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Home Model Engine Machinist > The Tools and Tips > Machine Modifications > Would replacing Q1 with a higher hFE transistor have any influence?

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Old 01-09-2018, 09:44 AM   #1
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Default Would replacing Q1 with a higher hFE transistor have any influence?

I have a phase shift oscillator circuit from a drum machine that I would like to reproduce as faithfully to the original as possible. My problem is that the 40 year old design uses a transistor (2N3394) that is no longer available. But... the others from the same data sheet (2N3391, 92 & 93: still are. The only difference between them is that the hFE is higher than the original.

Here is the circuit (as it is presented in the original schematic):

Note: I added S1 and the pulldown resistor (R11) to replace the original trigger. The output was originally fed to a summing bus, and then amplified.

The Intention : At this point, the plan is to faithfully reproduce the circuit on a breadboard (or PCB) and experiment with modification options and values without damaging the original device. I am considering a) only modifying the original drum machine; b) re-creating the drum machine with the modifications; or c) re-creating a few of the individual sounds as single units, and providing amplification for each one, rather than summing to a mono output.

So the main question is: Would replacing Q1 with a higher hFE transistor have any bearing on either the decay time, amplitude or frequency of the output?

Secondary to that is:
When seeking out a replacement, what specifications, if any, have a bearing on those same parameters?

Additional Note: While composing this question, I reproduced this circuit using the Falstad Simulator, and found that even changing the hFE within the range of the original part (hFE=55-110) presented major differences in the output. Butů from what I understand, the hFE can vary greatly within the given range, and is unreliable. This causes me to question the results I am seeing. If this were not the case, I would have taken the simulation result for my answer.

Here is a snapshot of the output at the C6/R9 junction:

Left: output at hFE=55. Right: output at hFE=110

The waveform result at hFE=55 seems to best represent what I hear in so far as the "sharp impact" which fades out, and also looks like the waveform image presented in the original schematic - down to the phase and number of peaks.

Schematic Source : Here is a link to the service manual that I referenced this design from. It's difficult to read due to the low quality of the scan, but the values in my own schematic were taken from a hard copy.

Thanks for any insight you may have on this.

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Old 01-09-2018, 10:23 AM   #2
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My problem is that the 40 year old design uses a transistor (2N3394) that is no longer available. --->See at the list of replacements of transistors in this link..

Jens Eirik, Bergen in Norway :)
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Old 01-09-2018, 05:27 PM   #3
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Default Would replacing Q1 with a higher hFE transistor have any influence?

If you have a simulator you could try tweaking a few components , the capacitors on the switch circuit - C1 & C2 possibly ? Looking at the waveform on the right it seems lazy to start oscillating but the frequency looks the same , amplitude is a little bigger . The Hfe will vary between transistors so you will need to test the one you end up using and run the simulator with the corrected values .
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Old 01-09-2018, 09:14 PM   #4
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Properly designed circuits for industrial application have sufficient feed back to be insensitive to beta variation. That said, the higher beta increase the gain and reduce the damping, thus a longer slowly decaying transient. You may be able to offset beta changes by changing R2. The gain of the stage is affected but not entirely determined by the beta, therefore is difficult to determine the ratio between the change of circuit gain and the change in beta but is less that proportional. I would start with being able to change R2 by +/- 25% use a 7.5K + 5K pot and play around until you get the twang you like. This circuit is non linear, the base bias changes during the burst. This make any manual calculation nearly impossible. Any component that affect the bias is critical. R2 is the only component that affect the gain but not the bias.
Austin, TX
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Old 01-09-2018, 10:36 PM   #5
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Wegger when you have a moment tell us a bit about yourself. Like where you are located, Machinery and main interests. It is a little unusual to enter a forum with a question as a first post.

Hallett cove
South australia
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