Output transformer specs for S.E.X. 2.0 Iron Upgrade

[Dr. Toobz]

I own a S.E.X. 2.0 that I built in February 2009, my first Bottlehead kit of many. For a cleaner product, I rebuilt the amp in 2014. In the process, I added some upgraded components, including an Alps potentiometer, Mundorf capacitors, the newer-style C4S boards, and the Bottlehead iron upgrade kit. I have not done any further work on the amp since then, but it still sees nearly daily use with headphones.

One of my favorite modifications was playing around with a bit of local feedback on the power triode sections to lower the output impedance for certain speakers and headphones. This was done with upgraded Magnequest iron, which I regrettably sold long ago, though I'm no less happy with the Bottlehead update. While not air-gapped, the MQ iron could take a bit of current on the secondaries before saturating, whereas the stock Specos could not. However, this was not really an issue with headphones, since it took so little to drive them. I have recently thought about revisiting that experiment to get my Grados to sound a bit cleaner in the bass frequencies, but wondered how up to the task the Bottlehead transformers might be. Since they are always used in parafeed designs, I am guessing they are not air-gapped. Will they tolerate any DC on the secondaries, and if so, how much?

[Paul Joppa]

I don't recall ever testing the DC saturation of an OT-2, though I did measure a Radio Shack line output similar to the Speco. (It ran out of steam at 1.7mADC in the primary. I would not advocate using more than 20% of that current, i.e. 0.34mA primary, or about 10mA secondary at 8 ohms.) The Iron Upgrade retained the W.E. split-load power supply bias topology, so it would have similar restrictions, possibly more restrictive due to the greater number of turns.

Lately I've been looking into what is called Schade feedback, among several other names. Are you familiar with this topology? This might be a better option both for performance and ease of modification. If you're interested, I'll run some numbers to see if that could work. Any idea how much feedback you are looking for?

[Dr. Toobz]

Per an old thread from around the time I had last experimented with this, I had estimated around 3dB of feedback by plugging numbers into an online calculator that no longer appears to be available. Since the amp and Grado SR-325x combo is already at about 90-95% of my ideal sound, I'm guessing I would again be aiming for just 3-5dB of feedback, as I am essentially just wanting to tweak its "voicing," if you will. For sake of comparison, I had briefly run the same DAC and headphones with a Schiit solid-state amp that I ended up selling. Compared to my S.E.X. build, the frequency extremes were slightly cleaner sounding, but the midrange was sterile and the whole thing just sounded boring and over-damped.

Isn't Schade feedback essentially plate-to-plate? I have seen some schematics where the plate of the output tube is connected to the plate of the driver tube, usually through a high-value resistor (average seems to be around 100k). I have also seen local feedback in the form of taking a resistor from the plate of the output tube into the cathode of the driver tube, but that just seems to be another version of cathode feedback. I wonder how much of my liking the small amount of cathode feedback was actually due to having the output transformer in the feedback loop?

[Paul Joppa]

Yes, you have an understanding of Schade feedback. (I called it "transresistance" when we used it  in the Seductor amp). It avoids a source of distortion; there is a recent thread on the Audio Asylum about it:

https://www.audioasylum.com/forums/tubediy/messages/28/283794.html

But first, to be sure, I have to ask if you have removed and/or shorted out the 120 ohm resistors in series with the headphone jacks? That would significantly improve the damping factor. We had to eliminate them from the latest revision because it needs a phone jack that was no longer available. Long story; there are, as always, pluses and minuses.

[Dr. Toobz]

Yes, the series resistors are long gone. I had the shorting jack kit in there in the early years, but found that lower-impedance headphones sounded odd (under-damped?) with the 120 ohm resistors in tow, especially Grados. The downside has been that I have a tiny bit of residual hum and tube rush with certain tubes, even after implementing a later trick to tie the center point of some 100 ohm resistors that I put in the power supply to ground and going to the 4 ohm taps on the output transformers (e.g.,  https://forum.bottlehead.com/index.php?topic=5215.30). This only became an issue when I moved to my current house about seven years ago, so it's likely a function of some variable other than the amp itself.  However, it can''t be heard when music is playing, so I never bothered putting the resistors back in.

I'd be wiling to try the Schade feedback trick, but would appreciate some help with the calculations.

[Paul Birkeland]

You can also apply feedback from the output plate to the driver cathode with a resistor for a similar effect.