Sunday, March 30, 2008

Harp Amp Project - Fender Silver Face Champ

Last year I bought a very nice 1970 Fender Champ guitar amplifier to use as a small blues harp amp. I made a couple of modifications to it right away to make it more suitable for harp (lower gain preamp tube and bigger speaker) and to make it safe to use (three-prong grounded power cord). The tone was a little ratty and boxy when I first got the amp, but those early modifications made it better without sacrificing its vintage collector value. Now I’ve decided to forget about keeping the amp original. Instead, I will make a series of careful modifications to make it the ultimate SF Champ for harp.

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

23 comments:

Anonymous said...

Rick, your Champ not only sounds great, but that's some beautiful playing!

Anonymous said...

Whoa, that sounds good.

Anonymous said...

Another mod which sounds even better is to rewire the amp back to the original tweed circuit. The tone controls will no longer work but the improvement in overall tone is incredible.

Anonymous said...

I am looking at a SilverFace Champ, but the tone controls do not work...is there any simple way to determine if it was modded back to Tweed specs? It is in a non-music shop (came from an estate), and they know no history on the amp. the amp sounds good, no hum, has the original speaker.

Gibbo said...

Rick what pot did you use in the feedback circuit?

Rick Davis said...

Gibbo, that was a B50K linear taper ‘B’ pot.

Anonymous said...

I have for the most part completed this mod and am very happy with the results. A word of caution - make sure you locate the new speaker as close to the base of the cabinet as possible. I had to permenantly remove the dust cap to leave room for the 6V6 tube

Rick Davis said...

Yes, the 10-inch speaker must be offset for clearance, as I said in the original article. But I was avoiding the can capacitor, not the 6V6 tube. Sorry you had to remove the bell cap on the speaker, but they are just for looks anyway.

If you make a mistake mounting the speaker you can always remove the speaker baffle and fashion a new one made of better-sounding wood, such as baltic birch plywood. I plan to do that with my Champ soon.

Still, the litte tweaked SF Champ is one of the best-sounding small harp amps I've ever heard. This post gets the most Google hits on the entire blog.

Rick Davis said...

NOTE:

As I was re-reading this post I noticed it was not clear that I was using the 5751 tube in the recording.

In its final configuration the amps is using a 5751 tube in the preamp socket. I like that tube a LOT better than the 12AT7 I originally tried (too bright) or the 12AY7 I tried later (to much of a volume suck). The 5751 sounds good and is a good compromise between gain and tone.

Anonymous said...

I switched my JJ 6v6 for a Tung Sol and the cap fits perfectly. Folks, this mod is easy to do and is a significant improvement. If you play harp and have a Champ go for it. Most of us own the Champ not because of its monetary value but because it is a great little harp amp. This mod makes it much, much better.

Rick Davis said...

Yep, JJ 6V6 tubes have bigger bottles than other 6V6s, and they present a clearance problem in this conversion. I don't care for JJ tube for blues harp anyway, and I don't recommend them. If you want inexpensive warm sounding tubes, use the Tung Sol new productions tubes.

Anonymous said...

Hi Rick, hi all,

I modified my SF Champ close to your description here (12AT7, 5V4, 6V6, Weber 10A125-O, cutted fb circuit), but when I put the vol more than 5, feedback is screamin' around anyway - before any tube distortion!
I've an amazing (harp-)sounding SF Super Reverb 1971, but it has a customized hidden Mastervol., so I think about a power-soak for the Champ. What do you think?

Martin
Austria

Rick Davis said...

Hi Martin-

A powersoak for a Champ seems pretty strange. Your band-mates would think you are crazy.

People are always asking me things like, "How far can you turn that amp up before feedback?" But there are so many things that impact feedback the question is meaningless. In a small room I can't turn any amp up much at all. In a large room I can sometimes turn the same amps up all the way.

It also depends a lot on your mic, and where you are standing, and your effects chain, and the phases of the moon. Feedback is a black art.

(BTW, the negative feedback circuit in the amp has zero to do with acoustic feedback you hear though the speaker.)

Having said all that, I've tried the 12AT7 tube in the preamp socket and didn't like it. It sounded harsh and, yes, induced feedback more quickly. The first think I would recommend is swapping that tube for a 5751 or 12AY7. That 12AT7 you have is more suitable as a Phase Inverter, not a preamp tube for harp.

Since you are having feedback problems I think I'd go with the 12AY7 tube. It has a bit lower gain...

Anonymous said...

Nice work Rick...could you post a picture of the wiring for the negative feedback pot you added? I want to make sure I get it right. Thanks in advance!

mreasy said...

Rick,got my Champ up& runnin recently
with help from your advice.I had Skip Simmons do the dangerous stuff and was impressed with his work.I put the upgrade speaker in and made some cool lookin tilt backs for it.
Rick gave me some good advice for a little more bottom end,enclose the back of the amp with a fitted piece of 1-4in. plywood. What a difference it makes.Check out the tilt backs I made on myspace.
Thanks Again Ron A.K.A. Mr Easy
www.myspace.com/easy46

bigernie said...

Interesting article, Rick. BTW, nice tone and playing on your audio sample. When you switched the rectifier for at 5V4, did the amp need rebiasing?

Anonymous said...

If you move the top board in the back down to where it touchs the bottom board it will sound even better! I've done this with amps of that type and worked real well. Seems to cut the high end a bit!

Enzore said...

Rick, now that you have lived with the Weber for a while would you do it again?

I am about to upgrade the speaker and am wondering what you think

Steve said...

I just did a search for a Weber 10A125-O and I did not get any hits at all. Do you know where I can get one

Thanks

Rick Davis said...

Enzore and Steve-

Yep, the Weber 10A125-O speaker is still the best sounding 10-inch harp speaker I've ever heard, especially for conversions such as this. You can buy it direct from Weber; there is a link to Weber Speakers in the Links section of the blog... over in the right-hand column.

bobby mac said...

Hello, you mentioned you put in a 5751 tube to replace the 12Ax7, i'm using a Au7 now, been looking at the 5751, my question is which company 's tube should i use, theres a few that make them??? thanks bobby Mac

Rick Davis said...

Bobby, my favorite 5751 is the U.S. made NOS JAN Philips 5751 available from www.TubeDepot.com for $24.95.

Rokplo said...

Hi !

just one thing i don't understand :

You say you use a B50K pot to adjust the NFB from original setting to none.

But, as it's a 50K pot, the maximum resistance is 50K, not infinite ? So you always have some NFB trough the pot ?

Is there a trick i didn't understand to wire the pot correctly ? I actually wired the NFB to center and side tap of the pot, leaving the other side tap of the pot unused. Is that correct ?

Thanx !!!!!