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Old 13th October 2011, 08:25 PM   #1
rongon is offline rongon  United States
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Default Tube amp OPT for Headphone Amp? (impedance matching question)

Transformer impedance ratios always kind of mystified me. Actually transformers mystify me. But I was thinking about them and came up with a question...

Let's take a typical push-pull output transformer designed for 6V6 or EL84 output tubes, which has an 8k ohm primary and a 16 ohm tap on its secondary.

Say I wanted to make a headphone amp, and I want to use a 6SN7 as the output triode pair (push-pull).

Say I have a pair of headphones with a quoted impedance of 50 ohms.

Now, if the step-down ratio for my output transformer is 8k pri to 16R secondary, that equals 8000/16 = 500.

Taking my 50 ohm headphones as the load, 50 ohms X 500 = 25k.

Does that mean if I put those 50 ohm headphones on the 16 ohm secondary tap, the primary of the OPT would "look" like a 25k ohm load?

If yes, then wouldn't 25k be a reasonable primary load for a 6SN7 triode? (Rp for the 6SN7 is about 10k ohms, so the load will be 2.5X the Rp of the tube).

So say I happen to have a pair of 6BQ5/EL84 OPT's... Could I use them as OPT's for a headphone amp with 6SN7 triodes as the output "finals"? Power out would be very low, I know, but this is for headphones...

-=|=-
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Old 13th October 2011, 08:30 PM   #2
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It'll work just fine. Go for it!

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Chris
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Old 13th October 2011, 09:15 PM   #3
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[QUOTE=rongon;2744875][QUOTE]
Now, if the step-down ratio for my output transformer is 8k pri to 16R secondary, that equals 8000/16 = 500.
[QUOTE]

500 is the impedance ratio; the step-down (winding) ratio is the root of 500 so it is 22,36.

Quote:
Taking my 50 ohm headphones as the load, 50 ohms X 500 = 25k.

Does that mean if I put those 50 ohm headphones on the 16 ohm secondary tap, the primary of the OPT would "look" like a 25k ohm load?
Right.

Quote:
If yes, then wouldn't 25k be a reasonable primary load for a 6SN7 triode? (Rp for the 6SN7 is about 10k ohms, so the load will be 2.5X the Rp of the tube).

So say I happen to have a pair of 6BQ5/EL84 OPT's... Could I use them as OPT's for a headphone amp with 6SN7 triodes as the output "finals"? Power out would be very low, I know, but this is for headphones...
Yes you can, but the problem will likely be that your headphone will sound bass shy.
The EL84 OPT's will have enough primary inductance for a pair of EL84's which, triode connected, have some 2k of Rp.
For PP 6SN7's with their Rp an order of about 4 higher there will not be enough primary induction, so there will be roll-off pretty much above 20 Hz.
But give it a try and listen.
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Old 14th October 2011, 09:54 AM   #4
DF96 is offline DF96  England
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Quote:
Originally Posted by pieter t
Yes you can, but the problem will likely be that your headphone will sound bass shy.
The EL84 OPT's will have enough primary inductance for a pair of EL84's which, triode connected, have some 2k of Rp.
For PP 6SN7's with their Rp an order of about 4 higher there will not be enough primary induction, so there will be roll-off pretty much above 20 Hz.
8k primary probably means pentode EL84, which has much higher anode impedance. The issue is the ratio of effective load to inductance. The effective load is the anode impedance in parallel with the transformed load impedance. As the triode has a lowish anode impedance it should be OK. The effective load will be 10k||25k=7.1k so quite near what the transformer was designed for.

The rolloff, wherever it occurs, will be below the corner frequency not above it.
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Old 14th October 2011, 06:51 PM   #5
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I don't agree.
Presume the transformer has a primary inductance of 60 to 80 Hy's which it should have at least, that will simply not be enough for a 6SN7.
In my experience 6SN7's and comparable tubes for Rp (26, 76) are about the limit of what you can load by inductance, be it choke or transformer. A 6J5 (half a 6SN7) needs at least 100 Hy's not to loose bandwidth above 20 Hz.
Maybe rongon will let us know how it performs.
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Old 14th October 2011, 07:13 PM   #6
DF96 is offline DF96  England
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Thre is not much difference between 80H (too small, you say) and 100H (enough). How much inductance you need depends on the anode impedance and the load. For an output stage you can get away with a bit less inductance than interstage because the load resistance will be lower.
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Old 14th October 2011, 07:17 PM   #7
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Although it's true that transformers' parasitic reactances do limit the range of impedances that they can reflect well, in this case (as DF96 carefully points out) rongon's use is quite similar to the likely original purpose. A good transformer for push-pull EL84's will have about 1000 Henry primary inductance:

Z565-48 17.5(35W) Watt Triode USA Dyna Clone Output Transformer for Dynaco MADE IN USA

Thanks to the OP for the inspiration to do something similar with an old pair of Eico ST70 OPT's and EL84's trioded. Time to get out the Greenlee punches!

Thanks,
Chris
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Old 14th October 2011, 07:23 PM   #8
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Please note the 60-80 Hy's is somewhat typical for an EL84 PP transformer.
My 6J5/100H is a SE situation; for a PP 6SN7 you won't get away with 100H, be it outputstage or interstage (I built these stages years ago to develop winding techniques).
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Old 14th October 2011, 07:34 PM   #9
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Chris,

My two cents that the specification of the Z565 having 1000H at 60Hz is a typo; I am sure it must be 100H.
Primary inductance values in the hundreds of Hy's can only be seen in specifications of Plitron output transformers, where a 250 or so AC voltage (50Hz) is used to measure inductance (please note that primary inductance depends on frequency and amplitude).
Normally inductance values are measured with 1 to 5 VAC or so at 100Hz, 1kHz and sometimes 10Khz with the typical L(CR) meters.
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Old 14th October 2011, 09:02 PM   #10
DF96 is offline DF96  England
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Yes, 1000H sounds too high.
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