Please help with calculations on Output transformer to get ohms l

[Stevi q]

Hi ,I am running 24 volts into the primary winding and I get .6 volts out of the secondary .Can you tell me how many ohms this output tansformer is?This goes to an amp from 1965 where the brand has been removed ,and there is no schematic in it .It comes with two 8ohm magnavox speakers that look original .so it should be 4 ohms in parallel ,or 16ohms in series ,and I don't wanna hook it up wrong .The wiring was disconnected from the speakers so that won't help.the spade 4 spade connectors on one speaker are covered in solder ,so that makes me think in series and its 4 ohms .I want to know what the equation says.I went on a conversion chart site and I got 24v /.6 =960ohms .I don't even know if that's right or what that translates too.Itbsaid current was .025 Amps .Please help me figure this out I don't have much hair left to pull out.😳

[Malcolm Irving]

24V to 0.6V means a transformer turns ratio of 24/0.6 = 40 to 1. But the impedance goes according to the turns ratio squared, so an impedance ratio of 40x40 = 1600 to 1.
With a 4 ohms speaker load that gives a primary impedance of 1600 x 4 = 6400 ohms
Or a 16 ohm load gives 1600 x 16 = 25600 ohms.
Don’t know what the output tube(s) are, but the 4 ohm version looks more reasonable.

[R.G.]

Malcolm is correct. You measure the voltages, take the ratio, square the ratio, and that's the ohms >ratio<.

I would add to that a couple of questions.
> How is the primary driven? Is it single ended or push pull? That is, one tube or two?
> How accurately can you measure 0.6V? That has a big effect on the calculations.

[Enzo]

Is there some reason we ought not run voltage through the other way? Would it not be easier to get good readings if we sent say 6v into the secondary, or even the 24v, and then expect a large voltage at the primary? Then you wouldn't need to try to interpret a tiny 0.6v voltage. SO 40 to 1 would be 6v in and 240v out. Easy to read. Am I missing something? No one seems to do it that way.

[Gnobuddy]

...expect a large voltage at the primary?
<snip>
240v out. Easy to read. Am I missing something?

Definitely lower error your way. But some of us get a little nervous around large AC voltages. And even more nervous about them when newcomers to electricity are involved.

I once built a strobe circuit for a physics demo in college. It took 230 - 240 V AC from the mains, and fed it into a voltage doubler. That produced somewhere around 650 volts DC to the Xenon flash tube. I was scared the entire time I worked on the bloody thing, but I built it, and got it working. (I was young and foolhardy. I wouldn't risk that today.)

-Gnobuddy

[Enzo]

Then don't work on tube amps, the power transformers put out more than my 240vAC, and as to DC, Music Man amps with their 700-800v B+ ought t be positively frightening. SVT with 660vDC B+. That newcomer already has his fingers in a high voltage circuit.

But my question was more the general case, I wondered if ther was some technical reason not to do it my way.

[Malcolm Irving]

Is there some reason we ought not run voltage through the other way? Would it not be easier to get good readings if we sent say 6v into the secondary, or even the 24v, and then expect a large voltage at the primary? Then you wouldn't need to try to interpret a tiny 0.6v voltage. SO 40 to 1 would be 6v in and 240v out. Easy to read. Am I missing something? No one seems to do it that way.

I understand what you mean. But a digital multi-meter doesn't read 0.6V and 240V, it reads 0.5973V and 240.2V. The accuracy of both readings (in %) is often the same.

[Stevi q]

Thanks for the help.I have 2 el84 tubes .

[Stevi q]

Thanks for the info.I have 2 el84 tubes.

[Stevi q]

Thank you for the info.I will Definately run the 24volts into the secondary and see what i get out of the primary.I am Definately comfortable inside a tube amp.Discharging caps etc....peace and love

[Dave H]

Thank you for the info.I will Definately run the 24volts into the secondary and see what i get out of the primary.

I always use the 6.3V heater winding. If it really is 24V:0.6V (40:1) putting 24V on the secondary will put 960V across the primary. That's 1357V peak!

[J M Fahey]

Thank you for the info.I will Definately run the 24volts into the secondary and see what i get out of the primary.I am Definately comfortable inside a tube amp.Discharging caps etc....peace and love

Please do not run *24VAC* into the secondary, for the very good reason that with expected 40:1 ratio you will be dealing with electric chair level deadly 1000VAC

Besides the very dangerous voltage,
a) the primary is not designed for such a high voltage and
b) most meters can not even *read* 1000VAC anyway ... and for good reason.

6VAC into the secondary will instead give you way more reasonable 240VAC at the primary side.

Guessing what voltage does that transformer expects to handle:
itīs a 2 x EL84 amp, thus, say, 15W RMS or so.
W=Vē/Z so
V=√(W*Z)
* 15W and 8 ohms give us: 11VAC
* into 4 ohms: 7.8VAC
* into 16 ohms: 15.5VAC

so we see that 6VAC into any normal tap gives us both high resolution (6V compared to 0.6V) and still safe values.

[Stevi q]

I haven't done it yet .I guess I will just stick with the calculations I got from using the primary to run the volts through.I am gonna find a 9volt and use that just to cross check things .I am a slow learner ,but I will get there .Thanks for the information ,it is greatly appreciated..peace and love .

[Malcolm Irving]

... .I am gonna find a 9volt and use that just to cross check things . ....

If you are thinking of a 9V battery it won't work - transformers only work with AC.

[Stevi q]

Appreciate that.I thing I have a plug that is 9 volts .I think it went to a phone I used to own years ago.I hope that will work .I got a different question .I just received some sprague atom caps .It looks like there is some hard silver material around the lead but on the cap.Not sure if that's damage from a test probe or that's some type of electrolyte leaking out.I will post results a little later .Thank you very much Malcolm.