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Thread: 1967 AB763 Bandmaster cap mods

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    Supporting Member Randall's Avatar
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    1967 AB763 Bandmaster cap mods

    What a trip this amp is! Customer drives to buy a brownface Showman 212 combo for $500. But when he brings it to me, it is actually a Bandmaster with a Showman face plate, in a custom 212 dirty blonde cabinet with a blonde style Fender logo on it. WTF? The filter caps were changed to IC caps 25 years ago, the trem roach was missing, the bias cap was changed, but everything else is stock except for two orange drops. In the Vibrato channel the .1 cap in the tone stack is changed to a .047, and in the PI the 500pF on the top grid is .02.

    The Vibrato channel sounds more bassier than the other, so I assume this bigger cap is why. Is the same thing going on with the PI cap? I'm torn between leaving it since it sounds good, or putting it back to stock. I don't want to put a lot of time into having the customer do listening test while it's here. He actually described it as sounding like a million bucks, so there's that.

    What say you?

    https://www.thetubestore.com/lib/the...-Schematic.pdf

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    Last edited by Randall; 07-29-2019 at 02:23 AM.
    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    First, where is there a .01 cap in the tonestack of an AB763? Did you mean the .1 cap? If you did, and the bass cap was changed from .1 to .047 I wouldn't even sweat it. I'd be surprised if you hear a difference between them.

    2) The .02 cap feeding the PI always seems like a good idea to modders. It's usually not. AB763 amps have AMPLE LF in the preamp without letting it all get to the PI. SF amps used a .01 here, but with other changes to the PI. I usually opt for something like .0022 in my own starter designs and I found a .001 sounded best (to me) in an AB763 build I did a few years ago. YMMV, but I'd change it a .001 and call it done.

    And C) Those IC caps are probably bad or going to be soon enough that a cap job should be a consideration.

    JM2C

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    Last edited by Chuck H; 07-29-2019 at 05:46 AM.
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    Quote Originally Posted by Randall View Post
    He actually described it as sounding like a million bucks, so there's that. What say you?
    With a review like that I'd leave the tone stacks & cap to the PI grid right where they are. Maybe freshen up hi voltage & bias filters, any other regular maintenance like clean pots & brite switches, clean pre tube pins & output tube socket electrodes. Check bias & power too, just for an observation. Unless the bias is way way off I'd leave it where it is too. "Sounds like a million bucks" maybe you can make 100 more or less.

    Decades in the past, when crustomers wanted more bass I'd swap out that 500 pF for a much larger cap. I'm guessing that's what happened to this one. But then the amps sound swampy. These days I'd max that at 0.0022 uF or 2.2 nF or 2200 pF any way you wish to write it. Since crustomer likes the tone, leave that alone.

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    Supporting Member Randall's Avatar
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    I meant the .1 tone cap, not .01, I fixed that.

    "sounds like a million bucks" was while he was checking out the amp before he bought it. It had a ticking noise, no trem, an intermittant oscillation, etc, so I'm not putting too much weight on that. After changing the electrolytics on the board (the originals were WAY off), a few out of spec cathode resistors, the drifted grid and screen resistors, cleaning, and tightening up a few dodgy solder connections (one in the HV section), the amp sounds quite a bit better than it did.

    I think I am going to go back towards the direction of the original PI cap value.

    edit: I put in a 0.0022, four times larger than the stock 500pF one, but way,way less than the 44 times larger 0.02 that I took out. I don't really think I hear a difference.

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    Last edited by Randall; 07-29-2019 at 03:23 AM.
    It's weird, because it WAS working fine.....

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    edit: I put in a 0.0022, four times larger than the stock 500pF one, but way,way less than the 44 times larger 0.02 that I took out. I don't really think I hear a difference.
    There's a difference. You might only notice it when the amp is pushed close to and beyond it's max current spec when conducting signal though. The 2.2n/.0022uf/2200pf cap rolls off at an appropriate rate below typical guitar frequencies where a 22n/.022uf cap remains essentially flat down to DC (even as the system approaches clipping and the input impedance falls). So the 2.2n cap is everything you need, and nothing you don't. Modern 7/8/+ string guitars not withstanding The consideration here is that overtones and "beating" in the lower frequencies can detriment performance since the speakers can't reproduce those frequencies anyway and they require A LOT of the amps potential to reproduce. Potential that could be preserved for frequencies the speakers CAN reproduce.

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    Thanks for explaining that Chuck. I opted to ditch the original modder's choice of such an extreme choice, but still give a nod to what he may have been going for, albiet at a much less and more reasonable value of 0.002. This amp has reconed speakers that look like low power CTS or maybe Oxford, so they are breaking up anyway, any making it difficult to hear the difference. I think this case is closed for now, at least.

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    It's weird, because it WAS working fine.....

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    Old Timer J M Fahey's Avatar
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    Don't overthink it.
    Donīt turn a cash cow into a bottomless time, money and effort pit; "sounds like a Million bucks?" ... ok, **just** solve the annoying stuff such as tickin tremolo, buzz, etc. and donīt touch any f*ckng thing which "might" change tone.

    Just remember dumbassbob and avoid getting there.

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    Juan Manuel Fahey

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    Supporting Member Randall's Avatar
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    Sorry Juan, I did change some things that might change tone. That enormous PI cap, the way off carbon grid and screen resistors, any cathode resistors that were more than 10% over, and a double cathode bypass cap that measured 5 and 8 nanoF (I can't live with that). I also fixed the tremolo, repaired a buzzy speaker, and got rid of the odd noises, scratchy pots and jacks, and slowed down the tremolo. It actually does sound very good now. I have never had anyone complain about putting a blackface back to stock, and I am confident with this one as well. I did leave in the 0.047 bass cap.

    But, thanks for playing!

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by J M Fahey View Post
    Just remember dumbassbob and avoid getting there.
    To be fair, DAB did okay once he got his amp to a technician instead of a marketing manager (to put it politely)...

    I have actually run into DAB on Farcebook, telling the sordid tale of the Bung-Fued Band Master & who NOT to send your vintage amps to. I answered his comment to check on his amp but didn't hear back.

    I remain confident that Randall will not let this become GW's BM, Round 2.

    Justin & Jusrin

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    I only fear Customer saying: "It does not sound like before, put it back as it was" and then you wasting the next 15 years endlessly modding it, finally saying: "look, I swear itīs exactly as before, I even put the shorted capacitor back" and him saying (for the 1000th time):"no, it does not sound as before".

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    Juan Manuel Fahey

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    "Send it to This Texas Guru Guy; he'll get it fixed up for you!"

    Jusrin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
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  12. #12
    Supporting Member Randall's Avatar
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    "I only fear Customer saying: "It does not sound like before, put it back as it was" and then you wasting the next 15 years endlessly modding it, finally saying: "look, I swear itīs exactly as before, I even put the shorted capacitor back" and him saying (for the 1000th time):"no, it does not sound as before"."

    That's possible I suppose, but I'm going to stick with my belief that if he thought it sounded good when it was compromised with bad parts, then he will like it even better after it was squared up. I like my odds here. This guy is knowledgeable, reasonable and experienced, and he only played the amp for a couple of minutes before I got it. I'm not worried about it.

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    It's weird, because it WAS working fine.....

  13. #13
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    Quote Originally Posted by Randall View Post
    he only played the amp for a couple of minutes before I got it. I'm not worried about it.
    Ah, we didn't know that part. I thought he'd played thru it for quite a while maybe years.

    A million bucks doesn't go as far as it used to. Let's hope his next review is "sounds like twenty million bucks!"

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    Supporting Member eschertron's Avatar
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    Just out of curiosity, you did write down the drifted values of those
    .. way off carbon grid and screen resistors, any cathode resistors that were more than 10% over..
    just for scholarly research purposes, of course. Could come in handy if the unthinkable happens and the customer balks

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by eschertron View Post
    Just out of curiosity, you did write down the drifted values of those

    just for scholarly research purposes, of course. Could come in handy if the unthinkable happens and the customer balks
    Or if you want to make the Randall custom "million dollar amp." Heck why not, Dumble's only 1/10 of the way there.

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    Bent Member Chuck H's Avatar
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    THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    I always wondered why there is so very little blueprinting of exceptional sounding BF amps. Because the really great ones are chimey, but full and really seem to get under the note in that way that makes the amp seem intuitive to your playing, most are really just "aight" with workman like tone. You can usually find a sweet spot with any particular guitar, but nothing to say "Hey! Now I get what all the fuss is about!" Only about five percent of those amps have the mojo. And I, for one, would like to know what it is. It may be different in each case, but... Some drifted values and/or some lead dress difference, maybe even a wrong cap value installed so Fender could fill the order (that happened). Inquiring minds want to know.

    I've been privy to two such amps. One was a BF clone built by a guy who claimed his amps captured THAT sound. They did and I was jealous as hell. The other was one you'd never suspect... A BF Bassman head. It was all stock, a little noisy, but it sang like a bird and growled like a lion. Big wet tone even without reverb, trem or the extra gain stage. I didn't have the opportunity to get inside either.

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    Last edited by Chuck H; 07-31-2019 at 02:11 AM.
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    Old Timer J M Fahey's Avatar
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    Sometimes the difference is an incredibly "unimportant" error, which happens to be the right way.

    John JR Frondelli who is a member here but doesnīt come often, works in a Tech Shop which is incredibly well situated in NY and rubs shoulders with the Gods on a daily basis.

    Among others, U2 sends *all* of their stuff twice a year for maintenance, both for Tours or recording.
    He had been intrigued by a certain revered and cherished small Fender amp (Harvard/PrincetonDeluxe?), **the** amp used by The Edge since forever.

    He has half a dozen of them but he swears by that one and does not take it on tour for fear of losing it.
    "None of the others sounds like that one"

    So as part of his maintenance job he opened all 6 chassis on his workbench and compared them to find any difference.
    As expected, basically the same (same schematic meaning same DNA) except for the cherished one: just repeating old information by heart, but apparently that one had, straight from factory, a .01uF cap where schematic asks for a .02 one.

    Probably a Friday afternoon job, the assembler must have run out of .02uF ones in his/her parts tray and didnīt want to wait for more.

    I suspect it was a coupling cap and the slightly reduced Bass made for aslightly cleaner distortion, so maybe usable somewhat louder for slightly improved "singing" or whatever.

    Only noticeable by somebody who uses them every day and has 5 other ones to compare; Iīm certain nobody will pick it by ear and scream "million dollar sounding amp" !!!!!!

    He was very careful NOT to "solve the error" by any means (Randall cough!! cough!! )

    But then he runs:
    https://www.dbmproaudio.com/

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    Differencies in blackface combos are often due to different speakers.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Differencies in blackface combos are often due to different speakers.
    As with any amp. Also the guitar, where the knobs are set, the player's technique etc. I've worked on amps that sound awful to me, but the owner plugs in and like magic, they sound terrific.

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    Quote Originally Posted by Chuck H View Post
    THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    I always wondered why there is so very little blueprinting of exceptional sounding BF amps. Because the really great ones are chimey, but full and really seem to get under the note in that way that makes the amp seem intuitive to your playing, most are really just "aight" with workman like tone....

    The other was one you'd never suspect... A BF Bassman head. It was all stock, a little noisy, but it sang like a bird and growled like a lion. Big wet tone even without reverb, trem or the extra gain stage.
    So... What particular BF Bassman? Could be any of 3 different circuits - quite different. At least the Bass channels are. Look at the AA864, AA165, & AB165. Most are either AA864 or AB165. Anyway, BF Bassmans all have 4 preamp tubes for a third "gain" stage on the Bass Channel. Look into it - especially the AB165. I've never tried lifting it, but that little tiny negative feedback loop might have something to do it... Also possibly the feedback on the 6L6s, though mine came without those (already removed). Mine also came with a Twin Reverb OT, so, fwiw.

    As far as your description of BF amps in general, to me it sounds like you're describing most of the SF amps I've played, up to when they switched to the UL. There was a thread here years ago about "silverface amp pull" that was an interesting read, theorizing on why the SF had something that the BF don't. I guess we could look for the thread...

    Justin

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    Juan,

    It was a Harvard. I do remember that.
    As for which cap, there's only 2 .02s to worry about, so I'll begin cloning that million-dollar amp (literally; it's gotten The Edge more than a few million) tonight!

    Justin.

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    As with any amp.
    Sure, but what I meant was that Fender used a number of different speaker brands in the 60s and those amps are rarely compared at the same speakers.
    I especially like the old 10 inch Oxfords.

    BTW, the AA763 Vibroverb (which appeared first in 1964 as a single 15" combo) is identical to the AB763 Super Reverb except for the smaller size 8 Ohm OT (and speakers).

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Sure, but what I meant was that Fender used a number of different speaker brands in the 60s and those amps are rarely compared at the same speakers.
    I especially like the old 10 inch Oxfords.

    BTW, the AA763 Vibroverb (which appeared first in 1964 as a single 15" combo) is identical to the AB763 Super Reverb except for the OT (and speakers).
    Deafinitely true. At the time, if you grabbed whatever was in the store, or ordered up a new Fender, you might find Jensen, Goodmans, Oxford, Utah. For instance some years ago I worked on a bone stock very early Deluxe Reverb, chassis date stamped December 1963. Mellowest sounding Deluxe I ever heard! Double checked the circuitry, it was exactly as the schematic said. Only possible explanation, the speaker. Now I admit forgetting, but it was either Oxford or Goodmans.

    Additionally, if the speaker failed, anything might find its way in there. A popular upgrade speaker at the time was the Jensen "Vibranto." Luv that speaker!

    If I'm not mistook, the black Vibroverb and early Vibrolux are also identical to the Pro Reverb, except for an 8 ohm (A7A) transformer in the Vverb or 4 ohm (A6A) in the Vlux & Pro. Great amps, all of 'em.

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    Quote Originally Posted by Leo_Gnardo View Post
    Deafinitely true. At the time, if you grabbed whatever was in the store, or ordered up a new Fender, you might find Jensen, Goodmans, Oxford, Utah. For instance some years ago I worked on a bone stock very early Deluxe Reverb, chassis date stamped December 1963. Mellowest sounding Deluxe I ever heard! Double checked the circuitry, it was exactly as the schematic said. Only possible explanation, the speaker. Now I admit forgetting, but it was either Oxford or Goodmans.

    Additionally, if the speaker failed, anything might find its way in there. A popular upgrade speaker at the time was the Jensen "Vibranto." Luv that speaker!

    If I'm not mistook, the black Vibroverb and early Vibrolux are also identical to the Pro Reverb, except for an 8 ohm (A7A) transformer in the Vverb or 4 ohm (A6A) in the Vlux & Pro. Great amps, all of 'em.
    Are you sure about the Goodmans as they were UK made? Could be found in AC30s.
    I know of Jensen, Oxford, Utah, C.T.S. (alnico and ceramic versions), Pyle and sometimes JBL. But there might have been more. Some of them even came in different (sounding) versions.


    The Vibrolux had a smaller PT than Vibroverb and Super Reverb. All of them are great sounding amps.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Helmholtz View Post
    Are you sure about the Goodmans as they were UK made? Could be found in AC30s.
    I know of Jensen, Oxford, Utah, C.T.S. (alnico and ceramic versions), Pyle and sometimes JBL. But there might have been more. Some of them even came in different (sounding) versions.
    Goodmans, yes. I can only guess at some time in the early 60's Jensen couldn't fill an order, then Leo and his crew had to punt to fill the demand for Fender amps, and landed on Goodmans. I was surprised as anybody what with having to import from England and all that. Can't let speakerless amps & cabs sit in the warehouse while you make excuses. Fill the orders & don't delay - or else our competition will eat us alive!

    Somewhere around 1970, Utah became Pyle. Similar to CTS speaker changing to Eminence around that time.

    One of my customers had the question, was the speaker outfit CTS the same company that made CTS potentiometers? Anybody know?

    JBL, on special order. At some point mid 60's they painted the frames orange for Fender, and added the suffix "F" as in D120F. I've also seen a 70'ish Twin Reverb with orange-frame Cerwin Vega 12's, with Fender 'special design' labels stuck on. Orange paint = special design, I guess. Word was, that amp belonged to Paul Butterfield at one time.

    Sometime in the early 70's, Radio Shack offered speakers intended for guitar. With orange painted frames! I guess they figured guitarists would associate the color with high quality speakers. (Insert laugh track here.) They weren't awful, but they weren't too great either. Chalk it up to marketing...

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    Wait, CTS speakers became Eminence? No wonder I've never had a complaint anout any CTS speaker in an amp, as long as said speaker wasn't blown...

    Justin

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  27. #27
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Justin Thomas View Post
    So... What particular BF Bassman? Could be any of 3 different circuits - quite different. At least the Bass channels are. Look at the AA864, AA165, & AB165. Most are either AA864 or AB165. Anyway, BF Bassmans all have 4 preamp tubes for a third "gain" stage on the Bass Channel. Look into it - especially the AB165. I've never tried lifting it, but that little tiny negative feedback loop might have something to do it... Also possibly the feedback on the 6L6s, though mine came without those (already removed). Mine also came with a Twin Reverb OT, so, fwiw.

    As far as your description of BF amps in general, to me it sounds like you're describing most of the SF amps I've played, up to when they switched to the UL. There was a thread here years ago about "silverface amp pull" that was an interesting read, theorizing on why the SF had something that the BF don't. I guess we could look for the thread...

    Justin
    Unfortunately I never even looked into that. It was a sort of "Hey, you like tube amps. Check this out." And I was handed a guitar cord. I don't even know what speakers were in the cabinet. Which is a good point made by the Helmholtz and Leo discussion. And you're right, there's a mix stage for the channels, so three gain stages (if you can call unity a gain stage). So nothing at all definitive here. What I DO know is that nearly every other BF amp I've plugged into left me a little flat. Which is confusing because you do hear THAT tone often enough to know it's real. I have modded some BF amps and gotten close to that tone. I analyze tracks I hear on the radio whenever I can tell it's a bf topography amp (though it's impossible to know just WHAT bf topography amp it might be) and sometimes you can tell that the amp is mediocre, but the player is outstanding (re: Sultans of Swing. Which I think was a silver face SR?). There's a lot of that. Once in a while you'll here an amp that is really on point. The frequency curve is right and it's grabbing notes in a way that implies compression, but isn't, because it's still dynamic. And sometimes that's in the hands of an otherwise mediocre player. I've trained myself to hear through the difference between playing and amp tone... I must have! Otherwise I could never plug into an amp and know it was special because I'm not a special player.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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  28. #28
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    Hence mike6158's quote in your sig?


    Jusrin

    1 Not allowed! Not allowed!
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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