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Simple Test Circuit for Tube Noise



 
 
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  #1  
Old September 10th 10, 08:17 PM posted to rec.audio.pro
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Default Simple Test Circuit for Tube Noise

Can anyone suggest a simple circuit to measure vacuum tube noise? How
about negative (emission) grid current? Also, what are some of the
quieter tubes out there? Thanks!

Sean Broderick
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  #2  
Old September 10th 10, 10:02 PM posted to rec.audio.pro
Scott Dorsey
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Default Simple Test Circuit for Tube Noise

> wrote:
>Can anyone suggest a simple circuit to measure vacuum tube noise?


There are some details in the Radiotron Handbook. You might also find
some information in a VERY early version of Motchenbacher's book on low
noise electronics.

However, you're better off testing in-circuit, or in a jig that emulates
the circuit that the tubes will be used in, because the different noise
sources are of different importance in different applications.

>How
>about negative (emission) grid current?


If you ever have to worry about grid current in an audio circuit, something
has gone horribly wrong. From the standpoint of the input, the tube is a
big capacitor. The only time the tube ever draws current is when the Miller
capacitance is so high that it's acting like a capacitive load on the input.
You might see that in a big power tube occasionally, but that's the last place
to worry about noise.

>Also, what are some of the
>quieter tubes out there? Thanks!


For what application? Part of the problem is that the input and output loads
have a lot to do with noise performance. Part of the problem is that some
applications have big issues with microphonics and others don't. And if you
are worried about RF noise, all the rules change.

In general, I'll point out that for low level signals with minimal drive,
nuvistors are hard to beat. And there are some really impressive compactrons
out there; people ignore them because they are viewed as TV tubes, but that
means the prices are low and the numbers are good. And in a pinch there is
always the EF86, the 6DJ8, and other frame grid types.

A lot of it has to do with quality of construction and cleanliness of
assembly too. There are very quiet 6AU6 tubes out there... but there are
also lots of very, very noisy ones.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #3  
Old September 10th 10, 10:16 PM posted to rec.audio.pro
Mark
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Posts: 966
Default Simple Test Circuit for Tube Noise

On Sep 10, 5:02*pm, (Scott Dorsey) wrote:
> > wrote:
> >Can anyone suggest a simple circuit to measure vacuum tube noise? *

>


if you want low noise, why are you using tubes...

just asking...


Mark


  #4  
Old September 10th 10, 10:45 PM posted to rec.audio.pro
PStamler
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Posts: 628
Default Simple Test Circuit for Tube Noise

On Sep 10, 4:16*pm, Mark > wrote:
> On Sep 10, 5:02*pm, (Scott Dorsey) wrote:
>
> > > wrote:
> > >Can anyone suggest a simple circuit to measure vacuum tube noise? *

>
> if you want low noise, why are you using tubes...


It's entirely possible to get low noise from tubes, IF you do the
design right and if the source impedance is appropriate (high).

As a rule, the quietest performance will be found in tubes with high
gm (mutual conductance); of course, since gm varies with operating
conditions, you need to be cognizant of them. There are typically four
noise sources in a triode circuit: the source resistance at the input,
the cathode resistor (if unbypassed), the plate resistor, and the tube
itself, which has an equivalent noise resistance of 2.5 / gm, where gm
is measured in mhos -- er, Siemenses. The contribution of the plate
resistance is Rp divided by the square of the tube's gain. Add that,
the tube's equivalent input noise resistance, and the cathode
resistance, again if unbypassed. If the total is 1/4 or less of the
source resistance, the circuit will add 1dB or less to the inherent
noise of the source resistance.

In theory. In practice, there are other issues, some of which Scott
has discussed above. But, if the source resistance is appropriate
(either inherently high, or made so with a transformer), tube circuits
can be as quiet as, say, a 5534 opamp.

Peace,
Paul
  #5  
Old September 11th 10, 02:54 AM posted to rec.audio.pro
Scott Dorsey
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Posts: 14,252
Default Simple Test Circuit for Tube Noise

Mark > wrote:
>On Sep 10, 5:02=A0pm, (Scott Dorsey) wrote:
>> > wrote:
>> >Can anyone suggest a simple circuit to measure vacuum tube noise? =A0

>>

>
>if you want low noise, why are you using tubes...
>
>just asking...


Freedom from overload?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #6  
Old September 11th 10, 02:56 AM posted to rec.audio.pro
Scott Dorsey
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Posts: 14,252
Default Simple Test Circuit for Tube Noise

PStamler > wrote:
>
>In theory. In practice, there are other issues, some of which Scott
>has discussed above. But, if the source resistance is appropriate
>(either inherently high, or made so with a transformer), tube circuits
>can be as quiet as, say, a 5534 opamp.


Unfortunately, the thermal noise in the transformers is sometimes a
bigger worry than the noise of the tubes themselves.... and certainly
the interwinding capacitance of the transformer is always a bigger
destroyer of bandwidth than the tube stage itself.

I'm a big fan of tubes, it's the transformers I don't like...
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #7  
Old September 11th 10, 06:49 AM posted to rec.audio.pro
PStamler
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Posts: 628
Default Simple Test Circuit for Tube Noise

On Sep 10, 8:56*pm, (Scott Dorsey) wrote:
> PStamler > wrote:
>
> >In theory. In practice, there are other issues, some of which Scott
> >has discussed above. But, if the source resistance is appropriate
> >(either inherently high, or made so with a transformer), tube circuits
> >can be as quiet as, say, a 5534 opamp.

>
> Unfortunately, the thermal noise in the transformers is sometimes a
> bigger worry than the noise of the tubes themselves.... *and certainly
> the interwinding capacitance of the transformer is always a bigger
> destroyer of bandwidth than the tube stage itself.
>
> I'm a big fan of tubes, it's the transformers I don't like...


Use the right transformer, and it works fine. Try the Jensen JT-13k7;
the thermal noise only adds 1 dB, and the bandwidth is good. It's the
tube used in Dan Kennedy's original preamp design, which is very clean
and neutral. It turns out the same transformer does a fine job with a
vacuum tube.

Peace,
Paul
  #8  
Old September 11th 10, 08:32 PM posted to rec.audio.pro
Scott Dorsey
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Posts: 14,252
Default Simple Test Circuit for Tube Noise

PStamler > wrote:
>Use the right transformer, and it works fine. Try the Jensen JT-13k7;
>the thermal noise only adds 1 dB, and the bandwidth is good. It's the
>tube used in Dan Kennedy's original preamp design, which is very clean
>and neutral. It turns out the same transformer does a fine job with a
>vacuum tube.


That's about as good as you can get, yeah!
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
  #9  
Old September 12th 10, 02:07 AM posted to rec.audio.pro
Mark
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Posts: 966
Default Simple Test Circuit for Tube Noise

On Sep 10, 5:45*pm, PStamler > wrote:
> On Sep 10, 4:16*pm, Mark > wrote:
>
> > On Sep 10, 5:02*pm, (Scott Dorsey) wrote:

>
> > > > wrote:
> > > >Can anyone suggest a simple circuit to measure vacuum tube noise? *

>
> > if you want low noise, why are you using tubes...

>
> It's entirely possible to get low noise from tubes, IF you do the
> design right and if the source impedance is appropriate (high).
>
>


true enough,

but if you put an equal amount of effort into the design and impedance
using a low noise FET, it will beat the tube hands down...

Mark

  #10  
Old September 12th 10, 02:54 AM posted to rec.audio.pro
Scott Dorsey
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Posts: 14,252
Default Simple Test Circuit for Tube Noise

Mark > wrote:
>
>true enough,
>
>but if you put an equal amount of effort into the design and impedance
>using a low noise FET, it will beat the tube hands down...


Not really. There's a really nice paper from Marshall Leach on the subject,
too. It's probably referenced on his website.

Tubes and FETs come out about neck and neck, with similar transformers.
Transformerless bipolar input stage comes out hands down, though, assuming
600 ohm source impedance.

That said, Fred Forssell makes a transformerless tube preamp that really
is shockingly quiet. It's not as quiet as the Great River or the Millennia
Media, but it's way more quiet than it has any right to be.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
 




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