RCA THEREMIN

Yes,  speaker and amp have a large impact on the color of the tone, but even when played through a regular amp (with an imp matching TX) the RCA still sounds "different" then say one of my Keppinger tube-based theremins.  Obviously 90+ year old tubes made at the dawn of the radio age are going to perform/sound differently than the "modern" compact tubes-  and both theremins have their own amp stages- but again...I 'd call that 'color'  

    The actual difference in sound is tough to describe.... really subtle and complex--- kind of a "baby bear" mix of  buzzy and smooth but also layered- like lot of little voices all coming together.   I always (mentally) equate it to a chorus of cicadas chirp/ droning away.   I "hear" it better in the low end. 

This quality is probably better (audibly) illustrated by Clara's theremin and also in the "Stout Theremin"  -Reid just plays a few notes as a teaser, but whoa..  that thing sounds gorgeous!   Someone who is friendly with Reid need to press him to:   1)   Post/ share some  info on this guy.Schematics? Provance?     and 2) record some more clips!

BTW-   Also Rca's can sound really shitty too!....actually seem to be the default setting (with mine anyway)  Buzz, harsh, and even glitchy.   Took a bit playing "tube swap"  to get her sounding pretty.  I eventually added some variable pots between the pitch osc's and the mixer  this made it more forgiving with most of my tubes.    Notice that both the Stout and Clara's have lots of knobs to tweak- (clara's has )  levels  to mixer, level to preamp, 10p trim cap to alter coupling.  

Id assume that even in a fet version  -the levels at these points will be key factors to finding "that" sound.

I have just found some waveform pictures here http://rcatheremin.com/tone.php 

They dont look anything like the waveforms I have derived from recordings .. If these are true, then the response of the speaker / microphone / recording is changing the waveshapes hugely.. I suspect.

"Also Rca's can sound really shitty....actually seems to be the default setting (with mine anyway)  " Chobbs

I dont understand what you mean by "default setting"  - what do you do to make it sound "not shitty" ? :-|

Fred

Should have said...   " Rca's can sound really shitty, without proper mixer tweaking"

  At first,  with most tubes i tried , the theremin sounded really gritty esp in the low end- definitely not the creamy smooth cello sound I was expecting.  I also had  1 or two tube pairings that sounded really good tonally but there were glitches , the scale was not smooth-  would jump a few tones.

Only One pair of tubes (out of a box of 10+ globe 27 tubes) sounded perfect!

I eventually found that by adding a variable cap to the  top of the mixer ( as i saw on RCAtheremin.com - I cant find it now tho')     I could balance the signal from each tube and dial in the tone.  So  ... at 0 ohms(pot on cap of 24 tube)  the tone is gritty and harsh.  This gets tamed as the resistance is raised but eventually starts to break and gets glitchy .    There is a nice sweet spot in the middle where it is nice and smooth, though has some of the dynamic "buzziness", and the scale in not glitchy.    Also I replaced the 10k resistor (from the other pitch osc to mixer ) with a 15k variable- though I have it  permanently set at 15K.   

Now that is all really interesting.. and this information could certainly be useful in terms of selecting the solid state replacement circuit for these triodes.. so I need to ask some questions: ;-)

1.) with the modified mixer, is it now possible to swap 27 tubes and tune the mixer so that it soungs good with any pair of tubes?

2.) You say: " by adding a variable cap to the  top of the mixer  " - Are you talking about a capacitor from the plate of V2 (I am using the schematic drawn by Art Harrison, Rev.11 3-4-04 modified by Mark McKeown 12-8-05) to ground? and what value was this capacitor ? - or did you mean a variable resistor - not really sure as you are talking about a '0 ohms - pot on cap'

3.) The 10k changed to 15k pot - presumably this is R16 (?) 15k gives a smoother tone? You see, this is somewhat mysterious to me - I dont actually understand the function of this resistor, as it connects to the grid which, as I understand things, is extremely high z.. the maximum combined g-c and g-p capacitances are less than 8pF - so the roll-off effects are going to be -3db somewhere above 1.2Mhz... In terms of harmonics, changing this resistor by a few k - well, I cannot see how it could have any effect.. it would reduce the 7th harmonic by perhaps 3db rather than by 1.6db with 10k fitted - have you tried increasing this resistance to even higher?

I am, I think, a little out of my depth.. Need some help from people who not only understand valve theory, but (perhaps more importantly) have a "hands on" feel for those damn evacuated test tubes with bits of metal in them! ;-)

Fred.

1)  pretty much- some tube are better than others, some are really glitchy-  but I imagine it would be possible .... possibly with larger pot values?- like clara's 

2) oops I meant:   a variable resistor to the  control grid cap of the 24 tube-  this simple 'mod' was posted on RCAtherein.com

3) yes...  R16  10K changed to 15K      (sorry, didnt have the schematic handy.) No I havent gone higher. I stuck with the more orthodox RCA adjustments.... ( Later RCA's were actually  factory calibrated and some have higher or lower values for r16, - 15k seems fairly common.).... but as I said above Clara's has a something like  50k and 25k...

Also probably varies a bit with the wide tolerance range  w/different 24 tubes

I dont know what it is actually going on , but by experimenting....I feel that that R16 and the other 15K pot (R16.5?) control the 'amount' of signal from their respective pitch osc.  When the proper ratio of fixed pitch and variable pitch signal mixes in the 24..... Voila.

1)  pretty much- some tube are better than others, some are really glitchy-  but I imagine it would be possible .... possibly with larger pot values?- like clara's 

2) oops I meant:   a variable resistor to the  control grid cap of the 24 tube-  this simple 'mod' was posted on RCAtherein.com  ** this is a 2k pot! I just remembered!

3) yes...  R16  10K changed to 15K      (sorry, didnt have the schematic handy.) No I havent gone higher. I stuck with the more orthodox RCA adjustments.... ( Later RCA's were actually  factory calibrated and some have higher or lower values for r16, - 15k seems fairly common.).... but as I said above Clara's has a something like  50k and 25k...

Also probably varies a bit with the wide tolerance range  w/different 24 tubes

I dont know what it is actually going on , but by experimenting....I feel that that R16 and the other 2K pot (R16.5?) control the 'amount' of signal from their respective pitch osc.  When the proper ratio of fixed pitch and variable pitch signal mixes in the 24..... Voila.

 Just found this from rcatheremin.com:

"Briefly, each of the two pitch oscillators sends its output to one of two grids in the 24A, to be equally blended. The screen grid in the 24A is physically much larger than the control grid in that same tube, so in order to equalize the level of the two inputs, RCA placed a 10,000 ohm resistor in line with the screen."

and:

"The Tone Resistor

During the course of production, RCA introduced a way to improve the tone quality by adding a carbon resistor added to the control grid of the 24A mixer tube. At the factory, the optimal value of this resistor was likely determined by trial and error (with measured progress) until a pleasing tone was achieved.

This tone modification is mentioned in the RCA Service Notes troubleshooting section. If your RCA Theremin does not have the resistor, you may find that you can obtain a sweeter tone by adding a resistor in series with the 24A grid cap, in the range of 500 to 2,000 ohms. In other cases where there's already a resistor there, you may find that further improvement can be achieved by experimenting with this resistance.

A non-invasive way to add a resistor or experiment with a number of resistors, is to make an adapter by using a metal grid cap salvaged from a dud tube (soldered to one end of your added resistor), and a spare grid cap clip (soldered to the other end of your new resistor). The new resistor can then simply be inserted between the factory-installed grid cap clip, and the top of the 24A without disturbing the factory solder joint on the original grid cap clip. Changing a factory-installed resistor requires unsoldering the 24A grid cap clip (and/or resistor) from its wire, followed by soldering in the new or replacement resistance and reinstalling the grid cap clip."

w0ttm .. Rob.. I need your help here! ;-)

How the hell does changing the value of a series grid resistor change the mix? I thought the grid was extremely high z - in fact, I know its extremely high z .. I built a VTVM back in 1969 which was based on this fact, had >10M input resistance, and worked..

Anyone ..  Thierry ... What am I missing ??

It should be high z, unless grid current is being drawn, which would not surprise me.

There are some high signal levels here, and driving the grid positive is a distinct possibility.

That changes everything. The load presented to the driving stage will be non linear, and all manner of distortions will happen when driven from a high z source.

My 1000 watt RF amp pulls grid current in normal operation. It uses a pi network on the input to keep the exciter from going nuts.

Grid current is on thing, especially since the screen grid is not biased at +90V as recommended for "proper" operation. Thus it catches all the secondary emission of the plate which makes that varying the series resistor will affect the DC level.

Another thing is the high screen-grid to plate capacitance which is amplified by the fact that there is a phase shift of 180° between both (the Miller effect) which forms a low pass with the grid series resistor. 

Thus the value of this resistor will affect the signal amplitude at the grid AND the bias. I don't remember where I've read it, but someone stated that varying the screen grid resistor had a much more noticeable effect on the output signal than varying the control grid series resistor (not in the RCA but in Clara's and Lucie's Theremins). I tend to see that as a confirmation of my above statements. :-)

While digging in my junk box looking for theremin parts, I found a few good ones.

Vintage tubes.

A pair of 27 triode's, one RCA the other Philco.

3 24A tetrode's from RCA, Philco, and "Super Airline".

And quite a few more that the markings are too faded to read.

I suspect these will come in handy. They have been in storage for about 20 years, and were in storage when I got them. They are probably good.

They were given to me by the same person that gave me my Kustom theremin.