Keeley Germanium Amplifier

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Keeley Germanium Amplifier

Postby apollomusicservice » 11 Mar 2018, 18:55

Thanks to mirosol photos http://www.freestompboxes.org/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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Manfred (11 Mar 2018, 23:18), mirosol (13 Mar 2018, 19:50), Wiseblood (14 Mar 2018, 03:45)
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Re: Keeley Germanium Amplifier

Postby Manfred » 12 Mar 2018, 00:12

With regard to these both schematics:
https://robertkeeley.com/woo/wp-content/uploads/2014/12/PSI-FUZZ1.png
https://i.pinimg.com/originals/e4/5e/41/e45e41a3bc8cfe9711dbdf4f9b2e5789.jpg
is my guess for following Values:
R8 = 4.7 Kiloohms
C5 = 10 Microfarads
C6 = 47 Microfarads
R1 = 51 Ohms.
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Re: Keeley Germanium Amplifier

Postby Manfred » 12 Mar 2018, 23:44

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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Re: Keeley Germanium Amplifier

Postby apollomusicservice » 13 Mar 2018, 13:13

Manfred, if you look at the photo, there is nothing else connect to base of the transistor.
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Re: Keeley Germanium Amplifier

Postby deltafred » 13 Mar 2018, 15:31

It must be relying upon leakage to bias it.
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Re: Keeley Germanium Amplifier

Postby Chugs » 13 Mar 2018, 20:07

Temperature stability will be an issue.
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Re: Keeley Germanium Amplifier

Postby deltafred » 13 Mar 2018, 22:39

Chugs wrote:Temperature stability will be an issue.

Just call it mojo. :lol:
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Re: Keeley Germanium Amplifier

Postby Manfred » 14 Mar 2018, 06:39

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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Re: Keeley Germanium Amplifier

Postby Manfred » 20 Mar 2018, 19:05

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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Re: Keeley Germanium Amplifier

Postby Manfred » 21 Mar 2018, 02:19

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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