Subject: 2.5V Voltage Regulator
   I have recently have become interested in vacuum tube audio. I'm an EE with mostly a digital design background, although most of my work has been designing audio products. I can design low level audio circuits with op-amps in my sleep, but designing circuits that you shouldn't touch when power is a applied is a different story. I do have some habits to break, such as soldering with the power applied to the circuit (yea I removed the ground from the soldering iron so I could do this). Anyway, I've been collecting info on 2A3 designs, and came upon "Tube CAD Journal". Not sure why I got stuck on the 2A3, just seems to be a lot of talk about it, and example designs seem simple.
   Lot's of good stuff in Tube CAD.... keep up the good work!
   Anyway, why I write; thought I would pass this information on: referring to "Voltage regulators for the 2A3" "Directly heated filaments benefit from DC voltage, just as indirectly heated cathodes do. Unfortunately, no fixed voltage regulators are available with a 2.5 volt output."
   There are a few different flavors of Linear LDO 1.5V/1.8V/2.5V/3.3V 3A to 5A regulators available from Unitrode, now a Texas Instruments Company. These are fairly new products, mainly used for powering/biasing high speed digital backplanes and the core of DSPs (Digital Signal Processors). I personally use a 2.1V @3A to bias a high speed backplane and a 2.5V @3A for DSP power. They do work well, and can deliver high current fast. These are surface mount parts, but for the DIY projects, they are large enough to solder by hand, mounting will be the only problem. There might be thru-hole versions; I didn't look. I would think DigiKey would carry these. These are about $3.00 ea (in 100s), so it's probably a wash as far as cost compared to your LM150/LM330 solution. I have no clue whether there is a performance difference. The Unitrode's are probably faster than the LMs. But, for this

(2A3) application, does it really matter?

They are:
      Unitrode (Texas Instruments)
      UC382-3, 2.5V @3A
      UC385-3, 2.5V @5A
The link to the datasheet is:
http://power.ti.com/docs/browse/productnavigation.jhtml?familyID=436

Regards,
Craig

   Thanks for the great tips. I stopped collecting solid-state data books about five years ago, figuring that I could always go to the net to find a PDF file. My shelf is less burdened, but I no longer know what is available to lookup on the web. A rethink is in order.

Subject: Output impedance of Para-feeds?
     Great webzine. I enjoy your free thinking, visual approach to (tube) electronics, by viewing circuits as building blocks all kinds of weird and wonderful structures are possible… I'm currently building my first tube amp, and am planning to use the SV572-10 DHT in SE transformer coupled configuration. But the thought of having 600 odd volts in the transformer seems a bit dangerous as only one small insulation breakdown could spell quite a disaster!?
    A para-feed arrangement seems like a logical way to isolate the transformer. Does the para-feed inductor, which in AC terms approximates a current source, raise the output impendence that the transformer sees to approximately infinity? And therefore lower the damping on the speaker through the transformer to zero?
   Also, if the answer to the above question is no, would it be an advantage to use a real current source, the bias current would be easily set then...?

Andrew