Subject: Design Idea: Safe Loftin-White
   Just wanted to let you know not only how much I like your output in Tube Cad Journal and in the previous GlassWare circuits of the month, but I used the Design Idea of May 2000 in an actual circuit. I had to play with the values, because I wanted to use a 5965 (low plate voltage) instead of 12AX7 and I used a 12B4 as power tube. I wanted to run the 5965 at 5 mA so the Rka came out at 4.4 K. It worked, but not too well, the circuit clipped before reaching full power ( about 1 watt). {left hand circuit in attachment} The clipping took place in the 5965 already and is one sided (only top caps of the sine wave get cut off).
   Not exactly sure which way to go I adjusted the resistor values closer to your design by simply soldering different value resistors in in progressive steps.
   I ended with the second circuit in the attachment and it is a very nice sounding amplifier indeed, now running into its third week. It's just right for the lower listening levels my wife likes and it doesn't heat the place as much in this summertime as my 811-10 amplifier does.
   I am working on a version with 6EM7 and 5965 that should get me closer to 2 watts output. ? What is the 0.1 µF capacitor exactly doing - how do I compute the value
of this capacitor?
  * I tried several other tubes in the 12B4 circuit. The 12AX7 did not do well. The next best to the 5965 was the 12AV7. I also tried 12AU7, 12AT7, 5963, 5964. The 5965 came out way best. This was only scope and waveform testing with maximum power output in mind. No listening tests.
   * I did give the upper 5965 and the 12B4 a biased heater supply at close to +120V B+
   * The different voltage measurements come from two dissimilar 12B4 - I only have 7 used ones around to try.
   * The higher B+ at the second circuit version results from the actually lower current draw of

the better version.
   What shall we call this baby? "Safe Loftin-White" doesn't ring too well with me - "Broskie-Loftin-White"????  I also would like to give proper credit, when using the circuit and talking about it with other tube amplifier builders.
    * I like to learn more about tube as current sources - both as plate loads and as cathode "resistors"- if you could find some space for it in the CAD magazine! As always looking forward to your next Tube Cad Journal

Herbert

     I am blushing too hard to respond.

Subject: Heater Concerns - some remarks
   Congratulations - I enjoyed reading this article. Heater supply is rarely discussed in recent publications about audio tube applications, but it is a very important fact. The directly AC-line powered series heating is quite interesting topic. In fact, at least in Europe special tube families were manufactured for this purpose. It started during W.W.II (for economical purposes) with the V-Series tubes (VCL11 e.g.). All V-Tubes had a heater current of .05A. Only the heater voltage varied according to the specific tube's demand. Due to the low current, high voltages were required. Thus, only 3 or 4 tubes with the filaments in series were enough to allow direct
connection to 110V or even 220V mains supply. Because this technique did not require necessarily a transformer in a radio set, it was possible to use those radio sets in areas with DC-line supply as it was common in some places in Europe at that time.
   After W.W.II, radio sets became more complex and contained more tubes. Consequently, more tubes had to be connected in series to the line voltage. This was achieved with a higher series current - the U-Series tubes (e. g. UL41) with a current of .1A were created. At the same time TV-sets were designed using