From: SPAM@not4me.com   
      
   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.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)