Finding OT impedance

[dude]

I know this has been discussed several times.


I just put a 10" speaker in my 67 Fender Vibro Champ, the OT has been upgraded to a 10 watt paper single ended unknown OT. I generated an input signal and the ratio winding is 34:1, I have a 6V6GT in the champ (6V6 tube data: likes around 8,500 load resistance, class A, single-ended). The 10" spk is 8 ohms: Is this math correct?


34 x 34 = 1156 x (6.5 ohms, speaker says 8 ohms but measures 6.5) = 7,514. 
So with a 6.6 ohm speaker and 34:1 wingdings ratio, impedance is 7K5, so the 6V6GT running  class A Single-ended and the 6.6 ohm speaker are a good match?


If I wanted to use a 6L6 (likes 5K, class A , one tube), I'd use a 4 ohm speaker...? 1156 x 4 = 4624, close enough.


al

[2deaf]

Did you measure the winding ratio with no load on the windings (other than the measuring equipment)?

You would use the 8r speaker impedance to calculate the primary impedance.  The 6.5r DC resistance isn't relevant for the primary impedance.

Tubes don't have likes and dislikes (within reason).  The data sheets just give examples of what happens under given conditions.  If you use a transformer with a lower primary impedance, the tube can use more current if the tube and the power supply are up to it so that the amp can put out more wattage.  You will see 6L6's with lower primary impedances in the examples because the 6L6 can handle more current than a 6V6. 

[sluckey]

34² x 8 = 9248 and that's fine for a 6V6 or EL84.

34² x 4 = 4624 and that's fine for a 6L6 or EL34.

[tubeswell]

34 x 34 = 1156 x (6.5 ohms, speaker says 8 ohms but measures 6.5) = 7,514. 
So with a 6.6 ohm speaker and 34:1 wingdings ratio, impedance is 7K5, so the 6V6GT running  class A Single-ended and the 6.6 ohm speaker are a good match?

The measured 6.5R you are getting is DC resistance. The 8R rating of the speaker is the nominal impedance (AC resistance, taking into account the physical effect of the cone, the electro-magnetic effect of the magnet pole on the coil etc etc on resistance). As we want to understand the impedance seen by the output tube through the output tranny, then the number you want to use for your speaker is 8R.

[dude]

Did you measure the winding ratio with no load on the windings (other than the measuring equipment)?

yes


The speaker is an Eminence Blue alnico 8 ohm, but reads 6.5 (I read all these blue alnico read low), so if that's the impedance of the speaker 6.5, why wouldn't I multiply by 6.5 instead of 8...?  Doesn't make much difference if I multiply by 8 or 6.5 still close enough, just closer to 8K using 6.5 ohms which is what the speaker reads...


 

[dude]

Thanks tubeswell, posted as you were posting. Got it.

[sluckey]

There will be a follow-up quiz tomorrow! 

[dude]

Ha, I'm sure I'll get a D, maybe a C, but that passing right? 

[sluckey]

OK Dude. Your up!

I have a 4Ω impedance speaker that measures 3.2Ω with my voltmeter. It's connected to an OT with a 34:1 turns ratio. What is the reflected primary impedance?

[dude]

34 x 34   = 1156 * 4 = 4624 primary impedance, bonus answer: put a 6L6 in that amp

[2deaf]

Now that you passed that quiz (you did pass, didn't you?), here's the final.

1.)  Two busses leave Philadelphia at 2:05pm EST.  One bus has a transformer under seat 32B with an 8r and a 4r secondary.  The turns ratio for the 4r tap is 94.868 : 3.   What is the turns ratio for the 8r tap?  Show your work.

2.)  The driver of the other bus has a 4K w/CT : 4r transformer.  What is the primary impedance using only half the primary with an 8r load on the secondary?

3.)  You missed both buses, but you have a transformer with a 4K primary and 4/8/16 Ohm secondary.  You connect an 8r load to the 16r tap and an 8r load to the 8r tap.  What is the primary impedance?

4.)  You have a transformer with a 4/8/16 Ohm secondary.  You connect a 16r load to the 16r tap, an 8r load to the 8r tap, and a 4r load to the 4r tap.  You want to get the same primary impedance using only the 4r tap with a 4r load and a load in parallel with that 4r load.  What is the value of the parallel load?       

[shooter]

I divided 8:37 by 4:20 came up with 1.976 and a lot more numbers after.  Close enough 

[dude]

Hey shooter I'm with you

[2deaf]

I divided 8:37 by 4:20 came up with 1.976 and a lot more numbers after.

You ain't cheating nobody but yourself when you copy.

[Colas LeGrippa]

If you have a 8 ohms speaker and you need a primary impedance of 4,3k, you might have to hook it up to the 4 ohm tap of the OT. Look at an OT as a winding ratio , squared multiplied by the load. Just do the math and hook up what you have on hand accordingly. So you can hook up almost any ot to any speaker load, if the impedance is matched ( I often double the load or divide it by 2 without any problems). Tubes amps are very tough on loads.

[shooter]

You ain't cheating nobody but yourself when you copy

 

It got me to 62 just fine 

[2deaf]

It got me to 62 just fine

I remember it like it was just yesterday.  I would be looking at the kid in question and I could tell what he was thinking: "you's 'bout a ig'nant fo'-legged sow. I just got me a B instead of a D- and you reckon I CHEATED MYSELF?"  I grew up in a rural area.

Anyways, time's up

[shooter]

I grew up in a rural area.

Yup,  I stole yur quiz to sell to da city kids

[ALBATROS1234]

You're not understanding impedance is a.c. resistance which is different from d.c. resistance. An 8 ohm impedance speaker normally has a d.c. resistance of 6 to 7 ohms. Dc resistance is measured when a multimeter puts a small d.c. voltage thru something like a speaker or resistor. It measures the voltage drop and calculated d.c. resistance. Speaker impedance is not d.c. resistance it's what the audio a.c. signal encounters.  Do you see what I mean?