Checking a mid-60s radio transformers

[lolbou]

Hi,

As described here viewtopic.php?f=7&t=414&st=0&sk=t&sd=a&start=60 I have two old transformers from an old radio. My aim is to build a Deacy-amp workalike.

One is a push-pull driver, the other is the output one. You can find the schematic in the above link (last page).

First of all, I'd like to say that I roughly understand the purpose of the transformers, but can't figure out what ratios should be used to achieve what they are designed for, so any help is gently welcomed...

But hey, I've made a few measurements, some of them might not be useful, but I may compute the ratio from them?

Driver: measurements done with a DMM connected on the secondary. Do I need an other load?

1kHz - 1V AC RMS at the input : 0.77 V AC RMS at the output (all the secondary winding)
Input DC resistance: 313 ohms - Output DC Res: 302 (all winding again, center is almost half the value)

Output T:

With stock speaker connected at the output: same signal as above: get sound. Output voltage is .0.09V AC RMS.
Input DC res: 0.2 ohms. Output DC res: 8.6 ohms (all winding again, center is exactly half the value, which is crucial according to one of my books))

From the values and sound, I guess that they are in good working order, even if the 0.2 ohms values scares me... I hope I can sort this to make a nice amp project from a "quite" common unit, or simply to update my knowledge and schematics...

Can this help to compute the speaker impedance too (not shown on my shematics)?

Thanks for any help on this, this oldest electronics of mine are the really newest to me...

[lolbou]

okay, using the speaker as a load, I measured primary current and secondary current in both transformers.

Driver: ip=4.15mA (between the centered winding extreme lugs); is=28.9mA (between two lugs).

OT: ip=0.8mA (between two lugs); is=1mA (between the centered winding extreme lugs).

With this formula for a n:1 transformer: n=is/ip, I get then

OT -> n=7, so I go for a 7:1 transformer

Driver -> n=0.8, so 0.8:1.

Is it appropriate for such situations? Is my method correct?

[moonbird]

Hi there -- any more info on this?? thx.

[lolbou]

I have to borrow a function generator at work tomorrow, for the one I have need repairing...

I'll check my above measurements (which still seem correct reading them again) and compute/measure the impedance using the half-voltage divider method...

Still on the job!

[moonbird]

Well -- sorry for the poke - do what you can do. thx much!!

[lolbou]

Well -- sorry for the poke

Don't be sorry, I need such pokes to make me go further...

I just measured the Output Transformer. Using this set up at 1kHz:

I get 0.50V at the primary winding for R = 820 ohms.

So Zp = 820 ohms. Considering this, and the fact that Vp²/Vs² = Zp/Zs, then I get Zs = 117 ohms.

Could any expert give its point towards these measurements (point the good or bad!!)?

I have the following question too (which may not be as off-topic as they sound):

- What do I choose for a load when testing the driver transformer (is the load value important anyway?)
- Do I have enough measurements here to compute the speaker impedance?
- Is 1kHz the standard frequency (or is it 400 Hz?) for such tests?

ps: for reference, the schem of the unit I got these from here

[lolbou]

BUMP!!

- What do I choose for a load when testing the driver transformer (is the load value important anyway?)
- Do I have enough measurements here to compute the speaker impedance?
- Is 1kHz the standard frequency (or is it 400 Hz?) for such tests?

Can anyone help / correct / give its feedback?

I'm stuck on this because I lack the theory, but I really would like to push it forward! I know that if you senior EEs were to answer each noob question this would keep you busy until 2042, but you can't really keep me stucked like this, can you?

I have plenty of time this afternoon and the equipement on hand to make it, so pleeeeeaaaaaaaaase...

[analogguru]

I don´t have time to read the whole thread, but here some infos:

The input transformer of this circuit typically has a primary winding between 350 Ohm DC and 500 Ohm DC (1, 5 Henry - 2,5 Henry). The two secondary windings each have typically between 50 Ohm DC and 150 Ohm DC (50 mH - 400 mH).

The output winding of the output transformer should have a DC-resistance below 1 Ohm. Typical values for the two primary windings are each between 10 Ohm DC and 20 Ohm DC (40 mH - 150mH). The secondary (speaker) winding typically has between 0,5 Ohm DC and 1 Ohm DC (4 mH - 20 mH).

analogguru

[lolbou]

Thanks a lot for your input AG!

I did not know that the DC resistance values were useful in such a case...

Here are the measured DC resistances on my transformers (though I had measured them [in the first post without separating the windings)

Code: Select all

Input primary : 308 ohms
Input secondaries : 143 ohms  / 153 ohms

Output primaries : 4.2 ohms / 4.3 ohms
Output secondary : 0.2 ohms

Though out of the ranges you mentionned above, they are pretty close, so I assume these are ok. This must mean that their impedances are taylored to suit the circuit. The 820 ohms I computed for the push-pull output seems alright.

BTW, here are the tranformers:

img_3013.jpg (32.56 KiB) Viewed 2122 times

These are looking the same, and are indeed pretty close to the Fryer's...

I'll measure the speaker impedance right now and will start looking at the different stages of the AF circuit of this radio... Thanks again!

[lolbou]

Input impedance for the speaker is 4.5 ohms at 1kHz. I used a speaker output on my function generator and measured current and voltage...

Sounds like this unit is close to what's in a Deacy indeed...

[moonbird]

This is extremely good info -- now we can look on Mouser and see if a replacement can be found with the same "DCR" value (DC measured resistance - which is what you get from a voltmeter). DCR values are shown for most inductors and transformers. Remember DCR is not the same thing as inductance it is often less because it does not take into account the additional inductance created by AC signals passing through the coils. For example, if you measure the DCR of an 8 ohm speaker is will typically measure LESS THAN 8 ohm - typical about 6+/- ohms. But at least it get you in the ballpark to do some testing. BRAVO!!

[moonbird]

Looking at these values again - it is more clear to me.

The "driver" transformer is a matched impedence transformer (probably 600:600 ohms) whose sole purpose is to do the phase inversion for the push-pull without any step-up or step-down apparently. The second transformer is a pretty straight forward OT seems like. If you measure the DCR of another OT with 4 ohm out the ratio of primary to secondary will likely be similar. However, the differences can help you estimate the primary impedence of the OT by looking at the turns ratio. Since this is a PP amp -- primary is measured plate-to-plate meaning 8.5 ohms DCR vs. .2 at the output = 1:42-45. When you are on Mouser look for that turns ratio. Wattage can be questimated by the comparative weights of the transformers. Get yourself a cheapo digital postage scale (50 pounder) - they are really handy for sleuthing transformers. Remember the weight "measurement" is only valid for comparing transformers when other values are ~equal. The heavier one will tolerate more wattage output *generally*.

[moonbird]

Hello all --

After more digging I came up with a vintage rosetta stone of sorts for transistor transformers. The attached spec sheet shows DCR values and impedence values for various transistor driver and output transformers from the late 50s / early 60s era. Please feel free to use it.

The question I have for this group is -- which OTs would folks recommended for the following transistors:

AC176/AC128 - PP
AC128/AC128 (if different than above)
AC128 - SE

AC187/188 - PP
AC188/AC188 (if different than above)
AC188 - SE

Also what driver transformer would be best as a PI for:

AC128/AC128 - PP pair
AC188/AC188 - PP pair

Thx to all --