Asymmetrical Clipping

What do your ears tell you about the sound?Your scope is a tool,yours ears are the final judge.

 

Try some different cathode resistors til you get what you want to hear.Does it have negative feedback?If so,tweak the resistor til you get the cleans you want.

 

It is very common to have asymmetrical clipping in SE amp. If you are going for maximum power with maximum unclipped symmetrical signal, adjusting bias might not be enough. Changing the load line (output impedance) might be required. Plus changing screen voltage might also be needed to change the curves. (Hopefully the transformer is not saturating and causing the clipping)

 

many single-ended amps clip asymmetrically because [as others have eluded], the B+ is too high. Because of how high it is, the current needs to be lowered to keep from cooking the output tube. The result of the lowered current is the asymmetry you see.

A blackface champ is a perfect example. B+ is way too high.

 

It sound like it's biased way too hot.

Just because it's a single-ended and Class A does NOT mean that you need to bias it up to 100% of its dissipation rating, or anywhere near it. The dissipation rating is simply a measure of how much power the tube will take before it starts to suffer damage, it tells you nothing whatever about the correct operating point - contrary to almost everything you will hear or see written about 'how to bias an amp'.

Correct bias is determined by the operating point and NOT the tube dissipation, but its difficult to get this across in the face of almost universal wrong information about it...

Biasing to 70%, 90% or whatever only works if the amp has been designed to use the maximum possible power for the tube, and even then are simply rough upper limits for tube reliability depending on the configuration.

If the amp is designed to put out far less than the maximum capability of the tube, you don't want to bias it anywhere near as hot as it will go, it does nothing other than increase the load on the power supply and the DC current in the OT, and you will also cause it to clip at the forward end first. You could build a 1W KT88 amp if you wanted, and you would NOT idle the tube at 40+ watts... would you? Then the signal would be like a tiny ripple on top of a flood of totally wasted power, and if the transformers were designed for a 1W amp, you'd certainly blow them.


Later BF and all SF Champs as Mark mentioned are a good example of a wrongly-biased amp too - but it's not because they're biased too cold to cope with the high B+, it's the exact opposite. The B+ is high, but actually within the usable range, and the reason they clip too soon at the forward end is because they are biased too hot as well. If you raise the cathode resistor value (usually to 820 ohms or 1K - and remember to change the cap to take the increased voltage), you can center the waveform and get both more clean power and better tube life.

 

Something is very wrong with either the bias or the voltage swing available. You will still get a high maximum power output with the bias way off, but only at the expense of having one side of the waveform clipped almost totally... but that's not the same as the maximum clean output.

You would normally expect the static dissipation to be just over twice the clean audio power output if the bias is correct.

Try raising the cathode resistor value far enough to center the waveform between cut-off and clipping (which is the correct operating point for Class A, FWIW) and see what you get. You will need a much larger value of cathode bypass cap too probably, since otherwise the audio signal will have to pass more through the resistor.

 

Hmmm....

If the positive voltage swing available is so much less than the negative, this means that the operating point is still far too high... but if reducing the tube current to under half doesn't cure it... or even make much difference... what OT primary impedance do you have?

 

590Va is WAY too high to be able to run true class A. The GEC sheets don't illustrate SE or class A operation, but the 6550A specs don't go over 400Va.

Champs and Vibro Champs aren't truly class A either, in the strict definition that both sides clip simultaneously. Voltage is too high.

 

Are you looking at the output waveform on the primary or secondary side? If you're looking at the secondary side, how do you know it's the positive waveform that's clipped?

 

Your Va is way too high. In order to keep the plate dissipation below limits, you're running on the 'cold' side of the halfway point.

The OT inverts the signal. You're clipping the negative side, and the only way you'll get symmetrical clipping is to get the Va down to around 400V or lower.

The voltage limits for true class A are really quite low, around 275V or so for EL34s and 6L6. You can run SE amps hotter but they clip asymmetrically - which violates the definition of class A.

 

That's correct, but the other side of the coin (as OTM mentioned earlier) is that for textbook class A, the idle point must be exactly halfway between cutoff and saturation - so any clipping occurs symmetrically.

Yours is idling below halfway, so that the tube cuts off at the negative input swing but stays clean on the positive. You can't bias it hotter to get to the halfway point because you'll exceed the limits of the tube....so the Va has to come down.

Or, you can live with the asymmetrical operation. Champ owners have been doing it for forever.