B&W-5100 resistor question

[K1ZJH]

My B&W 5100 transmitters have a 4.7-ohm resistor in series with the filaments
on the 6U8A audio tubes. What is R415 for, to reduce "shot noise" or some other
undesirable artifact?

Pete

[W3GMS]

Good question Pete!  I looked at several 5100 manuals I have and all version use the 4.7 ohm resistor in series with the 1st AF gain 6U8 tube.  That resistor represents about a 2 volt drop in filament voltage.  That will definitely lower the emission, but why do it that way?  If it was too hot of a front end, just change the plate and cathode resistors to lower the stage gain.  Sorry Jack is not around to answer that question.  I have never heard of "shot noise" being a problem with the Ham stuff. 

Maybe someone can come up with some possible reasons why they did that.

Have fun with your 5100.  Its a great transmitter and very well built.

Joe, W3GMS

[WA1ICI]

The 6U8 was designed to be a mixer/oscillator in TV tuners and thus was designed for high transconductance.  Part of this design involves running the cathode at the high-end of its temperature range.  Running hot increases the amount of metal vaporization and deposition in the tube, which increases leakage over time.  (This is the black deposit you sometimes see on the glass in tubes that is not related to the getter.) In an RF front-end tube, this isn't so critical, but in a low-level audio stage, it is.  It can cause hum and random crackling noises.  If the cathode current is well below what the tube was designed for, then the cathode can be run cooler without running out of current.  This reduces leakage and also dramatically increases the life of the tube.

In the 5100, the triode has a plate resistor of 270K and the pentode has a plate resistor of 500K.  I don't know the exact B+ voltage for the 5100, but these resistors imply a plate current of less than 0.5mA.  The RCA tube manual shows the recommended operating points for the 6U8 of 13.5mA for the triode and 13mA for the pentode (plate+G2 current).  Thus the way the 6U8 is used in the 5100 as a high-gain AF amplifier is in "starvation mode".  Running the cathode cooler is definitely a good idea here.  Other tubes that were originally designed for RF but are sometimes used for audio include the 12AT7, 6DJ8, 6EA8, 6AL5 (as a detector), and the 6AU6.  If they are being run in starvation mode (plate resistors > 100K), they are good candidates for running cooler - say 5.0 to 6.0 volts on the heater.

One thing to look out for in refurbishing vintage equipment that have filament dropping resistors is to check the value of the resistor.  I've found that the 1 or 2 watt carbon-comp resistors often used here tend to go up in value, thus cutting the heater voltage too much.  Also, some pro-audio and hifi low-level AF circuits used DC on the heaters, but used a selenium rectifier.  These almost always deteriorate over time, so should be replaced by silicon rectifiers.  However, an extra series resistor should be added to compensate for the lower drop of the silicon rectifiers.  This is hard to calculate, so I pick the value by cut and try.

- John Atwood, WA1ICI

[W3GMS]

John,
Many thanks for the great explanation.   Having never used a 6U8 in audio service, you information was quite beneficial. 
Regards,
Joe, W3GMS

[K1ZJH]

Thank you John. Another mystery is solved.

Peter