Sizing PP OPT - 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 8th June 2007, 06:46 AM   #1
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
Default Sizing PP OPT

I have been asking around on how to size push pull OPT and am getting various answers:

1. Size it plate-to-plage
2. Add the current through both tubes and add 20%.
3. One suggests to use the power listed for a PP configuration of the tube, and get a PP OPT that can handle the power.

In the Philippines, ready-made OPTs (Hammonds, Tamura, James, etc) are hard to get, so most of us are forced to have them custom wound, and when having it wound, I need to give the winder the specs.

Thanks.
  Reply With Quote
Old 8th June 2007, 10:54 AM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
There have been books (many) written on just this topic. Basically, you need to specify plate to plate impedance, power, and bandwidth. The last two will determine the size, the first will determine turns ratio.

As an example: Want full power at 10Hz? Prepare for a hunk of iron the size of a refrigerator. Willing to live with 20Hz? You've dropped the size by a factor of four and probably reduced the leakage inductance.

If you have to tell the winder how to size the core once you've given him the outline of what you want the transformer to do, that's a good indication that you're dealing with the wrong guy.
__________________
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 8th June 2007, 11:48 AM   #3
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
One more once

http://www.dissident-audio.com/OPT_da/Page.html

Look at RDH4 Chapter 5 before.
It contains unvaluable informations.

Yves.
  Reply With Quote
Old 10th June 2007, 07:08 AM   #4
alexg is offline alexg  Philippines
diyAudio Member
 
alexg's Avatar
 
Join Date: Jan 2005
Location: Philippines
What I really meant by my question is:

If I bias one tube of the PP pair at 40ma, should I get an OPT that is rated at 40ma or at 80ma? or some values in between?

Thanks.
  Reply With Quote
Old 10th June 2007, 08:02 AM   #5
diyAudio Member
 
dsavitsk's Avatar
 
Join Date: Jan 2005
Location: Hartford
http://www.audioasylum.com/forums/tu...12/123015.html
  Reply With Quote
Old 10th June 2007, 09:21 AM   #6
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Dsavitsk, that link discusses SE transformers, which are subject to core magnetization because of the net DC in the primary. With PP, that is not the case.

What is meant here, if I understand the question correctly, is what primary current should be specified so that the gauge of wire used will be sufficient. If 40mA is the quiescent current per OP tube, then that will also apply to each half of the primary winding. However, it's not as simple as that, because it also depends upon the class of operation of the OP tubes.

If the amp is Class AB or B, the current at full signal can be several times as great as the current with no signal. The primary winding needs to be able to cope with the maximum possible current. For example, EL34s in Class AB1 could be biased to draw 30mA per tube at zero signal but may well draw 100mA per tube or more at full signal. In such a case, it would be sensible to design the primary to tolerate at least 120mA (or even 150mA) in each half, to avoid the danger of overheating if the amp is run at full power. You don't want the OPT primary to be the weakest link in the chain - it's not a fuse!
  Reply With Quote
Old 10th June 2007, 11:20 AM   #7
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
Quote:
Originally posted by ray_moth

If the amp is Class AB or B, the current at full signal can be several times as great as the current with no signal. The primary winding needs to be able to cope with the maximum possible current. For example, EL34s in Class AB1 could be biased to draw 30mA per tube at zero signal but may well draw 100mA per tube or more at full signal. In such a case, it would be sensible to design the primary to tolerate at least 120mA (or even 150mA) in each half, to avoid the danger of overheating if the amp is run at full power. You don't want the OPT primary to be the weakest link in the chain - it's not a fuse!
IMHO, the best approach is to compute the DC resistance of the primary winding.
So you can find how much power it will dissipate according to the mean AC and DC current flowing thru it.

Much better that any rule of thumb based on "acceptable" current density, I think !

Yves.
  Reply With Quote
Old 10th June 2007, 01:10 PM   #8
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
I never mentioned current density. Alex wants to be able to instruct his transformer maker about the primary current that should be allowed for. Probably the easiest way to find that out will be to look up the max. plate current in tube data sheets for the type of tube, class of operation, load impedance, B+, etc. that he has in mind.
  Reply With Quote
Old 10th June 2007, 01:25 PM   #9
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
Hi ray_moth,

No, YOU didn't !

But guess what the winder will do ?
He should be informed about DC and AC currents to take its decision about wire gauge according to the way the heat is extracted from the windings.
It could be quite different if the design uses bells or if potted in some more or less temperature conductive material.

Not to tell about ambient temperature, probably much different in Manilla than here

Yves.
  Reply With Quote
Old 10th June 2007, 02:45 PM   #10
diyAudio Member
 
Join Date: Nov 2005
I've wound 5 output transformers for guitar amps and I guess the same applies to all OPT's, I use a figure of 3 amps/square mm for the current loading of both the primary and secondary. For the valve anode current I take the max anode current given in the valve data book and add a 25% margin for safety. So for an EL84 this works out at about 75mA, using the 3 amps/square mm figure, wire size is about 0 .18mm for the primary and up to 1mm diam for secondary depending on the secondary impedance. I don't have any 0.18mm wire so I use 0.2mm wire which I have plenty of. For guitar amps I don't use elaborate interleave just a simple P-S-P-S. I always test with a sine wave and run at full power for at least 2 hours, sometimes the anodes start to glow a dull red after an hour or so like this ( you can see it in the dark) but I have never had a transformer failure. They get warm but never so hot than you can't hold your hand on it. I size the core (which is a reclaimed power tranny core) to give me a 75%-80% fill of the winding window. When I wound my first OPT for a PP 6V6 amp I only had 0.16mm wire that I had pulled from an old telephone relay so I used that. The amp is still going after 2.5 yrs and being used weekly in a recording studio, although it almost never gets used at full power. Initially I was concerned about the wire size so I ran some tests on the current carrying capacity of enameled wire. I soldered about 5 cm of 0.16mm wire ( at 3 amps/square mm it will carry 60mA) to some leads and then wrapped the wire around a digital thermometer thermocouple, I slid this into an empty matchbox and then wrapped an old rag around it for insulation. Using a variable power supply I ran 60mA through the wire and after 2 hours temp had gone up only 0.5 deg C, I increased current to 100mA and after 2 hours temp had gone up only 2deg C. 3 amps/square mm is a safe figure to use I can't speak for anybody else but it works for me. I have unwound 2 failed OPT's and it looked to me that the failures in both cases were caused by arcing, in one instance between the laminations a primary wire an in the other instance between primary wires placed too close together.
  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
Help sizing psu smoothing cap riotubes Parts 5 8th July 2009 07:07 PM
Sizing capacitors preiter Power Supplies 5 29th May 2006 04:48 AM
Sizing a Regulator cantskienuf Tubes / Valves 5 17th May 2005 03:55 AM
Sizing thermistors eeka chu Parts 7 19th December 2004 07:03 PM
help w/ enclosure sizing mp3z24 Multi-Way 4 8th June 2002 12:36 AM


New To Site? Need Help?

All times are GMT. The time now is 03:10 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