Vox Repeater 2N2646 replacement? (kinda lost)

[moosapotamus]

FWIW, here is the build info on the VRP clone that I did a few years back...

http://moosapotamus.net/ideas/skippy-tremolo/

Includes info on swapping between the 2N2646 UJT and the 2N6027 PUT.

[Rob Strand]

Includes info on swapping between the 2N2646 UJT and the 2N6027 PUT.

Thanks, so it has all been done before!

Interesting you wired the PUT to emulate the UJT with the lower gate resistor going to B1.   Traditionally PUTs wire the lower gate resistor to ground.   I cannot remember why that's done or if it's better/worse/same.

I think the main claim to fame of the PUT is the behaviour is more predictable.  UJT circuits tended to have quite a bit of tolerance and invariably used trimpots.

There's a variation on the BJT version as well where the NPN is wired to ground, essentially giving the UJT 4-wires.  It doesn't look technically correct but it may or may not work better.   To me this version will have a very narrow pulse which is not dependent on RB1, unllike the UJT

[nothing about keeping the lower transistor gain low]
https://worldradiohistory.com/hd2/IDX-Consumer/Archive-Electronics-World-IDX/IDX/60s/1969/Electronics-World-1969-06-OCR-Page-0040.pdf
https://worldradiohistory.com/hd2/IDX-Consumer/Archive-Electronics-World-IDX/IDX/60s/1969/Electronics-World-1969-06-OCR-Page-0060.pdf

[moosapotamus]

Thanks, so it has all been done before!

Interesting you wired the PUT to emulate the UJT with the lower gate resistor going to B1.   Traditionally PUTs wire the lower gate resistor to ground.   I cannot remember why that's done or if it's better/worse/same.

Honestly, I couldn't tell you either. But, thanks to R.G. for helping me work that out.

[rankot]

Can I use 2N2160 for this? There's one available for cheap in my local store, but I can't find a datasheet for it.

[rankot]

There's readily available PCB for this here: https://shop.pedalparts.co.uk/Repeater_Vox_Repeat_Percussion/p847124_9673280.aspx, and i has the PUT option, too.

[PRR]

Can I use 2N2160 for this? There's one available for cheap in my local store, but I can't find a datasheet for it.

??

First hit out of Google:
https://www.datasheets360.com/pdf/1620933607216099197
https://4donline.ihs.com/images/VipMasterIC/IC/TXII/TXIID129/TXIID129-4-86.pdf?hkey=FB3F1F3F2A09A989A6BF9D772C3B8264
61kB PDF file




https://worldradiohistory.com/UK/Practical-Electronics/60s/Practical-Electronics-1967-03.pdf  9MB PDF, on PDF page "218".

[rankot]

Thanks PRR, somehow I've missed that. Maybe typo error in my search query?

[Rob Strand]

Can I use 2N2160 for this? There's one available for cheap in my local store, but I can't find a datasheet for it.

You would be pretty safe to chose a unijunction with a 'n' value which is close to the 2N2646, which it looks like is the case.
These things aren't precise devices.

https://worldradiohistory.com/UK/Practical-Electronics/60s/Practical-Electronics-1967-03.pdf  9MB PDF, on PDF page "218".

The guy on the cover looks like a drunked Rudy Giuliani stumbling into a hotel room.

[mozz]

I've found a 2n2646 here mixed in with a bunch of diodes and misc. It's a metal can GE. Test like two diodes on my DCA55 and MK328 testers. OP is welcome to it if still needed.

[pacealot]

Okay, completely OT, but I feel strongly compelled to call attention to the "Simple Sitar" circuit on page 192 of the Practical Electronics issue above:



Two headphones taped together!!! Brilliant? Insane? Why not both....

(and to make at least a perfunctory attempt to return to topic, you could run it into the repeat percussion—or vice versa—to really mess with the fabric of space-time)

[Rob Strand]

Two headphones taped together!!! Brilliant? Insane? Why not both....

I saw that too and thought exactly the same thing.
Some of that old DIY stuff had some very clever ideas.


One thing though.  In this era I suspect "ear pieces" could have meant Crystal ear pieces like this,



These things have a very thin, tinny sound.  Anyone who grew up in this ear would know exactly what I mean.  I suspect that sound is what is being used to get the sitar sound.   IIRC they were quite high impedance.

In the 70's these got replaced with dynamic ear pieces.  (you could still buy both)



The dynamic ones sounded a bit better and off hand I think they were 8 ohms but don't quote me on that.


This pics are very representative of the look of each type, right down to the cables.

[pacealot]

I grew up a bit later than the era they were common in, but I do still remember them! I had at least one hand-me-down transistor radio (ciggie-packet size) that came with one. In fact, I think I might even still have one or two of those earpieces in a box somewhere. Might be worth digging around to find them just to try this! (But maybe not to use up three OC72s on it!)

[duck_arse]

Okay, completely OT, but I feel strongly compelled to call attention to the "Simple Sitar" circuit on page 192 of the Practical Electronics issue above:



Two headphones taped together!!! Brilliant? Insane? Why not both....

(and to make at least a perfunctory attempt to return to topic, you could run it into the repeat percussion—or vice versa—to really mess with the fabric of space-time)

is a crystal earpiece going to provide a good collector load for TR3?

[iainpunk]

Okay, completely OT, but I feel strongly compelled to call attention to the "Simple Sitar" circuit on page 192 of the Practical Electronics issue above:



Two headphones taped together!!! Brilliant? Insane? Why not both...

been there, done that, but with shitty, 1-inch plastic 32ohm speakers instead. the powered speaker was cut to produce the kinks's ''you really got me'' vibe, but the output was more crossover distortion instead, which was really disappointing, not having the kinks's sound.
I'll do some domestic archaeology in my bedroom to find the foam-protected speaker pack, breadboard a new driver and output amplifier and make some sound recordings with my old phone.

sorry for the derailment.

cheers, Iain

[Rob Strand]

is a crystal earpiece going to provide a good collector load for TR3?

Good point.   Now you mention it I don't know if it works as intended with a crystal earpiece. 

Looking at the biasing, Q3 is DC coupled to Q2 but Q1 is AC coupled to Q1.
Since the base resistor of Q2 is low Q2 is likely to be on and means Q3 will be off.
So a dynamic speaker probably won't pull heap of current in the idle state.
A crystal load will probably work also. 

The output is going to be very on/off.

With a full signal there's going to be about 4.5V average across the load, so that's 0.5A with 8 ohm, which might not be so great.
A 32 ohm will pull that down to a more sensible figure.   Thinkig back there were 2000 ohm ear pieces and headphones around, often used with crystal sets.

So yeah, not so obvious.

[Knobby]

Can you get the same (Vox Repeat Percussion) tremolo effect using, for example, a conventional tremolo circuit with a STOMPLFO chip providing the necessary sawtooth waveform, or does the UJT add something special? Been thinking I could really do with a decent tremolo, and, being a Spacemen 3 fan, I do love the Vox sound, but it's a bit of a one-trick pony.

[kaycee]

Can you get the same (Vox Repeat Percussion) tremolo effect using, for example, a conventional tremolo circuit with a STOMPLFO chip providing the necessary sawtooth waveform, or does the UJT add something special? Been thinking I could really do with a decent tremolo, and, being a Spacemen 3 fan, I do love the Vox sound, but it's a bit of a one-trick pony.

If you build Charlie's 'Skippy' with the depth and tone and speed switches it does much more. It's been my only tremolo for years and I play many types of music. No other Trem I have built covers the ground of the Repeater, I love it.

[Axldeziak]

is a crystal earpiece going to provide a good collector load for TR3?

Good point.   Now you mention it I don't know if it works as intended with a crystal earpiece. 

Looking at the biasing, Q3 is DC coupled to Q2 but Q1 is AC coupled to Q1.
Since the base resistor of Q2 is low Q2 is likely to be on and means Q3 will be off.
So a dynamic speaker probably won't pull heap of current in the idle state.
A crystal load will probably work also. 

The output is going to be very on/off.

With a full signal there's going to be about 4.5V average across the load, so that's 0.5A with 8 ohm, which might not be so great.
A 32 ohm will pull that down to a more sensible figure.   Thinkig back there were 2000 ohm ear pieces and headphones around, often used with crystal sets.

So yeah, not so obvious.

In 1958 Koss introduced the first stereo headphones, the SP-3, In '60  the Sp-3X. They were 3 1/2' at 4 ohm speakers. In '62 they released the PRO/4 which was 250 ohms (They list the 1970 PRO4AA as being 250 ohm, 10-25,000 Hz, and a sensitivity of 95dB SPL/1mW.) I don't think it would be too far off the mark to consider someone having a trashed pair in '67 to recycle as an audio project.
I guess you'd have to try out multiple types of headphone speakers (or maybe even telephone type) fitting the time period until you hit across something that would work.

[PRR]

....In this era I suspect "ear pieces" could have meant Crystal ear pieces like this,

As said, violates DC conditions.

_I_ had assumed telephone receivers (earpieces). That's what the drawing suggests. The 100-200 Ohm impedance is a fair fit for a 3V system. Internally they are a fixed magnet and coil with moving iron diaphragm. Reversible.


https://physicsmax.com/wp-content/uploads/2014/08/1522-450x435.jpg

[iainpunk]

or does the UJT add something special?

the stomp LFO has straight edges, the analog oscillator has some more curve to it.

cheers