Hi,
a friend of mine want's the old VOX REPEATER circuit in his guitar (alla teardrop Vox guitars)... and it seems that the 2N2646 can't be replaced very easy with a PNP or NPN transistor
coudl someone explain me what type of transistor is, and how could i replace it?
small bear has them.
they are under the diode category.
they are unijunction transistors.
The 2N2646 can be replaced with the much more available 2N6027 PUT (Programmable Unijunction Transistor). The circuit is a little different, but has the same number of resistors and capacitors. You're undoubtedly doing this to make a Vox Repeat Percussion; I've done these with the PUT in both the pedal form and the version that's in the old Thomas Vox Beatle amp. They work.
Upon thinking about it, I realized I needed to provide more info.
The UniJunction Transistor has two emitters, E1 and E2, and a base, B. The base B is connected to the junction of a resistor pullup to the power supply and a cap go ground. The emitters have resistors in series with them. How it works is that the base junction is off, reverse biased, until the base voltage ramps up above the voltage in the middle of the channel between the two emitters. The resistances between the base and E1 and the base and E2 and the resistors in series with the emitters make the voltage where the base will start conducting be a voltage-divider fraction of the power supply. When the base gets high enough, it conducts, and this current causes the lower emitter (E2) resistance to drop, so the base conducts until it almost empties the capacitor. When it quits conducting, the channel resistance comes back up, the resistance to E2 is reestablished, and the cap starts charging again.
The PUT does the same thing, but in a different way. It is a latching thryristor like an SCR, but it starts conducting when its gate is more negative than the anode. So to make this act like a UJT, you tie the base to a fixed voltage divider to set the trigger voltage. The anode is tied to a resistor/capacitor like the base of the UJT, and the anode to ground, or nearly so. The anode ramps up as the capacitor charges until it gets high enough to conduct into the gate voltage. The gate current turns on the thyristor/SCR action, and the cap is discharged through the anode/cathode until the current drops below the holding current, which is quite small.
So if you connect the anode of a PUT where the base of a UJT was, the cathode of the PUT where the E2 used to be, and connect the gate of a PUT to resistor divider to ground, the resulting circuit acts very much like the UJT used to. The gate is not the exact equal of the UJT's E1, so the circuit is different there.
Quote from: R.G. on August 13, 2010, 08:27:10 PM
Upon thinking about it, I realized I needed to provide more info.
The UniJunction Transistor has two emitters, E1 and E2, and a base, B. The base B is connected to the junction of a resistor pullup to the power supply and a cap go ground. The emitters have resistors in series with them. How it works is that the base junction is off, reverse biased, until the base voltage ramps up above the voltage in the middle of the channel between the two emitters. The resistances between the base and E1 and the base and E2 and the resistors in series with the emitters make the voltage where the base will start conducting be a voltage-divider fraction of the power supply. When the base gets high enough, it conducts, and this current causes the lower emitter (E2) resistance to drop, so the base conducts until it almost empties the capacitor. When it quits conducting, the channel resistance comes back up, the resistance to E2 is reestablished, and the cap starts charging again.
The PUT does the same thing, but in a different way. It is a latching thryristor like an SCR, but it starts conducting when its gate is more negative than the anode. So to make this act like a UJT, you tie the base to a fixed voltage divider to set the trigger voltage. The anode is tied to a resistor/capacitor like the base of the UJT, and the anode to ground, or nearly so. The anode ramps up as the capacitor charges until it gets high enough to conduct into the gate voltage. The gate current turns on the thyristor/SCR action, and the cap is discharged through the anode/cathode until the current drops below the holding current, which is quite small.
So if you connect the anode of a PUT where the base of a UJT was, the cathode of the PUT where the E2 used to be, and connect the gate of a PUT to resistor divider to ground, the resulting circuit acts very much like the UJT used to. The gate is not the exact equal of the UJT's E1, so the circuit is different there.
Hello, Im really sorry to bother you by digging up some 11 year old thread and making possibly dumb questions, but just now I saw this and I didnt understand much, so I have some questions if that´s ok :) . In the beggining you said that the UJT has 2 emmiters and 1 base, but all I´m seeing online, and more specifically in this scheamtic (http://effectslayouts.blogspot.com/2015/10/earthquaker-devices-hummingbird.html) points to it having 2 bases and only one emmiter. You also said the PUT´s anode is tied to a resistor/capacitor, and then again that the anode is tied to ground. Which is right? Im really confused about the UJT pinout thing because you said we should connect the PUT´s anode to where the UJT´s base was, and etc., but if it turns out the UJT actually has 2 bases, how does the wiring turn out then? Do I assume that E1 and E2 are just B1 and B2 in reality?
You also said to connect the base of the PUT to a fixed voltage divider to set the trigger voltage. Is "base" the same as "gate" for the PUT? This brings us to the next point, which is where I really got confused. If our voltage divider will be sending in the current to the gate, and the voltage after the voltage divider being the tirgger voltage, why does the gate of the PUT go "to resistor divider to ground"? shouldnt it be receiving the current that passes through the voltage divider to teh gate, making there be less voltage at the gate than at the anode, so it starts conducting?
Im also really lost about how to get this voltage divider to work. Should I just add it to the circuit and let the rest inatct, or should I (Im really making guesses here) replace it for R12 in the schamtic I linked above/R10 in the Vox repeat Percussion schematic?
Also, sorry if Im pushing it here, but what results am I expecting with the voltage divider? should the voltage at the gate just become any number less than the voltage at the anode, or do I need a specific number? I have no multimeter yet and my breadboard is not with, so I dont really have a way right now of checking this out (even if I knew how lol)... so I´d really appreciate any info you could help me with!!! :)))
Thanks in advance and again sorry to be bothering you with these kinds of questions so many years after this post
I'll let RG fend off the question but if you can find a copy of the GE Transistor Handbook Manual it does a good job of explaining UJT's. There's different editions of the handbook and I'm not sure which edition the UJTs first appeared. [Seems like Ed 3 or later has UJT's]
FYI, from an electronics industry perspective the 2N2646 was largely replaced by the NE555 timer. Not a plug-in replacement but more a functional replacement.
i have wanted to build a tremolo like the repeat percussion for a while now, but with my own weird and wacky design features, does anyone know the amplitude of the oscillator?
QuoteFYI, from an electronics industry perspective the 2N2646 was largely replaced by the NE555 timer. Not plug-in replacement but more a functional replacement.
is that a chalenge to build an UglyFace based on the 2N2646 ?
cheers, Iain
Quotei have wanted to build a tremolo like the repeat percussion for a while now, but with my own weird and wacky design features, does anyone know the amplitude of the oscillator?
The emitter swings from VE_min to VE_max.
VE_min = (1/2) VEsat
VE_max = n VBB + VD
where device parameters are
VEsat = 3.5V, n = 0.65, RBB = 7k ohm, VD = 0.7V
VBB is more or less the supply voltage but we might do a bit better. On the RP ckt there is 1k + 470 to the 9V supply, that forms a divider with RBB so you end-up with VBB = 9 * 7 / (7 + 1 + 0.47) = 0.826 * 9 = 7.43V.
That gives a humble estimate for the emitter voltage of,
VE_min = 3.5 / 2 = 1.75V
VE_max = 0.65*7.43 + 0.7 = 5.53V
I suspect the 82K + 330k divider on the base of the transistor is about getting the base voltage below 0.55 to 0.6V when the UJT emitter is at 1.75V. 1.75 * 82/(82 + 330) = 0.2 * 1.75 = 0.35V.
It's worth someone else having a stab at an estimate.
Typically the pulses on UJTs are quite narrow.
Quoteis that a chalenge to build an UglyFace based on the 2N2646 ?
The replacement path is often only one way but in the bad old days you will find people doing innovative stuff with the parts they had.
Quote from: Rob Strand on February 17, 2021, 06:06:53 PM
I'll let RG fend off the question but if you can find a copy of the GE Transistor Handbook Manual it does a good job of explaining UJT's. There's different editions of the handbook and I'm not sure which edition the UJTs first appeared. [Seems like Ed 3 or later has UJT's]
FYI, from an electronics industry perspective the 2N2646 was largely replaced by the NE555 timer. Not a plug-in replacement but more a functional replacement.
Thank you so much!!!!! I´ll definitley look for that manual, really lookinf forward to it!! Also thank you for the heads up about the NE555, I´m not too good with the soldering iron so I´ve been trying to stay away from ICs of any kind (also beacause I havent done any research on them so I don´t really know what I´m dealing with lol) but it´s definitely something to think about.
Thanks again!!!
so the saw teeth ate 3.8v pkpk, thank you so much for that!!!
i think ill implement a triangle oscillator that has 2 controls, rise time and fall time. maybe even a gain and bias control so i can clip the wave from and dial in the asymmetry of that clipping
cheers, Iain
Quote from: Rob Strand on February 17, 2021, 06:06:53 PM....if you can find a copy of the GE Transistor Manual it does a good job of explaining UJT's.
http://www.introni.it/pdf/GE%20-%20Transistor%20Manual%201964.pdf 13MB PDF
Page 300
This is pure original UJT. The PUT came later after G.E. quit being useful.
The now-obsolete-too TIS43 in a TO92 package can be substituted for the 2N2646 pin for pin. It may be more available. There is at least one commercial pedal maker using the TIS43.
The 2N6027 PUT works, but it does require some wrangling to set the gate threshold right. Somewhere in the archives I have example circuits for how to apply the 6027 to the Vox repeater circuit. Not too tough - adjusted resistor values and one additional resistor.
All PUT/UJT circuits rely on the slow discharge of a capacitor, then the sudden dumping of that cap through the device. The capacitor voltage is an quasi-sawtooth (that is, a resistor-capacitor charging waveform) and a sudden, sharp return to nearly zero when the control electrode trips.
QuoteThe now-obsolete-too TIS43 in a TO92 package can be substituted for the 2N2646 pin for pin. It may be more available. There is at least one commercial pedal maker using the TIS43.
I pulled quite a few TIS43's out of industrial equipment as a kid. I can vouch that they are pretty much plug-in replacements.
In passing I've seen some criminal prices for the 2N2646's. The TIS43's plastic TO92 package might fend off some vintage dollars.
Quoteplastic TO92 package might fend off some vintage dollars.
i have noticed that! just before covid, i was designing someone a custom fuzz/distortion, and we were swapping a BJT and its collector resistor in and out of sockets to try some flavors, and he kept liking metal can transistors better than plastics, despite both being all over the map sound wise. he even liked the metal can 2n2222 's better than the plastic 2n2222 's despite them sounding nearly identical to me... people seem to listen with their eyes
cheers
Quote from: PRR on February 17, 2021, 09:58:05 PM
Quote from: Rob Strand on February 17, 2021, 06:06:53 PM....if you can find a copy of the GE Transistor Manual it does a good job of explaining UJT's.
http://www.introni.it/pdf/GE%20-%20Transistor%20Manual%201964.pdf 13MB PDF
Page 300
This is pure original UJT. The PUT came later after G.E. quit being useful.
Thank you!!!!! I was actually reading the 5th edition so this probably has some more stuff. :)
Quote from: R.G. on February 18, 2021, 11:40:12 AM
The now-obsolete-too TIS43 in a TO92 package can be substituted for the 2N2646 pin for pin. It may be more available. There is at least one commercial pedal maker using the TIS43.
The 2N6027 PUT works, but it does require some wrangling to set the gate threshold right. Somewhere in the archives I have example circuits for how to apply the 6027 to the Vox repeater circuit. Not too tough - adjusted resistor values and one additional resistor.
All PUT/UJT circuits rely on the slow discharge of a capacitor, then the sudden dumping of that cap through the device. The capacitor voltage is an quasi-sawtooth (that is, a resistor-capacitor charging waveform) and a sudden, sharp return to nearly zero when the control electrode trips.
Thanks so much for answering!!! Saddly TIS43 doesnt seem availiable around me either ://
Could you point me in the direction of said archives please?? I´d be very interesting in reading them ahah, epsecially since my knowledge is yet too small to fully understand what Im reading and how it would apply to making a PUT behave like a UJT. Your remark is really helpful though, I can now see it works like an oscillator of sorts, where the voltage is constantly rising and dropping, as the capacitor slowly fills up and when the voltage at the emitter becomes high enough for it to open, it discharges everything very quickly (is this right????)! If these archives arent something that you published and you just have it stored and cant find it then dont worry, I´ll just try to make do with the help up to this point. Thank you bery much!!!
> said archives please??
R.G. has much material at www.geofex.com Some of the linkage is broken. It will keep you busy a while.
Quote from: PRR on February 18, 2021, 09:48:47 PM
> said archives please??
R.G. has much material at www.geofex.com Some of the linkage is broken. It will keep you busy a while.
Thank you!!! Will definitely take a look :)
While I was thinking about this I did try to whip up something on paper, and just maybe I might have kinda got it right:
(https://i.postimg.cc/mPmF72t6/IMG-20210219-030534.jpg) (https://postimg.cc/mPmF72t6)
Can someone please check if this even makes any sense?? Seems kinda too simple to be right lol. R.G. did mention the resistor values would change and I have no idea what would be the calcaulations for the new one, so I would appreciate any help with that as well, if there´s some formula or something. (This was copied form this schematic - http://effectslayouts.blogspot.com/search?q=tremolo , it´s the bottom left part).
Thanks!!
For the VOX RP, you might be able to build a circuit using normal transistors,
(https://i.postimg.cc/bs7mKP8J/UJT-using-BJT-V11.png) (https://postimg.cc/bs7mKP8J)
The min VE voltage seems a bit low. I'd have to look into changing that.
Surely someone has already done this in the last 50 years?
QuoteCan someone please check if this even makes any sense?? Seems kinda too simple to be right lol. R.G. did mention the resistor values would change and I have no idea what would be the calcaulations for the new one, so I would appreciate any help with that as well, if there´s some formula or something. (This was copied form this schematic - http://effectslayouts.blogspot.com/search?q=tremolo , it´s the bottom left part).
The gate is used as the sense and conduction goes though A to K.
You can see a pic halfway down this page,
https://www.technocrazed.com/7-8-the-unijunction-transistor-ujt
The external resistors R1 and R2 are what programs the trip point. The resistors can be chosen by the user, so that's why the PUT is called programmable. For the UJT the trip point is built into the device and is specified through the parameter 'n', which is normally about 0.65. So to match the UJT you would choose R1+R2 = 7k ohm and you want R1 = 0.35*7k and R2 = 0.65 & 7k. More or less, as there's always a heap of evil finer points in reality.
Quote from: Rob Strand on February 18, 2021, 11:51:40 PM
The gate is used as the sense and conduction goes though A to K.
You can see a pic halfway down this page,
https://www.technocrazed.com/7-8-the-unijunction-transistor-ujt
The external resistors R1 and R2 are what program the trip point. The resistors can be chosen by the user so that's why the PUT is call programmable. For the UJT the trip point is built into the device and is specified through the parameter 'n', which is normally about 0.65. So to match the UJT you would choose R1+R2 = 7k ohm and you want R1 = 0.35*7k and R2 = 0.65 & 7k. More or less, as there's always a heap of evil finer points in reality.
THANK YOU SO MUCH!!!! This completely clears all my doubts!! Damn, I completely butchered that schematic ahah, I had just assumed from before that the gate would replace the emmiter in the UJT and went from there, so the whole thing is backwards.
This will make it all a lot easier, thanks!! That other schematic you showed with the bipolar transistors is also useful, something to think about, I see you replaced the PUT with the NPN and PNP which are basically what's inside of the PUT. This makes it very clear for me!!
FWIW, here is the build info on the VRP clone that I did a few years back...
http://moosapotamus.net/ideas/skippy-tremolo/ (http://moosapotamus.net/ideas/skippy-tremolo/)
Includes info on swapping between the 2N2646 UJT and the 2N6027 PUT.
QuoteIncludes 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
Quote from: Rob Strand on February 19, 2021, 06:16:46 PM
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.
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.
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.
Quote from: rankot on February 20, 2021, 12:15:07 PM
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://i.postimg.cc/tZhKNWG8/2-N2160-42.gif) (https://postimg.cc/tZhKNWG8)
(https://i.postimg.cc/R3GsSd9S/2-N2160-2-42.gif) (https://postimg.cc/R3GsSd9S)
https://worldradiohistory.com/UK/Practical-Electronics/60s/Practical-Electronics-1967-03.pdf 9MB PDF, on PDF page "218".
Thanks PRR, somehow I've missed that. Maybe typo error in my search query?
QuoteCan 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.
Quotehttps://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.
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.
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:
(https://i.postimg.cc/34jdfX8D/Simple-Sitar.png) (https://postimg.cc/34jdfX8D)
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)
QuoteTwo 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,
(https://upload.wikimedia.org/wikipedia/commons/thumb/9/91/Crystal_Earpiece.jpg/1200px-Crystal_Earpiece.jpg)
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)
(https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Dynamic_earpiece.jpg/1200px-Dynamic_earpiece.jpg)
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.
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!)
Quote from: pacealot on February 21, 2021, 02:54:07 AM
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:
(https://i.postimg.cc/34jdfX8D/Simple-Sitar.png) (https://postimg.cc/34jdfX8D)
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?
Quote from: pacealot on February 21, 2021, 02:54:07 AM
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:
(https://i.postimg.cc/34jdfX8D/Simple-Sitar.png) (https://postimg.cc/34jdfX8D)
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
Quoteis 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.
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.
Quote from: Knobby on February 21, 2021, 01:03:36 PM
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.
Quote from: Rob Strand on February 21, 2021, 09:30:40 AM
Quoteis 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.
Quote from: Rob Strand on February 21, 2021, 03:16:28 AM....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)
https://physicsmax.com/wp-content/uploads/2014/08/1522-450x435.jpg
Quoteor does the UJT add something special?
the stomp LFO has straight edges, the analog oscillator has some more curve to it.
cheers
Quote_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.
That's a good call. Those things were fairly available in the day.
We need some frequency shaping to get the thin sitar sound, which must come from the ear pieces. The telephone receiver (the "speakers") aren't where the telephone sound comes from, it's the microphone part. Using one of those ear pieces performs as a microphone might just do it. Butting the two ear pieces together with no damping will add to the effect.
or we find a way to make our won telephone style microphone, based on a sheet wit tension on it, and basically a guitar pickup. we could use a PET sheet and use the mountain banjo method of tensioning it to be drum-like, just add a piece of (ferro)magnetic material, like an iron washer, or simple magnets.
cheers, Iain
EDIT: forgot the apology for derailing the thread and the video link:https://www.youtube.com/watch?v=M5XBbv_A27c (https://www.youtube.com/watch?v=M5XBbv_A27c)
While trying to find info on old receivers (info which has vanished from the web >:( ), I was reminded that the improved Bell 500 came out before the investment in the Bell 302 was fully sunk. There was a back-room industry putting used 302 parts in 500-style housings. The 500 was better on very long lines, but the 302 was fine in town, but the 500 shape was so good people demanded "new phones" and accepted a 302 in 500 clothes.
Worn/abused 302 parts got dumped on the surplus market. Both 302 and 500 receivers (ear-parts) have strictly controlled resonant responses (which would be clear if the internet were not barren). The 302 more peaked than the 500. Damped so the response did not "ring", but put two in series and the Q soars, "ringy/singy".
In this hunt I discovered that the guy who has run Play Things Of Past since before the internet has passed away. He had some very nice telephone bits.
QuoteIn this hunt I discovered that the guy who has run Play Things Of Past since before the internet has passed away. He had some very nice telephone bits.
I found this UK version, which is from GE/GEC, but it would be nice to compare it against the Bell version. We had STC parts here in au but I don't trust my memory. I know I got some ear pieces from AWA equipment but I can't remember if AWA made there own. I had a feeling AWA started to outsource stuff in their later days.
http://www.samhallas.co.uk/repository/documents/gec_subscribers_telephones.pdf
Interesting. Balanced armature is about unknown in US Bell equipment (common pre-1922 'loud'speakers, hearing aids, and now musician in-ear monitors). The response curve in that paper is exactly typical for a BA.
The Bell 500 receiver is not built like the image above. The stationary signal coil is near the rim of the cartridge. The diaphragm rocks on the rim, edge-driven. It is a real bang-up design job and and a truly good transducer within its communication goals. Low distortion, damped intelligible resonances, and incidentally it can induce signal in an external coil (phone-tap, hearing aid).
Instead we have the image below.
(https://i.postimg.cc/BjCCbsk0/image-phone-tails.jpg) (https://postimg.cc/BjCCbsk0)
You might be able to make more sense out of the construction from this,
http://www.samhallas.co.uk/repository/documents/poeej_rocking_armature.pdf
As a kid I pulled a few earpieces apart and a few microphones. I honestly can't remember the fine details but I thought the speaker side had a fixed coil. In the back of my mind I remember two different (but similar) constructions for the speaker and two completely different constructions for the mic. The microphones where very different to the speaker. The earlier mics looked different to the ear-piece, they were bigger and I think they might have been carbon. The later ones looked a lot more like the earpiece except they had more holes. I'm pretty sure those were dynamic.
The later ones were something along these lines,
https://www.royalsignals.org.uk/inserts/rocking_armature_inserts_en.pdf
Note the peak in the mic response around 1.5kHz, like the GEC paper.
When I look at the UK stuff it seems to follow what we had here.