Keeley - Germanium Amplifier  [traced]

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apollomusicservice
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Post by apollomusicservice »

Thanks to mirosol photos viewtopic.php?f=7&t=28913 and some others from the internet, I drew my first version of this schematic. Some values missing.
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Keeley Germanium Amplifier.png
Keeley Germanium Amplifier.png (11.06 KiB) Viewed 3353 times

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Manfred
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Post by Manfred »

With regard to these both schematics:
https://robertkeeley.com/woo/wp-content ... -FUZZ1.png
https://i.pinimg.com/originals/e4/5e/41 ... 2e5789.jpg
is my guess for following Values:
R8 = 4.7 Kiloohms
C5 = 10 Microfarads
C6 = 47 Microfarads
R1 = 51 Ohms.

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Post by Manfred »

The schematic is right according the PCP trace, but there is something not right here.
The base of the AC128 has no DC connection, therefore the biasing cannot work.
A base potential divider or a base resistor connected to ground is missing on the PCB is missing.
KeekeyGeAmpBias.jpg

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Post by apollomusicservice »

Manfred, if you look at the photo, there is nothing else connect to base of the transistor.
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deltafred
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Post by deltafred »

It must be relying upon leakage to bias it.
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Post by Chugs »

Temperature stability will be an issue.

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Post by deltafred »

Chugs wrote:Temperature stability will be an issue.
Just call it mojo. :lol:
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Post by Manfred »

deltafred wrote:It must be relying upon leakage to bias it.
I see, an unusual ciruit, so that I trial is needed to find out the truth.

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Post by Manfred »

I ran a trail of a reduced circuit today.
I applied the collector and emitter resistor of the EQD Crimson Drive schematic.
I was really very surprised, the circuits works indeed without base resistor.
The input signal level varied from 100 Millivolts to 1 Volt RMS.
The clipping started at a input level of about 300 Millivolts.
KeelyGermaniumTrial.JPG
100mVto300mV_Input.jpg
350mVto1V_Input.jpg

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Post by Manfred »

I made a further trial adding the bypass capacitor.
The clipping started at much lower input levels.
Unfortunately, 70 Millivolts is the lowest output level of the integrated oscillator in my digital oscilloscop.
KeelyGermaniumTrial2.JPG
KeelyGermaniumTrialBypassCapacitor.JPG

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