Subject: regulated series heating of tubes - grounding
    I read your very interesting article regarding heater supply of audio tubes. I plan to build a preamplifier and in the current design I want to use serial heating of the tubes. For current regulation I will use the LM317. Tubes are 6DJ8, 12AT7 and 12AU7. The 6DJ8 requires .3A heater current as the tubes 12AT7 and 12AU7 will, if those are connected for 6.3V heater supply (both filaments per tube in parallel). So far this should work very well. The question I have is where to connect the heater circuit to ground. My guess would be that the best choice might be one of the heater pins of the first preamp tube, the most sensitive part of the preamplifier (6DJ8 in this case). Do you have any experience with grounding of series heaters?
Thank you and best regards
Hartwig

Subject: Power tube biasing
   Hi, I am a bit new to your site and find it extremely interesting, especially if one is given to some lateral thinking. I welcome it and find it an oasis of freshness. Now to the point: I read your article on biasing a power tube so as to protect it from excess dissipation resulting from a rise in the mains voltage.  In my power amplifier, the bias voltage comes from a tap on the secondary of the power transformer. I rectify it and use only a beta multiplier arrangement for the first filter section of the negative power supply. If the mains voltage rises, the plate voltage AND the bias voltage will track this. Thus the negative grid voltage will also increase and the current flowing through the tube will be lowered. Insofar as running my tubes full out. I tend to under-bias them any way, so as to have a good safety margin built-in. Am I completely wrong?
R. Guy

   No you are not. One mistake I have seen made is to use a zener diode to fix the negative power supply used for setting up the negative bias voltage. When this is done the B+ voltage can increase without finding a compensatory increase in negative bias voltage. The constant power circuit was a refinement of the usual auto bias circuit. It cannot be used in anything other than a honest Class A amplifier. One reader asked to see the stereo version of the schematic and it is shown below.

   If all tubes share the same heater current draw then you might want to try the LM317 configured as a current source, instead of a voltage regulator  (all that is needed is one resistor). Shown above is a trick used to elevate the heater voltage relative to the cathode and still allow an AC path to ground. Three advantages derive from this setup. The first is that since current can only flow from negative to positive, the interaction between cathode and heater is lessened. The second is the elimination of any chance for the ground to carry any heater current.  The third is that the tube may last longer with a slightly higher heater-to-cathode voltage (the jury is still out).