Oddball OT with extra secondary winding - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 13th September 2012, 02:18 AM   #1
diyAudio Member
 
Join Date: Sep 2012
Default Oddball OT with extra secondary winding

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:

Click the image to open in full size.


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.

Last edited by jonnycat; 13th September 2012 at 02:40 AM.
  Reply With Quote
Old 13th September 2012, 07:57 AM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
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!
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 13th September 2012, 09:20 AM   #3
palmas is offline palmas  Portugal
diyAudio Member
 
Join Date: Jun 2007
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.
  Reply With Quote
Old 13th September 2012, 02:55 PM   #4
diyAudio Member
 
Join Date: Sep 2012
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!
  Reply With Quote
Old 13th September 2012, 03:32 PM   #5
Struth is offline Struth  Canada
diyAudio Member
 
Join Date: Aug 2012
Location: books at londonpower.com
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
  Reply With Quote
Old 13th September 2012, 11:12 PM   #6
diyAudio Member
 
Join Date: Sep 2012
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.
  Reply With Quote
Old 14th September 2012, 03:25 PM   #7
diyAudio Member
 
Palustris's Avatar
 
Join Date: Aug 2006
Location: Cape Cod
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
  Reply With Quote
Old 15th September 2012, 12:20 AM   #8
diyAudio Member
 
Tom Bavis's Avatar
 
Join Date: Jun 2002
Location: Macedon NY
Fisher 100:

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

Bogen DB-20

http://members.shaw.ca/pacifictv4/bogendb20.jpg
  Reply With Quote
Old 15th September 2012, 06:06 AM   #9
diyAudio Member
 
Join Date: Sep 2012
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!
  Reply With Quote
Old 15th September 2012, 07:55 AM   #10
diyAudio Member
 
Join Date: Aug 2008
Quote:
Originally Posted by jonnycat View Post
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Grid Bias provided by transformer secondary winding, what is this stuff about? DualTriode Tubes / Valves 11 12th October 2010 09:22 AM
Using primary as the secondary winding in the power transformer. mishak Parts 10 30th May 2010 05:55 AM
Guitar tube amp OT Calculation/Winding HELP! mirkoslav Tubes / Valves 0 5th February 2010 10:11 PM
OT: Aluminum Magnet (Winding) Wire source - retail? tympani1d Planars & Exotics 23 25th December 2009 04:56 AM
NAD C320 secondary winding voltages? skenwrick Solid State 0 8th February 2006 09:29 PM


New To Site? Need Help?

All times are GMT. The time now is 10:20 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2