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 Post subject: Help rebiasing my bagpipes to Si!
PostPosted: Aug Mon 23, 2021 9:12 pm 
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No, you havent read the title wrong :mrgreen:
A while ago I stumbled across this circuit through the internet
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Everyday-Electronics-1974-05.pdf [590.54 KiB]
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Since finding it, I have wanted to build it. However, it calls for germanium transistors which are no longer made. I found an NTE replacement for the TIS43s, but I cant find a substitute for the OC71 or OC75. Does anyone have any suggestions for modifications to the circuit that would allow me to use modern Si transistors in place of them? Any help would be appreciated. This forum might not be the best place to discuss this sort of thing, but I feel you would have the best advice on transistor replacements.

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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Tue 24, 2021 2:06 am 
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A gentleman is a fellow who knows how to play the bagpipes, but doesn't!

:) Greg.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Tue 24, 2021 2:12 am 
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2n406 should be a close replacement for the oc75.

Digikey and tube depot lists the oc71n stock.

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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Tue 24, 2021 3:30 am 
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Bagpipes are best heard in Scotland and Scotland is about the right distance.

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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Tue 24, 2021 9:02 am 
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You may get answers that it is ok to substitute in silicon transistors for germanium and they will work, but there are caveats and this circuit would have to be a classic example of when it won't work.

Have a look at the phase shift oscillator TR-2 , the transistor has no base bias resistor, its floating ! If you put a silicon transistor in there it would not run. The circuit relies on it being a germanium and having a moderate amount of collector to base leakage intrinsic to the transistor, this leakage is three orders of magnitude higher than a silicon part. Even with the germanium part it would be better with a large value resistor between the base and collector to bias the transistor so its DC collector voltage is about 1/2 the power supply voltage.

Easier to get germanium audio signal transistors to replace the OC71 and OC75 are parts like the AC125 and AC126.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Tue 24, 2021 3:04 pm 
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Thanks for all the help. After doing some digging, I was able to pull up an old substitution guide that mentions 2SC-series transistors as replacements for the OC-series, and subsequently found that NTE makes replacements for said 2SC-series transistors. (I wonder why NTE doesn't list them as being compatible? :? ) The transistor substitutes both bring up the same transistor however, the NTE103. The 2n406 Dennis mentioned seems to have the NTE102 as an equivalent. Strange...
The circuit isn't super specific about transistors though, and says that similar transistors will usually work. I suppose I will just buy both NTE replacements and see which one works best.

If youre wondering why I am trying to go for new NTE parts instead of just using NOS transistors, its for 2 simple reasons:
1. I generally dont like mixing modern and vintage parts in projects due to weird psychological reasons
2. My dad (yeah, I'm 15) banned me from using ebay for some reason so I'd have a hard time finding a lot of them :mrgreen:

Thank you all for helping me in my quest to create weird music!

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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Tue 24, 2021 9:51 pm 
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I was able to pull up an old substitution guide that mentions 2SC-series transistors as replacements for the OC-series

They didn't get that right, not even close.

The Japanese low frequency PNP transistors which are generally Germaniums and similar to the OC71 etc are the 2SB series.

The 2SA series are generally for radio frequency applications and most of the 2SC & 2SD series are usually Silicon types. The occasional early 2SD part was Germanium, but NPN.

A 2SB71 or 2SB171 is close to an OC71 for example as are many early 2SB types, Look up 2SB types that are equivalent to the AC125 and AC126, these will all work for an OC71.

For early Germanium transistors ebay is essential, and most types are still easy to get.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Wed 25, 2021 1:20 am 
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Most common PNP silicon transistors will work in this circuit, with some changes to the base bias. 2N4403 or something. The output circuit may work as-is.

The PNP oscillator transistor can be biased with a single resistor from collector to base. Shoot for 4-5 V DC at the collector. Try 220k and adjust as required.

See if increasing R15 to 15k from 10k biases TR4 about right.

If you do not already have a stock of resistors, Jameco and Mouser sell kits of 1/4W resistors. You will probably need an assortment to get the chanter tuning right anyway.

The unijunction parts are silicon, but are not as common as they once were. It is possible to make substitutes for these using two resistors and either small SCRs, or a pair of transistors each.

Ted


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Wed 25, 2021 1:53 am 
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The circuit can be redesigned to work with common npn or pnp Si transistors, such as the 2N3904 and 2N3906, and more available programmable UT such as the 2N6027. You can get these genuine PUTs at Allied or Jameco. It shouldn't be very hard to modify the phase shift oscillator and CE amplifier for Si transistors. The PUT allows more flexibility than the UT.

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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Wed 25, 2021 4:46 am 
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bb.odin wrote:
The circuit can be redesigned to work with common npn or pnp Si transistors, such as the 2N3904 and 2N3906, and more available programmable UT such as the 2N6027. You can get these genuine PUTs at Allied or Jameco. It shouldn't be very hard to modify the phase shift oscillator and CE amplifier for Si transistors. The PUT allows more flexibility than the UT.


I agree with all this, but since the OP is 15 years old (they said), even obviously very advanced for this age, still, the highest probability of success for the project, is to stick to the original design as closely as possible, initially at least, with similar components, and once it is working properly, read up later on the differences in bias conditions for Germanium vs Silicon devices and the difference between UJT's and PUT's and how to easily make a PUT from an NPN & PNP transistor. The concept of negative resistance might be more challenging for a beginner.

A very good little book, to help the OP, is the; abc's of unijunction transistors by Courtney Hall, Howard Sams, 1973.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Wed 25, 2021 11:43 am 
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egg wrote:
A gentleman is a fellow who knows how to play the bagpipes, but doesn't!

:) Greg.


Nice...


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Wed 25, 2021 1:33 pm 
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555 timers will do the work of those unijunction transistors.

But yes, once you start adapting, you have to be sure to get the essence of the project. Including the right frequencies.

It's two oscillators putting out narrow pulses, so harmonic rich. Those are the unijunction transistors. One is fixed (or do you broadly adjust it while playing?), and the other is like a simple organ, switches to change frequency.

The outputs of those feed a simple bipolar preamp stage, which acts as a mixer for the two.oscillators.

And the bipolar phase shift oscillator, putting out a fixed sinewave signal, at a low frequency. That modulates the unijunction that has the "keyboard", continuously shifting its frequency by a very slight amount, vibrato.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Thu 26, 2021 11:31 pm 
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I think it would be an excellent experience to construct the project with original style components, including germanium transistors.

If one has any sense of pitch, it would probably be more satisfactory to employ multi-turn trimmer potentiometers for the tuning resistors in the chanter keyboard. This would facilitate tuning, and enable follow-up tuning when necessary. Multi-turn trimmer potentiometers, especially the horizontal rectangular style, with the adjustment screw on one end, were in use at the time the electronic bagpipes project was published. Bourns still make them.

I must agree with AcornValve that eBay is an absolutely essential resource for obtaining vintage and obsolete components. To-day, I easily find components that I was unable to get at major electronics retailers back in the 1960s and 1970s, when the components had been introduced not long before.

In my opinion, a parent who bars the use of an essential supply source to a technically-inclined and talented young adult who wishes to embark upon creative and instructive projects is doing him a grave disservice.

With regard to MBlack’s comments—the fixed oscillator is for the drone. If you listen to bagpipes, you will notice that there is a continuous note that plays throughout the piece. This “drone” is musically akin to a “pedal point” in organ and orchestral music—a note, often in the bass, that is held through changing harmonies.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Fri 27, 2021 2:50 am 
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This brings up visions of the Friends episode with the bagpipes!


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Fri 27, 2021 2:58 am 
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When he gets that one running he can look for a project for an electronic accordion.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Fri 27, 2021 11:49 am 
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I was always impressed the way AC/DC integrated the Bagpipes into Rock music, a good song, Long Way to the Top:

https://www.youtube.com/watch?v=nULs4JW3tPI


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Fri 27, 2021 9:21 pm 
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Try looking for the red hot chili pipers you will either love em or hate em.


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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Sun 29, 2021 1:36 pm 
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ACORNVALVE wrote:
I was always impressed the way AC/DC integrated the Bagpipes into Rock music, a good song, Long Way to the Top

I was blown away by the rendition of The Final Countdown by nine artists from around the world during the lockdown - four of which are bagpipers:
https://www.youtube.com/watch?v=P0Ys_E_dqKc

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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Mon 30, 2021 9:37 pm 
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Greetings, and thanks to all who helped with this. After a few days, I figured it would be good to address any of the concerns you may have had.
ACORNVALVE wrote:
The Japanese low frequency PNP transistors which are generally Germaniums and similar to the OC71 etc are the 2SB series.

Okay, I totally messed that up on my part. But yeah, the replacements I found were 2SB-series, not 2SC. Must've been a typo or something. Thanks for eliminating any confusion this may have caused.
mblack wrote:
555 timers will do the work of those unijunction transistors.

I suppose I could, although the UJT oscillators used originally will emit sawtooth waveforms rather than pulse/rectangular waveforms, and I want to keep it true to the original tonality as possible.
Philip Colston wrote:
I think it would be an excellent experience to construct the project with original style components, including germanium transistors.

If one has any sense of pitch, it would probably be more satisfactory to employ multi-turn trimmer potentiometers for the tuning resistors in the chanter keyboard. This would facilitate tuning, and enable follow-up tuning when necessary. Multi-turn trimmer potentiometers, especially the horizontal rectangular style, with the adjustment screw on one end, were in use at the time the electronic bagpipes project was published. Bourns still make them.

I was originally thinking of using trimmers, as I can imagine that the new UJTs will behave differently than the original. Plus this will allow me to modify the tuning if I so desire.

Once again, thanks to all who helped.

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 Post subject: Re: Help rebiasing my bagpipes to Si!
PostPosted: Aug Mon 30, 2021 9:38 pm 
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ACORNVALVE wrote:
I was always impressed the way AC/DC integrated the Bagpipes into Rock music, a good song, Long Way to the Top:

https://www.youtube.com/watch?v=nULs4JW3tPI

Frankly, this is one of the songs I looked forward to covering the most! :mrgreen:
Another one is the intro to In The Light by Led Zeppelin (because apparently they made prog rock at some point hehe)

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