Question about Pentode reverb drive circuit

[66 Kicks]

Here is an image of what is coming out of the other end of the tank. Clearly the current in the input coil was not just fine.

I never try to directly measure anything within a CL or RCL series circuit because the results will usually be erroneous. Instead I measure the AC voltage across the circuit, calculate the impedance, then calculate the current.

Unless you think that the impedance of the CL circuit is rapidly changing while the frequency is constant, the plate voltage is going to have the same waveform as the current.

[nickb]

Here is an image of what is coming out of the other end of the tank. Clearly the current in the input coil was not just fine.

I never try to directly measure anything within a CL or RCL series circuit because the results will usually be erroneous. Instead I measure the AC voltage across the circuit, calculate the impedance, then calculate the current.

Unless you think that the impedance of the CL circuit is rapidly changing while the frequency is constant, the plate voltage is going to have the same waveform as the current.

Now here's a funny thing. I looked in my stock and nearly fell over. There was a 6U8A looking back at me. You can guess what I did, right?

Ran it with 1.7pp in, supply was a little high at 260V. Monitored the plate voltage and the tank current. I used a Tektronix AM305 current probe ( what an astonishing instrument that is) to avoid any series resistive errors.

Here are the results at (about) resonance and 1KHz using the same input level.

I did learn that
(1) If you tweak the input level just right at resonance it can look clipped. Ratio of harmonics effect.
(2) Monitoring the tank output is fraught with problems due to the many nodes. Even a 1Hz shift can cause a 90% change in amplitude.
(3) The tank current behaves exactly as expected. It is not the same as the plate voltage, as explained in post #45

So given this, we do have a big question as to why you are seeing what seems to be a clipped output from the tank. Can you sketch your circuit and measurement set up? Especially the tank recovery. What happens if you make a small change in frequency?

Yes the impedance does change quite a bit with frequency. The frequencies of relevance here are the fundamental and 2nd, primarily. As you change the input drive, the ratio of these in the tube current changes and that has a big impact on the wave shape due to the filtering effect of the load.

[Helmholtz]

I used a Tektronix AM305 current probe ( what an astonishing instrument that is) to avoid any series resistive errors.

Did you mean AM503?

[nickb]

Did you mean AM503?

Indeed yes, an AM503.

[Helmholtz]

Indeed yes, an AM503.

Yeah, great tool. Used to work with it and would love to have one.

Your explanation of the observed effects is perfect. There will be also frequency dependent phase shift between voltage and current, further causing different wave shapes of voltage and current for signals containing more than one frequency (as distorted signals).

May I ask you to post the bypassed and unbypassed (cathode resistor) voltage gains of the pentode stage? I would like to test a formula for the lower corner frequency.

[66 Kicks]

Thanks a lot. Obviously the resonant bandpass is far wider - reaches down lower - than I expected. And the bass drop in the complete circuit starts at a much higher frequency than the 3dB frequency of the cathode circuit (25Hz).

The 56nF cap on the screen has a lot to do with it.

[66 Kicks]

So given this, we do have a big question as to why you are seeing what seems to be a clipped output from the tank. Can you sketch your circuit and measurement set up? Especially the tank recovery. What happens if you make a small change in frequency?

I didn't think that the signal was clipped. I interpreted it as having a massive second order harmonic that is in phase with the fundamental.

Changing the frequency around the resonant frequency changes the phase relationship between the fundamental and the second harmonic. My interpretation, anyways.

[Helmholtz]

The 56nF cap on the screen has a lot to do with it.

Oh yes, frequency dependent negative screen feedback with a corner frequency of 42Hz. I overlooked this, thanks.

[66 Kicks]

Yes the impedance does change quite a bit with frequency. The frequencies of relevance here are the fundamental and 2nd, primarily.

It's an interesting idea that the impedance of the coil may be responding to the second order harmonic. It occurs to me that such a change in impedance would move the rig off of the resonant and the plate voltage would increase. I'm not really seeing that on the screen.

[Helmholtz]

It's an interesting idea that the impedance of the coil may be responding to the second order harmonic. It occurs to me that such a change in impedance would move the rig off of the resonant and the plate voltage would increase. I'm not really seeing that on the screen.

The load impedance is minimal at the resonant frequency. The second harmonic sees a higher impedance. That's why it shows less in the current than in the plate voltage.

[66 Kicks]

The load impedance is minimal at the resonant frequency. The second harmonic sees a higher impedance. That's why it shows less in the current than in the plate voltage.

And so the second harmonic voltage cannot rise because it is a function of the fundamental voltage?

[nickb]

I didn't think that the signal was clipped. I interpreted it as having a massive second order harmonic that is in phase with the fundamental.

Changing the frequency around the resonant frequency changes the phase relationship between the fundamental and the second harmonic. My interpretation, anyways.

Yes, that's exactly what it is

I wanted to see your test set up just in case we were looking at the tank output after using an amplifier that was misbehaving. The only difference between our set up is I'm using 47nF for the screen grid bypass.

[Helmholtz]

And so the second harmonic voltage cannot rise because it is a function of the fundamental voltage?

Not sure if I am getting your point.

The tube load is a (highly damped) series resonant (filter) circuit. In these the impedance at resonance is minimal and the current is maximal. Because of the high series resistance, amplitude effects are small. But the current shape will differ from the plate voltage shape because of the phase shift of the second harmonic.

[nickb]

It's an interesting idea that the impedance of the coil may be responding to the second order harmonic. It occurs to me that such a change in impedance would move the rig off of the resonant and the plate voltage would increase. I'm not really seeing that on the screen.

This is what I trying to say back in post 45 ( and not doing it very well apparently ) . If you go to the load impedance plot from way back you'll see the impedance is 60 ohms at resonance (call it 500Hz) and 600 at 1KHz. So you can see that any second harmonic produced by the tube will be magnified by 10 on the plate voltage waveform. OTOH the current in the tank barely changes as 60 and 600 ohms are still small compared to 15K.

Here it is gain but zoomed in to the area of interest.

[SoulFetish]

This has been a great thread showing examples of expected design results, simulated results, and measured results.
You all have been really helpful. I think Helm is right, time to give it a listen. I just received a 6U8A and Im just waiting for my buddy to bring ovr the tank.
Anyone see any issues with the recovery stage? Its not a high gain triode, but I was able to get a gain pretty close to mu.