CCS Loaded Parafeed Output - Design Considerations?

[Paul Birkeland]

Here is my set of load lines, as I had considered 3K and 5K iron for this project.  This is swinging 520 to 70V, so 161V RMS. A 3K:8 is about a 19.5:1 step-down, so that gets you 8.25V at the speaker posts, which is just over 8.5W. 

[L0rdGwyn]

I see, thanks for the pic.

But the amp is going to hard clip asymmetrically on the positive peaks, is that not typically considered when quoting an amplifier's power output?  This is exactly what I see in my measurements, the positive signal is chopped off above 524V whereas the amp can continue to swing downward until the plate hits saturation at 70V or so.

[Paul Birkeland]

The 8.5W was at 10% THD I believe.  You'd have to make a plot of THD vs. power to determine where you'd say hard clipping starts.  The asymmetrical clipping will be pretty brief IMO in the grand scheme of things.

[L0rdGwyn]

Gotcha PB, I see now where I was making the mental disconnect.  Well regardless, this is basically a full A2 amp in this setup, going to give it some listening time, then decide how it compares to the LL9202 setup with the 6.5K primary.  Thanks for letting me pick your brain, yet again.

[L0rdGwyn]

I made an error in a previous measurement PB, my 0.38ohm output impedance wasn't reproducible, so something was off.  I was more consistently getting 2.8ohms output impedance with a 220K plate resistor on the EF37A.  I wanted to improve this, but using a bypass cap on the cathode of the EF37A degraded the sound quite significantly.  I decided to try the CCS setup.

I simulated in LTSpice to get ballpark values, then went for it.  I used 1.56mA out of a cascode CCS with a 470K resistor to ground. I figured maybe if I used a larger value without going crazy, I might not have DC bias instability issues but could still get a damping factor of 3 out of the amp.  EF37A bias point was Va 240 / Vg2 95V / Vk 2.77V.  Leaving the cathode degenerated, I was still able to get a open loop mu of 125 out of the EF37A.  I returned to my 6.5K LL9202, I think the sound is better than the full A2 setup with the 3.3K LL1620.

THD @ 1W 0.44%
Output Z 2.44ohm
-1.67dB @ 20kHz

Bandwidth improved as well.  I monitored the plate and cathode voltages of the EF37A with music playing at varying volumes, did several shut-downs and start-ups as well - the bias does not drift, it finds its way right back to the same point every time.

I think I'm gonna stick with this approach and improve up on it!

FR and unclipped full output below, this is a 5W amplifier

[L0rdGwyn]

Still working on this tricky amplifier, I was finding that adding capacitance to stabilize the feedback loop was hurting the HF response and there is none left to spare.  I pulled the trigger on a pair of 5K:8 60mA custom OPT from Sowter UK, no more of this bandwidth issues from the Lundahl LL9202, I'll have to repurpose them.  Specc'd the transformers for 60mA in case I want to go for a 0Vg bias point, something like 330V 55mA, either than or 360V 50mA.

[Paul Birkeland]

I kind of feel like I'm corrupting you...

I've had Sowter make plenty of iron for me over the last couple of years.  Spending the dough on their stuff lets you focus a lot more on the circuit itself rather than worrying about what the iron is doing. 

[L0rdGwyn]

No worries on the corruption I have an order pending with Sowter for another project, in truth I was already considering the switch after all the frustration with the Lundahl transformers.  I am very committed to this build, so I think it will be worth it, could always resell the Lundahls.  Good thing to be able to focus on the circuit, having some issues with ringing on square waves that I am working through.  Maybe the issue will disappear with the Sowter OPT in place, wouldn't that be nice.

[Paul Birkeland]

You could also experiment with a zobel across the secondary.  Heck, you're paying Sowter for a custom wind, Brian might be able to tell you the optimal values for R and C.

[L0rdGwyn]

Good idea, I'm wondering if it is an artifact of the Lundahls specifically.  I checked square waves on the pentode and source follower with the 801A out of the circuit (no feedback), no ringing whatsoever.  It is present on the primary and secondary, near the audio band based on the period of the ringing, roughly 50uS, so around 20kHz, don't want to shunt 20kHz to ground. 

Anyway, I'm probably going to shelve this beast until the Sowter OPT get here, probably not worth troubleshooting issues that might go away with a more optimal iron.

[L0rdGwyn]

Hey PB (and PJ if you happen to stop by  ) - still working on my 801A A2 amplifier, still waiting for the Sowter OPT, they are a bit hampered with COVID so I think building is slow, should have them soon.

Wanted to show you something pretty cool and also ask if you have any input on something else.  I have been hanging around diyAudio and discussing ideas for these high-gain CCS fed pentodes.  A certain someone who maintains a certain DIY tube blog suggested a fancier g2 supply for my pentode driver rather than the 1Meg resistor from B+ I was using before.  The idea is to use a local feedback mechanism with the DC voltage fed to g2 via FET, with the voltage derived from a divider from the mu output of the CCS.  Seems similar ideas are floating around a few places, but the one that inspired the suggestion to me was done by JC Morrison on his transconductance gain block here:

http://www.labjc.com/?p=5317

In this arrangement, any drift in the plate voltage is communicated down to the screen, I put together some Spice models, built out the circuit on a protoboard and gave it a shot, schematic below.

What I found is this setup provides VERY stable anode / screen voltages.  At maximum output into a 8ohm dummy load, sine waves as well as real music, I never see a voltage drift more than 0.5V on either, pretty nice, so this is going in the final design.  With the exact setup I have shown below, I am getting 0.14% THD at 1W out of the amplifier.

This design is 95% of the way there, just a matter of dialing in the feedback / output impedance / FR / harmonic profile when the Sowter stuff gets here.  The only niggling thing left is some somewhat ugly overshoot / ringing I am seeing on square waves, shown below as well, the output from the EF37A.  The amplifier is stable, perhaps it is a non-issue or will resolve with the Sowter OPT, still need to do some more troubleshooting, but the Lundahls seem quite prone to this type of ringing, we will see.


Stay safe fellas!  Hopefully 2021 is a better year.

[Paul Birkeland]

What happens if you adjust the value of C2?

[L0rdGwyn]

I changed C2 to 4.3uF without any change in the overshoot / ringing.  I discovered the ringing goes away with the feedback loop disconnected.  The outputs of the EF37A and source follower are clean, it originates at the output stage.  The ringing is 45-50kHz when open loop, roughly 20kHz closed loop.  Adding any feedback compensation capacitance starts to roll off the HF, so that isn't a viable solution, especially with it being so close to the audio band.

I wonder if this is just another issue with the LL9202 OPT and it will go away with the Sowter iron in place, maybe I should just wait it out and see.

Below is the 1kHz square of the 801A plate with the feedback loop disconnected.

[Paul Birkeland]

I think hanging around and waiting for the Sowter iron might not be such a bad idea.

[L0rdGwyn]

Agreed, these Lundahl OPT are so quirky, I think it's best to just wait it out.  Not sure what I will do with them afterward, despite their oddities they are very nice sounding transformers.  Thanks again, PB.