SE OT with separate windings connection doubt

[Miyagi_83]

Hi, everyone.
I'm building an SE guitar amp. I've got an output transformer which has two separate secondary windings that I'd like to connect to an impedance switch. I also want to get NFB from the 8 Ohm tap. Now, this may be a noob question, but I'm in doubt whether it's OK to connect it as shown in the attachment. Is it going to work? 
Any advice is appreciated.
Maciek

[sluckey]

That will work just fine.

[Miyagi_83]

Thanks a million :)

[tubeswell]

Hi, everyone.
I'm building an SE guitar amp. I've got an output transformer which has two separate secondary windings that I'd like to connect to an impedance switch. I also want to get NFB from the 8 Ohm tap. Now, this may be a noob question, but I'm in doubt whether it's OK to connect it as shown in the attachment. Is it going to work? 
Any advice is appreciated.
Maciek

If both secondaries have different Pr:Sec turn ratios, with one designed to reflect 4R and the other designed to reflect 8R, that will work.
But if the secondary windings designed to be  in series to get the other impedance, it won't work. (edited)

[PRR]

For various reasons it is unlikely to have two separate load windings. It is more likely to have, say, 100 turns and 41 more turns. The 100 scales to 4 Ohms and the 141 scales to 8 ohms.

[2deaf]

I've got an output transformer which has two separate secondary windings that I'd like to connect to an impedance switch.

There are output transformers with more than one secondary winding like the Hammond 1600 series and the Marshall Major.  I haven't seen a single-ended one like that, but then again I don't get out that much anymore.  If at all possible, I would like to know what particular transformer you have.       

[Miyagi_83]

Thank you all for your time and effort.
As far as the tranny goes, I pulled it out of an old record player where it worked with an ECL82. I don't know what make or model it is because any and all captions or labels it (might have) had are either illegible or simply not there anymore.
It has 6 taps connected in pairs of the following DCRs:
- pins 1-6 -> 2.4 R
- pins 2-5 -> 14 R
- pins 3-4 -> 630 R
No other connections on this one.
I also tested it by putting voltage through it and here is what I found:
- @240 VAC wall voltage were I live applied to pins 3-4
- 1-6 gave me 7.75 VAC which I figured means a turns ratio of about 31
- 2-5 gave me 5.45 VAC - a turns ratio of ca. 44.
Originally, the speaker on that record player was connected to pins 1 and 6 which has visibly thinner wire than pins 2-5.
Thanks in advance for any ideas or thoughts.

[DummyLoad]

3 & 4
240sq = 57600
1 & 6
7.75sq = 60.0625
2 & 5
5.45sq = 29.7025

57600/60.0625 = 959
57600/29.7025 = 1939

sec 1 & 6
959 x 4Ω = 3836Ω reflected   
959 x 8Ω = 7672Ω  " "

sec 2 & 5
1939 x 4Ω = 7756Ω reflected
1939 x 8Ω = 15512Ω   " "

if your build has a NFB loop and you end up with oscillation instability, then reverse pins 1 & 6 if you're going to use the 8Ω sec. for the NFB loop tap.


--pete

[Miyagi_83]

sec 1 & 6
959 x 4Ω = 3836Ω reflected   
959 x 8Ω = 7672Ω  " "

sec 2 & 5
1939 x 4Ω = 7756Ω reflected
1939 x 8Ω = 15512Ω   " "

Thank you, DummyLoad. This is exactly what my math gave me.
I want to use this transformer with an SE ECL84 output stage. From the load lines I drew, it seems it's going to work fine.
And thanks for the hint on reversing the 8 Ohm leads in the event of NFB squeal.

[Miyagi_83]

For various reasons it is unlikely to have two separate load windings. It is more likely to have, say, 100 turns and 41 more turns. The 100 scales to 4 Ohms and the 141 scales to 8 ohms.

What are the odds that one of those windings was meant for headphones? If it was, what are the risks of using it with a loudspeaker?
M

[Williamblake]

Wouldnt you need to measure the diameter to answer that question?

[Miyagi_83]

Wouldnt you need to measure the diameter to answer that question?

I'll do that in an hour or so. Any suggestions as to what the diameter of a headphone winding could be?

[Miyagi_83]

Wouldnt you need to measure the diameter to answer that question?

The wire diameter on the primary and one secondary windings is 0.6 mm. The other secondary is 0.4 mm and the speaker on the turntable I got the transformer from was connected to it.

[Williamblake]

The unedicated formula i go by is 3A/mm2, it would also depend on the physics of the transformer but for me 3A/mm2 has been good enough and in my experience fits what manufacturers rate their transformators at.