Keppinger Theremin Part Deux

Posted: 1/2/2011 11:23:28 AM

From: USA

Joined: 8/25/2009

See below...

Posted: 1/2/2011 3:36:32 PM

From: USA

Joined: 8/25/2009

OK - I made a page on my website here:

Theremin Troubleshooting (

It has the results along with a bunch of pictures of scope traces etc...

It looks like neither the fixed pitch, or volume oscillators are working. I even tried dropping the grid resistance, and increasing the cap values beyond 0.001uF - but this did not start them up.

Values are here:

d) Check the plate voltages of V1, V3 and V5: They should be identical now. Post values here.
V1 Plate : 74.0V DC
V3 plate : 78.3V DC
V5 Plate : 75.1V DC

e) Check the grid voltages of V1, V3 and V5: They should be identical now.Post values here.
V1 Grid : -0.006 V DC
V3 Grid : -0.6 V DC
V5 Grid : -0.3 V DC

Posted: 1/2/2011 5:36:09 PM

From: Colmar, France

Joined: 12/31/2007

OK, thanks for the details on your website. It seems that...

a) the fixed pitch oscillator is only pulled by the variable pitch oscillator through R5/R8 (I really hope that we are based on the same schematics concerning the denomination of the components)

b) at a first glance your 6J5 tubes seem to be more or less - sorry - crap. With identical grid and plate resistors there should not be such differences in the grid voltage (induced by the grid current and the grid resistors which are all identical = 100kOhms). It seems that the actual V5 is the only one which runs more or less inside its tolerance band.

c) Make another differential diagnose: Pull V1 and V3 out. Check if the circuit around V5 is still oscillating. If yes (and that should be the case) put it first in the place of V3 then in the place of V1 and check (still with disconnected antennae and shorted L3/grounded C1) if there is oscillation then in order to make sure that there are no other (wiring or heating) problems.

d) If you find that with this 6J5 tube all 3 oscillators are working and giving about the same amplitude (+/- 15%), throw the others away and buy new ones. But if the fixed pitch and the volume oscillator still weren't working post details. In that case check the previous V1 and V3 tubes in the place of V5 and see if at least the variable pitch oscillator is working correctly with them. Throw those away with which it doesn't oscillate.

Remark: The 6J5 has typically a transconductance of 3mA/V. So a change of 0.3V at the grid should give a change of 0.9mA at the plate which would translate by a difference of 9V with 10kOhm plate resistors, always spoken in DC terms without oscillation... Either the tubes are bad or badly heated (check heater voltage and current 6.3V and 300mA) or there are problems with your wiring or the plate or the grid resistors (in this order of probability).

Posted: 1/2/2011 6:26:41 PM

From: USA

Joined: 8/25/2009

Thanks - it's probably going to take me a while to get to that.

Although maybe I can sneak down to the basement later tonight and start work :)

a) Same schematic - that's the one I've been following and as far as I know the components are OK.

The tubes are NOS, so who knows how good they are - can you recommend a vendor?
Posted: 1/2/2011 8:36:20 PM

From: USA

Joined: 8/25/2009


I just noticed that the volume oscillator grid resistance was 100 OHMS not 100K. That's so easy to do with the meter I have - it puts a little 'K' in the bottom corner that is very easy to miss.

After replacing this I then noticed that I'd wired the coil up wrongly after the last strip down.

Once I corrected the wiring it started working - and actually sounded quite nice!

So the volume is now ticking over at 443Khz and works the VCA very well.

There is still something wrong with the fixed pitch oscillator - it's not putting out the same amplitude as the variable, but it's doing enough of a job to work (fairly) well.

There's still plenty of fizzing and buzzing along with the output, and the note range is a little high for my taste.

But it's actually working. :)

Thanks to Thierry, Philip, and everyone else that have helped me to get this far.

Posted: 1/2/2011 8:37:30 PM

From: USA

Joined: 8/25/2009

Which now makes me wonder if I put a 100K where a 100 ohm should be...

UPDATE : I've been tinkering for about an hour and I managed to tune for a good range of notes with a nice soft timbre.

There is some unpleasant high frequency fuzz that I've not been able to eliminate with tuning alone so I guess that'll be the next challenge!
Posted: 1/3/2011 12:50:31 AM

From: Richmond Hill, Georgia

Joined: 9/18/2005

Wiring errors will get you every time!
Posted: 1/3/2011 8:38:22 AM

From: USA

Joined: 8/25/2009

Philip - do you have any pictures of the scope output from the test point?

It would give me something to aim for!

Posted: 1/5/2011 11:01:58 AM

From: Brooklyn,NY

Joined: 12/1/2009

Hey Steve,
Im glad to hear you've made some progress. It sounds like you're on the home stretch
Good luck.
Posted: 1/5/2011 1:12:47 PM

Joined: 9/12/2009

Did you try Antique Electronics ( for the 6J5 tubes? Or maybe GrooveTube?

You must be logged in to post a reply. Please log in or register for a new account.