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Bought a Champ! Questions...

10/3/1999 10:49 PM
Michael Tousek
Bought a Champ! Questions...
I just bought a 1968 Champ to go along with my 1968 Bassman, and boy is it a neat little amp! Like all Champs, though, it turns into goo beyond about 6 on the volume knob. I've been thinking about a replacement speaker to help fix this and am leaning towards a Weber. I've heard lots of good things about their 8" alnico; does anyone have an opinion on the ceramic version (C8R?)?  
Also, where can I get a replacement for the multi filter-cap can?  
Finally, I notice that the smallest coupling cap in my Champ is the .02uF to the grid of the 6V6. A lot of Fenders have a .001uF going to the input of the phase inverter. It seems to me that a Champ -- which is the smallest Fender and the least able to handle low frequencies -- might benefit from some low-end rolloff. I'm thinking of trying a .001uF in place of the .02uF to the 6V6's grid. Any thoughts on this?  
Michael Tousek  
P.S. for Champ fans or trivia buffs, here's a little more about my Champ: the chassis code says it was made in May 1968, it has the earliest style of SF grillcloth and has the thin metal trim strip around the grill, it has the thin black lines on the faceplate, and it has an alnico speaker. I paid $200 for it which is a little high, but it was super-clean and I really like the early SF amps.
10/4/1999 3:05 AM
Tim C.

It has been my experience with Champs that changing the speaker doesn't keep the amp from mushing out around 6. I would consider changing the speaker more because original Champ speakers tend to blow rather quickly when played at higher volumes. Weber VST speakers would be a good choice and I would get the ceramic version. Thats just my opinion though.  
I would consider changing the .1 and .047 caps in the tone stack to .022 or even .01.  
Your thought of changing the grid cap to the 6V6 from .02 to .001 might sound alright. I haven't tried that before. You might get by with a .01 instead.  
Consider changing the 25mf@25v cathode cap of V1A to .68@25v  
to eliminate excess bass at the first input stage.That would be pin 3 of the first preamp tube.  
Just my two cents worth.  
Tim C.  
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10/4/1999 3:08 PM

My vote is to use a .047uF bass cap and a .022uf or .033uf mid cap.  
Tim is right, the coupling cap can be reduced to .0068uF to .01uf with not much loss of drive.  
Although you might like it, I wouldn't go down as far as .68uF on the pin 3, the cathode of the first preamp tube but I would drop it to 2.2uF or at least 4.7uF and change the second preamp triode's cathode cap on pin 8 to 10uf too.  
Then move over to the 6V6s grid load resitor and consider change it to a 180K. Sometimes that helps a little.  
Measure the screen voltage on the amp and see how far it is away from the plate voltage.  
I like them when they are close.  
The most obvious difference in sound after using a bigger speaker is to dump the tiny OT and use a much larger one.  
10/6/1999 4:02 AM
Michael Tousek
Thanks for the reply. Looking at the schematic, the bass cap is the one in the middle of the tone stack and the mid cap is on the bottom, right?  
Just curious: why do you prefer the 2.2uF to the .68 bypass cap?  
MIchael Tousek
10/8/1999 6:22 PM

Mike, I believe it works something like this;  
the 'standard' Fender cathode bias setup consisting  
of a 1.5K resistor in parallel with a 25uf/25 volt  
cap yields a low freq. rolloff of about 4.8Hz which  
is derived from the formula freq=1/2*pi*R*C.  
Leaving the 1.5k resistor and changing the cap to a  
.68uf would eliminate a lot of bass response in that  
the low-freq rolloff would now be about 156Hz -for  
comparision the low E string on a std 440A tuned guitar is about 82Hz. Finally a 2.2 would yield a low-freq rolloff  
of about 48Hz and a 1.3uf cap would give about a 82Hz  
10/9/1999 12:42 AM
Michael Tousek
Interesting. Seems to me like the 1.3uF would be the way to go because it would attenuate the useless, power-robbing frequencies below the low-E string, but it would let everything else through untouched.
10/9/1999 3:54 AM

Something else to think about is when combining two or more frequecies, you will hear the fundemental freqs, PLUS, the sum and differences of them too.  
If you are playing low and deep riffs, you actually get sub harmonic freqs mixing in that you can feel and hear that musclarity too. Maybe too much gets muddy.  
If you set the preamp up to sound like those freqs are rolled down too far, the amp will sound a little weak.  
I guess there is a point at which all the extra low gain is redundant but there is also a point at which not enough low freq gain is puny sounding.  
In any given amp design, I usually don't bypass the cathode resistors with much less then 2.2uF to 4.7uF on the first stage (lead channel tones are a little different) and, I try to keep it above 4.7Uf on the next stage... unless there are many more stages to follow, at which time I'd consider having NO bypass caps on the first and maybe even the second.  
A nice compromise on BF Fender amps is both the preamp stages with 10uF and 1200 ohms.  
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