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Modded my Fender Silverface Champ


badbluesplayer

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Sounds great. I see you got the right speaker for that amp , I used the same weber sig 8 alnico in 73 SF champ I had. I used the sig 8 S which had the ribbed cone which allowed for a bit less break up , seemed to hold the bottom end better. On the boost switch you have as you said many fellows remove that 15K resister to attempt to get that tween champ sound but doing so basically renders the tone controls inop. The 15k resister is a fixed mid on the tone since there is ony a treble and bass pot which I'm certain you already know.

 

I was looking at you chassis and you mentioned new caps which there is the cap can and then on the board itself I see the rather large orange cap on the left near the power transformer, I'm not sure if you have a three wire grounded power cord or not , if not remove that termed death cap and install a grounded three wire power cord. I am only making a suggestion and don't know how much you play the champ since you seem to have other amps. The Cathode bias resister looks stock and many like the stock value of 470 Ohms which does run the 6V6 power tube hotter on todays line voltage it's fine to leave it stock and allows more grit yet as you may know a 6V6 should run at not much more than 12 watts with the stock 470 ohm cathode resister mine champ was running at 17.76 watts. Many people have run them hot for years without issue , mine seemed to have the stock RCA 6V6 tube still going strong yet even Euro Tubes who sells JJ tubes which can take more than most tubes can told me it's best to run it near 12 watts as possible. It will raise the B+ and plate voltage close to 20VDC if you say use a 680 Ohm resister you need to experiment to get the exact resistence since each amp is a bit different but the end result is longer tube life and a more open sound. Also it's best to use a 5 watt cerment resister since they get real hot and raise then off the board and also the cathode resister electrolytic cap across the resister should be more or at least 50 volt rating.

Some people not all that long ago were using 1k cathode resisters but that was way to much and the amps sound suffered a bit.

 

You may already know this . I just thought I would pass it along to you and anyone else just in case. Another little mod you can try is to add a switch to break the NFB loop at the speaker jack that will offer a bit more grit as well.

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I may need some help adding a 3-prong cable to my Vox Student amp soon, probably should change the caps since it is such a simple circuit and it does have a little hum.

 

I am digging these low watt amps from the 60's, I may get some more.

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I may need some help adding a 3-prong cable to my Vox Student amp soon, probably should change the caps since it is such a simple circuit and it does have a little hum.

 

I am digging these low watt amps from the 60's, I may get some more.

 

 

Yes any tube amp with electrolytic caps over 10 years old should be replaced . If they leak of blow then they mess up the amp big time. Three prong cords are easy to install . same way as a two wire cord and the green lead goes to the chassis for the ground. Usually the other leads are black and white.

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I was looking at you chassis and you mentioned new caps which there is the cap can and then on the board itself I see the rather large orange cap on the left near the power transformer, I'm not sure if you have a three wire grounded power cord or not , if not remove that termed death cap and install a grounded three wire power cord.

 

Thanks for the heads up. The amp has a three prong power cord. Not sure if somebody changed it or what. So what you're saying is I need to just cut the big orange drop cap right out of there, right?

 

Edit - I answered my own question.

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Informative AND educational.

 

Perhaps, you can further my education a bit: Regarding the bright switch. I was under the impression that Fender amps (Blackfaced) that did NOT have the bright switch (Mainly the Deluxe Reverb) had the bright cap. Basically, always in "bright" mode. And, I seem to think I remember this was in the reverb channel and not the other.

 

I also remember something regarding the difference in tone between Silverfaced models with the "pull boost" sounding different than those without, and relating it to this. Also remember something about it being common to clip the bright cap in Blackfaced models, as something to be aware of when listening/buying a Blackfaced amp as to why some might sound different in that regard.

 

I could have this all backwards. I really can't remember if I knew it and forgot, knew it and somehow remember it backwards, or knew it backwards from the start. Or, if I am remembering something completely different. See, don't know what I knew, and don't know if what I knew was right. Or if I knew at all.

 

Anyway, good question, isn't it? I don't NEED the answer, but if you have it, it would be handy to know.

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Another question. This one is a bit deeper:

 

One MAJOR difference I have noticed between Blackfaced/Silverfaced Fenders and Tweed Fenders is the way the volume control acts. Specifically, the way the volume on the guitar works differently between the two.

 

Now, generally speaking (very generally), when you are in the "break up" realm, when you turn up the volume on the guitar, you get more break up as opposed to just more volume. Or, if thinking in reverse, lower the volume on the guitar and you get less. 2 separate things here: one, using the volume control on the guitar works to adjust volume. Good thing. But the other thing, is that when an amp is close to break up, the turning UP the volume results in more break up as opposed to just volume, as the amp is generally at the limit. Sometimes, no extra volume at all, just sweet distortion.

 

Said all that to say, that the MAJOR difference between Tweed ones and Blackfaced/Silverfaced ones is that, when you do this on a Blackfaced Fender, it gets BRIGHTER as you turn up. Much more so than the Tweed ones. So, often times using the volume on the guitar results in having to change the tone settings as you turn up. With the Tweed ones, generally the tone, or "brightness" levels, stay pretty much the same.

 

So...realizing also that the front end, the tone controls, on both amps are also different: Blackfaced Fenders the tone controls stop all volume when all the way down, as opposed to the Tweed models where you can turn the tone knobs (bass, treble, or whatever they have) and the volume level is much the same throughout the range.

 

I am wondering, how does this all relate? Does the mod here to the "boost" effect this? And, where are we at as far as differences between Tweed and Blackfaced Fenders where this mod is? (Or, does it have anything to do with it?)

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Informative AND educational.

 

Perhaps, you can further my education a bit: Regarding the bright switch. I was under the impression that Fender amps (Blackfaced) that did NOT have the bright switch (Mainly the Deluxe Reverb) had the bright cap. Basically, always in "bright" mode. And, I seem to think I remember this was in the reverb channel and not the other.

 

I also remember something regarding the difference in tone between Silverfaced models with the "pull boost" sounding different than those without, and relating it to this. Also remember something about it being common to clip the bright cap in Blackfaced models, as something to be aware of when listening/buying a Blackfaced amp as to why some might sound different in that regard.

 

I could have this all backwards. I really can't remember if I knew it and forgot, knew it and somehow remember it backwards, or knew it backwards from the start. Or, if I am remembering something completely different. See, don't know what I knew, and don't know if what I knew was right. Or if I knew at all.

 

Anyway, good question, isn't it? I don't NEED the answer, but if you have it, it would be handy to know.

 

All good questions.

 

You might be right about the bright circuit on the DR's. I guess it's "always on" on the reverb channel. I remember putting a bright switch in my DR, but I forgot the details. [crying]

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Cool mod John. From what I hear with good headphones on my laptop, the bright makes it a little more trebly. The boost works very nice at both lower level and, when you turned up the Les Paul's volume. It sounds clean as well, from what I was hearing. Good job! I have a 78 VC myself. I would love to get a second one or, better yet a blackface VC or Champ.

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So I went to put the amp back on the shelf and I flung the rectifier out of the back of the amp and smashed it on the floor. My nice original Sylvania black plate. [crying]

 

So I figured I'd better put in some new sockets with retainer clips and change out the cathode bias resistor and cap like catnine suggested. I'm hoping the ceramic resistor will give it a "heavier, more solid" tone. [laugh]

 

I'm going to put in a feedback loop interrupt switch, but I want to have 3 settings - on, off and halfway(or maybe adjustable). So I need to do some more homework.

 

DSC_0016.jpgDSC_0009.jpgDSC_0017.jpgDSC_0011.jpg

Previous mod by Fido:

DSC_0018.jpg

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So I went to put the amp back on the shelf and I flung the rectifier out of the back of the amp and smashed it on the floor. My nice original Sylvania black plate. [crying]

 

So I figured I'd better put in some new sockets with retainer clips and change out the cathode bias resistor and cap like catnine suggested. I'm hoping the ceramic resistor will give it a "heavier, more solid" tone. [laugh]

 

I'm going to put in a feedback loop interrupt switch, but I want to have 3 settings - on, off and halfway(or maybe adjustable). So I need to do some more homework.

 

DSC_0016.jpgDSC_0009.jpgDSC_0017.jpgDSC_0011.jpg

Previous mod by Fido:

DSC_0018.jpg

 

 

I heard a few people who suggested installing a much larger value Cathode bypass cap would add more bottom end , I never tried this. I did change mine to a higher voltage since these amps did originally run on 115VAC. I see you got rid of the orange Death cap . As far as the feedback loop if you want a variation you first need to find a place for a pot or switch. The only thing you are doing as far as variable is remove the lead to the pos side of the speaker jack coming off the 2.7K ohm resister on the board . If you decide to use a pot for variable a 50K linear like an Alpha B50K. Put the lead off the 2.7K resister on the center pot terminal and depending which way you want to add the resistance use either of the outside legs of the pot back to the speaker jack . The more resistance added basically opens the wire to the speaker jack lead like an on/off switch. The chassis I used had a spot for a fender DPDT slide switch so I used one side of that to either have it stock NFB or go through a pot . I used a 100K pot but past half way doesn'ty make any differece so 50K is more than enough. Or you can just not mount a pot and just wire one in so you can dial it to a spot or a few spots that sound good to you measure the resistance across the pot out of the circuit then use a switch with a resister with that value so if you had a spdt switch you could have one added resister and stock . main thing is keep the switch near the output side of the chassis. Any good mini toggle would work. The stock fender on/off slide switch is not high amperage. Perhaps even a 50k pot with an on/off switch on the back like old champs had would offer stock NFB and then turned on offer variable. My personal opinion is you may find that an on/off is all you may need because hearing wise I leave the pot full up or OFF then switch the slide to stock. I don't really hear a lot of diff adjusting it maybe a bit as I just add a bit of resistance then it seems to sound just about the same as off. I even used a rotary switch with 6 resister combinations and just jumpered it in and found it subtle besides finding a spot to place this rather tall large switch. I would just jumper a pot in series and dial it in to see what you like then go with that.

 

If you are after more bottom and you have a cab with one 4 ohm twelve or 2 8 ohm twelves wired parallel for 4 ohm try that I did and it's a different champ and won't hurt the OT.

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I added the negative feedback loop switch. Now you can disconnect the loop or engage it (like normal) or engage it with an extra 15k or so of resistance - for a kind of half way setting. It makes it a little more broken up or loose sounding when you cut in some more resistance. [thumbup]

 

DSC_0002-1.jpg

DSC_0004-1.jpg

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I added the negative feedback loop switch. Now you can disconnect the loop or engage it (like normal) or engage it with an extra 15k or so of resistance - for a kind of half way setting. It makes it a little more broken up or loose sounding when you cut in some more resistance. [thumbup]

 

DSC_0002-1.jpg

DSC_0004-1.jpg

That's probably the best solution. With a pot it's difficult to find any spot other than half way or off or on , witha switch you have a fixed setting. What did you use a spdt center off toggle? here is another thing you can do if you. want.

layout4.gif

This diag shows two 6V6's in a push pull amp you just have one so use one socket it's the same deal.

This shows two resisters added to the 6V6 socket , you see you need to swap pins to do this . Pin 8 remains as is because this is your cathode bias. The reason the wires to the socket pins are moved is because there are pins on the socket not used. one is called a grid stopper 1.5k ohm 1/2 watt or 1 watt ,this prevents instability at higher volumes , the cap you have there in a sense was put there to do the same thing, Some champs had them and some didn't as far as that cap goes. The other is a screen resister 470 ohm 3 watt. It's purpose is to keep the screen voltage or current draw from being higher than the plate , for some reason many champs seem to have the screen voltage a few volts higher than the plate and is considered normal , and if for some reason the 6V6 shorts out the screen resister saves the amp/OT basically. Neither were put on champs mainly to save money and many people never had an issue after 40 years. There was a forum with knowledable people who claimed if they used a 1k ohm 5 watt screen resister the amp would sing more . I meant to try it yet never did.

 

Here is something on cathode bias and why it's important. Now this is not to say you will ever have issues yet to be honest and you will hear many champ users say the like the 6V6 to run hot and it loses the crunch if you don't run it hot but this is not true . I have raised the value of the 470 ohm cathode resister on mine from 470 to 660 ohm and I am right at 14 watts with the 470 ohm you may very well be running the 6v6 at 18 to 19 watts and the amp will sound harsh . There is all sorts of info on this out there yet if you ask tube suppliers they will tell you not to run the 6V6 past 14 watts even JJ's. Closer to 14 watts the amp will have crunch but will also gain bottom end and the clean will be better at lower vol settings. It's all about the specs for 6v6 tube clipping and there is a good range but it's not 18 watts. By doing these changes the plate voltage will raise a bit maybe 10VDC the tubes can take it easily and it's not enough to harm anything and higher plate voltage = more output. The champ was meant to be a practice amp and sound as clean as possible thus the NFB loop and vol/treb/bass stack.

 

If you like your amp the way it sounds leave it as is , If you like to experiment try what I said. I know my champ sounded brittle and harsh biased so hot . Now it just breaks up

 

VintageJon03-14-2006, 07:24 PM

Here tis again.

 

Remove the 12AX7,(s), to remove any plate current being pulled.

(Thus only the screen current will be indicated for the 6V6...)

 

Measure the actual value of the 1K resistor coming from pin 8 on the rectifier. Write it down as Rs.

 

Measure the 6V6 cathode resistor, write it down as Rk.

 

Measure the voltage across the 1K coming from the rectifier pin 8.

Divide this Voltage by the resistors actual Resistance. Write it down as this is the 6V6's Screen current. Let's call it Is.

 

Turn amp on and let it warm up. Measure voltage across Rk. Write down as Vrk.

 

Measure plate voltage and write it down as Vp.

 

Get yer calculator and figure these:

 

Vrk / Rk = Ik or cathode current.

 

Ik - Is = Ip or plate current.

 

Ip X Vp = IPD or Idle Plate Dissipation in Watts. This should not exceed 14W.

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Ouch. 22 watts.

 

The voltages are high all over the place. Heater voltage is o.k., though. I have a solid 125 V here, so I guess that's an issue. I think I'll try to get the voltages down on the board and swap out the cathode resistor at the same time. I think it has some kind of intermittent connection thing going on with the heater wires but I may have solved it.

 

Lots of fun. I'm learning a lot. [biggrin]

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Ouch. 22 watts.

 

The voltages are high all over the place. Heater voltage is o.k., though. I have a solid 125 V here, so I guess that's an issue. I think I'll try to get the voltages down on the board and swap out the cathode resistor at the same time. I think it has some kind of intermittent connection thing going on with the heater wires but I may have solved it.

 

Lots of fun. I'm learning a lot. [biggrin]

 

There is no a lot you can do to get the volatage down across the amp with out using zener diodes. It depends on what voltages you are to as a source of reference. The PT is going to put out more on a silver face champ than on the blackface champ because fender upped the PT voltage on the silver face. My 73 SF champ ran 374VDC on pin 3 the plate and 385 B+ which is just at the point where the 5Y3 red lead off pin 8 connects to the board. You can with everything off and unplugged and the electrolytic caps drained check the value of the two resisters right after the cap can 10K and 1K and see what they are but they would really have to have drifted a lot to get high readings. My wall voltage is at it's highest 119VAC.

 

On my champ build I used a Weber PT and it has 2 leads for line voltage one is 120 VAC and the other 125VAC I used the 125 VAC to bring the line down a tad because that tap goes through more windings , I did this simply because the 6.3 heaters were a bit higher and this brought them down to 6.3 . It also has two OT secondary output taps 300-0-300 VAC and 330-0-330 VAC I used the 330-0-330 because a SF had close to 325-0-325 Volts and my B+ is close to 392VDC off the 5Y3. I also did not remove the 12ax7 when I took my readings not that it draws all that much . The Fender champ schematic shows 360 VDC after the 5y3 yet they allow + or - 20%.

 

If you could let me know what the voltages are the B+ and 6v6 pin 3 /plate and after the 1K first dropping resister and then after the 10K dropping resister . I wouldn't worry much about these voltages unless they are way out there. Mainy what I would do is get the 6v6 at or below 14 watts . The new cathode resister you have not might be lower or higher than 470 ohm so just based on what i needed try a 680 ohm 5 watt / You can't calculate these you have to experiment , I did this at first from what I had by using some in seriers or parallel and came up with 710 Ohm then it was 12.5 watt so 689 ohm brought it right at 14 watt.As you know they only come in certain values. Now you will see a raise on the B+ and plate voltage because the 6V6 is now not drawing so much current. 22 watts is sort of high . Some change the value of the dropping resisters but it taks a lot to drop the voltage a litte and most don't do this . Even each 6V6 will draw more or less even the same brand . like I said the main thing is the cathode bias and having the cap can rated more than the B+ so if you have 400 VDC B+ you want at least 475 or 500VDC cap can or filter caps. My voltages are not even across the board the 12AX7 is a bit higher than I would like yet it's still in spec. You can't go by the fender champ schematic because it is based off 115 VAC line. A JJ 6V6 cab handle 500 volts on the plate a JJ 12ax7 cab handle 300 VDC on the plate but I doubt you are over that or even near that. I have to look for the specs I got on my champ build but I did remember the B+ being 390VDC and the JJ 6V6 plate at 370 VDC JJ 12ax7 somewhere at 230 VDC . What you don't want to do is change values or resisters or caps because this will change the sound of the amp, You can try different brnads of 12xa7s but I found the jj to be a bit warmer and darker than NOS rca or EH because my champ was to bright . The JJ 6V6 has the most bottom end.

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I think I have this sucker licked. I put in a bigger 1.8k screen resistor and a 15k power resistor and then replaced the 100k resistors feeding the preamp tube with 150k's.

 

Then I put in a 680 ohm cathode resistor.

 

Now I have B+ = 411v. The plate voltage is 375 to 385v. I have about 220 to 225v at the 12ax7.

 

Now one of my JJ 6v6's runs at 15.3w and one of my RCA 6v6's runs at 13.9w. [thumbup]

 

The amp sounds great. The NFL switch works like a charm. Tweed, Silverface and in between. [thumbup] [thumbup]

 

Now I'm going to make the boost footswitchable. DaveinSpain gave me the idea. [thumbup] [thumbup] [thumbup]

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I think I have this sucker licked. I put in a bigger 1.8k screen resistor and a 15k power resistor and then replaced the 100k resistors feeding the preamp tube with 150k's.

 

Then I put in a 680 ohm cathode resistor.

 

Now I have B+ = 411v. The plate voltage is 375 to 385v. I have about 220 to 225v at the 12ax7.

 

Now one of my JJ 6v6's runs at 15.3w and one of my RCA 6v6's runs at 13.9w. [thumbup]

 

The amp sounds great. The NFL switch works like a charm. Tweed, Silverface and in between. [thumbup] [thumbup]

 

Now I'm going to make the boost footswitchable. DaveinSpain gave me the idea. [thumbup] [thumbup] [thumbup]

 

Screen resister? do you mean you changed the two resisters the 1K and 10k that sit at the left end on the board looking at the board from the top rear of the chassis ? If so you changed both the dropping resisters which is ok to do to a higher value which in turn drops the VDC down as does changing the 100K to 150K which further drops the plate VDC to the 12AX7 . Do you know what the reading were before the resister changes? Since my line voltage is never higher than 119VAC I didn't have to change the values and left them all stock other than the cathode . On the 12AX7 there are actually two 100K plate resisters that are pin 1 and pin 6 since a 12ax7 is actually two triodes in one tube , they are just to the left of the 3 caps that form the tone stack on the board and connect in a Y form one end to pin 1 and one to pin 6 where they tie together they are then wired to the one side of the 10k dropping resister. So you should check the voltage on both pin 1 and 6 on the 12AX7 . Basically you have the b+ from the 5Y3 then the one 20 MF cap of the three in the cap can then the 1 k dropper then another cap in the can cap then the 10K then the last of the 3 caps in the cap can this it the voltage dropping and filter stage , then after the last cap it goes to the junction of 2 100k plate resisters off to pin 1 and 6 of the 12AX7 . The plate pin 3 of the 6V6 comes right off the 5Y3 so there is no dropping resister until pin 4 the screen of the 6V6 . If you just changed one 100k to the 12AX7 then you either changed the Plate voltage for the preamp stage before the tone stack or the second gain stage after the tone stack, both are the 2 plates of the 12AX7. , pin 6 is the second gain stage of the 12AX7 after the tone stack. It won't hurt anything just changing one and if you like the sound let it be. many call this tweeking to get the amp to sound they way YOU like it. ie they build an amp from spec then change values of caps and resisters to either get more or less gain from a certain stage . As long as you don't go over the max plate VDC all is fine to do. The bias is cathode on the 12AX7 some play with values there as well for a different sound. I just never did it simply because you can't toss a switch in there to hear the difference and it becomes complicated. You can use a switch on the 12AX7 bias so you can change the bypass cap value or the cathode bias resister value or both but I wouldn't flip a switch while the amp is on. Most just add a bypass cap in parallel to the existing bypass cap and use a switch to add the extra one in these caps add gain to the stage , changing the cathode bias resister value sets the bias of each tride of the 12AX7 same as you did on the 6V6 and that relates to current draw . Pin 3 ands pin 8 are the cathodes on a 12AX7 . I just wanted a SF champ to run with in spec voltages so not to burn a tube and not to reinvent the amp. It gets all confusing to me even though I understand it . I have the champ I built and a 6V6 push pull and both sound fine . I did a few changes on the 6V6 push pull amp to get the tone and sound I was after but all that was was splicing the preamp section and tone stack of a 6G2 brown princeton to a tweed deluxe 5E3 power section and changing the cathode bias of a 5E3 to a adjustable fixed bias of the 6G2 . Many fender older amps are a lot alike plus on a push pull amp you have the phase inverter added in which a SE does not have. They are both very simple amps . I would like a tweed princeton 5F2A which is a champ with only a tone and vol pot basically I did build one years ago as just a head . It's just more gritty sounding . My 6V6 push/pull amp began as a 5E3 but I didn't have a place for 4 inputs or another vol pot but I wired it the same and didn't like the high gain sound. I did try a BF stack on it and that sucked all the tone out so I decided to use the tone stack Vol and Tone only and got where I wanted. Fender used the same vol and tone setup on many small amps brown and smaller tweed but the 5E3 deluxe is really only wired different with the vol before the tone pot.

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