Identification of unknown output transformer - 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 7th July 2010, 09:11 AM   #1
diyAudio Member
 
Join Date: May 2007
Default Identification of unknown output transformer

Hi there,

I'm completely new to tubes really but have been reading a bit and thought I'd attempt to build a guitar amplifier using salvaged parts from an amplifier that my flatmate bought from a garage sale. It was a stereo amp (Fountain, NZ's finest) but has had one output transformer removed. The remaining output transformer is very bulky (good thing?) so I'm trying to identify which lead is which but using Morgan Jones' method the numbers do not crunch. It has a total of 10 leads, 4 on one side and 6 on the other (not necessarily primary and secondary I guess). I'm fairly certain it's a push pull due to the amp topology and number of leads. The impedances are as follows:

Six wire side -
Brown/orange = 4.4R
Brown/green = 2.2R
Orange/green = 2.2R
Blue/yellow = 1R
Blue/black = 1.4R
Black/yellow = 1.6R

Four wire side -
Blue/red = 172R
Black/yellow = 153.9R

All other impedances measure infinite on my multimeter (cheap POS which measures 0.1-0.5 when leads are connected together anyway). You would think that maybe green is center tap with orange and brown being the anodes but the impedances are far too low. Anyone have any ideas?

Images of trans:

ImageShack Album - 2 images
Attached Images
File Type: jpg DSC00067.jpg (414.5 KB, 226 views)
  Reply With Quote
Old 7th July 2010, 11:21 AM   #2
diyAudio Member
 
Join Date: Apr 2003
Location: Portland,Oregon
Blog Entries: 4
Send a message via AIM to DigitalJunkie
To me it looks like there are 5 wires on one side,with 6 on the other. The higher resistance windings will be the primary,and the low resistance windings will be the secondary/secondaries.

It would seem that the side with 5 wires is the primary,complete with UL taps..But that's only an assumption on my part.

Edit: Nope,there are only 4 wires...The shadow underneath looked like a 5th wire. My bad!
  Reply With Quote
Old 7th July 2010, 11:30 AM   #3
Arnulf is offline Arnulf  Europe
diyAudio Member
 
Join Date: Feb 2009
1: Interleaved E-I core = no airgap so it is probably a PP transformer.

2: Oddball resistances, perhaps the primary is indeed asymmetrical ?

3: Grab any mains transformer with reasonably low (= safe) output voltage (say 12-48V), hook it up to mains, measure secondary voltage, feed secondary into one halfprimary of the OPT (say blue-red wires), connect one wire (say black) of the other halfprimary to either end the first one (say blue), then measure voltage across the remaining two wires (red-yellow). If you're reading close to 0V rather than what the output of mains transformer times 2, reverse the black and yellow wires. Measure voltage across outermost wires (now red-black) again to confirm that you're getting roughly twice the output of mains transformer. You have identified the primary taps, outermost wires go to tube anodes, the inner pair is connected together and goes to B+.

Note that you can also hook it up to mains directly if you don't have a suitable mains transformer handy and you feel confident you won't get electricuted in the process.

4: Now hook up your mains transformer output so that it is connected to two outermost wires of the OPT primary, as determined in previous step. Measure voltage across full secondary (brown-orange). Note the value and calculate impedance ratio:

Vin = mains transformer output voltage as measured in step 3
Vout = OPT full secondary as measured in step 4

Zratio = (Vin/Vout)^2

For example, if Vin = 15V and Vout = 0.6V then Zratio = 625

5: Repeat step #4 for all OPT secondary wire pairs that show different resistance to determine all avaliable Zratios. This will allow you to pick the winding that is most suitable to your purpose, considering your nominal speaker impedance. Note that secondary windings can be connected in various ways to get even more Zratios.
__________________
mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."

Last edited by Arnulf; 7th July 2010 at 11:34 AM.
  Reply With Quote
Old 8th July 2010, 03:01 AM   #4
diyAudio Member
 
Join Date: May 2007
Thank you so much for replies! I'm just about to go away on a trip but when I get back I'll hook up an 18VAC transformer and see how it goes. That makes a lot more sense if two wires connect together and go to B+. Will be awesome if this transformer works well.
  Reply With Quote
Old 13th July 2010, 01:12 AM   #5
diyAudio Member
 
Join Date: May 2007
Transformer behaves exactly as you predicted! Red and black go to anodes, blue and yellow together go to B+, brown and orange gives 0.7V from a 19V mains transformer giving Zratio=736.7



Three thumbs up for you, my friend!
  Reply With Quote
Old 13th July 2010, 07:40 AM   #6
Arnulf is offline Arnulf  Europe
diyAudio Member
 
Join Date: Feb 2009
This would be Zaa of 5900R : 8R, add DC resistance and you get approx. 6K combined load. Now you can select output tubes and design the circuit around the transformer to fit it best. Depending on transformer dimensions I think you'll be looking at 6L6 or similar tubes - you can always go for lower Zratio with different secondary taps and you should check them out so you can plan accordingly.

A very interesting tube choice would IMO be PCL805, a noval package TV vertical deflection pentode (with triode conveniently enclosed in same bulb) that rarely gets used for audio and which can usually be had for peanuts, you can also buy NOS Russian versions of this tube with 6.3V heaters (= equivalent of ECL805). Unusually high current for such a small tube, a pair of these would probably be on par output power-wise with a pair of 6L6. On top of everything you only need two of these and a fistful of passive components to make a complete PP stage as you only need 20V peak-to-peak to drive the pentodes to both extremes in class AB1 and triode section has mu of ~60 so it can easily cater to required gain of ~20x in either arrangement of the two triodes (if it was me I'd give LTP input over common cathode + cathodyne stage a try for starters).
__________________
mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."

Last edited by Arnulf; 13th July 2010 at 07:43 AM. Reason: typos
  Reply With Quote
Old 13th July 2010, 07:53 AM   #7
Arnulf is offline Arnulf  Europe
diyAudio Member
 
Join Date: Feb 2009
Just to clarify: above output power comparison was meant at the same supply voltage. PSU is easier (= cheaper) to design when dealing with sub-300V rather than with 400V and above.
__________________
mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."
  Reply With Quote
Old 13th July 2010, 08:51 AM   #8
diyAudio Member
 
Join Date: May 2007
Aah OK thanks for the advice. I was planning on using the other salvaged parts from this amp in order to build a guitar amp. The tubes I have are 4x RCA 7189 (basically an EL84 according to wikipedia), 3x 12AX7/ECC83 and 2x 6AN8A. The power transformer is 350-0-350 with 6.3V heater secondary. I was thinking of a Vox AC30 clone but I'm not sure at this stage. There are tonnes of schematics here for this amp and many others.
  Reply With Quote
Old 13th July 2010, 11:53 AM   #9
Arnulf is offline Arnulf  Europe
diyAudio Member
 
Join Date: Feb 2009
This might work as well, two tubes per side and one of the lower Zratio taps. For guitar amp you're going to need extra amplification, above suggestion was meant for audio (lin) input.

Tubes are cheap though, sockets usually cost at least as much as cheapo tubes, potentiometers can be expensive and so can be large capacitors, so this is the largest fraction of the cost once all transformers have been taken care of.
__________________
mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."
  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
transformer identification ChipTube Tubes / Valves 9 21st June 2010 03:45 PM
transformer primary wires - identification pheonix358 Power Supplies 5 6th October 2009 10:37 AM
Toroidal Transformer Identification Help krips Solid State 1 17th January 2009 01:01 PM
tube amp transformer identification iain42 Tubes / Valves 2 17th July 2007 10:13 PM
Torridal Transformer identification jeffshort2000 Parts 4 8th January 2007 07:13 AM


New To Site? Need Help?

All times are GMT. The time now is 07:58 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