[Patrick Turner]

On Wednesday, October 28, 2015 at 2:37:11 AM UTC-4, John L Stewart wrote:
Pink Floyd


A few, those of us that still dabble anyway. The long, cold winter is about to begin and I have half-a-dozen projects in the queue - mostly restorations.


Peter Wieck
Melrose Park, PA

Summer has just started here, with high probability we'll have a record number of days over 35C. ( 95F )

It won't be because every dude has an extra beer, and dozes off, leaving their tube amp on all night.

Anyway, I'm totally re-building an RF gene with tubes which I first made in 1997. This time I've armed my brain with Secret Knowledge known only by well paid HP boffins of 1955 who designed the HP606A RF gene. I'm at my limit, and I won't be trying to make an oscilloscope like the Tektronics 55 which starred in a video I watched recently. That Beast was reviewed by an oscilloscope collector, the truly compulsive collector of insanely complex old junk. All the other gathered oscilloscopes which sat around watching while this bloke told us all about the 55 all looked insanely jealous 'cos they were not getting the attention. Anyway, bloke rattled on and on and about the 110 tubes it had in two large boxes. He finally turned the darn Old Girl 55 ON, and it all came to life OK, and this sort of thing is the American Solution to Lonely Farnarkling in the Shed during your freezing winters which de-nuggets brass monkeys! Once the 55 runs, its T-shirt time while its -20C outside. Amazing what still lurks in countless basements and garages right across the USA. You can get every thing you want at the E lek Tronic resto ront.

My humble little gadget sure has tubes, but only about 8, plus a few bjts, because I'mm tryna combine benefits of tuned circuits with cathode modulation of a PP pair of 6EJ7 - or maybe EL86, and I should get a BD139 to do what one of a few 6B4 do in 606A, hopefully better, less THD in envelope shape..

I dunno how they'd do AM now, lots and lots and lots of utterly incomprehensible didgets & bizibots I guess.

I tried untuned circuitry with all SS, and frankly, the outcome compared to HP606A was quite appalling. Bandwidth stopped at 4MHz, and AM waves looked horrid above 2MHz, and probably, BF469 are NOT the ideal device to use for the modulator pair. NFB loops just would not work. I know - you don't have yell "Ain't ya heard about the latest HP function gene?" Much about HP I do like, but hasn't it all gone Chinese? Agilent stuff seems to have gone to China. I bought a new 117 Fluke DMM, better than oldun I had since '93. BUT, there's a bother with Vac reading, reads no lower than 22mV, and in mV range no lower than 4mV. BUT, another I tried at same shop reads down to zero with leads shorted OK. To get that Fluke fixed, its a pile of trouble I guess, or Sum Wonk in the Chines factory forgot to calibrate a pot properly.
so......... I was asked, "what do you think about Chinese Quality Control?"
and I answered "I was told they were to begin trials with that novel concept during next five year plan".

Oh, and if there's a whiff of Vdc while tryna read mVac with 117, she won't read properly, so I have to use an adapter I made for putting in 0.33uF and 330k to block the Vdc, then she's OK. Oldun read Vac OK regardless of presence of Vdc. Musta been a male voltmeter.

The Whirled won't disintegrate because of such trifling complaints.....

Patrick Turner.

[Big Bad Bob]

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)

[Big Bad Bob]

On 12/06/15 05:56, Patrick Turner so wittily quipped:
I dunno how they'd do AM now, lots and lots and lots of utterly incomprehensible didgets & bizibots I guess.

here's what _I_ would do, if it were solid state:

a) use a ring modulator with VERY heavy negative feedback

- or -

b) use a relatively linear MOSFET biased to its most linear zone with a
tube-like modulation circuit, i.e. source-feed from osc for carrier,
gate modulation.

- or -

c) use a "digital exciter" circuit. probably a ZILLION ways to make
THAT work. I'd give PWM a try. for 455khz IF modulator, you'd need
455Mhz with divide-by-1024 and 12-bit accuracy. THEN, modulate the
455Mhz with 455khz digital signal, with amplitude calculated based on
digital sampling of the input. Input is at 0.5V, output swing is +/-
256. Input is at 1V, output swing is +/- 512 [max modulation]. Anyway,
that might be kind of interesting.

- and -

You'd use an IF freq. for the modulator, maybe 455khz like AM receivers,
then drive this into a freq. synthesis converter, similar to old-style
"crystal oven" freq. synth transmitters the Navy used back in the day.


it should work for just about the entire range, and you should be able
to use tuned SSB mechanical filters on the transmitter IF, with an "add
carrier" level adjust.


but yeah, BEST way to do AM modulation (In My Bombastic Opinion) is with
an IF carrier, so you can do SSB and 'carrier injection' and whatnot via
off-the-shelf mechanical filters and THEN drive the transmitter through
a freq. synth. thingy. Super-accurate, good quality SSB, built-in
ACCURATE carrier generation for SSB reception, and so forth.

[that worked really nice with TUBES back in the day, it's how the Navy
transmitters (that I studied for ET 'A' school) worked, though they used
solid state switching diodes to generate harmonics for synthesis]