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How do you choose output transformer impedance?



 
 
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  #1  
Old August 25th 04, 04:11 PM
at
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Default How do you choose output transformer impedance?

Hello,

What things should be considered to choose the best output transformer for a
given tube and situation?

If low output impedance is important for good speaker damping and control,
why won't all amps just use output transformers with the biggest possible
primary winding impedance? That would give the biggest output transformer
impedance ratio and lowest output impedance of the amp.

Since it doesn't seem to be this way, what other considerations are there?
Tube type, Plate voltage, etc?

If I wanted to make a class A push-pull power triode amp with a 300B output
tube, run at 400V Vp, what would be the best output transformer primary
impedance to use, secondary impedance is going to be 4ohms.

Thanks!

-at


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  #2  
Old August 25th 04, 07:15 PM
Krzysiek Słychań
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But consider surge on valve anodes and OPT when the amp is "underloaded"
(the load's impedance is higher than the amp's output one)... It's not like
the transistor (whew!) amp. If you connect i.e. 16 Ohm speaker to 4 Ohm amp
output, the amp's likely to be damaged.

--
Pozdrawiam,
Krzysiek Słychań


  #3  
Old August 25th 04, 08:42 PM
at
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"Krzysiek Słychań" > wrote in message
...
> But consider surge on valve anodes and OPT when the amp is "underloaded"
> (the load's impedance is higher than the amp's output one)... It's not

like
> the transistor (whew!) amp. If you connect i.e. 16 Ohm speaker to 4 Ohm

amp
> output, the amp's likely to be damaged.
>
> --
> Pozdrawiam,
> Krzysiek Słychań


So, do you mean, that if I use 4ohm speakers, the output impedance should be
at least 4 ohms?
Then why do some people say that the output impedance should be at least 10
times lower than than the speaker impedance for good control of cone motion?
That would mean that with a 4ohm speaker, the Zo should be 0.4ohms.
I've also hear that the output impedance must be less than one ohm for best
performance. And they don't speak about SS amps, but tube ones.

If I understood it correctly, you mean that if the speaker impedance is
higher that the amps output impedance, the amp is likely to get damaged?
That would mean, that with an amp with a single ended 300B tube with Rp 700
ohms and a output transformer that has primary impedance of 4000ohms and
secondary 4ohms, the amp is likely to get damaged, since the output
impedance of the amp is now 0.7ohms with a 4 ohm load. Then to raise the
output impedance to safe 4 ohms, the output transformer primary impedance
should be 700 ohms. But people use higher impedances on the output
transformer primary, and the amps don't blow... or do they?

Or did I misunderstoond something?

-at


-at


  #4  
Old August 25th 04, 09:06 PM
Jon Yaeger
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AT,

I have a few suggestions for you.

1) Buy two books. #1: Audio Reality by Bruce Rozenblitz. Has quite a few
useful tube projects in addition to practical theory. Then find a copy of
the Radiotron Designer's Handbook." it's available on CD. Has more info
than you ever wanted to know about tubes and tube designs.

2) Find some reprints of articles by Norman Crowhurst on line. Excellent
info answering just the kinds of questions you are posing daily on R.A.T.

Best regards,

Jon

  #5  
Old August 25th 04, 09:18 PM
at
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"Krzysiek Słychań" > wrote in message
...
> But consider surge on valve anodes and OPT when the amp is "underloaded"
> (the load's impedance is higher than the amp's output one)... It's not

like
> the transistor (whew!) amp. If you connect i.e. 16 Ohm speaker to 4 Ohm

amp
> output, the amp's likely to be damaged.
>
> --
> Pozdrawiam,
> Krzysiek Słychań


Wouldn't that quadruple the impedance of the OPT primary? A 5000 ohm OPT
primary would became 20kohm. That would make the output impedance of the amp
four times lower. So, where's the safe margin?

I've heard that for best speaker control, the output impedance should be
lower than 1ohm, or 10 times lower than the speaker impedance. If it was 10
times lower, with a 4 ohm speaker it should be 0.4ohms.
A single-ended class A amp with a 300B tube with Rp 700ohms and a OPT
primary 4000 and 4 ohm speaker the Zo would be 0.7ohms [7004000:4)].
That's lower that 1 ohm but not 10 times lower. To get 10 times lower Zo,
the OPT primary should be 7000ohms. That would give Zo of 0.4 ohms.

Or have I misunderstood something?

Are there some problems? What things should be taken into account when
deciding upon a OPT and it's primary impedance?


-at


  #6  
Old August 25th 04, 09:23 PM
at
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Default


"Jon Yaeger" > wrote in message
...
> AT,
>
> I have a few suggestions for you.
>
> 1) Buy two books. #1: Audio Reality by Bruce Rozenblitz. Has quite a

few
> useful tube projects in addition to practical theory. Then find a copy of
> the Radiotron Designer's Handbook." it's available on CD. Has more info
> than you ever wanted to know about tubes and tube designs.
>
> 2) Find some reprints of articles by Norman Crowhurst on line. Excellent
> info answering just the kinds of questions you are posing daily on R.A.T.
>
> Best regards,
>
> Jon


Thanks. Ok, I'll try to find them.

-at


  #7  
Old August 26th 04, 02:26 AM
Tim Williams
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"at" > wrote in message
...
> What things should be considered to choose the best output transformer for

a
> given tube and situation?


Maximum power, or nearby with pentodes, minimum distortion. Triodes, just
max. Po (since min. distortion comes at infinite Rl, which is rather
useless).

Reason there's one "best load impedance" is because the tube will only draw
so much current at a given voltage and signal level. Same applies to
transistors as well, because they have the same curves as pentodes, but it
isn't thought of as such because they have such a contrasting design
culture.

Output impedance has absolutely nothing to do with it; that can be corrected
with negative feedback. The "perfect voltage source/source impedance" Zo
model works great for anywhere but when either voltage or current levels are
pressed to their limits for the amplifier.

Tim

--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms


  #8  
Old August 26th 04, 03:02 AM
Patrick Turner
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Default



at wrote:

> Hello,
>
> What things should be considered to choose the best output transformer for a
> given tube and situation?
>
> If low output impedance is important for good speaker damping and control,
> why won't all amps just use output transformers with the biggest possible
> primary winding impedance? That would give the biggest output transformer
> impedance ratio and lowest output impedance of the amp.
>
> Since it doesn't seem to be this way, what other considerations are there?
> Tube type, Plate voltage, etc?
>
> If I wanted to make a class A push-pull power triode amp with a 300B output
> tube, run at 400V Vp, what would be the best output transformer primary
> impedance to use, secondary impedance is going to be 4ohms.
>
> Thanks!
>
> -at


Learn to do load line analysis, and you will be able to
calculate all the impedances you will ever want.

At the moment, you show you know almost nothing and have a steep
learning curve to climb.

But you could start by reading a few books.

And when you try to build an amp, get back to us
about your problems.

But I can tell you that transforming audio voltages follow the same impedance
rules mentioned in the basic information in the theory books which you should
read
and study.

There is quite a lot on the WWW, so try doing a google search....

Patrick Turner.



  #9  
Old August 26th 04, 10:57 AM
Fabio Berutti
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Default

Many considerations:

- the higher the load, the lower the available power (you get V swing but
not A swing, and being the power VxA...)
- the higher the primary/secondary ratio, the more difficult is to wind a
good OPT. A 2K5 OPT is much easier to make (and therefore often better
sounding) than a 10K.
- a rule of thumb says that triodes work best into a load 2-3 times their
plate resistance (ie, a 2A3 with a plate R of 800R is usually used with a
2K5 OPT)

If You want to be more precise, You need the anode curves of the tube You
chose and a ruler. Choose a working point, then draw many loadlines and see
what happens. Usually a higher load is needed the more You "push" the tube
towards its limits (high V at working point).
But if You want to be sure, just find the original D/S of the tube You
prefer and stick on to recommended values for ALL parameters. After all,
these leaflets were written by professionals: they must have had some sound
reason to tell that ie. a 6V6 needs 8K a-a in PP, isn't it? Would You play
the same "it-seems-to-me" way with the braking pads of Your car?

By the way: the 300B needs a helluva driving voltage (some 250V p-p). I'd
start from something simpler.

Ciao

Fabio


"at" > ha scritto nel messaggio
...
> Hello,
>
> What things should be considered to choose the best output transformer for

a
> given tube and situation?
>
> If low output impedance is important for good speaker damping and control,
> why won't all amps just use output transformers with the biggest possible
> primary winding impedance? That would give the biggest output transformer
> impedance ratio and lowest output impedance of the amp.
>
> Since it doesn't seem to be this way, what other considerations are there?
> Tube type, Plate voltage, etc?
>
> If I wanted to make a class A push-pull power triode amp with a 300B

output
> tube, run at 400V Vp, what would be the best output transformer primary
> impedance to use, secondary impedance is going to be 4ohms.
>
> Thanks!
>
> -at
>
>



 




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