Tweed Champ tweak results, etc.

[Marky D.]

I did a couple minor tweaks to my 59 champ recently: Replaced the stock speaker w/ a Weber Signature P8RS 4 and put a 47pf cap across the volume pot. The combination of these 2 changes has really woke up the amp. It has always been a little boxy and dark sounding but now it's just sweeter and deeper sounding if that makes sense. Anyone else found similar results, especially w/ adding the cap? Next, I'll probably start messing around with tubes...any recommendations?

Also I swear I heard a difference w/ the preamp tube shield removed, but the amp picked up audio from "Diagnosis Murder" reruns..

 

[BrianF]

I've done the same mods

except I used the Weber Ceramic Sig speaker...great little speaker...

The addition of the 47pf cap across the Volume pot wiper is the exact same thing as adding a permanent Bright switch capacitor.

Without the cap, at Volume less than full....there is always some loss of high frequencies due to parasitic shunt capacitances and the high resistance of the pot...this forms a parasitic low-pass filter. This makes the amp sound dark sounding (lack of treble)...

Adding the cap across the top terminal to the wiper cancels this effect. Of course when the Volume gets louder and louder the whole parasitic thing disappears and so does the need, and the effect, of/for the Bright cap. You can experiment with different values of Bright caps to suit your taste. As you discovered it doesn't take very much capacitance at all to make a noticeable difference. Classic Fender Blackface amps use the Bright switch to turn this effect on and off.

Coincidentally....the cap that some people install across the volume pot of their Strat or Tele does the same thing.

 

[Marky D.]

Good explanation, and

The 47 pf cap was removed from the bright channel of my BF Deluxe Reverb. It was waaay to bright w the cap in place on that amp.

 

[BrianF]

as for tubes...

I've tried an EH 6V6, an OLD NOS Marconi big bottle 6V6 and a Chinese 6V6 and noticed no discernable difference. The only other tube to mess with is the 12AX7 and 5Y3 rectifier...you can try different tubes but not sure if it will make much of a difference. I guess you just have to experiment...

 

[jjmantele]

Homebrew

I Built a homebrew based on the '59 circuit. It was dark sounding. I did the vol bypass cap thing but this only works at lower volume settings. At full volume it does nothing. I took it off eventually and added a 0.1 bypass cap to one of the pre-amp cathode resistors. Adding one to both pre-amp stages was too much and caused feedback / oscillation. More gain in the upper eq range from this cap. I also put in a switch to turn off the negative feedback circuit. This is a popular "mod" to give more gain and a "rawer" sound. That will also allow for more tone coloration from different 6v6's since the feedback circuit seeks to eliminate 6v6 "coloring."