On 12/06/15 05:56, Patrick Turner so wittily quipped:
My humble little gadget sure has tubes, but only about 8, plus a few bjts, because I'mm tryna combine benefits
yep - use the BJT or FET where it makes sense. no problem here.
One thing that BJTs do VERY well are series voltage regulators. I found
some nice parts that should do nicely for pre-amps.
My *specific* issue is the switching power supply [driven by a
micro-controller] that would give you SEVERAL switching-regulated
voltages for various things, B+, bias, heaters, maybe screen volts too
for output stages [if not using ultra-linear].
The problem with a switcher is that you get very low frequency 'wobble'
in the voltage. After some testing, I discoverd that dropping 30V to
50V with a BJT (NPN) regulator with a zener on the base, emitter output,
virtually eliminates the LF bumpiness in the power supply. the
alternative would be an RC filter with a frequency cutoff of ~1Hz.
Yeah, impractical.
There are many reasons you'd want to use a microcontroller-based switcher.
a) variable amp characteristics [settings, basically]. Can include
constant B+ or 'voltage droop' like a tube rectifier for instrument amp
overload characteristics matching.
b) heater warmup using a current limiter (let's say 2A until it hits
6.3V, then 6.3V, or maybe ramp UP to 2A over 5 seconds, THEN 2A until it
hits 6.3V)
c) DC heater volts [cut back on hum-related issues in pre-amps and
modulators]
d) multiple voltages controlled by single "thingy" [my main reason, as
LTC-branded regulators are more expensive than the CPU!]
e) overall efficiency
f) potential for (efficient) battery operation
g) cost reduction [use laptop computer power supply that costs $29
rather than a $200+ linear power supply with transformer]
h) weight reduction (no super-bulky line-to-350VCT plus filaments;
transformerless switcher powered by laptop computer power supply instead)
I actually have a prototype for the power supply that's a "bench power
supply", though I can only get a pathetic 15-20ma or so at 400V - good
enough to test pre-amps of course, but that's about it. I need bigger
electros with lower ESR to get higher current.
http://mrp3.com/sftpowersupply.html
but it's got 4 voltages - +/- 0-12V, - 15-70V, + 35-400V
and so I can adjust it and test things, *like* a pre-amp circuit with
both halves of a 12AX7 and a linear regulator. Then measure output with
an o-scope, see if I "cure" the VLF noise from the power supply.
(it's not a LOT of VLF noise, but it gets amplified by the tube, and
makes a significant impact on the output when you amplify a very small
signal - yeah tube amps don't have very good power supply rejection)