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Post by Ron Rich on Aug 5, 2018 21:11:18 GMT
Hi All Does anyone have a copy of the modification needed on early AMi's that stop the "scratchy pot situation" ? I HAD a copy--may still have it--but can't find it, now that I need it
Let me know-- Thanks, Ron Rich
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Post by jukenorman on Aug 6, 2018 9:56:33 GMT
These were Wesley Dean's wise words on the subject -
"You can isolate the control from the DC component (which causes the noise) by using a load resistor in the cathode circuit and coupling the signal through a .1 mfd. If this this done, three wires must be used to the volume control. The shield as ground, the center tap used as the output of the control and the input to the high side. A 50 K log pot may be used. Wes"
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Post by Ron Rich on Aug 6, 2018 12:19:03 GMT
Thank-you Norman--Wes was, indeed, a wise man--and one hell of a nice guy too !
There is another way to do this, sent to me by Jay Hennigan---
You can do it internal to the amp or external. Connect a 150K resistor
between the original pins 1 and 2 of the volume control, or between the
junction of the 1.2K resistor on pin 2 of V2 and the .02 microfarad
capacitor going to pin 7 of V2 and ground. This is the same point
electrically.
Connect the + side of a 1000 uF, 50 volt capacitor to this point.
Connect the - side of this capacitor to the volume control.
+ 1000 50V
VC pin 2 ------------|(---> to volume control
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\
/ 150K
\
/
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GND (VC pin 1)
Ron Rich
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Post by robbytherobot on Sept 30, 2019 0:46:01 GMT
Does this apply to an Ami B ?
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Post by Ron Rich on Sept 30, 2019 12:31:20 GMT
Hi Robby,
Sure--tis an AMi, ain't it ?-- does it sound "scratchy" when you turn the Volume Control--then yes, it applies !
Ron Rich
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pault
New Member
Posts: 6
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Post by pault on Oct 22, 2020 9:06:04 GMT
Had the scratchy volume control. Also the two switches on the amp (equaliser and frequency) made an awful noise and affected adversely the sound output. Removed the amp and gave the two switches a good dose of switch cleaner. Put back and can now use the switches with no problems and now when operating the volume control very little scratchy noise.
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Post by Marcel Bogert on Nov 8, 2020 12:25:25 GMT
To put my 5 cents in the yar had a talk with Wes a long time ago about this same issue and some more tube substitute changes for the 5879 input and others.
I had a more complicated design changing the volume control all together which is not really relevant now.
BAL-Ami used the same amps in their machines and they used the setup as Ron described.
Only difference is the value's 39K and 50 MFD 50v.
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