Tee Hee! Made a voltage regulator today.

h-

| =A0 =A0 =A0 |

=A0 =A0 =A0 |

=A0 =A0 |<

=A0 =A0 |\

=A0 =A0 =A0 |

=A0 =A0 o----------o +135V

=A0 =A0 =A0 |

=A0 =A0 =A0.-.

=A0 =A0 =A0| |

=A0 =A0 =A0| | 150k

=A0 =A0 =A0'-'

=A0 =A0 =A0 |

| =A0 =A0 |

(snip)

I'm a bit surprised that the hum was from the B+ rail, only, or at least substantially. It certainly would suggest a cap deficiency there.

I only mention that because I have an olde Marconi TF995A/S sig=gen which used to have a hum issue. That was NOT on the HT rail, but actually introduced via the heaters. So we replaced the 6.3VAC feed with a 5V DC regulated supply, and sorted two problems - the hum and drift after initial warmup.

Many guitar amplifiers simply used a potentiometer across the floating

The only time I have seen a DC heater feed was one Geloso amplifier, with the front end tube heaters fed in series from a rectified and filtered DC bias. This negative voltage was also used to bias the power pentodes with a negative grid bias (thus allowing direct grounding of cathodes).

Wow! Such high tech >:-} ...Jim Thompson

Certainly where I used to see a fairly severe amplitude hum I now see a nice flat line. I was also fighting frequency shifts when loads turned on and off on the AC line (and I have a Weller soldering iron that clicks on and off...). There may still be hum from the heaters, but the overall performance is better.

It's going into service today; I'll see _how_ much better it is.

Hey, a voltage regulator in a vacuum tube circuit _is_ high tech!

Tim, as you know, gas regulators are quite noisy. This will modulate your oscillator and cause amplitude and frequency modulation in the output.

Perhaps you should put a large bypass cap across the NE-2, like 1uF?

Mike

I wondered that, but it would much exacerbate the overshoot problem, before the neon strikes. RC between neon and tr base would be better.

NT

That's in my on-paper schematic, but got left off because I was running out of room!

If I have problems I'll certainly shoe-horn that in there. Right now the signal generator is working loads better than it did yesterday morning, so I'm quite happy.

Too big a capacitor across the neon lamp and you'll get a relaxation oscillator!

That's probably why I didn't bypass the lamp.

Actually, I just looked and according to my "GE Glow Lamp" manual, 2nd edition published 1966, the way to determine if a certain combination of resistor,glow lamp, and capacitor will oscillate or not is to take the volt-amp characteristic curve of the glow lamp, and run a line with slope V/R from the supply voltage at the top left of the graph to the bottom of the graph. If the line intersects the negative resistance portion of the glow lamp curve, the circuit will oscillate.

So it seems it has to do more with the size of the resistance - different capacitance sizes will just change the oscillation frequency.

You are cheating ! !

A tube system should use a triode (or even pentode) as a series pass element :-) :-).

Of course, I understand that it would have required even one more filament winding for the series pass tube.

What frequencies are you needing to generate?

John S

It is not that simple - to oscillate, the feed resistor must be so large that the current from the resistor alone is not sufficient to maintain the glow discharge. If you feed enough current, there is no reason to skip the bypass cap.

he

Does it need to lose as much as 30v? I've generally resorted to RCRC in such situations, a brain dead solution that always works and doesnt lose many volts.

NT

First, that's what was there already (or at least CRC). Second, that doesn't stabilize against power supply fluctuations, and that was a good part of the problem I was trying to solve.

Second, dropping 30V was mostly intended to keep the B+ rail at the same voltage as it was before the change. It does make sure that I have plenty of headroom in case of brownouts, though (especially because it's an LDO regulator -- that's essential in a 135V supply!).

At the moment I'm generating 150kHz and dividing it down to a 15kHz sine wave. (My all-analog, el-cheapo audio signal generator just wasn't up to snuff in the drift department).

If I didn't want an excuse to mess around, or if it made a difference in how quickly I could get my work done, I'd just buy some modern plastic- encased synthesizer that works from DC to light and can be controlled over USB.

Oh. I was going to make you an offer, but my generator doesn't go below

John S