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76' Fender SF Champ mods and help!


NylesR

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I have a 76' SF Champ and I've recapped it, new Jupiter .022 caps new resistors, the whole shebang, put NOS tubes in it and replaced the 8" speaker and I want to love it but the reality is it sounds horrible with my Les Paul's anything I have with humbuckers (sold my Strat) so all I have is guitars with HB's. But I refuse to sell it so I want to mod it. I live in a small space and don't need or want anything larger than a 10' speaker in it so that will be mod #1, and either I buy a new cabinet for it or enlarge the existing baffle . Almost of my guitars are equipped with humbuckers and I swear they all sound horrible in anything other than three bridge position, it's so fat and farty, muddy and just drowning in bass,just waaay to much even with the bass on 1. So that's my biggest issue right now. Any advice or any mods that I can do would be awesome. Thanks!!

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Other then changing the tone stack caps to a different value, there's just not allot you can do.

 

I would try using different tubes. NOS means absolutely nothing. They are no guarantee the amp will sound good and the chances you simply bought someone else's old worn out tubes is extremely high. 20 years ago you may have come across some old ones some Hi Fi store had saved, but anything you see being sold today are likely to be used tubes and sticking them in an amp is like sticking used string back on a guitar. Tubes sound best when they are used. Sound quality declines as they age and that's when most people sell them.

 

The quality of new tubes being made today equal just about anything made in the past. There were many old tube versions that flat out sucked for guitar amps too. I know because I'm old enough to have grown up during that era and got an degree electronics when they were still teaching that technology. All my fender amps used to sound best using big bottle RCA power tubes. Allot of the others simply didn't hold up or became microphonic.

 

Not sure what you have in there now, but I recently upgraded my Bassman to Electro Harmonix tubes and it did an amazing job cleaning up the mud tones and tightening up the bass response. I'd definitely start there, especially the 12AX7 preamp tube. The EH is one of the brightest and tightest tubes, if the amp still sounds woofy, then try a different power tube

 

Going to a larger speaker is likely to increase the woffy bass tones. The amp is voiced to target a 6" speaker and make it sound big an bassy. Adding a larger speaker is going to make it sound even more bassy so I suspect you created your own problems here. If you do get a different speaker look at the speakers frequency response charts and be sure you aren't compounding the problem by choosing a speaker that lacks a good treble/mid response.

 

Wattage and SPL is critical in a small amp too. You want a wattage rating about double a tube the amps RMS value because tube amps are rated for clean tone, in your case 6 watts clean volume. The head can probably produce an additional 50~100% clipped volume however so you'd want a 10~20W speaker. And since you use mostly humbuckers I'd likely choose a 20W just to provide maximum headroom.

 

The other item would be the speakers SPL level. A high SPL speaker can make the amp much louder. A stock speaker for example may only be 90dB SPL. A 100~110dB SPL speaker can make the amp sound many times louder. +3dB is an actual doubling. a +10dB is a perceived doubling. Going from a 90dB speaker to a 110dB can sound as loud as a 50W amp in comparison.

 

What this does is allow you to run the volume lower, and have more clean headroom which in turn prevents those humbuckers from over saturating the power amp yet you still have a good volume level without the flab.

 

 

Again, the amp is only 6 watts and most of your saturation is going to come from the power tube and transformer. There are a couple of mods out there you can google up and try. I really cant advise you beyond what I have however. I'd have to hear a sound clip in order to decide where you can and cannot take the amp. I have heard some people getting improvements by putting in a higher quality output transformer. may be something worth looking at.

 

You wont be able to make this amp sound like its bigger brothers however. These were small budget practice amps, no more. They weren't designed to be driven hard, just produce enough volume to practice with or accompany an acoustic guitar or piano. Some people found the amps could be pushed a bit to get a decent drive but its really a matter of what guitar you use to do that. Because you own allot of guitars with humbuckers, doesn't mean this is the right amp for them.

 

As with most vintage Fenders amps, they sounded best with the pickups being produced at that time. Classic PAF's had around 7.5K resistance. Even those sounded Farty in smaller Fender amps. Many newer HB's are well up into the 10~20K ranges and would undoubtedly make a small champ sound like garbage.

 

When I bought my Blackface in 67 I tried Single coil guitars like Fenders couldn't make the amp saturate. Full sized HB's were OK for clean chords but they had way too much bottom end for my tastes. The ideal pickups wound up being P90/100 and Mini Humbuckers. You could push the amp hard and get saturated tones without passing gas.

 

You may just come to the conclusion as most guitarists do, that a Guitar and amp have to be a good match in order to get the best tones. The likelihood of getting that amp to sound killer with full sized humbuckers is low. It was designed in the 60's long before most humbuckers. They used cooler winds which give the sound a wider frequency response and allot more highs. When you consider what you've already done and considering on doing, I'd say you got the wrong amp for your needs. Either save it as an investment and let it go up in price to recover your investment. Then buy something more suitable to your immediate needs or sell it and get something more suitable.

 

Or you could start buying guitars that make the amp sound good. Guitars with various vintage single coil designs, low output Lipstick pickups, etc. Find another amp that can take HB's and not fart out. heck my little 15W Marshall with take HB's very well. Its designed to.

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Thank you for your reply. I have not put a 10" speaker in yet, was just an idea to possibly create more headroom.

The tubes I have I are a Sylvania mid 50's 6v6 that tested excellent 800 something where 600 was passing, I have a few mullard ax7s and a couple Telefunken's again all tested very high for quality and life. The 5y3 I can't remember I think it's an old rca but again excellent scores so I highly doubt it's the tubes given it sounds the same for the most part no matter which I swap out. I feel like the amp when I got it had way to much bass but it seems even worse now. Even with my Strat before I sold it the bass was so high it just seemed wrong, even with the bass as low as it will go. The tone caps are Jupiter .022uf and that should have helped cut out the bass compared to the orig ones I think they were .1uf caps. I really just want to have more headroom and volume for clean playing and use my pedals for grit/dirt and overdrive/fuzz. Not sure if I should just give up the amp and sell it or keep it. I understand everything else you said, just wondering if it's worth modding or not. I don't mind putting a couple hundred bucks into it if it will get me to where I want it. But if not I'll just sell it.

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I bought this amp a few months ago and when I open it up it's had all the caps and resistors changed out, looks like they used good parts, Jupiter and Sprague stuff. I bought some resistors from the vintage fender amp site thinking we'll maybe it just needs a new resistor or two. That came last week and today is the first time I've looked at a schematic of my amp next to the internals of the amp. Comparing it with the schematic it looks like whoever did it rewired a lot of parts different that the schematic shows. I'll try to add a picture of it

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The SF Champ is a great sounding amp - in spite of the 'small budget practice amp, no more' claims made in a previous post.

 

I've been recording with one for many years and even used it live a few times when my Twin Reverb suddenly stopped working.

 

The important thing with the Champ is getting the speaker impedance right. The amp is rated for 3.2 Ohms and will not sound its best driving an 8 Ohm load. I had an old 4 Ohm Jensen alnico radio speaker in mine and, on occasion, ran it through the two EVM12s in my Twin which presented a 4 Ohm load and sounded fantastic.

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The tone caps are Jupiter .022uf and that should have helped cut out the bass compared to the orig ones I think they were .1uf caps. I really just want to have more headroom and volume for clean playing and use my pedals for grit/dirt and overdrive/fuzz. Not sure if I should just give up the amp and sell it or keep it. I understand everything else you said, just wondering if it's worth modding or not. I don't mind putting a couple hundred bucks into it if it will get me to where I want it. But if not I'll just sell it.

 

If the .1 cap was changed in the tone stack its the wrong cap to mess with. The way the caps work with the pots are band passing. The higher the value the more bass is passed, the lower the value the less bass is passed.

 

With the treble pot turned to the 250pf cap only high frequencies are being passed. Bit mids make it through the .1 cap. When the treble pot is at the other extreme it gets mostly mids through the .1uf cap and less highs from the 250Pf. In the middle it gets some from both.

 

 

The bass pot turned towards the .1uf cap produces maximum bass, Turned towards the .047 it produces maybe half as much. In between of course gives you 50/50% of both caps.

 

The cap you want to try lowering is the .047 cap. Make it .022 and it should give the amp more bass attenuation when the pot is turned towards it. you don't want to mess with the .1 because that's your midrange/center frequency cap depending on where the treble pot is set.

 

There is one other possibility here. If the pots were replaced they may have chosen the wrong taper. The tone pots need to have the right taper for the tone controls to work properly.

 

The Bass pot should be a 250K audio pot. The treble pot should be a 250K linear. If the tape is wrong it would explain why you have to turn the pot way down to get any effect. The taper may simply be wring. The pots you want are here. The first two 250K pots on this page

 

http://amprepairparts.com/pots.htm#fendercts

 

the first is for bass

 

fetch?filedataid=121760

 

 

the second if for treble.

 

 

fetch?filedataid=121761

 

 

 

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Other then changing the tone stack caps to a different value, there's just not allot you can do.

 

I would try using different tubes. NOS means absolutely nothing. They are no guarantee the amp will sound good and the chances you simply bought someone else's old worn out tubes is extremely high. 20 years ago you may have come across some old ones some Hi Fi store had saved, but anything you see being sold today are likely to be used tubes and sticking them in an amp is like sticking used string back on a guitar. Tubes sound best when they are used. Sound quality declines as they age and that's when most people sell them.

 

The quality of new tubes being made today equal just about anything made in the past. There were many old tube versions that flat out sucked for guitar amps too. I know because I'm old enough to have grown up during that era and got an degree electronics when they were still teaching that technology. All my fender amps used to sound best using big bottle RCA power tubes. Allot of the others simply didn't hold up or became microphonic.

 

Not sure what you have in there now, but I recently upgraded my Bassman to Electro Harmonix tubes and it did an amazing job cleaning up the mud tones and tightening up the bass response. I'd definitely start there, especially the 12AX7 preamp tube. The EH is one of the brightest and tightest tubes, if the amp still sounds woofy, then try a different power tube

 

Going to a larger speaker is likely to increase the woffy bass tones. The amp is voiced to target a 6" speaker and make it sound big an bassy. Adding a larger speaker is going to make it sound even more bassy so I suspect you created your own problems here. If you do get a different speaker look at the speakers frequency response charts and be sure you aren't compounding the problem by choosing a speaker that lacks a good treble/mid response.

 

Wattage and SPL is critical in a small amp too. You want a wattage rating about double a tube the amps RMS value because tube amps are rated for clean tone, in your case 6 watts clean volume. The head can probably produce an additional 50~100% clipped volume however so you'd want a 10~20W speaker. And since you use mostly humbuckers I'd likely choose a 20W just to provide maximum headroom.

 

The other item would be the speakers SPL level. A high SPL speaker can make the amp much louder. A stock speaker for example may only be 90dB SPL. A 100~110dB SPL speaker can make the amp sound many times louder. +3dB is an actual doubling. a +10dB is a perceived doubling. Going from a 90dB speaker to a 110dB can sound as loud as a 50W amp in comparison.

 

What this does is allow you to run the volume lower, and have more clean headroom which in turn prevents those humbuckers from over saturating the power amp yet you still have a good volume level without the flab.

 

 

Again, the amp is only 6 watts and most of your saturation is going to come from the power tube and transformer. There are a couple of mods out there you can google up and try. I really cant advise you beyond what I have however. I'd have to hear a sound clip in order to decide where you can and cannot take the amp. I have heard some people getting improvements by putting in a higher quality output transformer. may be something worth looking at.

 

You wont be able to make this amp sound like its bigger brothers however. These were small budget practice amps, no more. They weren't designed to be driven hard, just produce enough volume to practice with or accompany an acoustic guitar or piano. Some people found the amps could be pushed a bit to get a decent drive but its really a matter of what guitar you use to do that. Because you own allot of guitars with humbuckers, doesn't mean this is the right amp for them.

 

As with most vintage Fenders amps, they sounded best with the pickups being produced at that time. Classic PAF's had around 7.5K resistance. Even those sounded Farty in smaller Fender amps. Many newer HB's are well up into the 10~20K ranges and would undoubtedly make a small champ sound like garbage.

 

When I bought my Blackface in 67 I tried Single coil guitars like Fenders couldn't make the amp saturate. Full sized HB's were OK for clean chords but they had way too much bottom end for my tastes. The ideal pickups wound up being P90/100 and Mini Humbuckers. You could push the amp hard and get saturated tones without passing gas.

 

You may just come to the conclusion as most guitarists do, that a Guitar and amp have to be a good match in order to get the best tones. The likelihood of getting that amp to sound killer with full sized humbuckers is low. It was designed in the 60's long before most humbuckers. They used cooler winds which give the sound a wider frequency response and allot more highs. When you consider what you've already done and considering on doing, I'd say you got the wrong amp for your needs. Either save it as an investment and let it go up in price to recover your investment. Then buy something more suitable to your immediate needs or sell it and get something more suitable.

 

Or you could start buying guitars that make the amp sound good. Guitars with various vintage single coil designs, low output Lipstick pickups, etc. Find another amp that can take HB's and not fart out. heck my little 15W Marshall with take HB's very well. Its designed to.

 

985 words on an amp that has something like 30 parts.

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I have a 76' SF Champ and I've recapped it' date=' new Jupiter .022 caps new resistors, the whole shebang, put NOS tubes in it and replaced the 8" speaker and I want to love it but the reality is it sounds horrible with my Les Paul's anything I have with humbuckers (sold my Strat) so all I have is guitars with HB's. But I refuse to sell it so I want to mod it. I live in a small space and don't need or want anything larger than a 10' speaker in it so that will be mod #1, and either I buy a new cabinet for it or enlarge the existing baffle . Almost of my guitars are equipped with humbuckers and I swear they all sound horrible in anything other than three bridge position, it's so fat and farty, muddy and just drowning in bass,just waaay to much even with the bass on 1. So that's my biggest issue right now. Any advice or any mods that I can do would be awesome. Thanks!![/quote']

 

 

The mud in the bass is produced by one primary thing:

Bypass cap of V1A preamp tube cathode.

By reducing the value of this, subsonic frequency does not enter the amp...and will not cause the amp to fart out in the lows.

See the calculator:

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

If you start w/ .68uF and increase from there till you get it just right, without being too much subsonics.

 

That's the first, most important starting point.

 

Second, You can put in a 10" speaker...I did this myself,

but changing the speaker does NOT increase the headroom.

The headroom is a function of power supply voltage....that's all folks. You cannot "increase" headroom, until you increase the power supply voltage.

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