LL1660S Anomaly

[arnoldc]

I was working on my new push pull amp with 6SN7 || -> LL1660S -> EL34 x 2

The 1660S is configured as ALT-B (SE to PP out). When I looked at the scope this is what I saw.

There is considerable ringing on the 1kHz square wave. This output is taken from the plate of the 6SN7.

The 6SN7 is running off 200V B+ and 750R cathode resistor, bypassed.

Substituting two green LEDs did not improve the output.

Any thoughts or suggestions as why this is happening?

[Tony]

leakage inductance of the ll1660 is responsible for that, not that bad from where i sit....

[jlsem]

You'll never hear that so-called ringing.

John

[arnoldc]

But my 2A3 can do this-

Waaah! Not what I expect from an expensive Lundahl!

Nothing I can do to fix?

[pmillett]

I haven't used the LL1660, but that looks fairly typical of unloaded interstage secondaries to me.

You can try adding resistors on the outputs (grid to ground), it will probably help. You can also use a series RC network. You should be able to damp the ringing, at the expense of frequency response.

Pete

[jrdmedford]

You could try to de-tune the LCR network by adding some
resistors across the secondary winding... Maybe 47K ?

Why dont you measure the period of those oscillations? You'll
find they are up around 60kHz anyways... Set your trigger to
leading edge and zoom in on the damped series of peaks,
count horizontal divisions to get the period, and thus the frequency of the oscillation mode... If you want to be smart, count
the number of periods as well to figure out what the damping envelope coefficients are.

-- Jim

[Gluca]

Got a very similar result with my LL1660PP time ago. I loaded the secondary windings with different combos R+C looking for the best 1KhZ. NOPE. There have been only slight improvements. At the end of the day I left 47kR+47kR. I didn't notice any difference in the sound.

Yes those rings are very typical with interstage trannies as Pete and Jim said ... but there is really nothing to worry with them as they are "local" instabilities. My experience with the ITs is that the output tranny usually cut these rings off flattening the square wave.

What about 10kHz square wave? Any issue with hi-freqs (say 50kHz)? You should notice a +3/+6dB boost there with the LL1660

Enjoy.
Gianluca

[Joel]

There is considerable ringing on the 1kHz square wave.

Welcome to the world of high impedance interstage transformers!

Gluca is correct that you need a 47k on each secondary just to tame it slightly, and also that you probably won't hear the difference even then.

The problem is not square waves that cause oscillations all of the same family which cannot "fold back" into the audio band as IM, but rather that high order distortion components from previous stages are given a 6dB or more boost. You may or may not be worried about that, depending on the circuit.

Joel

My push-pull circuit using the LL1660S is on my website:
http://www.joeltunnah.com

[Gluca]

Heya Joel ... I was looking at your very site yesterday (congrats!!). I saw your trans coupled EL84 project. You are using 47kR too. Great to see someone else found the same solutions as I did.

These interstages really like low Ra tubes. I used ECC99, 6H30 and 6N6. Probably you can get something better at 10kHz with these drivers.

Ciao
Gianluca

[arnoldc]

Hi Tony, John, Pete, Jim, Gianluca, thanks for your thoughts. I loaded it with 100K before but did not notice major "improvement" maybe 47K will do but I'll take the suggestions about not being able to get more improvemnt.

Hi Joel, I actually went to your site to have my waveform compared with yours