Quote:
Originally Posted by JohanBee
The 3pcs 6BA6 can also be swapped to 6BJ6, to save another 450mA heater current.......(for a lamp to the S-meter).....?
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Use of the 6BJ6 in place of the 6BA6 (with appropriate base wiring changes) will certainly reduce total heater current, but it might also reduce the large signal handling capacity of the receiver.
The 6BJ6 was intended both to reasonably replicate 6BA6 general performance at lower heater consumption, to suit mobile applications, and to provide improved VHF performance to better suit it for FM and VHF mobile applications.
The trade-off was a slightly lower transconductance, and, with a lower anode current, reduced signal handling capability. The latter was mentioned in RCA Application Note AN-127, available at:
http://www.one-electron.com/Archives...-AppNotes.html.
The 6.3-volt, 150 mA heater made the 6BJ6 suitable for use in both AC and AC-DC receivers. In the latter case, its 6.3-volt heater (as compared with the 12.6 volts of the 12BA6) made it advantageous for use in American AC-DC FM-AM receivers where the total heater string voltage was limited to 117 volts, but the valve count was reasonably high.
The improved VHF performance was obtained largely by reducing the mutual inductance between the control and suppressor grid leads-out by changing the pinout arrangement. The mutual inductance provided a feedback path including the suppressor grid-to-anode capacitance. This change resulted in something like a halving of the input conductance at 100 MHz. This VHF capability was obtained whilst retaining its suitability for narrow-band IF applications. There was the same relationship between the sharp cutoff counterparts 6BH6 and 6AU6. (See “Service” magazine 1948 August p.34ff for background; available at:
https://www.americanradiohistory.com...e_Magazine.htm. RCA AN-139 also provides some comparative numbers.)
Re pentagrids and the respective uses of g1 and g3, a further delve shows that the situation is quite complex, with for example the 6BE6 and 6BA7 sometime used the “wrong way round”. But that is getting off-topic…
Cheers,