From: ruffrecords@yahoo.com   
      
   Curator wrote:   
   > A few weeks ago there was a discussion regarding the use of pentodes in   
   > pre-amps. The following information is from an application report on the   
   > 6BR7 low-noise pentode by Standard Telephones and Cables Pty Ltd Sydney   
   > Australia and dated January 1955.   
   >   
   > Begin quote:   
   >   
   > Valve Shot noise (Hiss): A certain amount of random noise will be   
   > generated in the valve by the random arrival of electrons at the plate,   
   > and this is further increased by the partition of the cathode current   
   > between plate and screen, as the random collection of electrons by the   
   > screen must have its effect as an increased variation in the number of   
   > electrons arriving at the anode. This is inherent in the valve and cannot   
   > be entirely eliminated. A major contribution to valve noise, however, is   
   > noise produced by leakage between electrodes over the mica insulators and   
   > in the base. Also a poorly activated cathode can produce a noise voltage   
   > swamping the normal valve noise.   
   >   
   > The shot noise in the 6BR7 has been reduced to a minimum by careful   
   > design, and the noise due to leakage controlled by careful assembly and   
   > inspection. A certain amount of leakage noise is unavoidable in a   
   > mass-produced valve, and this reveals itself as a small variation in noise   
   > level from valve to valve.   
   >   
   > The curve No. 308.228 shows the maximum hiss voltage plotted against the   
   > percentage of valves, using a grid resistor of 100 k Ohm and a bandwidth   
   > of 10 kc/s. The curve is very steep and indicates that a hiss level on the   
   > grid always lower than 7uV is to be expected.   
   >   
   > The curve No. 308.230 shows the same parameters with the valve triode   
   > connected. Here partition noise has been eliminated. As would be expected   
   > the higher noise levels are unaltered because they are due mainly to   
   > leakage which is not greatly affected by whether the valve is triode or   
   > pentode connected. The lower noise level is reduced, and in fact falls   
   > below the thermal agitation noise generated by the 100 k Ohm grid   
   > resistor, which in a 10 kc/s bandwidth at 20 C is about 4uV.   
   >   
   > Conclusions: While it is not claimed that the 6BR7 has exceptionally low   
   > noise properties, it can be stated that it is a very great improvement   
   > over average normal valves. Individual samples taken at random from a   
   > large batch can be expected to give consistently good performance. As with   
   > normal receiving type valves, by selection, it is possible to find   
   > individual samples of outstanding performance, but whereas with normal   
   > types the average product is many times inferior to the selected samples   
   > in the case of the 6BR7 the average product is little different from the   
   > best samples selected.   
   >   
   > In spite of this, care must be taken when designing prototype equipment   
   > for high gain, low noise applications, to ensure that the design is not   
   > finalised from the experience acquired from a single sample of the 6BR7.   
   >   
   > End quote:   
   >   
   > Since I can't reproduce the graphs here, the table below shows the   
   > measured noise levels for a given percentage of valves extracted from the   
   > graphs. The bandwidth is 10 kHz. (use a fixed pitch font)   
   >   
   > Percentage  Noise (uV)   Noise (uV)   
   > of valves   triode       pentode   
   >             connected    connected   
   > 100%         < 7.10       < 7.10   
   > 90%          < 5.70       < 5.70   
   > 80%          < 5.20       < 5.36   
   > 70%          < 4.86       < 5.20   
   > 60%          < 4.60       < 5.12   
   > 50%          < 4.40       < 5.03   
   > 40%          < 4.20       < 4.95   
   > 30%             *         < 4.87   
   > 20%             *         < 4.80   
   > 10%             *         < 4.55   
   >   
   > * The triode graph stops just below 40% as the grid resistor noise   
   > dominates.   
   >   
   > Operating conditions   
   > Plate Supply 300V   
   > Grid resistor 100 k Ohms   
   > Plate resistor 220 k Ohms   
   > Cathode bypass 50 uF   
   > Cathode resistor 14 k Ohms (Triode) 1500 Ohms (Pentode) Screen resistor   
   > 1.5 M Ohms (Pentode)   
   >   
   > Published gain for these operating conditions is (following grid resistor   
   > 1 M Ohm) 14 times triode connected and 124 times pentode connected.   
   >   
   > Observations: The noisiest valve pentode connected is < 5 dB noisier than   
   > the best valve triode connected. You might be unlucky enough to find a   
   > valve that is just as noisy triode connected as pentode connected. 50% of   
   > valves pentode connected are only 1.5 dB noisier than the best valves   
   > triode connected. A pre-amp with 200 mV input sensitivity would have a   
   > worst case signal to noise ratio of 89 dB.   
   >   
   > Regarding the 6U8 (for Patrick and flipper), in the May 1960 edition of   
   > the AWV Radiotronics magazine there is a reprint of an RCA article titled   
   > "The 7199 in High-Fidelity audio equipment". The "in this issue" page   
   > describes this article as "An interesting discussion of the use of   
   > triode-pentodes in AF amplifiers, particularly the 7199, hifi version of   
   > the 6U8."   
   >   
   > Curator.   
   >   
      
   100K has a noise voltage of almost exactly 4uV in a 10KHz bandwidth. If   
   we assume the noise resistance of the tube is effectively in series with   
   this then the 100% triode case implies triode input noise voltage in   
   10KHz is always less than 5.9uV. This is not particularly good. The   
   worst 6CG7/6SN7 types I have tested will do better than 3.5uV (and true   
   also for most common audio triodes).   
      
   The 40% triode case implies the effective triode input noise voltage is   
   a mere 1.3uV in 10KHz which I find hard to believe. The best 6CG7/6SN7   
   types I have measured achieve only 1.6uV in that bandwidth (other common   
   audio triodes are similar).   
      
   What I am most curious about however is the low frequency filtering they   
   used because my own experience is that these sorts of figures are only   
   achieved in the absence of 1/f noise which usually requires a high pass   
   filter turning over at about 400Hz.   
      
   Cheers   
      
   ian   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)