TS-23 Tube VCO help

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Ceramic Pasty
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TS-23 Tube VCO help

Post by Ceramic Pasty » Sun Jul 14, 2019 9:09 am

So about a year ago I attempted to build the dual vacuum tube vco that Eric Metasonix had put up on the old CGS site.
TS-23 (Wayback archive)

I tried doing it point to point and couldn't get it to work, assuming bad wiring I left it alone while I finished university. Now I've got some spare time I thought I'd have another stab at it, this time using some tagboards I had from another project. I've attached my wiring layout on the tagboard as a picture here: TS-23 Tagboard Layout
(Heaters are DC)

Unfortunately, I still cannot get an output from it.

I've checked:
- The power regulation,
VR1 = 100V,
VR2 = 100V and
VP3 = 120V as expected.

The two 2D21 tubes (V2,V3) both heat up as expected, I can see them glow and feel the heat.
The 6BN6 does not warm up at all, I've tried two of them and get the same issue.
[EDIT: Incorrect]Swapping a 2D21 into the 6BN6 socket results in the 2D21 heating up so I think I've ruled out the wiring.
I checked the voltage at the socket at it's 5.9V with the two tubes connected. If I put a third 2D21 into this socket, it does not heat up. The heaters are connected in parallel.

There is nothing at the output, no matter what settings are selected via the controls. I've checked the jack wiring and there is continuity as expected.

Even if the 6BN6 was not working, I think the two oscillators should be producing a signal if the BN6 clipper is switched out.

Apart from that, I'm not really sure what to check next. Maybe there's something fundamental to tube design I'm missing? I don't want to fail twice so any help would be appreciated! :lol:

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The Peasant
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Post by The Peasant » Mon Jul 15, 2019 11:19 am

Have you measured the filament voltage at all of the tubes when they are all installed? Do the oscillators function when the 6BN6 is not in it's socket?

5.9V is a little low for the filaments but should be high enough to make them work.

Take care,
Doug
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Ceramic Pasty
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Post by Ceramic Pasty » Mon Jul 15, 2019 7:05 pm

Thanks for the reply!

I adjusted the power supply heater voltage up to 6.2V. Checking each of the heater connectors I measured 6-6.2V on each of the socket heater lugs with the tubes installed. No change. The two 2D21s glow and get hot as before but the third tube (6BN6) does not heat up.

The 2D21s do not oscillate with or without the 6BN6 installed, no sound, hum or noise at all at the jack socket. Voltages look fine at the pots and switches but I need to trace the output
When I checked continuity on the 6BN6 heater wiring, it was hard to get a clear connection so maybe the pins need cleaning. I'll try cleaning all of the tube pins and give it another go tomorrow.

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The Peasant
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Post by The Peasant » Mon Jul 15, 2019 8:23 pm

Don't worry about the 6BN6 until you get the thyratrons to oscillate, just leave it out for now. Do you have an oscilloscope?

Take care,
Doug
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The Peasant
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Post by The Peasant » Mon Jul 15, 2019 10:08 pm

Are you using the OP amp CV circuit? The voltage at the CV point before the 10k resistors should be somewhere between about +/- 6 volts and should vary between that depending upon the actual CV input that you have connected and the CV gain setting.

Do you have any voltage at point A in the V2 circuit?

Make sure that the out/in switch is in the out position and the output volume is not at zero. Also make sure that the sync switch is turned off.

As long as you are getting voltage through the tune potentiometer and the 10k resistor to pin 6 then V2 should oscillate. Make sure that the 4k7 resistor is connected correctly between pin 2 and pin 4.

Let me know what you find and we can go from there.

Take care,
Doug
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Ceramic Pasty
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Post by Ceramic Pasty » Tue Jul 16, 2019 7:03 pm

Thanks for the suggestions!
I have checked the pot and the 10k resistor, the expected voltages are there (around 94-100V iirc).
I have the cv circuit built but I just tied that to ground to test. I stimulated the cv section in ltspice and when the cv jack socket is grounded it shows 0.02 fV. Is that not effectively ground?
I get the same results with it connected or disconnected. I don't have a dual supply built for it yet (using smps for hv and heater) but will do that this weekend.

Everything else (switch positions/sync/pots etc) was tried, as you suggested but didn't help.

I'm away for work till Friday now but I'll be getting back to it this weekend, thanks again for your help! :sb:

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Post by The Peasant » Tue Jul 16, 2019 10:58 pm

The expected voltage at point A should be 100V when the tune pot is set to 0 ohms, but otherwise it should be changing with the tube oscillation, not a steady DC voltage. Checking here with an oscilloscope would confirm this and would be very helpful for troubleshooting. Measure the actual anode voltage of the tube at pin 6 with the tune pot at both extremes, if it is always close to 100V without oscillation then the tube is not firing, possibly due to an incorrect grid voltage.

You need to be able to vary the CV input in order to make the circuit work correctly, it may not oscillate at 0 volts. I have no idea if your SMPS is contributing to the problem as well, or if a DC filament is appropriate for this project. If you are not using the circuit as shown in the schematic that makes it really hard to troubleshoot, I had assumed that you had built it as shown. It is always best to follow a circuit as posted first, and then make other changes once you get it working, otherwise unexpected behaviour may occur and it is very difficult to determine which change is causing your problems.

The thyratron oscillator shown here:
http://www.synthpanel.com/tube/vco.html
is a lot simpler and would be a much better choice for a first effort for someone without experience with vacuum tube circuits.

Take care,
Doug
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Ceramic Pasty
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Post by Ceramic Pasty » Sat Jul 20, 2019 6:15 am

I've discovered the problem and (rather embarrassingly) I've wired the sockets backwards... I looked at the pinout on the datasheet and assumed it was top-down like an IC.

I only noticed when I connected the CV circuit up and found the heater voltage on what I thought was pin 5 :deadbanana:
If I had bothered to test the heater circuit before connecting up everything else I could have saved myself a large nuisance, lesson learnt!

Hopefully it hasn't wrecked anything else in the circuit, I've ordered some new bases and I'll tackle the rewire next week.

Thank you again for your advice!

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Post by Ceramic Pasty » Sat Jul 27, 2019 6:25 pm

[video][/video]

All working! Needs the cv driver and I might drop the summing resistor after vco2 to boost its volume. Haven't matched the thyratrons yet, I've got 6 to try.

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MARK27
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Post by MARK27 » Fri Aug 02, 2019 11:36 am

Beautiful! Gotta love that thyratron sound.

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Post by electricanada » Sun Aug 04, 2019 7:01 pm

Sounds amazing. Why’d Eric stop making those?
Eléctrica (electric) Nāda (the yoga of sound).

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metasonix
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Re: TS-23 Tube VCO help

Post by metasonix » Fri May 22, 2020 5:50 pm

I missed this thread the first time.

Yes, tube pinouts are numbered and shown from the BOTTOM, not like ICs. Tubes suck, okay? They do amazing things but hate you along the way. You were wise to use a switching supply to make the plate voltage. BTW I've discovered that Mouser now sells the Recom series of HV DC-DC converters. This one is perfect for running tube synth circuits--it takes 12v dc and makes adjustable 55 to 135v at up to 50 mA. They also have a 15v input version.

https://www.mouser.com/ProductDetail/RE ... QGlG7sc%3D

We stopped making the TS rackmount things because they were agony to make. Too much hand wiring. If you want a thyratron VCO get our new RK7 instead. (And DO NOT build the circuit the Peasant shows above. Driving the thyratron CV with a cathode follower does not work very well. Drive it with an opamp driver. You will be happier.)

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Ceramic Pasty
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Re: TS-23 Tube VCO help

Post by Ceramic Pasty » Sat May 23, 2020 6:00 am

metasonix wrote:
Fri May 22, 2020 5:50 pm
I missed this thread the first time.

Yes, tube pinouts are numbered and shown from the BOTTOM, not like ICs. Tubes suck, okay? They do amazing things but hate you along the way. You were wise to use a switching supply to make the plate voltage. BTW I've discovered that Mouser now sells the Recom series of HV DC-DC converters. This one is perfect for running tube synth circuits--it takes 12v dc and makes adjustable 55 to 135v at up to 50 mA. They also have a 15v input version.

https://www.mouser.com/ProductDetail/RE ... QGlG7sc%3D

We stopped making the TS rackmount things because they were agony to make. Too much hand wiring. If you want a thyratron VCO get our new RK7 instead. (And DO NOT build the circuit the Peasant shows above. Driving the thyratron CV with a cathode follower does not work very well. Drive it with an opamp driver. You will be happier.)
Since building this I'm half way through building the TS-22 and I've knocked up a few other tube circuits so consider me hooked :help:

Quick question on the VCO though, I've noticed on my build the 6BN6 clipper has a veerrry fine region where it is useful on the pot. It's either on and working or the oscillator is barely audible.
Is this usual? I was thinking of swapping a smaller value pot in with resistors either side to focus on the usable range.

Nice tip on the recom switcher, the supply I picked up is pretty good for £10 but I suspect the 30mA rating is generous as it gets quite hot.

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