OT impedance taps question

[flatfive]

How do I test the impedance of the yellow, green and orange leads on my output transformer? I can't find out for sure which is which.

[snoof]

google is your friend

http://www.radioremembered.org/outimp.htm

[katopan]

Any low voltage mains tranformer (heater winding is convenient but will give low levels to measure, AC plugpack, etc) can be used safely instead of the auto transformer mentioned in the ref (good doc by the way). Connect the secondary of the LV mains transformer to the primary of your OT. Connecting across the CT to one end of the OT primary will give you a bigger signal to measure on the secondary windings, but will give different winding ratios compared to connecting across the whole OT primary. Measure your input voltage and the various tapping output voltages with a multimeter. Ratios are covered in the theory ref from previous post. Basically the higher the nominal secondary load in ohms, the higher the voltage you'll measure. Double the ohms should give 1.414 times the volts. Do this test with nothing connected to the OT secondary tappings.

[olddawg]

I have a big old tube HP signal generator that will put out a signal up to about 20 volts. It will produce a 60 cycle tone but could I use say 1Khz at 10vpp on the primary measure it with my scope then measure the ouput taps with my scope and use the same calculations or ratios?

[katopan]

Being an audio transformer, yes this test can be done at audio frequencies. Using a signal gen at 1kHz will work fine as long as the signal gen unit can drive the magnetising current of the transformer. Some of the smaller bench test signal gens can't drive into the OT primary, seeing an impedance below what they are rated for. Doing it this way, the higher frequency is better as it gives you a higher OT primary winding impedance.

[flatfive]

Connecting across the CT to one end of the OT primary will give you a bigger signal to measure on the secondary windings, but will give different winding ratios compared to connecting across the whole OT primary.

What does CT refer to?

[snoof]

center tap

[flatfive]

I'm probably not going to try this, but I do appreciate your help. Thanks.

[Phil_S]

How do I test the impedance of the yellow, green and orange leads on my output transformer? I can't find out for sure which is which.

Best guess? Green=16, Yellow-8, Orange=4. If you know for sure they are 16-8-4, you can simply put your ohm meter between each color and the 0 or common (ground) lead. 16 ohms should offer the least resistance, 8 in the middle and 4 ohms the highest. That is because the 16 tap has the lowest turns ratio and the 4 has the highest. Therefore, the 4 ohm tap has the most wire and the highest resistance.

The readings are likely to be very low and differences could be subtle. Look for possibly less than one ohm up to maybe 2 or 3 ohms. For example they might be 0.6, 0.8, and 1.0. I don't know what you've got, so this is just a shot in the dark.

The only way to know for sure is to determine the turns ratio by running voltage in one side and seeing what comes out the other, or to find the long lost manufacturer's spec sheet.

Do you have any idea who made it and the model number? Post any numbers you can find. Maybe you will get lucky and someone will look it up.

[flatfive]

36-OT 9960412

[BooBird]

Phil, I agree with your post, but would'nt the lower resistance give you a lower ohmage load at the speaker, like lowest resistance in the sec. would be like a 2 ohm load and the most resistance would give you more like the 16 ohm reading, at least that's what i see when i look at a older amp Schematic like a Bogen which had lot's of multitaps out for various different speaker load's, you can also look at a new hammond, the more winding the higher the speaker ohmage you'll need !
Boo

[leslie]

for example Primaries around 200ohm continuity.
When you find half of that ..one of wire is ct..
to determine ct you must see about same resistance with other two primary leads.

Secondaries, if you know ground /0 than it's easy...
The most ohms reading is 16 ohm tap.
let's say ohm meter reads 1,6 0hm (biggest resistance)16 0hm
than lover than that let say 1,3 ohm is 8 0hm tap and so on.
The logic in most OT's is- 16 ohm tap is longest winding in series.

One more thing can be of help...usually there is some order when manufacturer is making tranny so if black is zero and is far left or right next to it is 4ohm tap, 8,16 respectively.
If there are only 3 lead wires on the secondary..it's either 0,4,8 or
0,8,16

Ground/zero lead wire is common to be in black ,brown or yellow/green colour(sometimes gray too).This are usual ground colours in electronics and people who repairs don't expect those colours to be hot, don't kill the service man lol.

[sub]

I tried to measure my heyboer OT pri and sec.
My DMM isn't a Fluke (unfortunately ) on 200ohm setting it isn't really accurate.
There is what i measured:
Pri:
Blue - red(CT) > 342ohm
Brown - red(CT) > 325ohm
Blue - Brown > 667 ohm

Sec:
Black - Yellow(4ohm) > 0.4-0.6ohm
Black - Green(8ohm) > 0.6-0.8ohm
Black - orange (16ohm) > 0.6-0.8ohm

Interesting thing i measured always the same on green(8ohm) and orange(16ohm)

Oh i forgot this is a 18W OT (not 36W)

[flatfive]

My taps come out of the OT left to right, Black, Yellow, Green, Orange. So it seems logical that it would be Yellow=4, Green=8, Orange=16.

[leslie]

My taps come out of the OT left to right, Black, Yellow, Green, Orange. So it seems logical that it would be Yellow=4, Green=8, Orange=16.

Measure it to be sure.

[leslie]

Interesting thing i measured always the same on green(8ohm) and orange(16ohm)

Oh i forgot this is a 18W OT (not 36W)

Hey sub,
yeah it's hard to measure low ohms and megaohms acuratelly with generic DMM...
The reason the 8 to 16 ohm tap is reading similar is because secondary wire is thick and you have only about let's say 30 windings or so inbetween end of 8 and 16...so it's hard to be accurate there...
the bigest windings (number of turns) is beetween Ground and 4 0hm tap around let's say 70 windings or so.

So winding turns progression looks something like this :

-70-30-30- (let' say complete winding is 130 turns)

"-" is your leads 0,4,8,16

Regards

[sub]

Thank you Leslie!

[flatfive]

I must have guessed right. The green (middle coming out of the OT) wire appears to be 8 ohm. I played it for 3 hours no problem. BTW, I really like the Vintage 30. It has a surprising amount of bass.