RCA - Signals / Waveforms etc - "Real" .

Fred, looking over the images over at RCAtheremin.com,  it appears that  (almost) all the lev-built theremins chassis-less. 

Charlie -

You are right! - I never noticed that before - LOL ;-)

And the construction of this theremin Amp-Rat has does look extremely like the Lev built theremins.

I eat my words (at least I said I was no expert - what I didnt say was that I was a complete ignaramus! LOL .. ;-)

Fred.

Another couple of signs are the green wire on the coils, and those capacitors stacked on a Bakelite block screwed to the chassis.

I also learned about this at RCA theremin.com

Yeah -

I have read all the stuff at RCAtheremin.com in the past - but somehow forgot it all! ... I think perhaps my focus on the RCA recently caused me to forget that this was an RCA and not a "Lev - Built" theremin.

Whatever.. This does look like it gould be one of the missing Lev theremins -

What a find! - I am particularly pleased that it has been found by someone extremely competent in electronics and tubes, as it should get the medical care it deserves!

And I have this hope that, in the hands of Amp-Rat some secrets and details will emerge which may otherwise have been missed. I do hope he shares his discoveries with us.. This is the sort of restoration which should be videod and turned into a documentary!

Fred.

Had the unit powered up a couple days ago, briefly. The original cloth power cord had been replaced long ago and miss-wired to boot.  Brought her up on the variac, nice and slow, no smoke, no hum, just the beautiful warm glow of filaments.

Put a 1.5K resistor across the output and one channel of the scope. I observed Theremin action moving my hands near the coils. (antennea not attached). I have high hopes that other than a newer 171A , not much more will be needed.

Chobbs, with your Hammond link, and after reviewing documetation from Andrew & Mike, I brought in a 125E, and am still searching for a JBL M31-4. Though, honestly, I think a Peavey Black Widow 4 ohm may do the job. I plan on also trying  a solid state amp driven from the 125E secondary.

Will have to see how it all sounds.

Also some more pic's. The notation on the tubes and both chassis, is in Lev's hand.   https://picasaweb.google.com/bellaudiolab/Theremin?authuser=0&authkey=Gv1sRgCNS3vJyi_OaeHQ&feat=directlink

Had the unit powered up a couple days ago, briefly. The original cloth power cord had been replaced long ago and miss-wired to boot.  Brought her up on the variac, nice and slow, no smoke, no hum, just the beautiful warm glow of filaments.

Put a 1.5K resistor across the output and one channel of the scope. I observed Theremin action moving my hands near the coils. (antennea not attached). I have high hopes that other than a newer 171A , not much more will be needed.

Chobbs, with your Hammond link, and after reviewing documetation from Andrew & Mike, I brought in a 125E, and am still searching for a JBL M31-4. Though, honestly, I think a Peavey Black Widow 4 ohm may do the job. I plan on also trying  a solid state amp driven from the 125E secondary.

Will have to see how it all sounds.

Also some more pic's. The notation on the tubes and both chassis, is in Lev's hand.   https://picasaweb.google.com/bellaudiolab/Theremin?authuser=0&authkey=Gv1sRgCNS3vJyi_OaeHQ&feat=directlink 

-Amp rat

All very exciting-I'm amazed that you sound so calm:-)
(yes I know it's bad form to rush in and all that...)

"All very exciting-I'm amazed that you sound so calm:-)
(yes I know it's bad form to rush in and all that...)"

Patience and calm is the difference between fixed or fucked when it comes to any repairs or modifications. Especially moreso with vintage gear.

Still looking for a suitable speaker.  The search continues. JBL D130f 4ohm, M31-4, maybe a real RCA 106? even if it's a loaner. If you are local I'll buy you lunch in trade for the loan, plus you can be hear for the first listening.

I dont want to hijack Dewsters thread, so .. From [here ]

"Fred,  have you noticed much of a difference (in performance, range, sensitivity, etc)  between using the different the RCA and Clara-min  antenna-coil - connection-points?    (make sense?)" - Chobbs

Hi Charlie,

Its quite difficult to be sure.. One problem I have at the moment is that I am winding my coils by hand, and have been using various different formers - Yes, there are differences between the grid connected and anode connected antenna topologies, and it does seem like the anode (Clara-min) connected scheme is slightly more sensitive, but at this time I am not sure if this is due to different winding capacitances or whatever.

I have now settled on a former, and am making a board which can be configured for either anode or grid connection (at present I have two completely incompatible boards, and coils for the grid variant are wound on cotton bobbins, while those for the anode variant are wound of the EFD former) - When this is done I can test the different configurations using the same coils and get a repeatable result.

Simulations say that there should be no difference.. But simulation of these sort of circuits are often innacurate.

The Anode connected topology, regardless of whether this increases sensitivity or not, is probably what I will go with - When using FET's, connection of the antenna to the Drain (Anode) is (I feel) a safer option - I never feel comfortable connecting a gate directly to the outside world (even though I add a neon and series resistance to protect it) - sometimes one must, but when there is no need to, I prefer not to!

I also think that the upper winding (between +V and Anode) is likely to be more robust, as it is actively driven, rather than the lower winding which is "driven" by inductive coupling - it just seems to me that in every way, particularly in a transistor implementation, the Clara-min is a better topology.

Funny thing is that I have looked at both the RCA and Clara-min schematics hundreds of times, but having studied the RCA first, had that topology in my brain - never even noticed the anode-antenna connection difference until a few weeks ago (after I had laid out my RCA board and built the first clone - complete with loads of extras which make coil substitution impossible)

Fred.

Very funny, I also  didnt realize that until just recently.  I've stared at those schematics for hours too.  makes me wonder what else I 'think' i know. 

  My experiences seem to fall inline with your simulations:  I've swapped the antenna circuit connection  back and forth and I cant really tell a difference in performance, range etc.   

"My experiences seem to fall inline with your simulations:  I've swapped the antenna circuit connection  back and forth and I cant really tell a difference in performance, range etc." - Chobbs

Thanks for that info!

I cannot see how it could make a difference, my simulations say it wont make a difference, and you have not seen a difference.. Laying out the board to be compatible with both topologies is a pain, and I want to settle on the Clara-min topology anyway..

So you have just saved me the hassle of making my board dual-topology compatible.. Clara-min it is! ;-) (this makes a big difference - I can get rid of the excessive fet protection circuitry if I only go the Clara-min route.. In fact, the effect of this circuitry could well be the cause of the minor differences I have been seeing).

Fred.