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Old 4th July 2009, 09:33 PM   #1
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Default OT primary impedance calculation

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
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Old 4th July 2009, 11:21 PM   #2
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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.
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Old 5th July 2009, 08:16 AM   #3
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I forgot to inform you that this is a guitar amp.
This one to be specific: http://www.ax84.com/index.php/oldpro.../ax84_m276.gif

In the schematics it 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?

Can you elaborate a little bit? 1:10 of what?

Thank you
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Old 5th July 2009, 08:51 PM   #4
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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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File Type: gif 12au710h.gif (79.7 KB, 343 views)
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Old 5th July 2009, 09:04 PM   #5
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The 20H primary.
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File Type: gif 12au720h.gif (80.2 KB, 325 views)
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Old 5th July 2009, 09:07 PM   #6
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I was hoping there would be a simpler calculation but that'll do, I'll just have to try to understand what you just explained thank you!
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Old 6th July 2009, 04:48 AM   #7
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Most folk are using 15K or 16K Raa for 12AU7 in Push Pull.

A Hammond 125A (3 Watts rated) would suit for an output transformer. Connect secondary as per
http://www.hammondmfg.com/125.htm

Some other links:
http://amps.zugster.net/file_download/11
www.geocities.jp/ja4cam/english12au7ppamp.html

Cheers,
Ian
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Old 6th July 2009, 01:03 PM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by cb951303
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.
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Old 6th July 2009, 01:09 PM   #9
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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
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Old 6th July 2009, 01:17 PM   #10
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Triodes are generally loaded with 2-4 times the plate resistance. Lower gives more power, higher gives less distortion. Double this for (class A) push-pull. Lower yet for AB. Pentodes have an optimum load of about V/I (again double for push-pull).
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