[ARC5] 10 meter BC-454 a bit more

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "Dennis Monticelli" <dennis.monticelli at gmail.com>
Cc: <Arc5 at mailman.qth.net>
Sent: Wednesday, March 06, 2013 9:36 PM
Subject: Re: [ARC5] 10 meter BC-454 a bit more


> On 6 Mar 2013 at 21:12, Dennis Monticelli wrote:
>
>> It does sound like the lower gain of the 12SK7 is 
>> exposing the input
>> to the noisy mixer.
>
> As I said, I don't "read it" that way: I think the 12SK7 
> is generating its OWN
> noise which is adding to that of the mixer, and even 
> masking it a bit.
>
> When I have the 12SK7 installed, what I hear is a higher 
> level of what one
> might call "static"....except that it isn't quite static. 
> I've always called that stuff,
> "shot noise".
>
> When I install the 12SG7, the "shot" type noise is pretty 
> much gone, but the
> steady hissing noise, which I am certain is what comes 
> from the mixer,
> becomes more obvious.
>
> From my reading of the tube data sheets, there really 
> isn't any measurable
> difference in the gain of the two tubes.
>
> But I could be wrong...
>
> Ken W7EKB
> ______________________________________________________________

     Which two tubes, if you mean 12SK7 and 12SG7 there is a 
lot of difference.  Pentode tube noise is calculated on the 
basis of Gm and what is called partition noise, which is 
dependent on the ratio of the screen and plate voltage.  It 
is the partition noise which makes pentodes noisier than 
triodes of the same Gm.  There are formulas for calculating 
the noise of a tube as an "equivalent noise resistance". 
This is the thermal or Johnson noise of a perfect resistor 
at the ambient temperature. Without calculating the 
partition noise the relative noise will be roughly the ratio 
of the Gm, which is considerable between these two tubes.
    Gain is also a matter of the Gm so a 12SG7 should have 
more gain than a 12SK7 provided both are biased for maximum 
gain.  The 6SG7 or 12SG7 is a very good RF tube. The 
miniature type 6BA6/12BA6 is similar but has lower 
interelectrode capacitances making it more suitable for 
higher frequencies.  RCA gives the limit for the 6SG7 as 18 
mhz although they were often used at much higher 
frequencies, at least to 30 mhz, while the 6BA6 is good to 
above the FM band.
    In conventional receivers the noisiest source is usually 
the mixer. The worst are pentagrid or hexode mixers.  The 
quietest are cathode coupled triodes but they have low gain 
and often load the RF circuits due to low plate resistance 
and miller effect. A cathode coupled pentode is not too bad 
but these were not often used. National did it in the HRO.
    I will have to dig out the pertinent formulas from the 
Radiotron handbook and calculate the noise of some of these 
tubes.
    FWIW, the best criterion of noisiness is noise factor. 
NF is a measure of the noise of a receiver (or amplifier) 
compared to the theoretical noise input.  A perfect 
noise-free receiver would have an NF of 0db. Many good HF 
receivers have NF of around 6db. Some radio-telescope 
amplifiers have an NF of a fraction of a db but many also 
used cooled input devices so that the theoretical noise 
floor is much lower than a conventional receiver.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com