If I were to change the cathode bypass cap on a SF champ second gain stage

[catnine]

to a higher value will it just increase the gain of that stage or will it roll off some highs? I can't tell if the second gain bypass cap has any affect on the NFB loop other than sharing the ground through a 47ohm resister . I thought I may make the added 25 uf bypass cap switchable using a fender dpdt slide switch , just add it in parallel on the + side and break the neg side with the switch.

 I was looking at the SF vibro champ and it has a 10uf cap and the SF champ has a 2 uf cap . Another confusing part is the Bronco has the same cap value as the vibro champ yet the PT has a higher B+ , seems to be a choice in PT's but they do not state which one but it seems they used the same PT as the other SF champs. Some PTs are like125P1B or 022 772 but these cross reference to the same thing , perhaps different manufacturers?

[PRR]

It would improve bass. In this case, the excess gain available for NFB to control speaker bass resonance.

10uFd is plenty big, unless you are doing bass in large speakers.

No cap will work, and the difference may be very subtle.

[catnine]

It would improve bass. In this case, the excess gain available for NFB to control speaker bass resonance.

10uFd is plenty big, unless you are doing bass in large speakers.

No cap will work, and the difference may be very subtle.

 I'm not certain what that means. Right now the SF Champ build I have has the same 2uf cap on the second gain stage after the tone stack so if I change it to 10 uf of 25uf since that's all I have on hand won't the higher value cap allow all the guitar frequencies through rather than focus on mainly the highs?

[Willabe]

Just try it and see what you think/hear.

It's about as easy a test their is.

The larger the bypass cap value the more bass. We can use the K bypass cap to tune frequency response in the preamp. Need less bass? Keep subbing in smaller bypass caps until your happy.

Tubenit and Geezer come to mind as having spent hours fine tuning their builds by changing bypass caps, plate R's, KR's, coupling caps, etc. They don't care about the math.

As long as the amp doesn't blow up and it sounds good to you, who cares.

With something like a K bypass cap (as long as your not having a problem with the amp like oscillation or wierd tone) it takes less time to swap in/out a few different values to hear what you like than to post a question about it. You'll learn more by your ears than with the math.


                Brad      

    

[Geezer]

Get some alligator clip leads (cheap from radio shack) and tweak away! That way you can parallel another cap with the 2uf to easily hear the results without desoldering anything. Parallel caps add, so a 10 or 22 or 25uf cap will give you close enough results to judge for your taste. Be sure to observe correct polarity on polarized caps.

We could try to describe the change all day, but you'll never know if it's something YOU will like until you hear it with your own ears.

Resistors in parallel divide, so you can only go DOWN in value with those with the clip leads (without firing up the soldering iron) but the same principle applies.

G

[catnine]

This bypass cap is inside the NFB loop . since it's the second gain on a SF Champ that is out of phase and that's where fender tied the NFB . The two resisters that made up the NFB are the 2.7K that is connected the a 47 ohm to ground . This same 47ohm to ground is the cathode to ground bias for the 1.5K resister bypassed by the now 2 uf cap.

 I suppose I can clip a higher value cap in parallel with the 2uf so see what changes yet I have a feeling it will change the NFB loop as well. The vibro champ has the same setup and uses a 10uf in the same spot . I just thought they had some reason for doing so. One the vibro champ they tie the second gain cathode between the cathode and the 1.5K bias resister to the intensity pot for the vibro then to the cathode of one tride of the 12ax7 . I thought perhaps this was a way to not make the vibro sound to thin.

[Willabe]

I suppose I can clip a higher value cap in parallel with the 2uf so see what changes yet I have a feeling it will change the NFB loop as well.

I thought perhaps this was a way to not make the vibro sound to thin.

Yes, Just try it.

Try it inside the loop and outside the loop and see/hear what you think for your self.

So what if it changes the NFB loop, see what it sounds like.

Do you realy think that Fender did all the circuit on paper and didn't try different values off parts to fine tune it??????    


                   Brad    

[HotBluePlates]

HBP has left the building.

[catnine]

 HotBluePlate:

 I will try it . I don't have any speaker flab now , I assume this refers to farting out . I don't have the cheapo 8 " speaker fender put in their champs . Those spaeakers could not hold bass together if their life depended on it. I took that same 73 SF champ with the stock OT and stuck in a tall cab I built with 2 twelves a fender 35 watt alnico fender used to made and a eminence 12" stonehenge they used to make and that champ had great bottom end so I think most of all it's the speaker.

 I asked on a few forums over the years and was told that the 2.7K NFB resister was in parallel with the 47 ohm to ground but I can't see how it's in parallel . So I got confused about NFB . The only way I can read the actual value of the 2.7K is if I break the connection with the OT speaker tap, simply because it's added to the OT to ground on the secondary side. So it seems to be in parallel with the OT secondary.

 I have played around with this build so much and last week I tried different 12ax7's I have here . which is just 2 and one is a sovtek 12ax7wa and the other is an old made in USA and they sounded the same to me. What they did was offer some nice balance ie  they lost some bottom yet if I did some double bends with finger vibrato they really hung in there . The 12ax7 I have in the amp is a JJ 12ax7 and it has more bottom end but the nice blend is not there . If I back off on the treb a bit then I lose the thing they brought.

 I kept the amp cranked to 7 all the time and the treb and bass set full up , I just shut the amp down and swapped the tubes .

 The speaker is just a jensen C10R which is not known for low end nor is the weber sig 10S ceramic but the jensen has a better bottom end.  

 If I could I would just go buy a better 10" speaker one that states it's focus is on bass and the mids are there and the highs are tame. Some tell me to get a larger OT . I know this would help . I'll tell You what really bugs me is that I had a 73 SF champ and went to great extremes to get the same value componants and voltages and I have that yet for some odd reason that SF cahmps witha weber sig 8 s alnico had a better bottom end than this built with a 10. I even tried a weber sig 10s alnico when weber made them in the 4 ohm offer and it had more bottom than the sig 8s  all I had to do was turn the treb 2 numbers down and it sounded just like the sig 10S alnico.

 Dang, Dang me , they oughta  take a rope and hang me . hang me from the highest tree, woman would ya weep for me.

[HotBluePlates]

HBP has left the building.

[catnine]

I tried the 25 uf cap but did not really hear any change . It was certainly not like the change I hear when I switch out the NBF loop and get an over all looser sound . Perhaps the bass notes were a bit tighter but again it was difficult to tell . But then it all seems to be a sume of the total .

 If the speaker can't produce more lows then the entire thing falls down. And if the OT can't pass the lows to the speaker then they won't be there either.

 Seems I can only get out with what I'm working with a SE low watt  practice amp with a small OT .

 It appeares to me a speaker that is voiced more in the low end will be all that can be done to balance the highs with the lows making the lows more pronounced.

 It's not bad as it is stock across the board , it does have lows , it's not like I'm working with a high powered amp to begin with . The speakers voice coil size and the magnet pull seem to make more difference than a larger value bypass cap.

 In my P-P type 6G2 /5E3 I had a 30 watt weber silver bell ceramic and it was so foucued on the bottom end that turning the tone pot full treble did not allow more higher so I replaced the silver beel with the big voice coil with a weber sig 12S alnico and then the tone pot worked well I could cut the bass and add highs and the sig 12s alnico only has a 1 " voice coil compared to the silver bells 1 3/4"  " voice coil and 40 ounce ceramic magnet. The weber sig 12S alnico is stated as a bright woody reedy tone but this is not the case in my P-P build. But then in that build I have a much better OT. Silver bell 30 watt is described as having a big bold base and the higher the watt rating the more bass it has . I chose the 30 because I was using a 20 watt P-P amp. David Allen told me he would try a different speaker first then prehaps go with a larger OT and then go with a 6l6  . I had his huge SE OT in a 5F2A build yet the 6L6 did not make that much difference to my ears. A bit more power and a bit more bottom end but not enough to be cost effective.

[HotBluePlates]

HBP has left the building.

[catnine]

I kept playing around and clipped the 25uf bypass cap in parallel with the existing 2uf bypass cap but this time I jumpered it through a fender style dpdt slide switch so I could campare the difference . This time I hear a noticable difference , With the added 25 uf cap there is a tighter bottom end maybe some mids to , the highs seem about the same as many have said they would.

 So soldered the new cap with the + side in the same eyelet that the 2uf cap is and then brought the - side of the cap to the switch just using one side of the dpdt slide switch center lug and one outside lug so it acts like an on/off switch. Then soldered a short lead from the switch outside lug to the same eyelet that the neg side of the 2uf cap is .

 I wanted it switchable so I can have both sounds . The only thing is when I flip the switch to add the 25uf cap into the circuit I get a very slight split second pop , guess that's the cap charging . the 2uf cap is always in the circuit so switch the 25 uf out of the circuit there is not pop and if I switch the 25uf back in right away there is no pop , just if I wait about a minute after it's switched out then back in then I get the slight pop sound . Isn't there a way to place a very low value none electrolytic cap across the switch terminals so the 25uf cap stays charged ? Or is the slight pop just nothing to bother with? There is 1.8 vdc on pin 8 cathode of V1 for the bias and a 1.5K ohm bias resister.

 I can't say if there is a way to avoid the pop yet it's not loud at all , like a pencil eraser tapped on a table top .  here is the champ schem.

[catnine]

I tried the 25 uf cap but did not really hear any change . It was certainly not like the change I hear when I switch out the NBF loop and get an over all looser sound . Perhaps the bass notes were a bit tighter but again it was difficult to tell .

Which was what we told you. It wouldn't be an obvious change, but would impact the operation of the feedback loop.

If the speaker can't produce more lows then the entire thing falls down. And if the OT can't pass the lows to the speaker then they won't be there either.

 Seems I can only get out with what I'm working with a SE low watt  practice amp with a small OT .

 It appeares to me a speaker that is voiced more in the low end will be all that can be done to balance the highs with the lows making the lows more pronounced.

Are you trying to boost the low end of this amp?? If so, you never stated that as an objective. Had you stated that was the objective, we would not have advised you to raise the value of that cap, because it would not have helped you reach the goal of boosted low end.

 Yes just a bit more freqency focusd on the bass .

[HotBluePlates]

HBP has left the building.

[catnine]

So soldered the new cap with the + side in the same eyelet that the 2uf cap is and then brought the - side of the cap to the switch just using one side of the dpdt slide switch center lug and one outside lug so it acts like an on/off switch. Then soldered a short lead from the switch outside lug to the same eyelet that the neg side of the 2uf cap is .

... The only thing is when I flip the switch to add the 25uf cap into the circuit I get a very slight split second pop , guess that's the cap charging. ...

Yes, it's the cap charging. Add a 100k or bigger (as much as several meg-ohms) resistor across the lugs of the switch, so that the switch shorts the resistor when you are adding the cap. That will kill the pop.

 So adding the resister across the switch lugs basically keeps the cap charged?

[ac427v]

Are you using 8 ohm speakers and the 8 ohm OT tap? The Champ NFB circuit is designed for 3.2 ohms and may provide excess feedback and reduce your power output if used with 8 ohm values. Someone experienced with the Champ feedback circuit could advise...in fact I've been hoping they would because I'm planning to do a similar build and have been wondering about this but haven't started my own thread.
--Craig

[catnine]

Are you using 8 ohm speakers and the 8 ohm OT tap? The Champ NFB circuit is designed for 3.2 ohms and may provide excess feedback and reduce your power output if used with 8 ohm values. Someone experienced with the Champ feedback circuit could advise...in fact I've been hoping they would because I'm planning to do a similar build and have been wondering about this but haven't started my own thread.
--Craig

 What I did is use weber PT and OT for a champ and since all I had were 8 ohm speakers I got his champ OT that has both 4 and 8 ohm taps but I use the 4 ohm tap for the NFB so it's the same value NFB .

[HotBluePlates]

HBP has left the building.

[catnine]

HotBluePlates;

 I'm a bit confused now with the resister so I ask . The stock 2uf cap is always in the circuit and is always charged and the added 25 uf bypass cap is added in parallel with the 2uf but is only added via the switch , all I am doing is breaking the ground of the 25uf .

 I took the 25uf and wired the pos side right to the same eyelet the 2uf is on and left the 1.5K as stock from the pin 8 cathode V1 to ground also the 2uf cap is always across the 1.5K bias resister to ground. At the switch and we can call it an on/off switch since it's wired like that ,  the neg side of the 25uf is one one lug and on the other lug there is a wire back to the same ground as the 2uf and 1.5K bias resister. So when the switch breaks the 25uf ground it then does not exist as far as the circuit is concerned, until I apply the ground via the switch.

  So   when I turn on the amp the 2uf is charged and since the 25uf has no voltage at that point it is not charged since there is no path to ground until I flip the switch , that's when I hear the slight pop. So I am confused as to which cap I am discharging . It seems to me that since the 25uf is just basically out of the circuit and I flip the switch to add it in this is when I get the pop , now if I leave the 25uf in circuit and wait a few seconds then switch it back out of circuit then switch it back in and it's now charged then I have no pop sound .

 I didn't remove the 2uf at all it's still wired in as stock across the 1.5K cathode bias resister and so is always charged . Only when I switch in the ground side of the 25uf then I can read the 1.5K bias resister , I read the 1.5k if I place the ohm probe on the pos side of the 2uf cap and to other probe to the ground side of the switch or the gorund end of the 25uf cap which is the same thing when the switch connects to ground.

  I thought the resister would charge the added cap yet it can't because the pos of the added cap and the DC comes from the 1.5K bias resister yet the DC can't flow through the added cap and never flow through the resister across the switch open or closed, once the switch is closed the resister is shorted and only the cap allows AC to pass. At that point the added cap is now in parallel with the in circuit 1.5K bias resister acting as a bypass cap and the resister across the switch is in parallel with the 1.5K bias resister so I guess while moving the switch from open to closed for that second the resister drains the 2uf cap, maybe ?

 I don't know , the more I think about these simple concepts the more I confuse myself. Main thing to me is will this pop hurt anything since it's very low current and volts . I have to actually draw these things out to understand them I try to do it in my head and that is not the way

[ac427v]

 What I did is use weber PT and OT for a champ and since all I had were 8 ohm speakers I got his champ OT that has both 4 and 8 ohm taps but I use the 4 ohm tap for the NFB so it's the same value NFB .

Great idea Cat. I'll do mine the same way. And thanks HBP for the math and the workaround in case my OT doesn't have a 4 ohm tap.

I have used the 100k resistor on a switchable 25u bypass cap and it works great. That resistor acts like a jumper across the switch but only allows enough current to charge the bigger cap. It doesn't allow enough current to let the cap function until the switch is thrown. Then...no pop-just boost!
--Craig

[HotBluePlates]

HBP has left the building.

[Ed_Chambley]

I have found in the champ circuit that cap values at any stage should reflect the speaker choice.  The circuit provides for an 8" speaker.  The question is "what frequencies do I want to provide the speaker for its range, frequency and efficiency"?  So if I want to run a 12 with a 5f1 circuit, then I consider the speaker(s) reproduction.  Just plugging in any 12, I have found the upper mids to sound metallic, but the bottom is sort of greasy which is sometimes cool.  All the matter of what you are wanting. 

It is the first bypass where I have found to have the most prominent change.  I like a scooped mid sound, so the champ I have has an Eminence Red Coat which is a very efficient speaker which to me is easier to get a strong bass response, which I like.  I have a number of common value resistors soldered across various caps.  I just clip them in until I get the bass response I am wanting which I do at v1.  In small watt amps this is the most difficult thing for me. 

From this point I can halve the second stage resistor and run it to a switch with the other half resistor wired in the "off" position to maintain a stock value.  From the other side of the switch I can jumper any resistor/cap value until I find the upper mid I am searching for.  This will give you one "trick switch" which I have on a couple of builds.  For the champ I took some ques from bass amps, I use rotary 6 position switches with different values starting from smaller to larger cap values.  Then according to the room I am playing in at home I can dial in whatever I want.  My champ has no tone knob, this is where I put the 6 position switch.  I use the amp exclusively for home playing and moving room to room the tone changes a lot.  Also, it allows for me to easily change from single coils to humbuckers.  Hardwood, slate, concrete and carpeted rooms or simply how I am feeling that day.

It is difficult to offer different values as I do not know what you are wanting to hear, but it is a great investment and fairly cheap to get some various 50v caps. 1 and 2.2 and 3.3 and 4.7, well you get the idea.  Doing the math helps me to understand why the change is happening and is important as this understanding benefits your next build.  With the help I have received in understanding this, my "testing" time has really been reduced because I know what I am looking for in tone and the math makes it so I don't have to haphazardly try cap values.  I usually have it narrowed down to one or two values and I do not have to shoot in the dark.

[catnine]

So when the switch breaks the 25uf ground it then does not exist as far as the circuit is concerned, until I apply the ground via the switch.
[/quote]

Right. And you get popping when you throw the switch to connect the cap in the circuit, right?

Right

So   when I turn on the amp the 2uf is charged and since the 25uf has no voltage at that point it is not charged ...

Okay, call the resistor a "charging path" then if it helps you visualize it.

The point is when you connect the 25uF cap, it is not charged and tries to draw some amount of current to charge the cap and allow it to function in the circuit. That quick pulse of current is what you're hearing as a pop.

The resistor slows the current, changes its shape from a quick step or shelf of current increase, and creates a slower ramp-up. That kills the pop.

Look up Resistor-Cap charge/discharge. Whether the pop is solved by changing the charging or discharging is irrelevant, because the resistor performs the same function in either case.
[/quote]

 So since the cap can't pass DC the the resister charges the cap with AC and the resister is always in the circuit SW open or closed. In series with the switch open and in parallel with the cap switch closed. here is a crappy drawn schem

[catnine]

Ed:

 I just left the stock 25uf bypass cap across the stock 1.5K bias resister . since my goal was the create a stcok SF Champ. I only went for the second gain after the tone stack and added a 25uf in parallel with the stock 2uf so it would pass all freqencies . I felt the 2uf was not passing the lows . I just added a switch to go back to stock value just in case. I know a better 10" speaker than the jensen C10R that offered freqency response focused more on the low end and mids with a tame high end would help. I thought perhaps a jensen MOD 10/50 would be btter.

[catnine]

 Well I put a 1 meg resister across the switch and the pop went away . Thanks for the help on this. I have to wait a few minutes before flipping the switch or the pop is still there. Perhaps I should have used a 100K instead of 1 meg but it's fine I can wait as I usually do before playing through the amp anyway.

 It didn't change the bias one bit on the cathode pin 8.

 I must say it was a real pain soldering that resister in there  because there is not much room at all , the switch is right next to the input jacks by a 1/4" and maybe 1/2" away from the vol pot , just sandwiched in between . Here are a few photo's.



just the area were the switch is and the full view of the chassis .

[HotBluePlates]

HBP has left the building.

[catnine]

HotBluePlates:

 I read what you just wrote and understand it (basically). I sure as heck know that an electrolytic cap stores DC voltage .

 I was thinking along the lines of a cap not passing DC thinking of coupling caps yet they're not the same thing as an electrolytic cap .

 So then the added bypass cap I added was discharged to begin with and when I had the amp on it was not charged at all , so when I flipped the switch it made the pop because it was not charged with DC and the resister added keeps it in the circuit charged or discharged the switch just breaks the ground so the cap is not in parallel with the existing 2uf at that point it's just the 2uf cap . Even if I did not have the 2uf cap and just switched in a bypass cap across the 1.5K bias resister to say add gain,  I would still get the pop without the resister .

 What confused me most is that I only got the pop when switching in the added cap it never happened when I switched the cap out or broke it's ground.