60 Year Old Tube Instrument Resurrected!

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60 Year Old Tube Instrument Resurrected!

Postby HideawayStudio » Mon Apr 20, 2015 10:52 pm

Almost as a means to psych myself up for the Ondioline restoration which is due later in the summer I decided to have a go on one of its somewhat simpler relatives first.

I have spent the last few days resurrecting what has turned out to be a wonderful little 1950s all tube Jennings Univox model J6. I say "turned out" as although I have always been very curious about these in truth my expectations were always rather low and even feared it might turn out to sound little more than a glorified Stylophone!

Well my fears have been put to rest and after some toil the biggest surprise was returning the thyratron based percussive envelope generator to life... I had half thought this was simply a click generator at first but in fact this little beasty actually sports a crude envelope generator driving a tube VCA circuit with three different decay rates available.

The first hurdle was that when she turned up almost every key top had separated from its wooden base!

Ahh - The Perils of 60 Year Old Wood Glue... Dig the Bizarre Trapezoid Mechanism on Each Key!
Image

A Quick Test Fit of The Glued Black Keys To Determine the State and Alignment of the Contacts..
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The next task was to strip down the PA chassis in the speaker cabinet and recap it prior to attempting to power up the PA in isolation under controlled conditions.

That wasn't a raving success... after some consultation of the schematics and tentatively poking around 250Volt DC laden circuitry with a scope it turned out that the 6BR7 phase splitter tube was not exactly doing anything and very little audio was getting to one side of the push pull amplifier. Luckily I found one of these slightly less easily obtainable tubes in my collection and within seconds the PA was singing loudly again but quickly transpired the original 10" loudspeaker was knackered so I fitted a temporary unit out of a 1967 Revox G36 whilst I wait for a replacement to arrive.

Much better!...

So I turned my attention to the main synth chassis which is built into the keyboard assembly.

Looking Much Happier After Removing 60 Years of Grime Inside - Lots of Nice Mullard Tubes Inside!
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The construction is a little unusual with the tubes mounted horizontally over the top of much of the passive components. This turned out to be a very frustrating layout as each sub-chassis is soldered directly to the wiring harness and held in with tiny wood screws in each corner. I had to remove the smaller left hand chassis to repair the thyratron based envelope generator and decided to fully recap it since it was already out of the unit.

The instrument arrived without the 7 pin umbilical cable that runs between the PA and the keyboard. The connectors are really very difficult to find so I decided to make my own umbilical which turned out quite neatly in the end.

Having completely rebuilt the now glued back together keyboard assembly it was plugged into the synth section and a quick test power up resulted, to my delight, in some life and fueled me to carry on.

By day three much was working again bar the dreaded thyratron circuit. Much of the issues were centered around the usual array of components drifting out of spec with age and badly tarnished switch contacts.

The final hurdle was returning the percussive envelope generator to life which turned out to be mainly a calibration issue now doubt as everything had become way out of kilter due to aging components.

I eventually managed to get the thyratron triggering on each key press and producing some nice looking decay curves but they didn't appear to be going anywhere near the tube VCA. In the end this turned out to be be due to two issues. Firstly the contacts under the tab that enables percussive envelope was stuck in the disable position. Once this was resolved only a very small amount of amplitude modulation was heard. This was traced to a pot that controls the biasing to the VCA. A couple of minor adjustments later with the aid of the scope and envelope control was returned!

The Removed and Recapped 2D21 Thyratron Chassis:
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Hearing the percussive envelope for the first time was a revelation. The few demos and known recordings I'd ever heard of this instrument usually left me thinking this is little more than a portable electronic organ but I have to say this adds a whole dimension to the instrument and with the aid of the sub-oscillator mode and a reverb its possible to create some nice hall plucks with this 60 year old instrument!

Looking So Much Happier For Herself Than When She Turned Up In Bits!
Image

At this point I just had to return to the PA and fit a line output so I could record her directly.

Here is a quick test recording I made shortly after returning the J6 to life...

Last edited by HideawayStudio on Tue Apr 21, 2015 9:29 am, edited 1 time in total.
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Re: 60 Year Old Tube Instrument Resurrected!

Postby Blue Monster 65 » Tue Apr 21, 2015 12:25 am

Very nice! Now you need to haul it over here for the annual Del Shannan Days and do a set of covers!
Muhahahahahahahahaha!
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Re: 60 Year Old Tube Instrument Resurrected!

Postby rhino » Tue Apr 21, 2015 12:45 am

Good work! Gives me incentive to start on the stack of SoloVox's in the corner.
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Re: 60 Year Old Tube Instrument Resurrected!

Postby Walter Ego » Tue Apr 21, 2015 3:23 am

Those tones are beautiful. That would be a gem for someone to record with in a studio. Definitely more than a glorified stylophone. I'm curious how the solovox compares. Demos I've heard have sounded much more raw and not nearly as sonorous as that J6. Anyway, wonderful work for a rare piece of electronic instrument history.
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Re: 60 Year Old Tube Instrument Resurrected!

Postby madtheory » Tue Apr 21, 2015 10:20 am

Beautiful sound.
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Re: 60 Year Old Tube Instrument Resurrected!

Postby seamonkey » Tue Apr 21, 2015 12:39 pm

Very sweet sound, thank you for sharing the demo and restoration process with us.
I love this stuff, and to see and hear them being brought back to life is a true joy.

Thank you again for the time, energy and passion you put into restoring these wonderful pieces of history. :D
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Re: 60 Year Old Tube Instrument Resurrected!

Postby meatballfulton » Tue Apr 21, 2015 11:22 pm

They just don't make thyratrons like they used to :agree:
I listened to Hatfield and the North at Rainbow. They were very wonderful and they made my heart a prisoner.
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Re: 60 Year Old Tube Instrument Resurrected!

Postby Walter Ego » Wed Apr 22, 2015 2:15 pm

meatballfulton wrote:They just don't make thyratrons like they used to :agree:


I think Eric Barbour agrees with you.

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Re: 60 Year Old Tube Instrument Resurrected!

Postby Dr. Phibes » Fri Apr 24, 2015 12:43 am

Kewl! Seen these now and again but had I no idea what they actually sounded like.

Mind if I ask how the thryratron envelope generator works?
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Re: 60 Year Old Tube Instrument Resurrected!

Postby HideawayStudio » Fri Apr 24, 2015 9:54 am

Dr. Phibes wrote:Kewl! Seen these now and again but had I no idea what they actually sounded like.

Mind if I ask how the thryratron envelope generator works?

The 2D21 is a heated miniature thyratron used as a threshold device resetting an RC timer in this case.

Image

A trigger pulse is capacitively coupled to the grid of the thyratron and biased about the trigger set point by a trim pot which is strung between ground and -80V bias rail. In the idle state the thyratron is not in conduction and there is no voltage across the 220K anode resistor (R60). When a trigger pulse is received the thyratron triggers and momentarily goes low impedance which causes a large voltage to develop across the anode resistor. This resets the voltage across C24 and the selected timing components C25 thru C27 on the "circuits" control knob AKA decay rate. As soon as the thyratron returns to its high impedance state the capacitors exponentially return to their normal idle voltage. The resulting exponential curve is applied to the VCA tube which makes up part of the same degenerative volume control as the swell lever.

To my utter amazement the 2D21 tube is still in manufacture to this day!...
http://uk.farnell.com/richardson-electr ... 0000004637
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Re: 60 Year Old Tube Instrument Resurrected!

Postby StepLogik » Fri Apr 24, 2015 4:24 pm

I genuinely love your restoration threads!
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Re: 60 Year Old Tube Instrument Resurrected!

Postby HideawayStudio » Sat Apr 25, 2015 12:02 am

StepLogik wrote:I genuinely love your restoration threads!

:geek: <== one of these through and through I'm afraid...

Will's arrived today which, interestingly, has all the same faults and issues mine had when it originally turned up and Adrian's is due soon too so three in one month!

The infamous Four Voice is on the slab next - a back breakingly heavy monster that one!!
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Re: 60 Year Old Tube Instrument Resurrected!

Postby Dr. Phibes » Sat Apr 25, 2015 12:17 am

HideawayStudio wrote:
Dr. Phibes wrote:Kewl! Seen these now and again but had I no idea what they actually sounded like.

Mind if I ask how the thryratron envelope generator works?

The 2D21 is a heated miniature thyratron used as a threshold device resetting an RC timer in this case.

Image

A trigger pulse is capacitively coupled to the grid of the thyratron and biased about the trigger set point by a trim pot which is strung between ground and -80V bias rail. In the idle state the thyratron is not in conduction and there is no voltage across the 220K anode resistor (R60). When a trigger pulse is received the thyratron triggers and momentarily goes low impedance which causes a large voltage to develop across the anode resistor. This resets the voltage across C24 and the selected timing components C25 thru C27 on the "circuits" control knob AKA decay rate. As soon as the thyratron returns to its high impedance state the capacitors exponentially return to their normal idle voltage. The resulting exponential curve is applied to the VCA tube which makes up part of the same degenerative volume control as the swell lever.

To my utter amazement the 2D21 tube is still in manufacture to this day!...
http://uk.farnell.com/richardson-electr ... 0000004637


Interesting, thanks.
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Re: 60 Year Old Tube Instrument Resurrected!

Postby HideawayStudio » Mon May 04, 2015 12:30 pm

Image

The last couple of weeks have turned into a bit of a blur to say the least - not to mention the collective dust and grime that has been cleaned up...

Two more Univox J6 came in for the same treatment - and it really was too - all three instruments were suffering the same faults ie. failed thyratron percussive envelope generators due to leaky caps and calibration issues, separating wooden keys, leaky caps, misaligned/dirty key contacts, tab switches heavily tarnished and out of alignment and wonky precision resistors in the keyboards.

All three instruments (dating from 1955 to 1959) are now signing nicely now complete with direct output jacks fitted but talk about deja vu by number three!!

I am more convinced than ever that the Univox is poorly known not least because so many are clearly not working to their full potential. What makes it worse is they are so often attributed to famous hits that they were never even used on (prior to contrary belief although the Univox and Clavioline may look very similar they are not as close in design as some suggest and in fact don't even share the same master oscillator waveforms). The oboe and plucked instruments are the highlights of this instrument in my opinion and yet it seems to be best known for its rather cheesy organ type timbres using the sub oscillator.

It makes me grin that this 60 year old box of tricks can also make a percussive bass sound that's kind of reminiscent of the infamous Lately Bass and features a repeat mode just like the one found on the Korg Micro-Preset two decades later.. hard to think that there is 180 years shared between these three instruments!
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Re: 60 Year Old Tube Instrument Resurrected!

Postby metasonix » Mon Feb 15, 2016 4:46 am

HideawayStudio wrote:To my utter amazement the 2D21 tube is still in manufacture to this day!...
http://uk.farnell.com/richardson-electr ... 0000004637

Sorry to dredge up this thread but I had to comment.

The 2D21 IS NOT "in manufacture" and has not been made since the late 1980s. Please believe me. Richardson is notorious for making any claim to get business. Also, that price is an outrage, which is also typical with Richardson. I guarantee any 2D21s they sell are merely relabeled New Old Stock. Because NOS 2D21s are still quite commonplace and easy to purchase, you do not have to pay that price. I would suggest buying them on eBay rather than from Farnell, where they usually sell for less than $5 apiece.
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