Toroidal output transformers with screen drive- why not? - 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 5th May 2008, 03:26 PM   #1
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
Default Toroidal output transformers with screen drive- why not?

Just had an interesting thought.

Toroidal output transformers seem to have one drawback... the problem with very quick magnetic saturation due to primary DC current mismatch between the two sides, using push-pull output tubes.

Well... seems that most screen-drive amps can get away with VERY LOW bias current... to the tune of 3ma to 5ma DC on each tube, for a push-pull pair!

This seems to be an ideal situation for toroidal outputs, it would seem to me. With that little bias current, even large-percentage shifts in bias current would make for very small actual changes in DC current differential between the two sides...

Anyone tried this, or have any ideas as to why this might or might not work?

Thanks!

Regards,
Gordon.
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 5th May 2008, 04:53 PM   #2
JoshK is offline JoshK  Canada
diyAudio Member
 
Join Date: May 2004
Location: Toronto, ON
Neat observation. However, who makes reasonable priced toroid OPTs? Plitron's are very expensive IIRC.
  Reply With Quote
Old 5th May 2008, 05:58 PM   #3
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
Default Re: Toroidal output transformers with screen drive- why not?

Quote:
Originally posted by GordonW
Just had an interesting thought.

Toroidal output transformers seem to have one drawback... the problem with very quick magnetic saturation due to primary DC current mismatch between the two sides, using push-pull output tubes.
I am using a 120VA mains toroidal transformer as a PP OPT for my KT88 amp and I am measuring DC imbalance of 6ma and I am not getting any magnetic saturation on them.

The KT88 are cathode biased and triode strapped, producing around 20 watts per channel. Each power tube has its own cathode resistor and bypass, I previously configured it to have a shared resistor and when I installed individual cathode resistors it seems to be more balanced.
  Reply With Quote
Old 5th May 2008, 09:21 PM   #4
GordonW is offline GordonW  United States
diyAudio Member
 
GordonW's Avatar
 
Join Date: Mar 2005
Location: Marietta/Moultrie GA
Default Re: Re: Toroidal output transformers with screen drive- why not?

Quote:
Originally posted by alexg


I am using a 120VA mains toroidal transformer as a PP OPT for my KT88 amp and I am measuring DC imbalance of 6ma and I am not getting any magnetic saturation on them.

That's kind of the direction I was going with my thinking, when starting this discussion.

The lower impedance ratios of power transformers could be just what is needed for a PPP screen-drive amp...

Regards,
Gordon.
__________________
Speaker Design, Restoration and Repair- since 1985.
  Reply With Quote
Old 5th May 2008, 10:48 PM   #5
diyAudio Member
 
Join Date: Jan 2006
Location: Ireland
A user here Shoog has used power toroidals in a few amps and I can tell you they sound great. I think he goes for 4-5 times over the wattage.

Fran
  Reply With Quote
Old 5th May 2008, 11:01 PM   #6
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
I would think that you'd have to have the transconductances matched pretty tightly for this to perform well.
__________________
The more you pay for it, the less inclined you are to doubt it.- George Smiley
  Reply With Quote
Old 6th May 2008, 01:30 AM   #7
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
Quote:
Originally posted by woodturner-fran
A user here Shoog has used power toroidals in a few amps and I can tell you they sound great. I think he goes for 4-5 times over the wattage.

Fran
I got the idea of using toroidal OPT from Shoog, and have been closely following his work.

I did a 6as7/6080 parafeed using a 40VA mains transformer and it sounded excellent!
  Reply With Quote
Old 6th May 2008, 09:29 AM   #8
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
I am encountering the first serious issues with the Tabor clone I am currently building. I get big overshoot on all square wave leading edges. I needed to install a Zobel on the primary to tame this. I haven't got the same issue with other amps using the same transformers. I can only presume that the very low output impedance of the plate to plate design is interacting with the interwinding capacitance to over excite the transformer.
Certainly an issue to watch for and a good scope is needed to diagnose. Aparently there are no free lunches.

I also find that wiring up a transformer in one direction will induce ringing and then reversing the wiring will make it go away.

Shoog
  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
Output (beam) pentodes / tetrodes suitable for screen grid drive ilimzn Tubes / Valves 19 15th June 2013 06:23 AM
FS: Toroidal Transformers BobG Swap Meet 2 25th August 2009 07:00 AM
WTB/S/T: Toroidal Transformers - UK deadparrot Swap Meet 5 16th June 2009 09:34 AM
Toroidal transformers muddy Class D 3 24th February 2005 11:23 PM
Best Transformers (Encapsulated Toroidal or Open Frame Toroidal) Rixsta Pass Labs 10 23rd April 2004 03:41 AM


New To Site? Need Help?

All times are GMT. The time now is 07:31 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