What transformer trickery is this? (RG?)

[Helmholtz]

If you re-watch the video, at the 27 minute mark he shows the anode voltages at both ends of the OT primary and explains that "if we go to the other side, the other pair of 6V6s, you can see that kink is there but it is obviously inverted."

That's just how a transformer works.
No matter on which side of the CT the kink is caused, it will show on both sides mirror-inverted.

[nuke]

That's just how a transformer works.
No matter on which side of the CT the kink is caused, it will show on both sides mirror-inverted.

This particular YT popped up in my feed, thought it is months old. I decided to watch it, since I've heard a few player friends of mine rave over the Divided-by-Thirteen amps. I was hoping for some gut shots to have a look-see of what's in them.

The schematics available online, just hand drawn mostly, show interesting pre-amps, but really, nothing exciting going on. Mostly just Voxy and Fendery preamps, all nice enough but nothing really jumped out.

So I put the YT vid on highest resolution, and freeze framed, skipping over most of the narration (lengthy). There's enough quality there, I could just about draw you the schematic from the gut shots.

The output section is mostly unremarkable. It uses both, fixed bias and cathode bias. It has two pairs of output push-pulls, and a half-power switch. In half power, one pair of 6V6's is connected to single 270-ohm and 220uf capacitor. The full power switch simply connects the other pair of push-pull 6V6's it's shared 270-ohm/220uf cathode circuit.

Everything else visible in the output section is quite normal, screen resistors, grid stopper on the sockets, just like old Fenders.

I think the output transformer was made to simulate one with a partially failing primary winding. Maybe the build had run across a peculiarly interesting amp where the winding would partially fail under load. So he decided to build one that did it on purpose.

All the iron in this amp is purely custom made. So, he could get coils wound however he wanted.

I can think of a few ways to do such trick, but it is kind of fascinating.

I'm pretty sure this is an "on-purpose" and not a "something-broke".

[nworbetan]

If you re-watch the video, at the 27 minute mark he shows the anode voltages at both ends of the OT primary and explains that "if we go to the other side, the other pair of 6V6s, you can see that kink is there but it is obviously inverted."

That's just how a transformer works.
No matter on which side of the CT the kink is caused, it will show on both sides mirror-inverted.

Okay, it took a few people pointing out my mistake for that to actually sink in, but now I see that the kink is actually only happening on one side of the OT.

As it turns out I already had a power amp schematic with both cathode and fixed bias set up, but that one is based on a Park with KT-88s so obviously the voltages and transformer parameters are going to be completely different than the FTR 37 with 6V6s. I was able to introduce crossover distortion by adjusting one set of cathode component values but the distortion was more of a temporary decrease in slope and not the sharp kink where the voltage suddenly changes from increasing to decreasing.

I found (on scribd.com) the exact same FTR 37 schematic the guy in the video was using, so I'll put that power section into spice and see if I can come up with any results closer to what was happening in the video. I'm not holding my breath though.

[Helmholtz]

I'm pretty sure this is an "on-purpose" and not a "something-broke".

What makes you think that all those amps show the same artefact?
To me this just looks like a bias issue on one side of the OT primary, i.e. tubes staying in cutoff until the grid voltage exceeds some threshold.
I can't see from the schematic in the vid how the fixed bias is applied.
Could also be a screen pin contact problem.
Also would be interesting to see how the signal frequency affects the kink.

BTW, I watched the demo vid on the :13 website and wasn't impressed. Not bad but not special either.

[nuke]

What makes you think that all those amps show the same artefact?
To me this just looks like a bias issue on one side of the OT primary, i.e. tubes staying in cutoff until the grid voltage exceeds some threshold.
I can't see from the schematic in the vid how the fixed bias is applied.
Could also be a screen pin contact problem.
Also would be interesting to see how the signal frequency affects the kink.

BTW, I watched the demo vid on the :13 website and wasn't impressed. Not bad but not special either.

Can you replicate this particular waveform?

Find some videos with clean sound from an FTR37, especially older clips and those with clean sounds. There's a lot of thick harmonics at clean settings. Very thick and rich sounding.

What the Rift amps guy says, the owner of this amp says it has always been that way, and he bought it new in the early 2000's.

[nworbetan]

It looks like a strong second harmonic might actually be pretty easy to reproduce.

All it takes is connecting the plates on one side to the UL tap instead of the correct connection at the end of the winding. A couple people (both here and in the youtube comments) suggested a mis-matched primary winding and that seems to roughly fit the description.

edit: I just found an error in my biasing that when corrected made the output waveform less easy to recognize as a fundamental + second harmonic. So I removed that original screenshot, and I'm about to look into Fourier analysis so I don't have to rely on eyeballing the waveform.

[nworbetan]

Screenshot 2026-07-12 011653.png

First pic is with the OT primary connected "correctly".

Screenshot 2026-07-12 011842.png

Second pic has an asymmetric connection to the OT primary via a 40% UL tap on one side. Other half of primary connected normally and B+ connected to center tap as usual.

Lots of even order harmonics, not just the secondary.