[ARC5] 10 meter BC-454 a bit more

Thu Mar 7 10:18:35 EST 2013

At the levels to be expected in that receiver a simple diode noise generator 
can give pretty accurate results and circuits in the HB are very useable and 
I believe some include correction charts.
Simple diodes become inaccurate when used at VHF and up and NF's of the low 
single digits and less.  I would expect your 454 to be around 12dB.

I used a HB 5722 noise diode for decades until I was able to obtain a HP 
8970A with the HP noise head and manual for a very nice price at a local 
auction 11 years ago. It also had a current cal sticker at the time. The 
first thing I checked was that modified 75A4 and found the 5722 was about 
1.5db optimistic. After a slight tweak of alignment and a new 6GM6 the 10M 
NF was 6.8dB (the 7360 mixers were still like new) and far better than the 
rebuilt R-390A with a string of 6BE6 mixers and the rather deaf 6DC6 RF amp.
By comparison a stock TS-940 is 12dB or more on 10M. Mine has PIN diodes in 
the bandpass filters plus a 1.5dB NF SS preamp with 11dB gain can be 
switched in for a system NF of 4.8dB.

Since about 1-1.5dB is detectable by ear little changes can open up another 
level of signals.....even on 160 with low noise receiving antennas.

Carl

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "Dennis Monticelli" <dennis.monticelli at gmail.com>; "Richard Knoppow" 
<1oldlens1 at ix.netcom.com>
Cc: <Arc5 at mailman.qth.net>
Sent: Thursday, March 07, 2013 1:22 AM
Subject: Re: [ARC5] 10 meter BC-454 a bit more

> On 6 Mar 2013 at 21:55, Richard Knoppow wrote:
>
>> Which two tubes?
>
> The two we have been talking about.
>
>> If you mean 12SK7 and 12SG7 there is a
>> lot of difference.
>
> Gm for the 12SG7 is almost exactly double that for the 12SK7.
>
> Transconductance is 2000 for the 12SK7, and 4000 for the 12SG7.
>
> It is 4900 for the 12SH7, 9K for the 6AC7, 11K for the 6AG7, and over 13K
> for the 7963 submini dual triode.
>
>>  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.
>
> Yes.
>
>>  It
>> is the partition noise which makes pentodes noisier than
>> triodes of the same Gm.
>
> Yes. The more grids, the more noise, according to Terman.
>
>>  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.
>
> 2X. Partition noise is identical according to the tube manuals.
>
>>     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.
>
> Yes. Provided. In this case, they are biased identically, so I maintain 
> that the
> gain is not substantially different.
>
> One added factor is that the screen voltage for both tubes is 1/2 that 
> listed in
> the tube characteristic manuals: i.e. 75 VDC instead of 150 VDC.
>
>>  The 6SG7 or 12SG7 is a very good RF tube.
>
> Not too shabby, but certainly not perfect either. I wish there was a tube 
> with
> the transconductance of the 6AC7 (9K) or even the 6AG7 (11K), or better
> yet, the 7963 (13K), but which was a remote cutoff pentode type.
>
>> 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.
>
> This is one reason I was surprised that the 12SK7 works as well as it does 
> at
> 30 MHz.
>
>>     In conventional receivers the noisiest source is usually
>> the mixer. The worst are pentagrid or hexode mixers.
>
> And the 6/12K8 is one of the noisiest mixers available. As I remember it,
> ENR is something like 360K ohms, which is WAAAAAAY up there.
>
>>  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.
>
> Yes, but even Collins used straight triode mixers in some of their better
> receviers....like the R-390.
>
>> A cathode coupled pentode is not too bad
>> but these were not often used.
>
> The ARRL Handbooks had several receivers which used a 6AC7 as such a
> mixer. Those receivers worked quite well. I have never seen a commercial
> receiver which had one of those and always wondered why.
>
>> National did it in the HRO.
>
> Another unusually sensitive and quiet receiver.
>
>>     I will have to dig out the pertinent formulas from the
>> Radiotron handbook and calculate the noise of some of these
>> tubes.
>
> Terman's book has those.
>
>>     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.
>
> S+N/N ratio is a pretty fair comparison and is fairly easy to do. Ultimate 
> NF is
> not as easily measured with amateur equipment, as I remember it.
>
> Again, I could be wrong.
>
> Ken W7EKB
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