Class AB2 - Kink, how and why?

[tricomp]

Hi,

I was doing a small audio-amp with Compactrons and ended up with what you see in the schematic attached. Sorry for the rather chaotic way it's been drawn.
In order to extract maximum output I choose to drive the 6AD10's in AB2.
It turned out that this is a though little tube. Spec's say 12W combined A/G2 dissipation but I ran it with Va around 350V and dropping ~26W without red-plating. After the abuse I went to safer grounds as you see in the schematic. Rather fancy having the CF built-into the OP-Tube
With some heavy-duty Edcor OPT is sounds very good indeed, but seeing the waveforms on a scope I wonder about just one thing:
What is the small 'kink' you see when you feed the amp a sinewave and amplify the section just about when 6AD10 G1 starts dropping it's impedance way down to a few 100 Ohms? Scope on 6AD10/Grid1.
It occurs when G1 reaches around +1V and is very slight and barely noticable. Obviously a distortion-analyzer will notice it.
The CF in my construction will happily swing G1 to +12-15Vpeak without any further distortion. The 6AD10 give in at around +3 to 4V at G1 so this amplitude is not needed, just notiched.
Setting another CCS-current has no influense on the 'kink' as well as changing Va of the CF.
Actually the ONLY way I found to influence the 'kink' was by using another OPT..?!

Please shed some light, thanks...

rgds,

/tri-comp

[SpreadSpectrum]

Did you have a picture of the scope screen?

[tricomp]

I will tonight.
Thanks.

[SpreadSpectrum]

My first thought is that the driver may not be keeping up. If this is the case, distortion should be detectable on the grid of the output tube. If not, the screen current might be high in the G1 positive region of your load line and the screen supply might be sagging. If both of those are not the case, maybe the tube in question just doesn't have very good positive grid characteristics. That probably isn't likely, though.

[tricomp]

The distortion IS detected on the CF/G1/CCS common connection.
That's where I notiched it in the first place.
Not reasonable to think B+ will sag. Look at the cap's

I'll post a scope-pic tonight and check for G2 variations.

rgds

[Osvaldo de Banfield]

Is the supply line steady when clipping? Normally it is used a choke input supply for Class AB2 amplifiers because of its voltage stability.

[Michael Koster]

I think I know what's happening

The 6AD10A section 2 (CF driver) unit has 2500 uMho transconductance. As a cathode follower with CCS the equivalent drive impedance is 400 ohms. This is equivalent to a 400 ohm resistance in series with an ideal voltage source.

As you drive the grid positive, it begins to draw current. The higher the positive grid voltage, the lower the effective impedance. It's like a diode and will reach a few mA of current with just a few volts positive. at 1mA you're already losing 0.4V of drive...

So what is happening is that the slope of the drive signal at the section 1 grid is changing as you drive into the grid current region of A2. This will be audible and seen as both even and odd harmonic distortion.

I'm not sure without testing how much grid current you'll need to source, but 400 ohms seems way to high driver impedance for a practical A2 amp. I would add a power drive circuit. One MOSFET, one resistor, one zener diode to protect the gate. Solder right over the tube socket ;-)

Look up my thread on the Meteor amp for some example grid current measurements and clean class A2 operation.

[tricomp]

Michael,

CF is being run as a Triode.

I get your point, but it's ONLY the instant when G1 starts to draw current the distortion is apparent.
Immediately after the 'kink' or distortion-point the curves look very normal up to around +14V.

FET-point taken. No biggie if it helps, but then no combined CF/OP-Tube which I think is rather nice.

I'll check-out your Meteor later. At wrk now..

[Michael Koster]

Originally Posted by tricomp

Michael,

CF is being run as a Triode.

I get your point, but it's ONLY the instant when G1 starts to draw current the distortion is apparent.
Immediately after the 'kink' or distortion-point the curves look very normal up to around +14V.

FET-point taken. No biggie if it helps, but then no combined CF/OP-Tube which I think is rather nice.

I'll check-out your Meteor later. At wrk now..

I guess I'll need to see the scope shot to be sure. In my experience there is a kink if the grid circuit has significant resistance. The top looks normal because the grid load is progressive and hardly impacts the sine wave shape. If you used a triangle or ramp you may see the slope change.

The MOSFET is about 20 ohms equivalent impedance and will pretty much eliminate the kink. Triode CF + CCS load plus MOSFET follower may be a really nice A2/AB2 driver. You load the previous stage with tube grid, you still have the triode characteristic due to the CF changing plate-grid voltage with signal swing, the MOSFET gate is well driven by the low impedance CF, and it has super low output impedance.

Another benefit of CF + MOSFET for class AB/AB2 drive is the output tube grid need not be driven negative beyond cutoff at the highest plate voltage excursion. If you're using a MOSFET drive you need to provide for the full negative grid excursion anyway because of the -Vgs limit. With a CF plus MOSFET you can get away with a low voltage (-) driver supply rail. I'm planning to use this in my next G1+G2 dual grid drive amp, which needs only positive grid voltage.

[tricomp]

Sorry for the late posting.
I had 500+ vacation-pic's from The Nile that needed to go from the camera before taking these
Anyway, here we go.
Yellow = G1
Blue = Speakerterminals with dummyload, 8R, Full-OP = 12.25W
First 3 pictures show 1KHz Sine with the 'kink' developing when turning up volume
Next 3 shows 1KHz Ramp, same development.

You'll clearly see the distortion in the 'Exaggerated' pictures with volume beyond reasonable.
G1-curves are 'bending' just around +2V and just before the output starts flat-topping.

Next posting shows relations with G2.