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Thread: Fender BF Vibro Champ mod clarification

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    Fender BF Vibro Champ mod clarification

    I need some experienced guys to explain what I just did.
    I read on fenderguru.com about a mod to make on the Vibro Champ. I did this and happy with the results but can someone explain what I did and how this effects the amp.
    I would like to know, as a learning experience, so that I know more about how these amps work.

    The website says that I will bypass the tonestack. What happened is the treble knob turned into a second volume knob and the bass control works but only in a very limited fashion.

    Here is the mod:

    " The default 250pF treble cap should be replaced by a 0.22µF cap. Remove also the horizontally lying 100K resistor that goes from the input of the 250pF cap (now 0.22µF) to the mid and bass caps 0.1µF and 0.047µF."

    This is what I did but the tone stack was not bypassed, but only modified, as far as I can tell. Or was it?

    Please someone tell me what exactly was achieved with this mods by changing the cap value and removing the 100K resistor.

    Much appreciated.

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    Supporting Member The Dude's Avatar
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    By changing the treble cap to the higher value, you are passing more than just treble, so that's why the treble knob "turned into a second volume knob". By removing the 100K resistor, you've unhooked the signal feeding the bass control. It's now just a pot with a .032 cap across it which will act like a variable filter of sorts.

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    This makes total sense and I get it. Thank you for your time. Just the skilled person I was hoping for.
    Can you tell me if there is a further improvement to this mod/situation that I can or should do, in order to make the amp even more useable.
    Should I connect the bass control back with just a wire, where the 100K resistor was? What would this do?
    My goal is to make the amp have a fuller thicker, warmer tone, with early distortion. A bedroom monster.
    In stock form, the amp was thin and brittle in the high end with minimal bass response. Frankly, I did not like to play it. Now it is better but still could use more bass and more gain to make it perfect.
    I may not have mentioned that I unhooked the first wire on the intensity control of the Vibrato. I read that this takes the Vibrato function out of the circuit and adds more gain to the amp.
    What would you do?

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    Supporting Member The Dude's Avatar
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    IMO, the best solution for you would be to get a distortion stomp box with EQ and just leave the amp stock.

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    Speaker? Geez, I'd have to take a ton of pictures ansmd measurements but my 79 had almost no headroom. It was awesome with a chorus...

    Justin

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    Quote Originally Posted by ES225 View Post
    Can you tell me if there is a further improvement to this mod/situation that I can or should do, in order to make the amp even more useable.
    Should I connect the bass control back with just a wire, where the 100K resistor was? What would this do?
    My goal is to make the amp have a fuller thicker, warmer tone, with early distortion.
    A simple way to make it thicker with more gain is to disconnect the tone stack's 'mid' resistor (15k?) from ground or add a 50-100k pot in series with the mid resistor to make it adjustable. I used to have the pot in a foot pedal so I could stomp on it for a preset boost.

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    I don’t think that disconnecting the trem will make much difference with the arrangement used in the VC. It has a completely different way of implementing the modulation (bias variation) than optocoupler used in the 2 channel BF amps (for which that mod would provide a gain boost).

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    According to Gerold Webber in his first book. He recommends disconnecting the Tremolo and also the tone section by removing a resistor and changing a cap value.

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    Dave H - This may be too complicated for my needs. At first I did disconnected the cap which goes to the back of the bass pot from the left lug. Then I reconnected it but removed one resistor on the board and changed the connected cap value to .2 I believe. This actually did remove the tone section, and also added more gain and bass. Then I also changed the value of the filter cap connected to pin 6, V1 and this gave it a lot more gain but my headroom is all gone as I turn it up past half way. I don't mind however. Its for bedroom playing. I did as per Gerold Webber suggested in his first book or a Vintage Guitar article.

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    Last edited by ES225; 10-05-2018 at 07:13 PM.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by ES225 View Post
    According to Gerold Webber in his first book. He recommends disconnecting the Tremolo...
    As pdf64 pointed out in post #7 that makes no sense for a Vibro Champ. Either you misinterpreted or GW was...well... "mixed up."

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    I don't recall any mention of VCs in Weber's book. All of that was addressed to 2-channel amps.

    The tremolo in the VC works quite differently in the VC compared to other amps.

    Justin

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    THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    Disconnecting the trem in two channel push/pull Fenders with the roach trem removes the trem pots 50k load from the signal chain. The VC uses a bias vary trem and disconnecting it wouldn't yield the same results.

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    Quote Originally Posted by Tom Phillips View Post
    As pdf64 pointed out in post #7 that makes no sense for a Vibro Champ. Either you misinterpreted or GW was...well... "mixed up."
    It was actually Ken Fischer who mentioned disconnecting the vibrato. Read pg 133 of the Weber book "A Desktop Reference"

    I am not here to debate anything. I simply want to increase the bass response and gain of the Vibro Champ.
    If by any chance there is an amp expert in the room, please speak up and tell me which cap and/or resistor to change.
    It should not be so difficult

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by ES225 View Post
    It was actually Ken Fischer who mentioned disconnecting the vibrato. Read pg 133 of the Weber book "A Desktop Reference"

    I am not here to debate anything. I simply want to increase the bass response and gain of the Vibro Champ.
    If by any chance there is an amp expert in the room, please speak up and tell me which cap and/or resistor to change.
    It should not be so difficult
    As noted, that only applies to models with the roach trem. Ken simply wasn't clear on that. Trust me. It absolutely does not apply to any Fender amp that doesn't use the roach trem. On the VC disconnecting the right leg of the trem pot will ONLY disable the vibrato and nothing more.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by ES225 View Post
    It was actually Ken Fischer who mentioned disconnecting the vibrato. Read pg 133 of the Weber book "A Desktop Reference"

    I am not here to debate anything. I simply want to increase the bass response and gain of the Vibro Champ.
    If by any chance there is an amp expert in the room, please speak up and tell me which cap and/or resistor to change.
    It should not be so difficult
    As to the changes you want... Yes it IS difficult. There simply isn't any opportunity for a lot more gain in in the VC without aggressive circuit modification. And since the amp is limited to power tube overdrive the bass response is relative to the output transformer and the speaker. Both are expensive parts that also require aggressive modification. What you want simply cannot be had by changing a couple of capacitors and resistors or bypassing the tone stack. Champs have $h!tty output transformers and extra $h!tty speakers. They were cheap amps when they were made. The OT and speaker is where the most cost could be cut. I don't expect any opposition here from seasoned builders and designers that those two components are the weak links in those amps. Achieving gains in the areas you desire requires upgrading both of those components.

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    Also, re: bass response on a Champ/Vibro-Champ...
    What speaker are you using? If you're talking the Original or vintage-style 8", there's half the problem. Try it through a 15" or two. The amp should have as much bass as any Fender amp; what it doesn't have is a large output tranny or a very good speaker.

    I had a 20W Eminence PA speaker in my 79 Champ. The amp had enough bass to give the famous Fender fart.

    Justin
    EDIT: to clarify: my Champ has LOTS of bass. What it DOESN'T have is PUNCH or snap. You'll have to go bigger for that.

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    Last edited by Justin Thomas; 10-06-2018 at 04:19 AM.
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    Quote Originally Posted by ES225 View Post
    It was actually Ken Fischer who mentioned disconnecting the vibrato. Read pg 133 of the Weber book "A Desktop Reference"

    I am not here to debate anything. I simply want to increase the bass response and gain of the Vibro Champ.
    If by any chance there is an amp expert in the room, please speak up and tell me which cap and/or resistor to change.
    It should not be so difficult
    You've changed this and that through the course of the thread. Where are you with the amp at this point and what are you looking for? This thing won't ever be a gain monster- not enough gain stages. You could ditch the vibrato and use that tube for an additional gain stage, but that's not something as easy as just changing or adding a part. One thing you might try would be to add a small cap in parallel with the NFB resistor. That'll give you a cut in the high frequency, effectively a bass boost.

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    Last edited by The Dude; 10-06-2018 at 04:17 AM.
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    Quote Originally Posted by Justin Thomas View Post
    ......famous Fender fart.......
    Click image for larger version. 

Name:	rofl-face.jpg 
Views:	29 
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ID:	50625

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    In another thread you have regarding a Harvard, which is the same amp as a VC by the way, you are asking about the same improvements. What has already been covered here is accurate. But I'll mention another option. You could restore the tone stack to stock so that you have EQ control at clean settings and then use a dirt box for the distortion.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Chuck H View Post
    In another thread you have regarding a Harvard, which is the same amp as a VC by the way, you are asking about the same improvements. What has already been covered here is accurate. But I'll mention another option. You could restore the tone stack to stock so that you have EQ control at clean settings and then use a dirt box for the distortion.
    Edit: Correction. It's the Bronco that's the same as a VC. Not the Harvard. My bad.

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    As noted, trying to get real bass out of a Champ, Vibro- or otherwise, is like trying to get blood from a stone. I put a Weber replacement speak in mine, and the overall tone improved some, but bass? Nah. And don't boost it with a full range booster either: it'll just fart out on you.
    Yeah, you could run it through a bigger cab with bigger speakers, but what's the point? If you are recording with it, you can squeeze a little more out with mice placement, or by blending it with DI (although phase cancellations can be an issue). With some radical surgery, you might be able to fit a 10" speaker in the cab--not that that will set the world on fire with bass response either.
    Otherwise, accept it for what it is, with its limitations--a very cool, sweet sounding, old school living room amp. (JM2˘.)

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    Supporting Member Randall's Avatar
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    In my experience, a re-cap and a Warehouse Guitars G8C 20 watt replacement speaker will bring a VibroChamp up a level or two. I have done four this year with satisfying results.

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

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