Transformer single stage output ratios

[AdrianPH]

Just wondering if anyone could say what the typical primary:secondary ratios would be on typical old valve gear?

Say for example the small output transformer on Vidor picnic radios, and ECL86 output stage and any others.

At the moment I am just using what I have to hand with a best guess, I could always work it out later with a scope, but if a table could be made up before hand I could look on the auction site for what ever ratio I was after.

Adrian

[ppppenguin]

It's not very critical unless you want absolute maximum output power. Speaker impedances are nominal and will vary widely with frequency. Also you're unlikely to do any harm even if the ratio is quite a long way off. The impedance match is the square of the turns ratio. So a 20:1 transformer will give a 400:1 impedance match.

[AdrianPH]

I guess Ed would know better than me as he must have rewound a few audio transformers for many sets, but for example
I have an audio output from a Vidor picnic radio it uses a DL96 which has a range of Ra 13 to 15 K Ohms and a 3 ohm speech coil so I would expect a ratio of SQRT(14000/3) approx 68:1 ratio, but that seems high?

From reading a spec sheet I get the impression that an ECL86 will be between 7 to 10 K Ohms as a single ended class A amp and to a 3 Ohm coil would be a 53:1 ratio.

These all seem to be high to me so was wondering what they actually were?

Adrian

[Mike Watterson]

About 40:1 might be more normal.
You can measure the transformer with 1 kHz.

[Ed_Dinning]

Hi Folks, a wide range of ratios are used from about 20 to 1 to 80 to 1 depending on the sat and the speaker impedance.
For any given valve the ratio for a 16R speaker is half that for a 4 R speaker

Ed

[ppppenguin]

For ordinary radios if you stick a 30:1 transformer in a circuit that needs 60:1 or vice versa it's most unlikely to do any harm. The worst that will happen is that it will be less loud than you might expect, or there might be more distortion.

For high quality amplifiers it does matter, though you're still unlikely to do harm. For sets with feedback or hum cancelling windings it's a lot trickier. You'll need to get close to the original.

If you try a transformer designed for push-pull in a single ended circuit it will probably saturate due to the DC and sound horrible.

If you try a mains transformer it may work but will tend to saturate in a single ended design. Its frequency response may leave a lot to be desired. But you still won't do any harm.

So try what's to hand. If it works well enough to sound OK then you've got a result.

[Mike Watterson]

Using a larger mains transformer (x2 wattage or more) may avoid single ended saturation.

On push pull the bias currents should be the same and thus a centre tap or dual mains winding for dual mains voltage will work as the DC currents cancel.

[AdrianPH]

It just for me trying to get some better transformers on the 1920/30's style radio, there is not a lot of volume to play with so trying to minimise the losses.

The simple triode output valves I am using are more than likely to have an Ra between 3.5 K to 6 K Ohms so although it works with higher ratio transformers I am just not able to drive the speaker coil, hence wondering what output transformers had what ratios so I could scan the auction sites for suitable single ended output ones.

At present I think I would need something like an 0.75 Ohm speaker!

So really looking for the likes of 30 to 40:1 ratios. From what I can glean many of the speakers of the time would be quite high impedance in the region of a few hundred Ohms, which I do not have, only the typical 4 to 8 ohm jobs to hand so the old 5:1 ratio transformers would not help as I would be loading it heavily, not that I had any.

Hope that makes sense and to why I was asking about transformer ratios.

Adrian

[ppppenguin]

Try a 240V to 9V mains transformer (or whatever you have to hand), bigger than you need to reduce saturation. See if you get enough volume. If you don't, then your problem is likely to be an inefficient speaker. Low power output stages relied on efficient speakers.

[AdrianPH]

Yes there is that on the speaker drive. Please forgive me if it takes some time to respond, I am no longer receiving notifications again from the forum and I am sat here at the PC doing other work, it happens from time to time.

Adrian