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OT primary impedance calculation

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I researched how to determine the OT primary impedance but all I get is how to calculate reflected impedance, or maybe I don't get it.

For the sake of simplicity let's say I have a 8 Ohm speaker. And my amp uses an 12AU7 for PP output.

How do I determine the primary impedance of the OT?

Thank you
 
Output transformers don't have any intrinsic primary impedance. They act like mechanical levers which reflect and multiply the impedance of the speaker load.

I don't understand your example. The 12AU7 is not an output tube, it is a small signal tube. It might be useful with a resistive load as a phase splitter or something, but it isn't likely to be seen connected to an output transformer. It has a high plate resistance relative to a typical power tube. I would expect if one were to attempt to use it as an output tube, you would either need a very large primary impedance (80K?) or you would require many pairs of 12AU7 in parallel to get the effective plate resistance down. The schoolboy rule is 1:10, but allowances must be made for the plate characteristics.

To determine an appropriate primary load impedance, refer to the datasheets for the output tube. They will always specify a recommended load resistance.
 
Np/Ns is determined by the inductance needed for the primary to effectively load the Rp down to a specific frequency. Example (not accurate) suppose we have one tube with an effective Rp of 5k and another with 10k. The primary would need to have twice the inductance for the 10k to load it down to the same frequency of the 5k tube.

The pic in this post and the next are examples, first loaded with a 10H primary and with a 20H primary in the next post.
 

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cb951303 said:
Iit says 22500 Ohm primary.That makes Np/Ns = sqrt(22500/8) = 53
Even though OT transformers reflect the impedance of the speaker we must still decide on a Np/Ns. I'm asking how we define that?
It sounds like you have already answered your own question. You know the impedance ration, and you know the turns ratio.

Are you asking how to calculate how many turns of wire are required? Are you trying to wind your own transformer?
That calculation rerquires knowing the primary inductance you want, and the core permiability.
 
Sorry for not being more clear. I was asking how do we define that ratio if we don't know that the tube requires a resistance load of 22500 ohm in the first place. I wasn't asking for that specific schematic but in general.

EDIT: Actually, to make it even more clear, all I need is how to define load resistance of an output tube for PP output.

Jerluwoo answered it though

thanks
 
hello there everyone, im new on this forum and in tube world. in my country specially in my place, tube amplifier parts are hard to find or to buy specially OUTPUT TRANSFORMER.. at first i build some jcm 800 preamp powered by 2x6L6GC parallel SE amp and using PT 220Vac primary and 9V secondary as OT for experiment and walla it really have output sound. my real plan is to build jcm 800 2204 push pull using 2x6L6GC but my problem is the output transformer. i have some PT that i salvage lately, E I core dimension 2''1/2 x 3'' x 1'' is this a good core to use as OT?? and how to determine the primary turns and secondary turns??? here is the data sheet of 6L6GC beam tube..Vp: 450 v
Vscr: 400 v
Grid bias or res. -37 v
Ip idle: 116 mA
Ip max: 210
Iscr idle: 5.6 mA
Iscr max: 22 mA
Load: 5600 ohms
Pout: 55
THD (approx) 1.8%.

hoping for some answer and advice for it.. godspeed all :)
 
@ mangekyo24, where you from? I am also a diyer of tubes here, at first I tried using power Trafo as opt with correct impedance ratio, yes it sounds but only good on midrange area or maybe for guitar ... so i have tried to calculate opt values based on lowest freq 20hz and have somebody wind for my specs and i got good results. Ask sir Tony, one of the moderator here and he will help you. the link given by prairieboy is helpful too.

Look for transformer calculator software. usually for power trafo but, just change the 60hz to 20hz then your close to it. just follow some techniques on balanced winding and interleaving for PP and the gapping and insulation for SE opt.
 
importing opt from other country is costly..the price itself can be affordable but the shipping cost and custom taxes in the Philippines will make it triple or 4x time the item cost.Just recently i had ordered some tubes worth 50 USD but i had paid another 100 usd for taxes....how sad....

You can save much money if you will able to do your own OPT.
 
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