6600 ohm primary and 3.6 ohm secondary OT

[Hebert]

Hi, based on the subject line info does anyone know what ohm speaker to use?  I think a 4 ohm would work well but I am not sure if the 6600 primary comes in to play for the choice.  thanks

[vampwizzard]

well.. the 6600 ohm primary means that its likely designed for a tube amp impedance which is good.

Transformers are tricky beasts though.

They dont use the same size wire on the primary and secondary sides to get the voltage transformation which means that its impossible to figure out how many turns are on each (and thus the turns ratio).

Is there a model number on the OT anywhere?

[Hebert]

no it does not have a model on it. It is from a 1948 RCA radio.  I am using it in my new build, testing with an 8 ohm speaker. sounds pretty good but I think it is supposed to be 4 ohm. my understanding is a 4 ohm would have more volume

[vampwizzard]

Do you have a model or a picture of the radio?

Youll be ok if the amp was designed for a 4 ohm speaker only. If it was designed of 8 you can use a 16 or an 8. If the amp feels too quiet and the OT is dead cold you may be looking at a 4ohm OT but likely youre ok. If you use too low an impedance youll burn the OT up. I wouldnt put the 4ohm speaker load on it without knowing what the original speakers used or deriving it off the OT

[vampwizzard]

more info: https://www.youtube.com/watch?v=5jUitplchok

[Hebert]

I just found the manual for the radio and it says the speaker is a 5 by 7 inch 3.6 ohm speaker

Do you have a model or a picture of the radio?

Youll be ok if the amp was designed for a 4 ohm speaker only. If it was designed of 8 you can use a 16 or an 8. If the amp feels too quiet and the OT is dead cold you may be looking at a 4ohm OT but likely youre ok. If you use too low an impedance youll burn the OT up. I wouldnt put the 4ohm speaker load on it without knowing what the original speakers used or deriving it off the OT

[tubeswell]

6300/3.6 is 1833:1,


so if you run a 4R speaker you'll get 7.3k

[sluckey]

They dont use the same size wire on the primary and secondary sides to get the voltage transformation which means that its impossible to figure out how many turns are on each (and thus the turns ratio).

Size of wire has nothing to do with it. The impedance ratio is 6600:3.6 or 1833:1. The turns ratio is equal to the square root of the impedance ratio, so that would be 42.8:1. But knowing the turns ratio is not important since you already know the impedance ratio.

The important thing is that a 4Ω speaker would work just fine with this transformer.

[PRR]

There's *TWO* ways to spec a speaker.

1) DC resistance. 3.2 Ohms was a common value.

2) Audio impedance near midrange dip. "4" Ohms is a common value.

These are the same thing. The mechanical and acoustic audio elements act-like a part-Ohm to the electric side. Add the dead DC resistance to get the effective resistance.

I have no idea where RCA got "3.6 ohms". Sometimes RCA just liked to be strange. This is of course same-as 3.2/4r speaker, since it is all -20%/+50% approximations.

No, you won't burn the OT. Something like 4r may be the most power. Something over 40r and sustained clipping may arc the OT. A kilowatt amp, a dead-short "could" heat the OT, but that class of amp something else would burn first.

[vampwizzard]

They dont use the same size wire on the primary and secondary sides to get the voltage transformation which means that its impossible to figure out how many turns are on each (and thus the turns ratio).

Size of wire has nothing to do with it. The impedance ratio is 6600:3.6 or 1833:1. The turns ratio is equal to the square root of the impedance ratio, so that would be 42.8:1. But knowing the turns ratio is not important since you already know the impedance ratio.

The important thing is that a 4Ω speaker would work just fine with this transformer.

It does if you're trying to determine a turns ratio based on dc resistances of coils (impossible), which doesn't appear to be what the initial problem was. I assumed the 6600 was a dc resistance measurement not a known impedance. My apologies.

[kagliostro]

I have no idea where RCA got "3.6 ohms".

I too, I can oly say it was a common impedance, I've 5 or 6 (6W) NOS SE OT with a label on the box that say 3.6 ohm

Franco

[finkaudio]

Hi,

no need to be so picky with the speaker impedance. A 4 Ohm speaker can have plus/minus 20% and is still a 4 Ohm speaker. So the minimum is 3,2 Ohm  . The impedance is only halfway flat in the lower mid-band. At low frequency, it raises at resonance frequency to few times the value ( resonance frequency 50-100Hz and the inductance of the voice coil raises the impedance at higher frequencies depending on voice coil diameter and winding length.

Best regards

KH