Power Transformer Issue - 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 31st January 2013, 07:43 PM   #1
diyAudio Member
 
Join Date: Dec 2010
Location: Amesbury, MA
Default Power Transformer Issue

I have a 100 watt EL34 Push Pull amplifier here that has some strange symptoms. A friend of mine said his amp was dead quiet until the other day when there was a noise though the speakers, even in standby (typical standby that just opens the B+ to the amp but leaves the heaters on).

Ok so I asked him if he ever noticed his power transformer being so noisy and he said he isn't sure. Well to me the noise through the speaker is the same noise that power transformer is making, you can even feel the chassis to the amp vibrating.

So here is where I am at. TUBES ARE REMOVED AND NO HELP

I disconnected the B+ to the standby switch taped it up and left it floating and no help. Next I disconnected the primary taps from the output transformer and that didn't help. Next I measured 500mA of AC current on the primary side of the power transformer. I disconnected the power transformer high voltage secondary taps, taped them up and left them floating. I measured about the same current on the primary of PT and the noise is still present. Next I disconnected the 6.3vac heater taps for the EL34's, taped them up and left them floating, I measured about the same current on primary side of PT and the noise is still present. Next I disconnected the 6.3vac taps for the preamp tubes, taped and left them floating, I measured about 400mA of current on primary of PT (the on bulb is on this circuit) and the noise is still present.

If I relocate the output transformer off the chassis the noise goes away through the speaker.

Ok so with nothing but the primary side of the power transformer connected I see almost a half amp of current. Is that normal? I would think not. Is this a power transformer or output transformer issue? I would think power.

It seems as though the noise is being injected from the power transformer into the output transformer and on to the speakers.
  Reply With Quote
Old 31st January 2013, 07:47 PM   #2
diyAudio Member
 
Frank Berry's Avatar
 
Join Date: Sep 2003
Location: Midland, Michigan
Turn the power or the output transformer (but not both) 90 degrees and see if the hum goes away.
__________________
Frank
  Reply With Quote
Old 31st January 2013, 08:00 PM   #3
diyAudio Member
 
Join Date: Feb 2008
Location: Jackson,michigan
Sounds like bad transformer to me!!!

If there is noise coming out of the speaker with all of those lines disconnected then there is possibility that the primary maybe shorted to the core.
BE CAREFUL IT MAY BE LIVE!!!!!!

Especially if the sound goes away by just removing it from the chassis.

500ma of Idle current seems kind of High to me if there is no load on it,I may be wrong as it has been a while since I have messed with a tube power supply transformer.

FWIW

jer
  Reply With Quote
Old 31st January 2013, 08:14 PM   #4
diyAudio Member
 
Join Date: Dec 2010
Location: Amesbury, MA
Quote:
Originally Posted by Frank Berry View Post
Turn the power or the output transformer (but not both) 90 degrees and see if the hum goes away.
I tried to move the output transformer 90 degrees and no help. But if I move it off the chassis the noise goes away. The layout is good, the power transformer is mounted on the total opposite side of chassis a good foot apart. This is an old amp from the 60's and my buddy has owned it for 6 years or so and he said the noise was never there, and he is total sound nut and would have noticed.
  Reply With Quote
Old 31st January 2013, 08:34 PM   #5
12E1 is offline 12E1  United Kingdom
diyAudio Member
 
12E1's Avatar
 
Join Date: May 2012
Location: Near Huntingdon, Cambridgeshire, UK
Quote:
Originally Posted by famousmockingbird View Post
Ok so with nothing but the primary side of the power transformer connected I see almost a half amp of current. Is that normal? I would think not.
For a 100W amplifier, that could be normal. The key point being that with the secondary disconnected, the current is almost 90 degrees out of phase with the voltage - it's just acting as an inductor rather than as a transformer in this case - and so the power lost in the transformer is very low.

As for the main problem, I am inclined to suspect magnetic coupling. What is the chassis made from?
  Reply With Quote
Old 31st January 2013, 08:48 PM   #6
diyAudio Member
 
Join Date: Dec 2010
Location: Amesbury, MA
"As for the main problem, I am inclined to suspect magnetic coupling. What is the chassis made from?"

It is made from steel.
  Reply With Quote
Old 31st January 2013, 09:23 PM   #7
Tajzmaj is offline Tajzmaj  Slovenia
diyAudio Member
 
Join Date: Mar 2011
Location: Ljubljana
Almost half amp of idle current without anything connected to secondary seems like 100V primary (maybe some old Japanese transformer?) or overheated and so shorted turns. If you have variac you can slowly turn up voltage and measure input current. If it goes up linear something is shorted. If you will notice some point where it will start to increase rapidly you will find out for which primary voltage was calculated. 0.5A is probably too high even for 2KW unit.
Cheers
  Reply With Quote
Old 31st January 2013, 10:03 PM   #8
12E1 is offline 12E1  United Kingdom
diyAudio Member
 
12E1's Avatar
 
Join Date: May 2012
Location: Near Huntingdon, Cambridgeshire, UK
Quote:
Originally Posted by Tajzmaj View Post
Almost half amp of idle current without anything connected to secondary seems like 100V primary (maybe some old Japanese transformer?) or overheated and so shorted turns. If you have variac you can slowly turn up voltage and measure input current. If it goes up linear something is shorted. If you will notice some point where it will start to increase rapidly you will find out for which primary voltage was calculated. 0.5A is probably too high even for 2KW unit.
Cheers
Thinking some more, and re-reading, you could be right. I missed the bit in the original post about the noise coming from the mains transformer. That does begin to sound like an internal short somewhere.
  Reply With Quote
Old 1st February 2013, 03:01 AM   #9
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
I would split the chassis from the wall socket safety ground, and measure current chassis (transformer shell) to safety ground. Hammond Clock Co suggested you put a 10k 2 w resistor parallel to a .01 uf cap from chassis to safety ground, and measure AC voltage. More than 4 v they said your organ needed a new power transformer.Use clip leads and one hand at a time, don't touch the chassis while making this measurement.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 1st February 2013 at 03:09 AM.
  Reply With Quote
Old 1st February 2013, 04:21 AM   #10
diyAudio Member
 
Join Date: Dec 2010
Location: Amesbury, MA
Quote:
Originally Posted by indianajo View Post
I would split the chassis from the wall socket safety ground, and measure current chassis (transformer shell) to safety ground. Hammond Clock Co suggested you put a 10k 2 w resistor parallel to a .01 uf cap from chassis to safety ground, and measure AC voltage. More than 4 v they said your organ needed a new power transformer.Use clip leads and one hand at a time, don't touch the chassis while making this measurement.

I will try this out and post my readings
  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 Transformer Issue??? howe0168 Tubes / Valves 9 6th September 2012 02:59 PM
Power Transformer Issue Sperrino Power Supplies 3 14th September 2011 01:16 AM
Power Transformer issue dosilegecko Tubes / Valves 5 12th April 2010 07:23 PM
Transformer Issue? litezoner Solid State 36 5th October 2008 12:07 AM
Transformer issue filholder Chip Amps 29 25th March 2006 06:31 PM


New To Site? Need Help?

All times are GMT. The time now is 02:07 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