Sola Sound - Tone Bender Mark I (1965)  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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MarkL
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Post by MarkL »

This is a strange post... you have everything set up to go and you're asking the board - is this ok?
No, I'm asking for clarification on conflicting information throughout the thread regarding Q2, and if low leakage on that transistor is okay in this circuit. It's one thing to experiment, and it's another thing to understand. I don't want to just plug-and-play, but to have a sense of theory before putting it into practice, and the thread discussion leaves me confused about the leakage required for Q2.

Please bear in mind that I have limited resources, and I'm just trying to avoid mistakes that I may have to undo and, in the process, do damage to the few parts that I have.

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modman
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Post by modman »

MarkL wrote: 03 Feb 2022, 12:24
This is a strange post... you have everything set up to go and you're asking the board - is this ok?
No, I'm asking for clarification on conflicting information throughout the thread regarding Q2, and if low leakage on that transistor is okay in this circuit. It's one thing to experiment, and it's another thing to understand. I don't want to just plug-and-play, but to have a sense of theory before putting it into practice, and the thread discussion leaves me confused about the leakage required for Q2.

Please bear in mind that I have limited resources, and I'm just trying to avoid mistakes that I may have to undo and, in the process, do damage to the few parts that I have.
I'm sorry, I sounded like you were sitting in front of your build waiting to try it out... because that's ultimately the only benchmark: do you like the sound?
In general, you want as little leakage as possible, but it's unavoidable with these old devices. Use transistor sockets and try whatever you have. Gain may be and indicator, but know that some of the gain is leakage... the two are intertwined.
How much leakage is too much? 100uA is common, 200 happens pretty often. More than 300uA means the device is suspicious, and more than 500uA I would say is bad.
Let's say the device really leaks 93uA, and has a gain of 110 - a prime specimen. What happens when we test? We chuck the thing in the socket, and read (93uA)*(2472) = .229V. Then we press the switch, and read 1.330V. To get the real gain, we subtract 0.229V from 1.330V and get 1.101V. The true gain is just 100 times the reading.
http://www.geofex.com/article_folders/ffselect.htm
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MarkL
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Post by MarkL »

I'm sorry, I sounded like you were sitting in front of your build waiting to try it out... because that's ultimately the only benchmark: do you like the sound?
In general, you want as little leakage as possible, but it's unavoidable with these old devices. Use transistor sockets and try whatever you have. Gain may be and indicator, but know that some of the gain is leakage... the two are intertwined.
Thank you...this is the explanation for Q2 I was hoping for, and I will try the few devices I have with relatively low leakage in that spot.

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andregarcia57
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Post by andregarcia57 »

after reading all the topic I found a starting point, really the selection of transistors is really fundamental, despite being working with some problem, now I can play a little until I make some kind of fine adjustment now.

https://www.4shared.com/s/fpXgZLDDBea
bat 9,58v
Q1 - HFE 68 - leakage 74uA ( 0vc, 8,66vb, 9,48ve )
Q2 - HFE 139 - leakage 0.41mA ( 0,32vc, 9,53vb, 9,56ve )
Q3 - HFE 142 - leakage 0.52mA ( 6,48vc, 9,35vb, 9,56ve )


470k cq1

what next point so I can improve the sound? thanks

(This really is a hard pedal to tame.)
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