[ARC5] 10 meter BC-454 a bit more

Thu Mar 7 10:50:48 EST 2013

I have a curious thought.   Since Ken is saying he is experiencing what
sounds to me like white noise masking the atmospheric, that implies either
Johnson or Shot origin. What if the filament voltage of that RF tube was
cranked down to where the gain is not yet affected but the cathode is
running cooler?   Johnson noise is a function of the square root of
absolute temp and the temp of the filament is a very strong function of
applied voltage.   If the tube is strong (has plenty of reserve emission)
it should be possible to cut back its cathode temp without adversely
impacting gain and improve the noise figure through Johnson noise
reduction.   Has anybody heard of this being tried?

Dennis AE6C

On Thu, Mar 7, 2013 at 7:18 AM, Geoff <geoffrey at jeremy.mv.com> wrote:

> 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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