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Old 11th February 2003, 04:27 PM   #11
dhaen is offline dhaen  Europe
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Hi Bas,

How did I know?
Well, it is just my opinion that it just looks useful for the intended application.
And when fet's first appeared, they were depletion types. It's nostagia, but not as we know it...

Cheers,
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Old 11th February 2003, 11:21 PM   #12
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Could some engineer translate this message from a tube god? (Paul Joppa of bottlehead fame, and parafeed expert.)

Is that not the Japanese word for engineer..i.e. one who translates the god's work into human language. Question for you Dhaen. ;-)

Quote:
Don't be fooled by the data sheet. Look at the curves at the operating point you are contemplating, and you will see that the plate resistance is much higher than advertised. At 250v/8mA it is around 8k per triode, for example.
To spec a transformer, determine the load impedance for which the amplifier is designed assuming a resistive load. That is the "nominal" transformer impedance, meaning that the inductive reactance should be larger than the target impedance above whatever minimum frequency you want to operate to. Tell the transformer designer the source resistance (plate resistance) also, so that winding resistances can be chosen for properly damped HF resonances. Tell them the design maximum signal voltage also, so the the core can be optimally sized.
Posted the same question at the AA.
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Old 11th February 2003, 11:26 PM   #13
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Hi Bas,

I could explain that for you...however I feel John is in dire need of a little challenge so I'll stay out of it.

Ciao,
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Old 11th February 2003, 11:50 PM   #14
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Hi Bas ,
I think what Paul is saying is that transformer design is a black art that needs to be specified carefully . He is right about the 6N1p (especially Svetlana) curves , particularly misleading . Why use a 6N1p if the Ra is 8k ? Why not use a 6SN7/7N7 instead ?

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Old 11th February 2003, 11:50 PM   #15
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Hi Frank,

The tube related question is for everyone..so feel free..don't keep me in suspense???!!??



The only question specifically for John is on the Japanese thing.

Regards,
Bas
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Old 12th February 2003, 12:21 AM   #16
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Hi Bas,

Quote:
Don't be fooled by the data sheet. Look at the curves at the operating point you are contemplating, and you will see that the plate resistance is much higher than advertised. At 250v/8mA it is around 8k per triode, for example.
What Paul Joppa means is that Ra varies according to the operating points you choose, hence when you need to specify a xformer, keep in mind these points and don't go for the Ra given in the datasheets as if it were a constant for it is not.

Not that it is necessarily all that critical...I'd need to read the whole article to see if this is actually what he meant.

Hope this clarifies it a bit,
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Old 12th February 2003, 08:50 AM   #17
dhaen is offline dhaen  Europe
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Hi Bas,

If you mean "gishi" ( ?? ), I think "performance master"

Or, "kikanji" (???) "machine concerned gentleman"

I like the first one best

No gods as far as I know...

Cheers,

PS I don't think this browser is handling kanji properly...another thing to sort out.
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Old 12th February 2003, 09:05 AM   #18
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Hi Frank,

The part you explained I could still understand...it is this part that is greek to me.

Quote:
To spec a transformer, determine the load impedance for which the amplifier is designed assuming a resistive load. That is the "nominal" transformer impedance, meaning that the inductive reactance should be larger than the target impedance above whatever minimum frequency you want to operate to. Tell the transformer designer the source resistance (plate resistance) also, so that winding resistances can be chosen for properly damped HF resonances.
Cheers,
Bas
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Old 12th February 2003, 11:52 AM   #19
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Hi Bas,

Quote:
To spec a transformer, determine the load impedance for which the amplifier is designed assuming a resistive load. That is the "nominal" transformer impedance, meaning that the inductive reactance should be larger than the target impedance above whatever minimum frequency you want to operate to. Tell the transformer designer the source resistance (plate resistance) also, so that winding resistances can be chosen for properly damped HF resonances.
From what I read here I assume Paul is referring to a SE OPT:

Quote:
To spec a transformer, determine the load impedance for which the amplifier is designed assuming a resistive load.
As an example,let's assume the amplifier was designed to work into an 8 Ohm load.

Quote:
That is the "nominal" transformer impedance, meaning that the inductive reactance should be larger than the target impedance above whatever minimum frequency you want to operate to.
So the inductive reactance should be larger than the 8 Ohm impedance for a given bandwidth,say from 15 Hz to 25 kHz.

Quote:
Tell the transformer designer the source resistance (plate resistance) also, so that winding resistances can be chosen for properly damped HF resonances.
And in the last part he says it all:

The xformer designer can now choose the winding resistances from the data you provide in order to keep the xformer HF resonances well damped.

In other words: give the designer all the operating parameters you determined for the outputstage + the load into which this should work best for a given bandwidth so he can avoid ringing of the OPT inside this band.

I hope this is what he meant,
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Old 12th February 2003, 01:20 PM   #20
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Thanks Frank,

What is a typical load that a preamplifier "sees" is that 47k? In my case..the power transformer input rca has a grid leak resistor of 47k..

Cheers,
Bas
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