Oddball OT with extra secondary winding

[jonnycat]

Hello,

I have an OT I pulled from the power amp of an old organ tone cabinet. The power amp used two 6L6 tubes in P-P configuration, with an 8 ohm speaker, but the OT has an extra set of windings on the secondary which are connected to the cathodes of the power tubes:




The DC resistance between the outside of those extra windings are about 30 ohms, while the DC resistance of the secondary output (speaker) is an expected < 1ohm.

Now here's my question: If I connect the speaker to use both the speaker output and the "extra" windings (like a buck or boost configuration), I can get a wide range of reflected loads to the power tubes.

While I don't think this would be something I would want to do with the original 6L6 tubes running at 60W, would this still be a terrible idea if I was to use the OT in a low-power (< 5watt) amp, such as one of the low power 6SN7 P-P guitar amps that likes to see ~22k reflected load?

Any perspective on this would be greatly appreciated.

[SY]

That "extra" winding provides feedback to the cathodes of the output tubes. It won't be rated for the current you need for speakers unless your speakers are very high impedance. Likewise, the turns ratio for this winding is such that if you try to buck or boost the secondary, its voltage and impedance will almost certainly overwhelm that of the speaker winding.

Use it for feedback- it can not only work for the output tubes, but you could conceivably use it for a feedback winding to one of the driver stages. Potentially VERY useful!

[palmas]

cathode feedback is a great way to do it. Audio Research does that using the speaker windings (grounds 4Ohm and uses 16Ohm and 0).
If you have it, use it, it works great.

[jonnycat]

Okay, thanks Sy, and Palmas. I'll restrain myself from using the winding as a speaker lead.

Can you point me to any examples (schematics) of topologies that use such windings? This amp is the only place I have seen such a thing (any other NFB scheme I have come across uses a take-off from one of the speaker leads).

Thanks!

[Struth]

Hi Guys

The cathode windings also contributes power to the output. It is a primary winding not a secondary winding. Yes, you can have dual primaries on an OT, just like for a PT.

When taken to an extreme with the plate winding present, you get the unity-coupled output developed by Frank MacIntosh. You can take it a step further and have only cathode drive which provides very good output damping but requires very high drive voltage.

Have fun
Kevin O'Connor

[jonnycat]

Thanks Kevin; I'm thinking if I had another one of these I could probably have the beginnings of a pretty neat stereo amplifier. Hopefully I'll find a use for it some day.

[Palustris]

Here's an example of how one can use this transformer. It would be great to find another for stereo.

Feral Eye schematic

Feral Eye amplifier

[Tom Bavis]

Fisher 100:

http://www.audiophool.com/Schem_A/Fisher_100_schem.gif

Bogen DB-20

http://members.shaw.ca/pacifictv4/bogendb20.jpg

[jonnycat]

That Fisher 100 even has the same DC resistance value as mine; the only difference is in the color of the leads, and my lack of multiple speaker taps. Looks like it's from the same era, too!

[LinuksGuru]

Originally Posted by jonnycat

Okay, thanks Sy, and Palmas. I'll restrain myself from using the winding as a speaker lead.

Can you point me to any examples (schematics) of topologies that use such windings? This amp is the only place I have seen such a thing (any other NFB scheme I have come across uses a take-off from one of the speaker leads).

Thanks!

Sansui AU-111 and Quad II are classic examples.

Schematics freely available on the net. BTW, you can improve virtually any penthode PP by means of converting it to CFB.