Question about determining OT Speaker Ohms

[Platefire]

On this Baldwin Organ amp I salvaged, without a schematic or no marking on the chassis I have no idea what the OT output speaker Ohms is. I previouly detemined speaker Ohms on some heathkit OT's by injecting an small AC voltage (approx 15VAC) to the primary taps, reading the output on the secondarys with a multi meter and applying a formula was able to determine Ohms on the transfomer taps. I had that formula wrote down but can't find it. Please refresh me on how to determine this. Thanks, Platefire

[RicharD]

Turns ratio = voltage ratio  If you put 10vac in and get 1vac out, then your turns ration is 10:1, just call it 10.  To calculate reflected impedance ratio the equation is turns ratio squared times load impedance.  TR^2 *Rl

Here's a video.

[Geezer]

And here's written directions>  http://www.radioremembered.org/outimp.htm
~~ 1/2 way down the page

[Platefire]

Thanks! Little confused about how to apply the AC. Have the primary red for DC to OT and then the
two leads to the EL84 plates. My AC test Transformer is providing 15 VAC. I'm not sure of hookup?
With my DMM set at 200 volts AC I get the following:

A-If I apply the AC supply to the two EL84 plate taps=I can put the MM lead on the secondarys and get .38 VAC

B-If I apply the AC to the red (DC input) and Green (EL84 Plate) I get .75VAC, also if I switch to the other EL84 plate I also get .75VAC. This is almost exactly 2x.38=.76!

15:.38  15x.38=5.7x5.7 (squared)=32.49

15:.75   15x.75=11.25x11.25(squared)=126

This is sure not looking right. This was done with the primarys still connected to the circuit, tubes removed but DC input still connected to filter caps/power supply. I'm a dummy when it comes to this so I hope you can make some sense of of this. Platefire

 

[jojokeo]

15/.38=39.47  call it 39.5
39.5*39.5=1560.25  call it 1560
1560*5.7 ohms (actual speaker Z) = 8892 call it 9k reflected impedance

[Platefire]

Thanks! So if this OT was designed for a 4 Ohm speaker would it be a little less than 4 Ohms, like 3.8 Ohms and the 5.7 Ohms reading would put it in the 8 Ohm speaker range?
I hope so, because I don't have any 4 Ohm speakers. Platefire  

[jojokeo]

You got it. The actual measured speaker DC resistance is less than the speaker Z rating listed. Going a rating up or down a "step" is fine. Meaning if you have an 8ohm tranny but use either a 4ohm or 16ohm cab/speaker - you won't hear much if any difference at all. My personal experiences on this is that it isn't a big deal tonal wise in most "everyday" situations.

[Jack1962]

IMHO 16 ohm loads have a better tone however , in my limited expereince , with tube amps as long as the load doesn't drop below 4 ohms your ok.

                                                      Rock On

[Platefire]

OK, Good! My cabs I use with these conversion heads I build is either an 8 Ohm 12" or a 8 Ohm 10".
So glad to know it will work fine with that. I guess I will proceed to make this into a project---18watt Marshall or something similar. Thanks, Platefire

[chocopower]

Hi,

Please check my maths for a push-pull U.L. transformer. Cathode biased power secction. 320vdc into the plates.

I put 36.6vAC into the plate leads and measured 1.65 in the secondary.

so...   36,6/1,65 = 22,2
that means a Turn ratio of 22:1

so...  Impedance Ratio is 484.

Wich speaker load should i use for a pair of EL34?
And for 2  6V6?

thanks in advance,

david

[jojokeo]

There's more to consider from doing a couple quick calculations to get optimum reflected primary Z but 8 ohms for el34 and 16 ohms for the 6v6s on this tranny will get you where you want.

[chocopower]

Thanks.

Silly me, i was connecting it to a 16ohm load with 2 EL34.
O.T. get really hot, but the amp cames with that configuration, so i never suspected about a wrong load.

Yesterday i used a 8 ohm load and the trafo was just warm after 2 hours playing really hard.

thanks!

[Platefire]

I'm still not understanding this formula and it's driving me nutty! I need to know Ohm impedance

On a differnt OT I'm testing I got 14.65 VAC in and getting 1.34 VAC out.

so the ratio is 14.65:1.34=10.93

10.93*10.93=119.46*4 Ohms=477.84 Impedance

This just isn't making sense to me and not coming out right. Help Please! Platefire

[jojokeo]

Plate, that tranny seems to be a line transformer and you're calcs & thinking all seem okay. If that (the voltages) is true, you don't want to use it.

[Platefire]

The Transoformer is a 100 Watt Twin Reverb OT with I think a single 4 Ohm tap. So my calculations are right and the transformer is bad? The voltage in and voltage out are correct because I double checked it---so my calculations are right and the transformer is bad? I need to know for sure because I was going to sell it. Platefire

[Platefire]

Bump--If you could give me varification on the OT that my calcs are really good and my tranny is bad that I was going to sell but may need to trash can it. Just have a look a my finding on the previous post. Thanks, Platefire

[sluckey]

Bump--If you could give me varification on the OT that my calcs are really good and my tranny is bad that I was going to sell but may need to trash can it. Just have a look a my finding on the previous post. Thanks, Platefire

Nothing wrong with your calculations. I have no way to verify your voltage readings though. I would never label a OT bad based solely on that test.

[Platefire]

Ok! Thanks---because you questioned my readings, I went back and did the test over again and found one bad mistake. My input was connected incorrectly to the primary red voltage lead and one of the plate leads producing an output of 1.34 VDC. When I realized my mistake and connected the input 14.65 VDC to the blue and brown plate leads and now getting an output of .66 VDC on the green and black secondarys--50% of preveious readings. So let me re-calculate and see.

14.65:.66=22.20

22.20*22.20=493

493*4=1972

So would this be good for a 4 Ohm output?

[sluckey]

Sounds right for coupling a 4 ohm speaker load to a quad of 6L6s like a TR would want.

[Platefire]

Ok, Good! Thanks a bunch for your support. Platefire