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Thread: Jet city JCA20H mod - can't find schematics or instructions

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    Jet city JCA20H mod - can't find schematics or instructions

    Hello,

    I have searched the forum and found some threads but the attachemts won't open.

    Also I have been looking for any info of this mod all over the web, but all the threads are really old, the fles won't open, or the links are down.

    Maybe someone has kept the schematics or info about it, and could share it.

    Thank you in advance.


    (sorry for the bad english)

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    Supporting Member Jazz P Bass's Avatar
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    What specific 'mod' are you looking for?

    Try the HC Jet City JCA20H page: https://www.harmonycentral.com/forum...la-28/1376468-

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    Quote Originally Posted by Jazz P Bass View Post
    What specific 'mod' are you looking for?

    Try the HC Jet City JCA20H page: https://www.harmonycentral.com/forum...la-28/1376468-
    Thank you, already been there. All the links and pics are down. No pdf files.

    I want to make the amp cleaner

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    Old Timer J M Fahey's Avatar
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    OK.
    Mods in general do not make amps "cleaner", just lower gain.
    Maybe they clip some cathode capacitor off, replace 12AX7 with lower gain tubes, etc.
    So after one maybe you can set amp to 7 before clipping, while before it started at 4 ... not a real change; amp puts out, measured by ear, exactly as before, you could simply have turned volume down a little.

    If when , say, playing along a drummer (which are NOISY) you had good distortion but not clean sound, no matter what you do that wonīt change either, youīll still have around 15/16W RMS.

    If you want bto improve real headroom, as in acoustic headroom, not just "a number on a knob"; I suggest you use more sensitive/efficient speakers.
    That is a real change.

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    Juan Manuel Fahey

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    Senior Member Pedro Vecino's Avatar
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    with gain on 2 and master on 5/6 the amp is clean,if its too dark just add bright cap on the 1M before the gain pot,for less gain put a 100k in parallel of gain pot.

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    Supporting Member Jazz P Bass's Avatar
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    Mike Soldano did a pretty decent design on this amplifier.

    Unfortunately, the Chinese manufacturers got a hold of it.

    I had one of these in for a signal dropping out issue.
    It was the head version.
    Turned out to be a crappy 1 uf bypass film capacitor on the first stage cathode that was shorting.

    I was not impressed with the soldering quality at all.

    The clean channel starts to get rounded at a gain setting of 3 (100 mv input).
    The dirty channel was over the top gnarly in a good way.

    All in all, with a little TLC, I found it to be a nice amp.
    With a good speaker.

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    Mine has too much HISS, even with the gain pot at 4 and volumen at 7.

    Changed the chinese tubes with tunsols in pamp and EHX in pre. It is the same.

    I haven't tryed a 12aY7, the only one I have is 12aT7 but I read that is a kinda sterile, used on reverb circuits. is that correct?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    ***Yikes! That schematic is full of errors Suggest you look at what you actually have and feed back a corrected schematic.
    **Scrub that. It's just sheet one that is misleading.

    Turn vol to zero - how is the noise?
    Vol at max, pre gain at zero. How is the noise?
    Raise the pre gain. How is it now?

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    Last edited by nickb; 07-08-2019 at 08:50 PM.
    Experience is something you get, just after you really needed it.

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    Old Timer J M Fahey's Avatar
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    I read that is a kinda sterile, used on reverb circuits. is that correct?
    Its basically like 12AX7, somewhat less gain, somewhat higher current (so itīs preferred in Reverb drivers and Phase Inverters), "sterile" is Marketing Dept. babble (just not to call it BS)

    Please do the tests suggested by nickb and post results.

    But in general Mods are basically not much more than gain adjustments; if yopu post a trusted schematic we might suggest a couple .... generally not needing much more than a resistor or two.

    Way more flexible and powerful than tube rolling and definitely cheaper, in a good way.

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    Juan Manuel Fahey

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    Quote Originally Posted by nickb View Post
    ***Yikes! That schematic is full of errors Suggest you look at what you actually have and feed back a corrected schematic.
    **Scrub that. It's just sheet one that is misleading.

    Turn vol to zero - how is the noise?
    Vol at max, pre gain at zero. How is the noise?
    Raise the pre gain. How is it now?
    vol 0 gain 0 - no noise
    vol 10 gain 0 - no hiss, just normal noise
    vol 10 gain 4 - a little hiss, but lots of gain
    vol 10 gain 5 - lots of his, lots of gain

    Quote Originally Posted by Pedro Vecino View Post
    Pedro uploaded the schematics, thank you.

    How can I mod it to clean the sound, or lower the gain?

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by spunko View Post
    vol 0 gain 0 - no noise
    vol 10 gain 0 - no hiss, just normal noise
    vol 10 gain 4 - a little hiss, but lots of gain
    vol 10 gain 5 - lots of his, lots of gain
    Thanks, thats about what I expected, meaning amp is not broken, just high gain.

    How can I mod it to clean the sound, or lower the gain?
    Start by adding 150k in parallel with R33 (330k).
    Itīs about 10dB attenuation, I guess that should be enough.

    If you want more, add 39k instead, that should be about 20dB attenuation.

    If still too much, then simply a Soldano is not for you, trade it for a small Fender tube one or a Roland SS (maybe a Katana)

    Please do the tests and comment results.

    Hint: you may add a small toggle switch, (like those phase/split ones on Guitars) to the front panel, to switch the attenuator in-out at will.

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    Quote Originally Posted by J M Fahey View Post
    Thanks, thats about what I expected, meaning amp is not broken, just high gain.



    Start by adding 150k in parallel with R33 (330k).
    Itīs about 10dB attenuation, I guess that should be enough.

    If you want more, add 39k instead, that should be about 20dB attenuation.

    If still too much, then simply a Soldano is not for you, trade it for a small Fender tube one or a Roland SS (maybe a Katana)

    Please do the tests and comment results.

    Hint: you may add a small toggle switch, (like those phase/split ones on Guitars) to the front panel, to switch the attenuator in-out at will.
    Thank you!! I made the changes you suggested and also something from another forum.
    Tha amp will stay like this.

    - R28 lower 220k to 100k
    Lifted C27
    Lifted C15
    R33 330k in parallel with 150k


    I want it to be switchable, and I noticed that the three changes in bold go to ground, so that can be switchable.
    The problem is that R28 doesn't go to ground, so it can't be in the same switch, at least as I imagine. Position one everything lifted, position two everything grounded, with a dpdt on/on switch.

    Is there any way to make this with one swicth?

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    Don't forget the joker g1's Avatar
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    Adding another 220K in parallel with R28 will make it 110K.
    Use a DPDT switch, one side as you described, the other side add 220K parallel with R28.

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    Old Timer J M Fahey's Avatar
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    You can have 3 attenuation settings with a single switch and no high voltage involved

    You need an SPDT Center OFF switch.



    it means (as seen in this picture):
    * toggle has 3 positions: left, center and right.
    * when toogle is at left, center and right legs are shorted
    * when toggle is at right, center and left legs are shorted (so far, like on any other switch)
    * when toggle is at center, center leg is connected to neither left leg nor right leg (i.e. "center OFF")

    Connection:
    * 150k resistor to left leg
    * 39k resistor to right leg
    * free resistor ends twisted and soldered together, go to R33 top end.
    * centerb leg goes to ground.

    Effect:
    * toggle center: unmodded JCA20
    * toggle right : 10dB attenuation
    * toggle left: 20dB attenuation (same as 10dB attenuation + 3 Mods you already made, which are now unnecessary)

    You can try it before fully committing to it.

    The beauty of the resistive attenuator is that:
    * it can get any attenuation you can get by using other, more complex methods, simply use different resistor values.
    In general: smaller resistor value: higher attenuation.
    * simple and cheap.

    I said "right and left" to match the picture I found online, I guess youīll mount switch so toggle goes Up and Down.

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    Juan Manuel Fahey

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    Senior Member vintagekiki's Avatar
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    Links to Jet City Schematic

    Quote Originally Posted by spunko View Post
    Maybe someone has kept the schematics or info about it, and could share it.
    http://starin.info/Product%20Info/-%20-%20Archive/Jet%20City/

    http://starin.info/Product%20Info/-%20-%20Archive/Jet%20City/Manuals/
    Jet City manuals & schematics

    https://music-electronics-forum.com/showthread.php?t=21418
    Jet City JCA20 Mods

    http://valvesphere.blogspot.com/2012/01/jet-city-333-jca20h-hsg-mod-high.html
    Jet City 333 JCA20H HSG Mod (high saturated gain) with schematic circuit diagram.

    https://valvesphere.blogspot.com/2012/01/triode-pentode-switch-in-jca20h.html
    JCA20H with EL84 Triode / Pentode Switch and schematic circuit diagram.

    https://valvesphere.blogspot.com/search?q=Jet+City
    Jet City JC20H output harshness

    https://docs.google.com/document/d/15nDuz_whyORWFFBaNSn74szUwBzMu8kpZ-InX6-Poag/edit#heading=h.gg08ch9zpnsk
    Biasing your Jet City Amp

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    Last edited by vintagekiki; 07-09-2019 at 11:13 PM.

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    This is the way I think it might work. Haven't tried on the amp yet.

    What do you think?


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    Last edited by spunko; 07-10-2019 at 08:14 AM.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by spunko View Post
    This is the way I think it might work. Haven't tried on the amp yet.

    What do you think?

    Sorry but no.
    C27 , C15 and the attenuation resistor need a pair of switch contacts each.

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    Juan Manuel Fahey

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    Why it wouldn't work?

    I have connected all the parts/mod into the switch, and into the amp and tested with my fluke (without power on) and it apparently works, or at least it does what I intended with that connections.

    1) the upper strip just opens/closes grounds of C27, C15 and the 150K attenuator.

    2) the lower strip puts 220K in parallel with R28 (I noticed I put 100k in my diagram, typo)

    The two strips don't have continuity between each other (neither the lugs, just when engage them).
    When I flip the switch, the two strips do what intended with the diagram.

    I haven' tryed with the amp powered on yet, and I want to make sure it is ok before the real test.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    Why it wouldn't work?
    Because ...
    Quote Originally Posted by J M Fahey View Post
    C27 , C15 and the attenuation resistor need a pair of switch contacts each.
    After all, if you're so sure in your design, why you not try it how it works.

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    *Edit: Sorry I didn'y understand the post at first. Bad english.

    All the caps are going to ground. Why they need a single a terminal for each? Sorry, I want to understand.


    Quote Originally Posted by spunko View Post
    I haven' tryed with the amp powered on yet, and I want to make sure it is ok before the real test.
    I hope someone gives a hand here and point my mistakes in the diagram. I want to understand the way it should work.

    This could help a lot of people, not just me.

    Also, don't want to fry the amp

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    Last edited by spunko; 07-10-2019 at 05:23 PM.

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    Senior Member vintagekiki's Avatar
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    All the caps are going to ground.
    That's okay. What happens when the switch is in a different position, when the caps C27 , C15 and resistor 150k are not at ground?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	1.jpg 
Views:	19 
Size:	105.2 KB 
ID:	54261  

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    Quote Originally Posted by vintagekiki View Post
    All the caps are going to ground.
    That's okay. What happens when the switch is in a different position, when the caps C27 , C15 and resistor 150k are not at ground?
    When the caps are not grounded, they seem to lower the gain. Also your question make me notice something wrong with my diagram.
    The switch is supposed to lift from ground C27, C15, and "connect" to ground the 150k resistor so it could be in parallel with R33, but it is doing the opposite with the 150k and the way I connected it.

    Quote Originally Posted by J M Fahey View Post
    Sorry but no.
    C27 , C15 and the attenuation resistor need a pair of switch contacts each.
    Maybe that is why the response of JM Fahey

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    Last edited by spunko; 07-10-2019 at 06:06 PM.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    Also your question make me notice something wrong with my diagram.
    Schematics that you have attached, do not match the description you provided.



    useful links

    http://www.guitarstudio.tv/documents/Designing-V-T-Amplifiers.pdf
    Designing-V-T-Amplifiers

    http://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_i d=14475
    The Electric Guitar Amplifier Handbook

    https://www.4shared.com/get/jmzf8Vxb/Designing_Tube_Preamps_For_Gui.html
    Designing Tube Preamps for Guitar and Bass

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    So I edited again the diagram. I think this should work as I intended to:

    Position 1 (normal):
    - upper strip Grounds C27, C15 and R33 (330K)
    - lower strip does nothing

    Position 2 (clean):
    - upper strip lift C27 and C15 from ground, and grounds 100k instead of R33
    - lower strip puts 220K in parallel with R28


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    Don't forget the joker g1's Avatar
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    When the caps and R33 are not grounded by the switch, they are connected together. That is a problem. You are creating circuit paths that should not be there.
    You need a 4PDT on-off-on so that the parts do not interfere with each other when not grounded.

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    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

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    Old Timer J M Fahey's Avatar
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    In a nutshell, you are connecting Audio at the third tube cathode straight to the first tube one ... what could go wrong?

    Imagine sticking the singerīs microphone against its own PA speaker cone and you end up with about the same.

    Thatīs why you need separate switches so each element has its own.

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    Juan Manuel Fahey

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    Senior Member vintagekiki's Avatar
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    Good intentional advice

    In order to practically do mod it is assumed that the user is minimal knows how amplifier works, what is expected from mod, and only at the end drawing schematics and practical realization on the amplifier.
    With How can, What do you think, Why it wouldn't work, do not repair amplifier, and the way to moding is is far away.
    In this Thread there is an experience of members who want to help beginners.

    A lot of effort has been invested to find the links that deal with the Jet City JCA20H mode problem, and we are always at the beginning.
    Dear Colleague, I see you have a lot of enthusiasm to deal with the amplifiers, please begin by hearing some of the people who have passed the same path as you, from the beginning to get to the experience they now have.

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    Thank you very much for all the hel and info provided.
    I have checked some of the URL posted in here and downloaded the pdf files to read them all, it will take a while. Unfortunately some URL have broken links, but there is a LOT of useful info.

    Also I have a question related about those URL articles, like this one:
    https://valvesphere.blogspot.com/201...harshness.html

    About the harshness he says: "The solution was with the power amp section. A 330pf capacitor across the 47K negative feedback (NFB) resistor marked R12 in the service schematic"

    But it doesn't tell what type of capacitor, and if it goes in parallel or series.



    Also I tried my diagram, and it didn't work well as expected.

    The clean part sound great IMO, also a friend came to hear the amp, and he likes it a lot. We compared it with his "unmoded" head, which is the exact same model and tried to clean it like the "moded one" and it couldn't get that clean.
    Also we noticed that the unmoded amp when trying to get clean, it lacks of bass; while the moded amp in clean sounds with good tone (despite what a member of another forum told, that the clean will sacrifice tone)

    The "normal" or crunch does not work well, as all of you already know. I think is oscilating, dunno how to describe it well. If I hit a chord, it will sustain but in the end oscilates the trail. Sorry for my bad english.

    I have another news too.
    I wrote an email to Dr. Olaf Krampe, who helped a lot with these mods in those threads of 2009 or so. He kindly repplyed to my email with these instructions of how to clean it:

    "Cut out the preamp gain pot and get a new 1M log one.

    From middle of pot (reconnect left/right pins to the board, flipped because the pins are now upside down) run into a switch (on/on).
    One side back to the middle of the pins on the PCB (the original channel)
    The other side to R36. This way you create a cleaner channel with just two gain stages"


    Unfortunattely, with my bad english I'm a little lost.
    Does he mean I have to install the pot backwards?
    The left lug to the rigth pcb hole, and rigth lug to left pcb hole?

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    Last edited by spunko; 07-11-2019 at 06:06 PM.

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    Supporting Member Jazz P Bass's Avatar
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    Quote:"About the harshness he says: "The solution was with the power amp section. A 330pf capacitor across the 47K negative feedback (NFB) resistor marked R12 in the service schematic"

    But it doesn't tell what type of capacitor, and if it goes in parallel or series."
    ACROSS implies a parallel connection.
    Any capacitor type will do. (ceramic disc is fine)

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    Quote Originally Posted by spunko View Post
    "Cut out the preamp gain pot and get a new 1M log one.

    From middle of pot (reconnect left/right pins to the board, flipped because the pins are now upside down) run into a switch (on/on).
    One side back to the middle of the pins on the PCB (the original channel)
    The other side to R36. This way you create a cleaner channel with just two gain stages"
    I'm lost with these instructions. Do I have to install the pot backwards? The left lug to the rigth pcb hole, and rigth lug to left pcb hole? I don't understand what he means with "flipped because the pins are now upside down"

    Can I use the original pot? it says A1M, I guess is audio taper or logarithmic as well.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by spunko View Post
    I'm lost with these instructions. Do I have to install the pot backwards? The left lug to the rigth pcb hole, and rigth lug to left pcb hole? I don't understand what he means with "flipped because the pins are now upside down"
    Your English is good enough to understand you, and here is friendly translate.google.com.

    Middle pin on the potentiometer is always a slider.
    If the potentiometer is not foreseen for the PCB, when wiring potentiometer, end pins are cross when with the wire connect to the PCB.


    http://sound.whsites.net/pots.htm
    Beginners' Guide to Potentiometers

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    Last edited by vintagekiki; 07-11-2019 at 10:50 PM.

  34. #34
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    Quote Originally Posted by vintagekiki View Post
    [COLOR="#000000"]If the potentiometer is not foreseen for the PCB, when wiring potentiometer, end pins are cross when with the wire connect to the PCB.
    Oohh ok, I get it. Thank you very much.

    what I don't get now is why do I have to get a new pot?
    He said to use a Log pot, and the original is marked as A1M, which stands for Audio taper or log taper, that's what I've read; and B1M stands for linear taper. Is this correct?

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  35. #35
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    Quote Originally Posted by spunko View Post
    ... which stands for Audio taper or log taper ...
    There are 'old' and 'new' standard for marking potentiometers.

    http://sound.whsites.net/pots.htm
    Read Pot Markings

    https://www.vintage-radio.net/forum/showthread.php?t=132235
    Markings on Potentiometers ('A,B,C,L,F')

    http://www.potentiometers.com/potcomFAQ.cfm?FAQID=29
    Potentiometer Frequently Asked Questions

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