Why does my RCA transmitting tube manual recommend fixed bias and not cathode bias? My RCA receiving tube manual gives typical grid resistors for both types of bias for the 6L6.
Thanks!
Thanks!
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I think the reason is that in RF applications the tube operates in class B or class C. For those classes you don't use a cathode bias resistor.
PFL200 is right.
807 in typical RF circuits can really smoke, when the drive signal goes away.
More accurately, it is deformed metal or deformed glass.
Go find a 1950s to early 1960s ARRL Radio Amateurs Handbook if the library has one,
or go to a radio amateurs swap meet.
I built a low power single ended 807 amp, triode wired, for Summer operation.
It used a cathode self bias resistor with a bypass cap.
807 in typical RF circuits can really smoke, when the drive signal goes away.
More accurately, it is deformed metal or deformed glass.
Go find a 1950s to early 1960s ARRL Radio Amateurs Handbook if the library has one,
or go to a radio amateurs swap meet.
I built a low power single ended 807 amp, triode wired, for Summer operation.
It used a cathode self bias resistor with a bypass cap.
The RCA transmitting manual data sheet that I have dated Nov 5, 1954 clearly states maximum grid circuit resistances for both fixed (.1 meg) and cathode (.5 meg) bias for TRIODE connection as an AF power amp and modulator in class AB1 with 400 volts maximum plate voltage.
For PENTODE connection as an AF power amp and modulator in class AB1 or AB2 only fixed bias at 0.1 megohm is specifier with cathode bias not recommended. The obvious changes from triode connection is a CCS (continuous commercial service) maximum plate voltage of 600 volts and an ICAS (ham radio) maximum plate voltage rating of 750 volts.
Limited class B RF power amplifier ratings are given, but the usual case for RF amps provides a low grid circuit resistance.
An 807 from 1954 is a 6L6GA or 6L6GB with a different base, a plate cap, and some RF shielding. The 1957 spec for an RCA 6L6GB shows a maximum plate voltage of 360 volts in pentode, and 275 volts in triode mode.
I'm sure that any 807 can run in cathode bias with 360 volts on it's plate.
Not so sure that some of todays 6L6GC's will run for long in cathode bias with 500K of grid resistance due to poor vacuum, or for even a minute at 750 volts due to an arc over inside the base between pin3 (plate) and pin 2 (heater) or due to runaway from the residual gas inside (poor vacuum).
For PENTODE connection as an AF power amp and modulator in class AB1 or AB2 only fixed bias at 0.1 megohm is specifier with cathode bias not recommended. The obvious changes from triode connection is a CCS (continuous commercial service) maximum plate voltage of 600 volts and an ICAS (ham radio) maximum plate voltage rating of 750 volts.
Limited class B RF power amplifier ratings are given, but the usual case for RF amps provides a low grid circuit resistance.
An 807 from 1954 is a 6L6GA or 6L6GB with a different base, a plate cap, and some RF shielding. The 1957 spec for an RCA 6L6GB shows a maximum plate voltage of 360 volts in pentode, and 275 volts in triode mode.
I'm sure that any 807 can run in cathode bias with 360 volts on it's plate.
Not so sure that some of todays 6L6GC's will run for long in cathode bias with 500K of grid resistance due to poor vacuum, or for even a minute at 750 volts due to an arc over inside the base between pin3 (plate) and pin 2 (heater) or due to runaway from the residual gas inside (poor vacuum).
Check out the STC application report for the most complete 807 data - http://www.tubezone.net/pdf/807stc.pdf