Single end OT question

[dude]

How do you determine what ohm speaker to use on a single ended OT?

Is it possible to find out with the OT out of the amp? I have two old OT's that came out of an old tube stereo, I assume left and right channels each had their own OT's. I believe EL84 where in the stereo but not sure. Hoping they are 8 ohms each?

Thanks in advance,
al

[kagliostro]

The necessary impedance of the OT depends on tubes that are used and voltage of B+

if you have a pair of old OT and you want to discover the impedance, you must take some measure using an AC voltage source as to identify

the turn ratio of the transformer then, assuming you want to use speakers that follows the new standard in speaker impedance (4 - 8 - 16 ohm - time ago there were speakers with impedance of 3.2ohm, 5ohm and, may be other)

you can do your math to identify the impedance reflected to the primary using one of the standar impedances of today

Oh, remember that an OT has'nt a specific impedance, it has a turn ratio and the reflected impedance depends on the impedance of the speaker that is used

K

[HotBluePlates]

How do you determine what ohm speaker to use on a single ended OT?

Is it possible to find out with the OT out of the amp? I have two old OT's that came out of an old tube stereo ...

What Kagliostro said.

And the hard lesson everyone keeps having to re-learn is document everything before you remove anything from a donor chassis.

My opinion is it saves more time and money to keep the power supply and output section of the donor chassis completely intact, or at least put together exactly the way it was originally. That's because the tubes, supply voltage, OT primary impedance, power supply setup and the PT were are designed to work together as they were. The individual parts are unlikely to be well-suited to a design that alters any of those elements in a way that's radically different than the original circuit.

[dude]

Thanks

[kagliostro]

Give a look here


http://el34world.com/Forum/index.php?topic=18637.msg190654#msg190654


K

[dude]

OK, I used a 7.3 vac transformer hooked to the the singled ended OT's primary got .20 volts on the secondary.

Doing the math, I get 36.5:1 turns ratio, squaring the ratio, 1332

Now at 8ohms impedance that's 10.5K (8x1332), at 4 ohms impedance that's 5.3K (4x1332)

The amp I want to put the OT in is a singled ended 6V6 running the plate at 315/325vdc.
What does the tube, 6V6 want to see at 8 ohms impedance?  Want to use a single 8 ohm speaker and looks like the OT is at 10.5K then, OK?

  http://www.r-type.org/pdfs/6v6gt.pdf

al

   

[kagliostro]

With anode 315v - G2 225v - 317R cathode resistor and a load of 8500ohm you get 5.5W output from a 6v6 tube

10k is a bit high to me

K

[shooter]

Look at the last graph for this data sheet, it's based on a B+ of 250vdc, but go to the 5k point at the bottom, then straight up.  I think 4ohms and B+ @250 will yield better results than 320v, 10K into 8ohms.  How that goes from a graph to your ears is beyond my paygrade

[kagliostro]

If you want to use the PT and OT you have I think you must accept the compromise of a lower power

8500ohm isn't so far from 10500ohm, but isn't so close

K

[dude]

If you want to use the PT and OT you have I think you must accept the compromise of a lower power

8500ohm isn't so far from 10500ohm, but isn't so close

K

OK, But maybe if I lower the B+ with a few 10 watt Zeners to around 290vdc, the 10.5K OT at 8 ohms would be better suited and I won't sacrifice less volume or power....?

I get this from looking the 6V6 specs, seems that the higher the plate voltage, the OT wants to see a lower ohm reading. I'm trying to learn here more then a single application.

Give a man a fish and feed him once, teach him how to fish and feed him for a lifetime.....     

Anyone feel free to chime in....

sincerely,
al

[kagliostro]

I'm not sure you can drop 25-35v with zener, may be you can do it with big zener

(or better, I'm sure you can drop the voltage, but seems a big waste on energy)

Can you tell us something about the PT and OT ?

How did you get it ?

Have you any news about the circuit where it was mounted ?

Are you sure the OT is an SE OT ?

Do you know for certainly the power that can afford the OT ? How may W ?

K

[shooter]

OT wants to see a lower ohm

The OT doesn't care what speaker you wire up, the tube on the other hand, cares a lot!
Look at that last page again, all those graph lines and values along the X and Y axis are based on specific things.  so by using 290 on the plate each of the lines gets "tweaked" and this is Analog so you can't assume they all change by a "fixed" ratio.  Get close, + or - say 10%, listen, adjust, listen, repeat as needed til you wanna slit your wrists, then start a new build based on what you just learned, now go fish

[2deaf]

OK, I used a 7.3 vac transformer hooked to the the singled ended OT's primary got .20 volts on the secondary.

A little problem with using 60Hz on a low-fidelity transformer is that you get a lower secondary voltage than you would get at 1 KHz.  Maybe you would get 0.22 volts at 1 KHz and your primary is 8.8K ohms with an 8 ohm load.

[2deaf]

OK, But maybe if I lower the B+ with a few 10 watt Zeners to around 290vdc

I'm not endorsing this idea, but if you try it you don't need 10 watt Zeners.  I used three 8.2V 1W zeners in series to drop the voltage with a SS rectifier in a Champ with no problem.  For peace of mind, 5W would be plenty.

[PRR]

> from looking the 6V6 specs, seems that the higher the plate voltage, the OT wants to see a lower ohm reading.

That does not make sense. Cite the datasheet you are looking at.

I see:
250V  - 45mA - 5K -4.5W
315V - 34mA - 8.5K - 5.5W

Also note that tube V/I is very nearly the best load condition. 250V/0.045A= 5.5K. 315V/0.034A= 9.26K. (Going 10% low may give better THD numbers but is unimportant in loudspeaker work.) And that tube V*I can not exceed 12W-14W.

Ah--- ignore the 180V 5.5K condition. This is for when you want LESS power, 2 Watts, instead of the 4W-5W possible if you work the 6V6 hard.

(Why would you want less? Maybe that's all the PT and OT the customer can afford, or you also offer a higher-price model with "more", or you are in a system where only 180V is available and 2W is ample. Tubes smaller than 6V6 would work, but 6V6 is/was super-common, cheap, in-stock everywhere.)