[ARC5] 10 meter BC-454 a bit more

Thu Mar 7 12:07:16 EST 2013

Not at HF but studies reducing the B+ to achieve a SNR improvement have been 
around since the mid 30's when James Millen suggested it with the original 
HRO. Ive been running mine at 180V versus 250 and it is better.

Carl

----- Original Message ----- 
From: Dennis Monticelli
To: Geoff
Cc: kgordon2006 at frontier.com ; Richard Knoppow ; Arc5 at mailman.qth.net
Sent: Thursday, March 07, 2013 10:50 AM
Subject: Re: [ARC5] 10 meter BC-454 a bit more

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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