is grid current such as AB2 or B must be transformer coupled instead of R-C coupled.

Not RC coupled is correct, but only an interstage transformer is too restrictive. A voltage follower DC coupled to the O/P tube control grid is perfectly satisfactory, as it has the requisite low impedance and high current. FWIW, I like the IRFBC20 MOSFET for its low reverse transfer capacitance and excellent current handling capability. The high transconductance is nice too.
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Old 13th November 2008, 04:36 AM   #15
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Chris,

The Tamura F-684 is rated for only 30 W. You will not be able to go "whole hog" with this idea. UL mode "finals" operating in Class "AB1" should work out for you. Triode wired Class "AB2" is a possibility.

DC coupled MOSFET drive can be a distinct plus in an "AB1" amp. There are ZERO worries about blocking distortion.
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Old 13th November 2008, 05:43 AM   #16
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Quote:
Originally posted by SpreadSpectrum
Victor, are you sure you aren't confusing AB2 with a unity-coupled output stage?
Not really. Unity coupling is just a term meant to describe the way the output stage is coupled through both the anode and cathode equally and tightly...thus in unity. It does not, in itself, determine class of operation.

Quote:
Is there something else about AB2 mode that results in worse crossover distortion than plain old AB?
Yes. It has to do with how much of the cycle a tube is conducting or is cut-off. In class A the tube is biased so that plate current flows constantly for the entire cycle. In class B the tube is biased at or near cut-off and plate current flows during the positive cycle only. In between these two is class AB. it is biased so that plate current flows for more then half, but less then the entire cycle.

Class AB operation may be subdivided into classes AB1 and AB2. Subnumber 1 indicates that no grid current flows during any part of the cycle. Subnumber 2 indicates that grid current does flow during some portion of the input cycle.

Now it should be realized that the positive peaks that produce grid current also have negative peaks that extend the cut-off period. In a push-pull transformer coupled circuit, it is this extended off time during the cycle's transition that produces greater crossover distortion. In transformer-less circuits (like SS) it is often refered to as notch distortion.

Victor
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Old 13th November 2008, 06:06 AM   #17
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The first thing I will do is admit that much of this is above by head!

I understand that the transformer I have is limited to 30 watts. I don't think that a 30 watt maximum is going to be a real limitation regarding my speakers etc. Not so interested in ultimate power output, wanting good quality...

Anyway, Eli, I am sure that your advice is sound. That is why I am asking on these forums So AB1 UL 6L6 output, IFRBC20 mosfet follower, ECC99 LTP, driven by 12AT7? Will have to wait till I get home to consult Morgan as I am a bit clueless as to how to set this all up ...

Thanks so far, I am glad this appears to attracting some interest. I am hoping there are others wanting to start a similar project to share the development questions and answers!

Cheers,

Chris
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Old 13th November 2008, 11:44 AM   #18
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Quote:
In a push-pull transformer coupled circuit, it is this extended off time during the cycle's transition that produces greater crossover distortion.
Couldn't disagree more. If both tubes are conducting through and around the zero crossing, there's no crossover distortion. If you have a distortion analyzer that shows residual, it's easy to see the effect of underbiasing and proper biasing. The glitch at zero crossing just isn't there when the amp is biased properly in AB.
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Old 13th November 2008, 12:17 PM   #19
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The glitch when one tube cuts off doesn't occur at zero crossing, so I would call it "notch" distortion rather than "crossover". Ultralinear or triode operation usually avoid the cutoff region and this type of distortion.
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Old 13th November 2008, 03:20 PM   #20
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Quote:
Originally posted by SY
Couldn't disagree more. If both tubes are conducting through and around the zero crossing, there's no crossover distortion. If you have a distortion analyzer that shows residual, it's easy to see the effect of underbiasing and proper biasing. The glitch at zero crossing just isn't there when the amp is biased properly in AB.
For the condition you have stated above (biased properly in AB and conducting through and around zero) you are correct. But you have missed my point. Class AB2 is a lot closer to class B, especially on strong peaks, then it is to A. A cheap O-T with less then the best coupling may perform poorly at this level. That is my point. Not that it can't or won't work. BTW, clipping is also greater in class AB2 operation then class AB.
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