RCA THEREMIN

Swapping tubes is what's recommended in the RCA service manual.

RCA Theremin

Even modern tubes have quite a bit of variation between tubes of the same type. I often sort between various 12AX7's in my guitar amp first stage to find the one with the best gain and least microphonics. This always results in a compromise.

All tube type theremins, receivers, and transmitters will drift a bit. Their frequency determining parts are made of metal that expands when heated, and tubes generate quite a bit of heat. Most will settle down in an hour or so. Some take longer, some are faster.

Rob.

Mine is pretty drifty too.  first 5 minutes are unplayable-   takes about 20minutes to  or so  to stabilize -pitch and tone-    .Even then  I will still  have to occasionally tweak the pitch to reset "null"    - Perhaps some of your 24 and 27 tubes are more stable than the others.?

 Congratulations on acquiring an RCA (and having the creepiest avatar image!)   Did you have it restored, or did she arrive in a playable condition?

Sweet I won creepiest avatar image! Im honored.

She came bruised and battered but playable. I'm loving her up though. So going with the manual I swapped the tubes and found a good combination. About 20 mins to warm up and she stays pretty darn steady. I set her and left her playing with a tuner and she was stable for a good 45mins, then I had to leave. I am thoroughly thrilled with it. She sounds beautiful. The 106 speaker is nice. Very warm and full. 

So I guess logically the next question is the 120 tube.

I like to play the staccato and my 120 has a bit of a lag on the heat and cool. Any suggestions as to what brand of 120 has the quickest heat up and cool down cycle so as to obtain a more punchy staccato?

Thanks for all the help you guys and dolls

Staccato + stock RCA = :(

The only way I'm aware of is modification of the volume circuit into something resembling Clara's.

It may be possible to make such a mod reversible, as in no drilling or cutting. Chobbs would know more about this.

More complete analysis of the oscillator, and my latest (hopefully correct) understanding of its operation follows:

:

The above original triode oscillator ran at about 173kHz - this requires a tank inductance of ~ 600uH with tank capacitance of ~ 1000pF, and a small tuning cvariable capacitor was in parallel with C5.

The tank inductance consists of a transformer type coil, each winding having an inductance of ~168uH (wrongly specified as ~ 600uH by previous analysts of the RCA theremin, who believed the design was parallel and that L1:B served only as a "tickler" winding - I nearly became one of this group, and simulated my first Lev oscillator as a standard parallel Armstrong oscillator - it was only Hobbs bugging me and quoting Uncle Howie which caused me to look again..) the combined inductance (seperated by the series tank capacitance C5) is ~600uH

The waveform shows the all important 180 degree phase shift which is due to the coupling of the tank inductances. The oscillator, on first examination, seems to have both inductances grounded at one end - L1:B appears grounded (from AC analysis) through C2, and L1:A is directly grounded (in the case of the original Triode schematic, grounded via parallel 5k0 and 500nF capacitor) - I believe it was this "grounding" at both ends which caused the incorrect assumption that this was a parallel resonant circuit.. and values for the tank components were wrongly calculated to fit this misconception.

However, if one 'drives' the top of this network, one gets the same frequency response as from the actual circuit..

A passive frequency analysis is shown below:

It can be seen that the actual mode of operation is effectively "driving" he same point as the passive schematic above - this is because the fet / triode is pulling current through L1:B which induces current flow through L1:A, C5 sits in the middle with 180 degree voltage phases across it, but with current in phase - and therefore the circuit behaves like, and is, a series resonant circuit.

Below is the oscillator with its gate driven from a swept sine, and the resulting frequency responses - I have also run the simulation with two different inductor coupling values - as expected, this causes a change in resonant frequency:

Fred.

"Staccato + stock RCA = :("

Yeah ...not going to happen and I dont think swapping tubes is going to provide much if any difference in speed.   Clara's uses a different tube entirely and has different circuitry so I do NOT recommend modding your rare antique.

FYI here is a passage from Mark Mckeown's "Restoring the RCA theremin"  

'A Type 31 tube can be substituted for the Type 120 tube but the response time is a little slower. I made a socket adapter and used a Type 3Q4 miniature tube, and it works about as well as the 120 and is easy and cheap to obtain. A Type 01-A tube can be substituted for the Type 171A audio amplifier and actually works better. A 171A is necessary in the volume oscillator because the 01-A will not oscillate.'

maybe a small 'modern' tube can be found w/ a faster response?

Can someone confirm my understanding of the strange volume circuit? It looks like the filament is heated by the induced current from the volume antenna ? Is this correct? - the UX-120 requires 125mA filament current for normal operation - is this kind of current really available from the antenna resonant circuit ? - or is this tube being severely under-driven?

I am not surprised that the volume response is sluggish! - there must be a substantial warm-up / cool down time.. Perhaps it needs a little help - how about strapping a peltier device (or several small ones) to the tube, and get these to heat up / cool down actively in sync with the volume current.. (only joking ;-)

Best solution I can think of (but you wont like it ;-) is to build a MosFet circuit onto a plug you can drop into the tube socket, take the signals from the volume windings into a simple rectifier / filter, derive whatever DC voltages you need from these signals, and get the MosFet to supply the current to V4 via T4.

Other than the history, this is one of the bits where retaining an inefficient antique circuit relic serves absolutely no purpose - using a heated wire to control volume was almost certainly used because, at the time, it was all that could be used..  Replication of this particular bit of the RCA in modern tube theremins is complete absurdity!

Fred.

Quick stab at a circuit - R4 is load to match filament loading if required (probably can be reduced) - Depending on voltage which may depend on R4 a bit, assume > 6V peak, D2 may need to be low Vf diode. C3 integrates rectified volume voltage, RV2 provides discharge for C3, and allows trimming of base voltage, Q2 acts as follower, R5 sets current.

Quick and dirty - PNP could be used instead by inverting D1, this would reduce Vdrop of circuit..

This circuit probably wont work! But I have no doubt that some simple solid state circuit will work as a plug-in replacement which is better, in every way, than the UX-120!

Fred, thanks for your Lev pitch oscillator simulation above, that really helps me understand it!

For the antenna (17.5" long x 0.438" dia from the schematic bits you posted on page 1 of this thread) I'm calculating 6.2pF.  So to resonate with the ~200kHz tank, the linearizing coil would have to be around 100mH, but I'm seeing 20mH on the schematic.

"For the antenna (17.5" long x 0.438" dia from the schematic bits you posted on page 1 of this thread) " - Dewster

Yeah, I pasted these, but I never produced them (Perhaps I shouldnt have used them - they are from Art Harrisons work)..

I dont trust any of the values given - But I also think it extremely unlikely that the antenna has 6.2pF - There are some pretty huge construction "circuit" capacitances - so I expect its probably more in the order of 12pF (?)

Also, the antenna and "concentrator" coils are coupled, which will increase the total antenna inductance..

I suspect the total antenna inductance will be in the order of 70mH, with the antenna having a capacitance > 10pf.. IMO, antenna capacitance lower than 10pF is rare, even with the most carefully constructed theremins - I wouldnt be surprised if you need to add about 8pF to the calculated (based on length and diameter) 6.2pF

Fred.

I have now removed Art's schematic from page one, and replaced it with one I have drawn - Mine correctly shows connection of oscillators to the correct grids.. As per the horrible RCA schematics.

I've also been doing some head scratching on the volume circuit.

The volume antenna coil looks like a step down transformer with a high ratio. With some serious voltage across the primary, I can see the potential for substantial current in the secondary.

The VCA itself looks to be a transformer coupled, single ended triode, with the supply voltage / current to the transformer controlled by the 120 triode heater in a stock RCA, or bias on a dual triode as in Clara's.

Am I close?