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SE amp: Please suggest a better operating point

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Well, I built this 12B4A amp which sounds very nice but every now and then, I get this nasty distortion on loud transients, .. sort of a sharp buzzzzzzz sound.

The schematic:
An externally hosted image should be here but it was not working when we last tested it.


The input is clean (PC sound card outputting sine waves from winIsd, ... scoped it out).

I scoped the driver (past the coupling cap) and the output (at the secondary of the OPT) and it seems that the driver cuts off early (see pic). The driver is biased with a red LED @ 1.7 V .
The output tube eventually clips (or is that ringing?) and the cut off wave form of the driver is superimposed upon this.

I'd ideally like to increase the bias on the driver to avoid the cut off, but am not really sure what to do, as I don't have that much voltage to play with (about 210V). I have a CCS in place of the plate resistor. I tried increasing the current through the CCS
but that didn't help the issue. I'm currently at around 2mA through the 12AT7 driver.

Sorry for the poor cell phone picture quality.

Driver cutting off
 

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zobsky said:
The driver is biased with a red LED @ 1.7 V .
The output tube eventually clips (or is that ringing?) and the cut off wave form of the driver is superimposed upon this.

I'd ideally like to increase the bias on the driver to avoid the cut off, but am not really sure what to do, as I don't have that much voltage to play with (about 210V). I have a CCS in place of the plate resistor. I tried increasing the current through the CCS
but that didn't help the issue. I'm currently at around 2mA through the 12AT7 driver.

At a constant 2mA through the 12AT7 and 1.7V on the cathode, you should expect about 110V at the plate.

If you get too close to the Ug=0V line on the curves, you're going to cutoff the tube. I'm sure this happens before you actually hit the 0V line, but I don't know how close you can get. I think you are on the right track with wanting to increase the bias on the 12AT7, but if you have only 210V to play with at the plate you're probably out of luck. Increasing the bias voltage even half a volt will put the idle voltage of the plate around 170V, and your power supply will have no volts left for the upswing. Get a higher voltage supply, or keep your input signal under 0.7 Vrms.

You might also want to think about what might happen if you ditch the CCS plate load and replace it with a resistor. If you use a relatively small resistor (10K?) I think you'll significantly reduce the gain of the driver stage. Maybe that's not what you want, but it'll probably keep it from clipping.

 
Thanks
to summarize my earlier posts, I would ideally like the output to clip before the driver

reducing the gain on the driver might not be the best way to do this. I'll see if increasing c1 in the filter gets me any significant gains in voltage

I'll also see if I have any other 9 pin dual triodes that might work and deliver at least 3 ma
 
The drive can only rise 25V before the output's grid conducts.
With driver Mu=60, that equates to only .417V at the input.

Your flat spot may simply be the drive signal shorting to the
output's cathode through that grid's forward bias. That seems
the most likely.

If it was your input's grid doing the distorting, wouldn't your
drive signal's flat spot have been on the lower side of the trace?
 

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Kenpeter spotted your "problem". It must be the output grid that's drawing current that causes the clipping at the driver output.
At 2mA and 1.7V bias and a load line of 470K (a ccs parallell with the 12b4 grid resistor should not add significant loading) the 12at7 should be able to swing at least +/-100Vpk.
There is NO way the driver clips before the output tube in this setup.
Easiest way to check is to diconnect the output tube and measure the driver output swing. Compare this with the bias of the 12b4.

!Just make sure that you don't overload anything when you remove the current draw of the output!

Edit:
Sorry was too quick. the ccs will run out of voltage, but you should at least be able to swing more than +/-50Vpk which shoulb be more than enough.

Regards
/Olof
 
Drive from the IXYS's K terminal instead of directly
from the 12AT7's plate. And perhaps increase the
value of the drive coupling cap.

Should behave as a Mu Follower with 100ma of
drive capability. Of course this is NOT A2, the cap
will "pump" negative away from 0V conduction.

The original problem will still be there, but less
severe, and perhaps more tolerable. Requires
only a very minor circuit change to give it a try.
 
Why a grid that can source 100mA driven by a CCS that
sinks only 2ma, would look anything but flat when it clips?
Especially if you measure on the driven side of the coupling.

I quote Zobsky, "I scoped the driver (past the coupling cap)"

Again, the drive signal is clipping hard and flat on TOP.
Explain your alternative theory how it got that shape if
it isn't clipping on the output's grid's forward conduction.
 
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