First a little background on the Fender Champ amps. The Champ is the smallest line of Fender tube amps, generating about 6 watts of power. The Champion amp first appeared in the late forties, and in the fifties the Champs – and other Fender amps – were covered with Tweed. In the early sixties the design of Fender tube amps was known as Black Face, for the black panel around the volume and tone controls. In 1968 this changed to the Silver Face design, which was produced until 1982. Fender Champ amps after that are not really desirable as harp amps for a variety of reasons. So the good Fender Champs are known as Tweed, Black Face, and Silver Face. The Silver Face is the most recent and thereby least valuable. You can buy good examples all day on eBay for $200 - $300.
The Champ amps have a very simple circuit called Single-Ended Class A. In general that means the amp has a single power tube that drives the speaker. The standard tube compliment in all Champ amps is: 12AX7 preamp tube, 6V6 power tube, and 5Y3 rectifier tube. It is the simple circuit that makes these amps sound cool with blues harp. When you turn up the volume knob on a Champ, the sound does not get any louder after you pass 5 (out of 10) on the knob. After 5 the tone just gets more distorted and compressed… more colorful. A well-sorted Champ near full crank is a wonderful thing to hear. Eric Clapton recorded the song “Layla” this way.
So then… the first modification I made last year was to swap out the 12AX7 preamp tube with a lower gain 5751. The biggest challenge in playing amped blues harp is audio feedback though the speakers, and the lower gain preamp tube will allow you to get to the sweet spot in the power tubes before you get to the squealing feedback level. It is a very careful and tricky balancing act that changes in every venue you play. A lower gain preamp tube is the required first step. I prefer the 5751; other harp players like the AY7 or the AU7. Each has a different level of gain reduction. It is a matter of taste, playing style, and your harp microphone.
The next step was to install a larger speaker. I chose the Jensen vintage re-issue P10R, a 10-inch 4-ohm 25-watt guitar speaker with a ribbed, seamed cone and alnico magnet on a 1-inch voice coil. I picked this speaker because the original Jensen P10R was legendary as original equipment on some fantastic vintage Fender amps used for harp, like the tweed Bassman or black face Super Reverb (Paul Butterfield in particular). The reissue P10R speaker, however, lacks much of the warmth of the original.
At that time, I decided to preserve the originality of my Champ by mounting the 10-inch speaker over the 8-inch opening in the baffle board for the original speaker. That way the mods were reversible and I could restore the Champ to vintage condition if I wanted. But the small opening in front of the bigger speaker made the tone a little boxy and nasal, and after more than a year of playing through this amp and tweaking it around its vintage margins, I have decided to sacrifice any collector value it may have for the sake of getting the ultimate SF Champ tone for harp.
Here are the steps I have chosen in my quest for tone:
-Cut the baffle board to properly mount a 10-inch speaker. The opening needs to be offset slightly so the speaker will clear a can capacitor that sticks down from the chassis.
-Swap out the 5Y3 rectifier tube for a 5V4. The rectifier tube converts AC power to DC, which is what the other components need to see. For a variety of technical reasons, the 5V4 rectifier tube will make the Champ sound louder and punchier with more note definition and better bottom end. The trade-off is it gives less sag. Sag is a cool sound a harp amp makes when its power tubes momentarily run short of DC power when a loud note is played. This may seem like a bad thing, but it sounds cooler than shit on big harp amps like the Fender Bassman. With small amps like the Champ, sag almost never occurs because the little Class A amps are almost always cookin' at full heat anyway, and the sound is so compressed you cannot hear the sag. I have ordered a NOS (new old stock) 5V4 tube from Tube Depot in Memphis. [Update: Got it! NOS Sylvania JAN 5V4G]
-Alter or disconnect the negative feedback circuit in the Champ. Fender’s goal for its guitar amps was a sparkly clean tone, and one way to do this was with a negative feedback circuit. It was thought that power tubes introduced distortion, so Fender engineers looped an out-of-phase portion of the output signal back to the preamp section. That way the distortion cancelled itself (sort of), but it meant they were siphoning off some of the Champ’s power for the sake of clean tone. Amped harp does not need sparkly clean tone, and distortion is our friend. Many pro players swear by this modification, saying it makes the Champ louder and warmer, while a few others say it can make the tone gritty and harsh. (Heck, I live for gritty and harsh tone!) So I have decided to install a potentiometer in the NFB circuit so that I can dial down the negative feedback from the factory setting to nothing – or anything in between. Old vintage amps actually have wires and resistors and things; not just chips and printed circuit boards. This is a pretty simple mod that requires drilling a hole in the chassis and mounting the pot, and connecting two wires.
-Install a 10A125-O Weber speaker. This is Weber’s best vintage-style speaker for harp. It is 20 watts, 3.2 ohms, alnico magnet with 1.25 inch voice coil. The cone is not ribbed. This is a punchy speaker known for warm tone, early breakup, loud, compressed at high volumes. Since it is 3.2 ohms (which is what the Champ expects to see) it may be marginally louder than the 4-ohm Jensen I have in the amp now. The Weber speaker is $95 (!)
Those are the four steps I plan to make on my quest for tone. As you read though the list you may notice that all the steps promise to make the amp louder, but that is really not the goal. This amp is mic’ed through the PA in almost all gigs anyway, so volume is not an issue . The point of being slightly louder is to give me a broader range of tonal textures before the Champ’s output tube just compresses it all down to that lovely honk. As it is now, my Champ is kind of a one-trick pony, but it is a beautiful pony that can do one helluva fine trick.
Above is the original Champ the day I bought it. Pretty, eh?
Above is the Champ cab cut for the 10-inch speaker opening. That is the Jensen P10R speaker, soon to be replaced by the Weber 10A125-O.
This is a photo of the Champ's guts. The negative feedback loop is the yellow wire that goes from the speaker tap (top, second from right), around the tube socket, and then connects to the resistor on the circuit board.
Updates: More project photos. The amp is finished and sounds sensational.
I replaced the old RCA speaker tap with a 1/4 inch Switchcraft phone jack. This is a very simple mod. The phone jack fits perfectly in the hole in the chassis for the RCA jack.
This the the 10-inch Weber 10A125-0 speaker mounted in the Champ cab. The Weber speaker is the most important step in the project, having the biggest impact on the tone.
The Presence control on the back of the Champ is the potentiometer installed in the negative feedback circuit, another simple mod. 10 = factory setting of negative feedback; 1 = zero negative feedback. For blues harp, 1 sounds best, so the thing to do on your Champ is to just disconnect the NFB loop. This involves unsoldering one wire. You can see the 5V4 recto tube on the left. The power tube has since been swapped for a NOS Blackplate GE 6V6.
Update: Here is a link to a short sound clip of this Champ amp in it's final form... My ultimate SF Champ for harp. In this recording my friend Miles Nichols is playing a Fender Tele through a 1964 Magnatone amp; the Champ is mic'ed with a Shure SM58: Magnatone Blues