300b lovers

I just cannot resist.  This is a poor photo of the square wave on the Blackbird interstage transformer between the 6V6 and the 300b at 1 KHz and about 30% power (7 or 8 watts).  There are NO grid resistors or grid stoppers or networks of any kind.  The primary of the transformer is wired directly to the driver plates and the secondary is wired directed to the 300b grids.  The KT88 looks as good or better....  Lynn and I were quite impressed.  Dave Geren at Cinemag knows what he is doing.....   This is an all tube amp with no feedback anywhere.

Yeah, it sounds like it looks..... 

Now’s a good time to discus the difference between overshoot in feedback vs non-feedback amplifiers. Despite similar appearance on the scope, they are caused by completely different mechanisms.

* Overshoot in a feedback amplifier is quite malign, since it indicates the onset of oscillation, something that can destroy the amplifier and the speaker it is connected to. It is caused by the amplifier running out of phase margin, possibly the result of a reactive load, but also the result of design oversights in the feedback loop.

* Overshoot in a non-feedback amplifier is quite different. Now, it might be the result of high-transconductance tubes self-oscillating, but this can prevented by grid-stopper resistors and good layout practices. Normally, though, it is merely transformer overshoot, the result of phase shift at the edge of the passband, and has nothing to do with stability. That is what we are seeing here.

Something to be considered about ultrasonic behavior in the time domain: if there is no spectral content in the frequency range of the overshoot, it will never be stimulated in the first place. It never happens.

This is the difference between overshoot in a feedback amplifier and a non-feedback amplifier: in a feedback amplifier, it is a warning sign, like the LOW OIL light in a car. You ignore it at your peril. In a non-feedback amplifier, it has nothing to do with stability, since there is no feedback loop to induce oscillation. It is simply the behavior of passive parts, in this case, the input and interstage transformers.

As you can see, Cinemag has done a very nice job here. (Same photo as above, just tidied up a little.)

Hi @donsachs ,

Yes Dave Geren at Cinemag do great job!

I use Hashimoto A-305 IT between 6f6 (driver) and 300B. The square wave was almost perfect at 10KHz, 20KHz, 30KHz. I use an 80K Ohm grid resistor on 300B. But I’m not sure this resistor is needed! The -3dB frequency response is from 6Hz to 95KHz. This IT is extraordinary on measurement and sound.

For 6sn7 I use Hashimoto A-107 that is not as perfect but has more primary inductance that is essential for higher input impedance 6sn7.

Hashimoto A-305 is designed specially for SET, but A-107 is universal IT that can be phase splitter or SET 1:1 or SET 1:2.

I just installed A-107 and it has zero break-in time. So I can’t comment how does it sound compared RC (V-Cap CuTF, AN Silver Tantalum) that were before. It need more break-in time.

@alexberger Please report back on the sonic diff between the A-107 and RC coupling.  Completely different amps, but I much preferred all IT coupling.  Curious to hear your impression

I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and 47KHz.

Should I decrease the load resistor more to remove overshoot completely?

If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.

@alexberger As you have noticed, if you are using a coupling (interstage) transformer, it will be needing proper loading to prevent ringing (distorting). You are nearly there with your technique so far; put a potentiometer across the output of the transformer, run a square wave through the active circuit prior (6SN7) and adjust the pot for minimum ringing (critical damping). IME its probably best if you leave a very slight amount of overshoot as opposed to rounding the square wave.

Its important that the driver to the transformer be active, since transformers transform impedance: Whatever impedance on the primary side, if it varies, will affect the critical damping value on the output side. Conversely, whatever is loading the output side will also load the input thru the ratio of the transformer. So you want to feed the squarewave to the active 6SN7 circuit so your loading value will be correct. Best to have the 6F6 running also for this very same reason, although the loading resistor will likely dominate that side of the transformer equation.

Once that is sorted out, you might find it interesting to measure the impedance at 1KHz on the primary side (you won’t need the 6SN7 in the circuit for that, but it would be a good idea to have the 6F6 in place and active) just to see what the load on the 6SN7 actually is. You may find that you have to adjust the operating point of the 6SN7 to obtain greater linearity (by adjusting the cathode resistor value if you have one); if you do that then you may have to readjust the loading value of the transformer since the source impedance of the 6SN7 varies a little with the operating point; in this way zeroing in on the optimal values.

Obviously you can stop at any point (call it ’good enough’), but in a zero feedback design I’ve found that the more you pay attention to refining things like this, the more it pays off in the end!

@lynn_olson I agree overshoot in a circuit using feedback is bad!

But if the feedback is applied properly you’ll get no overshoot at all. Our OTLs don’t exhibit any squarewave overshoot, being zero feedback and free of inductors in the signal path. The 10KHz waveforms are pretty good since they have wide bandwidth; our class D, which has less bandwidth owing to the output filter, nevertheless has a very similar 10KHz waveform, despite (well, actually because of) running 37dB of feedback; the difference being the residual sine waveform imposed by the switching frequency.