12AU7 / 12AX7 push-pull - Page 2 - 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 2nd October 2010, 12:20 AM   #11
diyAudio Member
 
lazzer408's Avatar
 
Join Date: Sep 2010
Location: Chicago
Quote:
Originally Posted by Merlinb View Post
The impedances are the same (symmetrical), but by using the centre tap you end up with 4ka-a rather than the 8k you expected.

For small-signal valves you really want one of those really small line OPTs, the ones that offer you several primary taps up to about 1W (usually 0.1W, 0.25W, 0.5W, 1W). That way you can get impedances up to 10k, which are more appropriate.
Right. So wouldnt I want 0-4000-8000 with 4000 as the center tap? That leaves 4000 in each direction. Tom said 2000 is the center tap. How can that be? That makes 0-2000-8000. I'll have 2000 on one phase and 6000 on the other.
  Reply With Quote
Old 2nd October 2010, 12:52 AM   #12
diyAudio Member
 
Join Date: Jun 2004
Location: Indiana
Interesting. I have a boat load of 12AU7s from and organ pull. Might be fun to make a little computer amp or headphone amp using this approach. I assume that a 70V line tranny could be used in a similar way, just different turn ratios at the same power level taps.
__________________
If We The People refuse to hear the truth we will be ruled by liars.
  Reply With Quote
Old 2nd October 2010, 03:29 AM   #13
ChrisA is offline ChrisA  United States
diyAudio Member
 
Join Date: Jan 2008
Quote:
Originally Posted by lazzer408 View Post
Right. So wouldnt I want 0-4000-8000 with 4000 as the center tap? That leaves 4000 in each direction. Tom said 2000 is the center tap. How can that be? That makes 0-2000-8000. I'll have 2000 on one phase and 6000 on the other.
OK this is how I'd explain it..

What you really want is an equal number of TURNS on both sides of the center tap. Think volts not impedance.

So the number on turns from one end to center is 1/2 the number of turns from end to end. Doubling the number of turns increases the impedance by a factor of four. Remember impedance is proportional to square root of the number of turns. So the center tap to end is always 1/4 the total impedance in any transformer with a center tap.

Try this experiment: Connect a signal generator to 1/2 of you output transformer and look at the volts on the secondary. (Use a 12V heater transformer or wall wort for the signal generator if you have to.) Then switch the signal so it is across the other half of the OPT. You should see the same volts on the secondary (give or take a few percent) if you have the "correct" center tape. This way you can pick the correct tap even without having too understand this. Just pick whichever one give symmetric results.

Last edited by ChrisA; 2nd October 2010 at 03:36 AM.
  Reply With Quote
Old 2nd October 2010, 09:43 AM   #14
Merlinb is offline Merlinb  United Kingdom
diyAudio Member
 
Merlinb's Avatar
 
Join Date: May 2006
Location: Lancashire
In other words, the 4000 tap is NOT half way between the 0 and 8000 tap. The 2000 tap is the TRUE centre tap.
  Reply With Quote
Old 2nd October 2010, 02:33 PM   #15
diyAudio Member
 
lazzer408's Avatar
 
Join Date: Sep 2010
Location: Chicago
Quote:
Originally Posted by mashaffer View Post
Interesting. I have a boat load of 12AU7s from and organ pull. Might be fun to make a little computer amp or headphone amp using this approach. I assume that a 70V line tranny could be used in a similar way, just different turn ratios at the same power level taps.
That's what I was thinking. It seemed to work much better single ended then it did push-pull. If single ended, you could even grab a UL tap and use a tetrode. Maybe use a 12AU7/6V6 or EL84. Check this thread. EL84 SE advice'n'schematics needed

Quote:
Originally Posted by ChrisA View Post
Remember impedance is proportional to square root of the number of turns. So the center tap to end is always 1/4 the total impedance in any transformer with a center tap.
So 2 2000ohm coils in series is 8000ohms because they share the same core right? Otherwise 2 inductors in series equal the sum of the inductors. So a center tap IS the center tap of the coil but the measured series inductance is 2x the sum of each seperate coil due to the coils sharing the same core.

Last edited by lazzer408; 2nd October 2010 at 02:40 PM.
  Reply With Quote
Old 15th November 2010, 03:38 PM   #16
diyAudio Member
 
Join Date: Feb 2005
Location: Gelderland
i wanted to build a el84 triode PP amp, but meanwhile accidentally saw that 5 sections of 12AU7 ~ EL84 triode quite near, regarding , rp, transconductance, down to heater current, so i will give that a try. they will be able to dissipate slightly more than EL84s,
because 5 units are inside 2.5 glass bottles, while a EL84 sits in a tube about 4/5 the size. (still, the AU7s are rated a bit cooler and not dissipate as much as that factor relatively, but little less).
first complete ;p

Last edited by jechentau; 15th November 2010 at 03:48 PM. Reason: have not decided a name yet lol
  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
"Push-Pull" for push und "Push-Pull" for pull (like Gamnut) - whats the right name? tiefbassuebertr Solid State 70 15th August 2012 08:08 PM
6L6 / 12AX7 push-pull lazzer408 Tubes / Valves 115 2nd October 2010 07:55 PM
modify parallel push-pull EL84 to single push-pull chungtat Tubes / Valves 12 3rd November 2005 11:25 PM
12AU7 or 12AT7 instead of 12AX7 ? Gubby Tubes / Valves 5 25th June 2004 11:31 PM
Double push-pull versus single push-pull jraraujo Tubes / Valves 19 19th October 2002 12:23 AM


New To Site? Need Help?

All times are GMT. The time now is 06:08 AM.


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