Advice Request

Hey guys, thanks for all the advice to date.
There hasn't been as much productivity as I'd like so far but thought I'd give a lil update on what I'm doing.
Basically I'm now planning on making a valve/tube theremin based upon some of the original designs. Also I'm planning on using a 6JH8 sheet beam tube to add a modulation effect if the user so wishes, and maybe a 6BN6 gated beam tube to give an optional distortion to the sound (haven't decided on the last).

just had a meeting with my personal tutor and it has been decided that due to time-constraints and the difficult in parts that I should change my plan to me making a transistor theremin with a valve wave shaper on the output.
my new complication is that I need to research power supplies so that the mains voltage can be dropped down for the theremin to operate but also be at 140V for the valves. obviously, with the inherent dangers this is something I need to get right preferably first time

The transistor oscillators are pretty easy to get working if you use off-the-shelf inductors instead of winding your own.

As for 140Vdc being "dangerous" ... I don't agree. Sure you can get a nasty shock if you find yourself between +140V and GND, but the current is very low in those sort of tube circuits. Just follow the basic safety rule ... don't have both hands in the thing at the same time while working with live power. The reason for it (preventing current flowing through your chest and stopping your heart) is scary sounding, but is not very likely for this sort of a circuit. Still, though, it is better to be safe.

Voltage alone is not "the killer". You can burn yourself very badly shorting out a 12V car battery! In the days of working on old TV sets, the worst "shock hazard" most people experienced was getting zapped by the 25kV and ripping your hand or arm open on some jagged chassis part as you involuntarily jerked away from the source of the shock!

Back to the topic of tubes: http://www.nj7p.org/Tube.php

This is an excellent place to go to find basic data about vacuum tubes, knowing the number stamped on the tube.

Don

thanks don. i've been trying to find info on how i can make a power supply with dropping the mains voltage down to 12V and then back up to 140V. sadly i'm not having much luck on finding info on the latter part of this power supply design. don't suppose anyone knows of any useful sites i may have over looked?
i've been hoping i might be able to find a tranformer with 2 output taps (one for each of the voltages i require), but its currently looking that i'll have to find a way of raising the voltage back up in a seperate stage

Not sure what you are trying to do there. If you just want isolation, use a transformer with a 1:1 turns ratio ... commonly called "an isolation transformer".

Variacs are not isolation transformers, by the way. They are autotransformers ... a single winding with a movable wiper that sets the "tap" at a position of interest.

If you are attempting to have multiple DC outputs ... 12Vdc, 140Vdc ... then you would use a transformer that has either a winding with 3 taps (center-tap, taps at 12.6Vrms above and below center-tap, and beginning and ending connections for 140Vrms) or separate 12.6Vrms and 140Vrms windings. You will also want a separate filament winding (6.3Vrms). That winding should always be separate!

If you can find a copy, the Voltage Regulator Handbook from National Semiconductor is an excellent reference source for basic power supply design.


Don

sorry, I mis-read the post.

Yes, it does look like you'd have to buy 2 transformers unless you want to have one custom wound, or are lucky enough to find what you want at a surplus electronics store.

You might look at some older, non-functioning test equipment at a surplus house. You might find a vacuum tube-based scope calibrator or other piece of odd test gear that you wouldn't otherwise have a use for. They sometimes have multiple windings that might be more like what you want.

Another option would be to look into the various buck-regulator chips that National, TI, Linear Technology (LT) and others have now. You could start with 140Vrms, then use the buck-regulator circuit to drop that down to +12V. LT has some freeware (SwitcherCAD) that will help you design your circuit.


Don

Perhaps...
You could start with a transformer dropping mains to 12V AC.. Feed the 12V AC into the secondary of a seperate transformer with 120V AC primary..
Rectify / regulate the 12V AC from the first transformer to give you low voltage DC..
Rectify / regulate the 120V AC from the 2nd transformer to give you high voltage DC..

All transformers should be isolation type.. 1st transformer must be rated to supply total current for both HV and LV. 1st transformer could have more than 1 secondary if required.

Potential problem: The secondary voltage rating of the 2nd transformer will be greatly exceeded - it is possible (but I think unlikely) that the winding insulation will break down causing catastrophic failure.. So put a fuse between the 1st transformer's output and the 2nd transformers input - this way damage will be restricted to the 2nd transformer.

EDIT> Ignore the above (Potential problem).. it is nonsense.. I was thinking about a different HV circuit.. Voltage into T2 secondary is 12VAC, which is what it should be rated at!

I have done this sort of thing occasionally without problem - but there are no guarantees..

Deleted because out of context - sorry

hey guys, thanks for all the help.
i'm currently using a very basicly constructed test circuit, but need to find out what i can use for a very basic antenna and was wondering if anyone could help?
i tried just using thin solid core wire and 24-strand wire but this showed no change in the signal unless i actually touched the wire. i then thought i'd try using aluminium foil rolled into a tube (VERY basic i know) - this would only make a difference to the signal if a put my finger inside the tube. i'm rather confused as to what i could do to rectify this :-S

[i]"i tried just using thin solid core wire and 24-strand wire but this showed no change in the signal unless i actually touched the wire. i then thought i'd try using aluminium foil rolled into a tube (VERY basic i know) - this would only make a difference to the signal if a put my finger inside the tube. i'm rather confused as to what i could do to rectify this"[/i]

Without having a schematic of your oscillator, it is difficult to give advice.. if you can put this online somewhere and give a link to it, that would help.

It sounds like your oscillators sensitivity to capacitance change on the antenna may be way too low - This could be due to some circuit or build error.. Alternatively, you may be losing sensitivity because the antenna is being shunted by a much larger capacitance than the changes..

You could try removing the antenna and fitting a small capacitance - you could make a small variable capacitor using 2 pieces of unetched circuit board, or foil (with insulator between the sheets) and connect one to your antenna point and one to ground.. moving these sheets over each other will form a crude variable capacitance.. If you get no change in output you know with certainty that you have a circuit problem.
Edit ->
BTW - It might also be as simple as the oscillators being detuned too far apart, so no audio is produced... Need more details about what you are measuring and what you have tried