Output transformer and shock from the speaker jack - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

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
Thread Tools Search this Thread
Old 11th June 2012, 01:07 AM   #1
diyAudio Member
Join Date: Mar 2011
Default Output transformer and shock from the speaker jack

I have a couple of questions:

1.) I got a little shock from the speaker jack when I pulled it out to switch speakers in my cabinet. (I know you're not supposed to run the transformer without it being plugged in but I'm fast enough that I can do it.) Is this possible? I then plugged an 8 ohm resistor (dummy load) and tested the voltage at ~18.5v which is what I expected. Then I switched the meter to current, and got 4.2 amps. That is not right according to the math.

2.) The transformer is 4 and 8 ohm for a push pull setup, the power section is close to a Bassman clone. I am having trouble determining which lead is which. The resistance between the two secondaries is .7 ohms as is the resistance between the green secondary and ground. This tells me that would be the center tap and such would be the 8 ohm lead. The resistance between the yellow and ground is .9 ohms which tells me it is longer and should be the 4 ohm tap. It definitely sounds better in my 8 ohm speaker with this configuration, but with the little electric shock mentioned above I'm really not sure. Or sure that it's related.

Any help would be great,

Attached Images
File Type: jpg The Mänster Schematic.jpg (472.9 KB, 104 views)
  Reply With Quote
Old 11th June 2012, 01:13 AM   #2
diyAudio Member
Join Date: Mar 2011
Default ignore first schematic

there is no speaker polarity switch anymore...
Attached Images
File Type: jpg The Mänster Schematic.jpg (457.8 KB, 105 views)
  Reply With Quote
Old 11th June 2012, 03:05 AM   #3
kevinkr is offline kevinkr  United States
diyAudio Moderator
kevinkr's Avatar
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Good way to fry your output transformer, when unloaded abruptly the way you are doing it the primary will go into flyback mode as the field collapses and the voltage across the primary will go way up - this happens in milliseconds. The end result could be to destroy your output transformer. Think about what an inductor with a large amount of current flowing through it wants to do when you suddenly remove the current. Turn the volume/master gain down first!

Measure current by inserting the meter in series (in current measurement mode) with a load resistance or just calculate it based on 18.5V/8ohms which is a little over 2.25A.. Switching your meter to current is essentially placing a short across the output of your amplifier - hardly benign..

Might want to bone up a little on basic electronics before going too much further - imho you've been lucky so far.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
  Reply With Quote
Old 11th June 2012, 01:44 PM   #4
diyAudio Member
Join Date: Mar 2011
Thanks Kevin, I won't do that anymore. But what about the questions...
  Reply With Quote
Old 11th June 2012, 07:30 PM   #5
diyAudio Member
chrispenycate's Avatar
Join Date: Aug 2006
Location: W.Sussex
Eighteen volts is enough to give you a tingle, but it's more likely you cut in the middle of a cycle, and the collapsing field in the transformer gave you fifty or sixty volts. Just think of what was stepped up and reflected back to the tubes, and be gentle with your output stages.

That diagram is wrong. The eight ohm tap should have higher voltage than the four, thus more turns. If you've got an oscillator (doesn't have to be a great one) you can run it into the output of the tranny (with the amp turned off; the anodes aren't going to give any reading while cold. Great, tubes, aren't they?) and you can measure the voltages at the various taps. Cold to eight ohm is the highest voltage, cold to four about two thirds of that (yes, I know one over square root two, but I'm not calibrating to a tenth of a dB for broadcast authorities here.)
But speaker impedances are rarely all that precise (or regular), and amps put up with this. Make it sound right.
  Reply With Quote
Old 11th June 2012, 07:38 PM   #6
diyAudio Member
Osvaldo de Banfield's Avatar
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
To staying secure that the amp will not be run unloaded, replace the output jack with some that short the amp output. It is preferable a short for pentode output than an open circuit.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.
  Reply With Quote
Old 11th June 2012, 08:41 PM   #7
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Originally Posted by dscottguitars
I'm fast enough that I can do it
You can switch output plugs in less time than the transformer insulation takes to break down? I'm not sure if that is ms or us, but you must be very fast!

If you get a small shock from the secondary, think how much bigger the primary voltage will be.
  Reply With Quote
Old 11th June 2012, 08:45 PM   #8
diyAudio Member
Join Date: Mar 2011
Thanks everyone...

I now understand how I can get a little more than a tingle of shock with it being unplugged and in the middle of the cycle. Sounds like that would be normal as I shouldn't pull the speaker plug with it on, for more reasons than OT frying.

Chris: about the diagram, are you referring to the drawing of the OT and the way I have the 4 & 8 ohm taps drawn in? I didn't really think about correctness, only to show that it has two taps. And can one tell the impedance of the tap being used by measuring the voltage coming out if I use an 8 ohm resistor and select the four ohm tap on the switch and then the 8 ohm tap? I don't have an oscillator to use. So I am thinking that the voltage coming out will be different if I have it switched in 4 ohm mode with a 4 ohm resistor, than in 8 ohm mode with an 8 ohm resistor.
  Reply With Quote
Old 11th June 2012, 10:09 PM   #9
diyAudio Member
Join Date: Apr 2010
I wire a small 100 - 200 ohm resistor across the output jack. The voltage will climb of disconnected but not as bad as an open circuit.
  Reply With Quote


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
Jack For Windows Output Config Goto PC Based 5 29th August 2011 04:28 PM
Newbie question about output transformer to speaker matching rss388 Tubes / Valves 5 15th May 2011 08:05 AM
4ohm output transformer with 8ohm speaker nige838 Tubes / Valves 2 2nd November 2010 07:21 AM
running a transformer backwards(anti-hi-jack) seventenths Parts 10 6th October 2007 01:15 AM
Lucite for output jack cover? mytool4u Instruments and Amps 3 10th July 2005 02:43 PM

New To Site? Need Help?

All times are GMT. The time now is 11:21 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2