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30th August 2011, 03:09 AM  #1 
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island

Unknown output transformer
Hi , I have an old Hammond Output transformer (123D) that i don"t know the specs of and was wondering if there is a way for me to figure out the Winding Ratio/Impedances using my DMM ??
I contacted hammond and they said that it doesn"t show up in any of their cataloges and don"t know anything about it .... I got it off a homemade tube amp that I found in a junk pile that didn"t work , it had really old and obscure Japanese tubes in it (One recifier tube and triode and a pentode) and the Power transformer was also japanese (it had 90v and 100v primary taps) ...... The output transformer has a primary(2 wires) and 2 secondaries (2 wires and places to solder 2 more numbered 1  4) I was hoping to use it in a 6V6 power amp project ..... Thanx guys ....... 
30th August 2011, 05:58 AM  #2  
diyAudio Member

Quote:
You have already figured out the winding connections and figured out that the primary will have more windings, thinner wire and higher resistance. With a low voltage AC source (AC wallwart/variac/filament transformer/transformer from cheap junked SS gear) and your DMM you can find the voltage aka winding ratio, and thus (by squaring that ratio) the impedance ratio. Google:'measuring output transformer impedances' Remember that the OT is an 'impedance reflector' , so the speaker impedance and the connections you choose for the speaker, control the impedance reflected back to the tube hanging on the primary. 

30th August 2011, 10:05 AM  #3  
diyAudio Member

Quote:
Just hookup the primary of the 123D (with no load) to the secondary of a little low voltage transformer, measure the voltage at the primary and at the secondary (also between the unused taps if you like). Then divide the primary voltage by the secondary one, this will give you the voltage ratio to 1 of the transformer ... The impedance ratio is the square of the voltage ratio, just multiply it by your speaker impedance to see what impedance you will have at the tube plate. One of the secondary wire must be a common and the two unused taps must be for other speakers impedances, most of those four wires secondary transformers was made for 4, 8 and 16 ohms (when all the taps go to the same winding). For a 6V6 you need a 5000 to 8 ohms, that mean you need a voltage ratio of the square root of ( 5000 / 8 ) = 25 : 1. For example, with your test setup, if you read 12 volts on the primary and 0,48 volts at the secondary, you have a 12 / 0,48 = 25 : 1 voltage ratio and it is just perfect for your project. Now, suppose you have 15 volts at the primary and 0,85 volts at the secondary, 15 / 0,848 = 17,689 ... For a 8 ohms speaker, the impedance ratio is 8 x the square of 17,689 = 2503,2 ohms ... It is good for a 50C5 or a 2A3 power triode $$$ but much too low for a 6V6 ... But for a 16 ohms speaker, 16 x the square of 17,689 = 5006,4 ohms, that mean it is a 16 ohms tap for a 5000 ohms transformer ... Good luck with your project ! Alain. 

30th August 2011, 11:47 PM  #4 
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island

Thanx , that helps a lot , I"ll try testing it tonight with a 12v AC transformer I have .....
Cheers 
31st August 2011, 02:49 AM  #5  
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island

Quote:
I just hooked up the Transformer to a 12.3v AC source , First I connected it backwards (oops) and got 350v AC on the output , then connected it the proper way and got 0.48v AC which is exactly what you said it should be for a 6v6 amp ..... So Did i just get lucky ?? It is actually a quite large transformer , about 3x as big as the one in a different 6v6 amp i have ...... Thanx 

31st August 2011, 07:46 AM  #6 
diyAudio Member

The math is pretty straightforward just follow Alain's calculation.
12.3/0.48 is about 25.6 which is the voltage or windings ratio. Square 25.6 and you get (25.6*25.6=657) the impedance ratio. So, whatever impedance you hang on the secondary will be multiplied by 657 to the primary. 8 ohm speaker will reflect about 5.3k to the primary, 16 ohm speaker will reflect 10.6k etc.... Were both secondary windings the same? 
31st August 2011, 07:55 AM  #7  
diyAudio Member

Quote:
8 x square of ( 12,3 / 0,48 ) = 5253 ohms ... It is perfect for your 6V6 project ! Notice the impedance don't have to be exact, +/ 5 % is OK, even +/ 10% is acceptable ... There is also some power loss in every transformers and the speakers impedance vary a lot anyway, at the resonance frequency, a 8 ohms speaker can have over 50 ohms impedance and more than 16 ohms at high frequency ... Hammond make transformers since almost 80 years, they "know how", since your transformer is bigger than average one's, you will get very good bass because the DC magnetic saturation will be lower. I use Hammond transformers very often since 40 years for projects and replacements, some Montreal store near my village sale them, a good thing to save on shipping but they are a bit expensive. The 5000 ohms SE transformers was very common, they can be use with a lot of tubes types like the beampowers or pentodes 6AQ5, 6BQ5, 6GK6, 6L6, 807, 5881, 7408, 6550, power triodes like 50, 300B, 211, SV5723, SV57210, SV8113 and many "dissimilar twin triodes" made for B&W TV VerticalDeflection Oscillator and Amplifier, they sound very good and are not very expensive ... You can also use two tubes requiring 10000 ohms in parallel, this will double their power ... Take care with high voltages, they can hurt or even kill you ... Cheers, Alain. Last edited by Alain Poitras; 31st August 2011 at 08:11 AM. 

31st August 2011, 11:45 PM  #8  
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island

Quote:
I just finnished building a 2 channel guitar amp , it had 2 x 12ax7 per channel preamp with a 3 band tone controll all running at 300v DC going into a lm3886 chip and I used a couple 5v relays to build the switching circuit activated by a footswitch and it has a couple LED"s to show what channel is in use , and it has an effects loop ..... Another question , I have this 115v in 115v out ac power transformer and was wondeing if it would be possible to use a voltage doubler with it to get 300v ish DC for the 6v6 ?? I think the transformer can probably put out about at least 500 ma .... And I think the 6v6 only needs like 45  50 ma ...... Thanx 

1st September 2011, 12:02 AM  #9  
diyAudio Member

Quote:
Grab a free copy of PSUD (PSU designer) from Duncan amps site it's a great tool for simulating PS circuits. It showed 290v out of a 115VAC transformer, when I checked just now. Now that caps are cheap (and compact) doubler circuits make a lot of sense and can use up iron from the parts box. You could even put a couple of bell transformers (or filament transformers) backtoback if you didn't have a 115/115 transformer. 

1st September 2011, 08:37 AM  #10 
diyAudio Member

If you are interested using a voltage doubler, you must know there is two kinds of them, "full wave" and "half wave". I make the next Spice simulation many months ago to see the differences between them.
This "equivalent" circuit is for 120V supply and 250ma load and the DC output is more than 290V ... If your transformer give you 115V and your maximum load is around 60ma, you can get easely about 290V like VictoriaGuy said or even more than 300V, it depend on the value of the electrolytic capacitors. That is more than enought because the datasheet's of the 6V6 say the typical operation with a 5000 ohms transformer need 250V between plate, screen grid and the cathode ... You have to take care the dissipation don't exceed the maximum of 14 watts for the plate and 2,2 watts for the screen grid or your 6V6 will toast ... There are some parts in the circuit just for the simulator, they are not there in the real circuit ... The 10 ohms resistor represent the resistance of the transformer windings or current limiting resistor for direct AC line operation (dangerous), the two circles represent the DC load, it is a constant current symbol, the 100n capacitor and 100k resistor are just there to measure the ripple with Probe2NODE ... The only "real parts" you need are the diodes and the electrolytic capacitors. The ripple of the two doublers are about the same but not at the same frequency ... You will have to use a good filtering anyway, a choke or a resistor follow by another electrolytic capacitor. Personnally, I prefer the "half wave" one because the "working voltage" of the capacitors can be 200V, the "full wave" need 350V one's but the final ripple will be a bit lower at 120 Hz than at 60 Hz with the same second filtering capacitor. When you evaluate the supply voltage you need, you have to calculate the voltage drop on the primary winding of the output transformer, if the current is 45ma (with no signal) and the DC resistance of the primary is 100 ohms for example, the voltage drop across the primary will be 100 x 0,045 = 4,5 volts. If you use a autobias resistor (and a coupling capacitor of 100 or 220uF 25V) for the 6V6 (that is the best), for a 12,5V bias, since the current with no signal of the plate plus the screen grid is about 50ma, you need a 12,5 / 0,05 = 250 ohms resistor (2 or 3 watts OK). Of course, the voltage drop across this resistor is 12,5 volts ... So you need a supply voltage of 250 + 4,5 + 12,5 = 267 volts ... If your supply give you 290 volts, you need a resistor in the filtering circuit with a voltage drop of 290  276 = 14 volts ... The current is 50ma plus the current of the driver tube, let said 2 ma, a total of 52 ma, the resistor have to be 14 / 0,052 = 269,23 ohms (270 ohms is OK) and is dissipation is 14 x 0,052 = 0,728 watts, so a 2 or 3 watts resistor will not heat too much ... I hope I don't all confuse you with those details but they are important to get the right voltages for the tube. This is just an example but if you like to design your amplifier completely, it is not very difficult. If you use the typical operation of the datasheets, you will have no problems, just add a highmu driver/preamp like a 6AV6 or a 12AX7 triode and you will have a lot of gain, add a  6dB negative feedback and wou will have a much lower distortion. Here's a good 6V6 GTA datasheet from RCA Here's the important part for your project, I put red rectangles around what is important for you, notice the 4,5 watts is the one that use a 5000 ohms transformer ! If you have any questions about all that, I will be happy to help you, the DIY part I prefer since 40 years is the design and maths ... Have a good day ! Alain. Last edited by Alain Poitras; 1st September 2011 at 09:06 AM. 
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