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How to transform a Fender Champ 12 amp into a Champ Deluxe with a few mods

fender_champ12_guitar_amplifier_320px — Manufactured from 1987 to 1992, the Fender Champ 12 is an all-tube 12-watt, 1×12 combo based on two 12AX7 preamp tubes and a 6L6 power tube. Photo courtesy of telecaster.com

For sometime I’d been on the lookout for a small tube amp for use as a rig for audio testing the subminiature tubes that are fitted in our effects pedals. I had my sights set on a ‘Blackface’ Fender ‘Champ’, however these vintage amps were way above my budget and the prices of ‘Silverface’ Champs have soared over twice what they were just a few years ago. So, I widened my search to encompass newer tube amp models and it wasn’t long before I came across a nice looking, and more reasonably priced, Fender ‘red knob’ 1980s ‘Champ 12′ on Ebay. The amp seemed like it might just do the job, but before taking the plunge I got hold of a copy of the schematic diagram, just to make sure the ‘suits’ and bean counters that had infiltrated the R&D department of the Fender corporation from the 1970s onwards hadn’t sneaked any silicon in the signal path.

Well, there was a FET amplifier stage to boost the output from the spring reverb, but other than that the signal path was all tube and the amp had switchable clean/gain channels—not a bad deal at all for a £200 amp by any measure. However, although this Champ’s circuitry bears some similarity to Fender’s vintage amps it also has a few of it’s own, unique idiosyncrasies. Firstly, the spring reverb pan is driven directly from the speaker output transformer, a clever idea which negates the additional overhead of the 12AT7 tube and transformer driver circuit found in Fender’s ‘Twin’ and ‘Deluxe Reverb’ amplifier models. Secondly, the channel switching is achieved with opto-couplers as in the Mesa Boogie ‘Studio 22′ combo and the old Effectrode‘Blackbird’ preamp, which is why there is small amount of latency when switching between the clean and overdrive channels on these amps (the new model Blackbird utilises reed relays instead of opto-couplers to eliminate this switching delay). The LEDs within the opto-couplers require a low-voltage DC power supply from which to operate so the designers of the Champ 12 took a tap off the cathode biasing circuit in the 6L6 output stage as a low-voltage source to power them. Being a budget amp its designers were obviously trying to get as much bang for their buck as possible.

vactrol_vtl5c1_320px — Channel switching in the Champ 12 is achieved using an opto-coupler. The body of this device houses a light dependent resistor and LED, which, when illuminated, reduces the resistance from several megaohms down just to a few ohms.

Anyway, when the amp arrived I gave her an inspection, clean-up and retube with a set of new JJ tubes. She was now ready to be fired-up. So, I took my guitar, plugged it into the now gleaming Champ and clicked the power switch on. As the amp warmed up she began to hum. A little more mains hum than I would expect but perhaps that could be rectified (pun not intentional) later. At least there were no sparks or smoke. Everything seemed to be in order so far. But then, after a fair bit of noodling around on my old Strat, it began to dawn on me that I was not at all impressed by the tone of either the clean or overdrive channels. No, not impressed at all. Now, I’ve owned several Fender amps in my time, Twin Reverbs, Deluxe Reverbs and currently my go to amp is a Fender 1 x 12 ‘Tweed’. All these vintage models are beautiful sounding guitar amps and I have a great deal of respect for Leo Fender’s work, but this Champ 12 sounded weak and pale in comparision.

champ_12_schematic_320px — The Champ 12 features an all tube signal path with the preamp section bearing more than a passing resemblance to Fender’s older designs such as the ‘Deluxe’ and ‘Twin Reverb’ amps.

The clean channel wasn’t too terrible, just lacking volume that would make unusable in a live gig situation with drummer. It was the overdrive channel that really let the amp down. The quality of the distortion was harsh, possibly the result of blocking distortion in the latter gain stages of the preamp section. Additionally the tone was thin and overly mid-focused, lacking the body and depth I’d come to expect from tube circuitry. I was astounded that a tube amp could sound so unmusical. In all fairness the amp was made in the 1980s, a time when mainstream pop and rock guitarists were seeking icy clean chorused clean sounds for chord/arpeggio work and mid-focused overdrive sounds to ensure their guitar solos could punch through the thick layers of synths in a mix. In short, the 1980s was not the best era for guitar tone in my opinion.

But maybe this Champ 12 was a good amp born in a bad time. Although it was only going to be utilised for test purposes, I could not resist the challenge of getting under the hood to perform some fine-tuning, make a few modifications to squeeze some extra performance from this little amp, after all, hot-rodding tube amps is what got me into the the business of making tube effects pedals in the first place. A few resistor and capacitor changes here and there might just open up the sweet spot and make the Champ sing as a tube amp should.

As with Fender’s Tolex covered Deluxe and Twin Reverb amps the circuitry is easily accessed be removing the four screws on top of the amplifier which keep the chassis in place (note: don’t forget to disconnect the loudspeaker leads too). The whole chassis can then be slid out of the amp revealing the internals. The passive components are mounted on a single layer circuit board, not a turret board as in older amps. Don’t be put off by this though, as an amp based on a good circuit board design performs just as well as a boutique hand-wired or vintage amp, and the circuit board in the Champ 12 isn’t bad at all. The solder joints are bright and shiny, indicating good joints and the amp circuitry seems well laid out and well put together.

fender_champ12_circuit_board_320px — Components are mounted on a single sided circuit board within the ‘Champ 12′ and Fender utilised polyester capacitors in the tonestack circuitry and for interstage coupling—an excellent choice for audio.

I had hoped all my mods could be made by tacking resistors and capacitors on the solder side of the circuit board, which was visible and easily accessible, however as I got further and further into the tweaking process I realised the circuit board would have to be removed to gain full access to the component side. This wasn’t a big deal and just involved popping off the red knobs and unscrewing the nuts that secured the pots and jack sockets.

fender_champ12_red_knobs_320px — Access to the circuit board is achieved by popping off the red knobs and unscrewing the pot and jack sockets nuts.

It has to be said, modifying the overdrive channel of a tube amp really is a kind of art—sculpting the tube clipping circuitry to remove the deadwood, the sound you don’t want, to uncover and enhance the sounds you do want. Designers often curtail the amount of bottom-end entering the early stages of the overdrive circuitry in an amp as too much of this can make the drive ‘flappy’. However, the Fender Champ 12 lacked the body of vintage amps, which I love, so what I did was decrease the value of C1 from 22uF to 10uF and increase C8 from 0.68uF to 10uF so that the amp wasn’t losing power in the bottom-end. Changes were also made to cathode bypass resistors to increase the gain in earlier amplification stages and reduce it in latter gain stages—R15 changed from 2K7 to 1K5 and R24 from 2K7 to 3K3. The aim of this was to spread the signal clipping across all stages in the overdrive channel and reduce blocking distortion.

More modifications were made to the clean channel in an effort to increase its output level and match the tone/level of both channels relative to one other. But there was only so far this process could be taken without altering the fundamental nature of the clean channel circuitry, causing it to distort and no longer be ‘clean’. The changes revolved around components in the channel switching and the low-pass filtering circuitry, which takes the edge off the additional harmonics overdrive introduces (a.k.a. signal clipping). This quest to squeeze as much volume as possible from the clean channel resulted in an increase in its sensitivity—a nice bonus, as the amp became more responsive, breaking up just a little when pushed to add some colour and a little “hair” when hitting the lower strings hard with the pick. The following sound clip demonstrates what I’m getting at

All the sound clips were recorded direct from the headphone output on the amp and no post equalisation or speaker emulation plugins were used, in fact the only post-processing is in the second sound clip where I added a deep reverb to enhance the clean sound.

The process of making all these modifications to sculpt the clipping response and timbre of the amp was like chasing shadows. Altering the value of a certain component to obtain a desirable tonal improvement would more often than not introduce some negative tonal effect, which would then lead further adjustments having to be made to other component values. The work required a certain dogged persistence to make any headway, however after several hours of tweaking and listening my Champ 12 was slowly beginning to shine. This schematic shows the mods made. The modified Champ 12 will never really work for high volume level gig situations, but it does make an excellent high calibre recording amp. With this in mind I uprated the power supply smoothing capacitors to reduce mains hum. The result of my labours was a beautiful sounding and quite useable little amp for practice, recording and testing subminiature tubes!

A few words from a happy experimenter.

Dear Effectrode,
I am only writing to THANK YOU for the valuable information you have given me regarding the modification of the FENDER ‘CHAMP 12’ amplifier.
In these difficult days due to the epidemic that hit the world, I have completed the changes on the amplifier.
I am amazed at the great improvement I have achieved !
Phil said that with the change of the CLEAN channel he could stand up to a drummer. I think two drummers can now hold it too!
Remarkable is the headroom and the sound is beautiful.
In addition to the changes written on Phil’s scheme I have:
– replaced the front grill;
– open the grids for better cooling in the upper part where there is the carrying handle;
– replaced the small internal transformer PN 024038;
– installed a new spring reverb on the bottom behind the cone;
– brought the 8 Ohm output of the amplifier on the rear panel in order to connect any power attenuator or to connect another speaker;
– installed a jack on the rear panel to connect the footswitch;
– install new JJ valves.
I am particularly attached to this amplifier because it was from a dear friend of mine who learned to play on this amplifier when he was still young. Today he is an excellent guitarist and has played countless times with his groups in Northern Italy in Pubs, events and in every possible situation. It is not excluded that if one day he wants to have it back I will be happy to return it to him and I would like to see his amazed face when he tries it for the first time!
I hope you and your family and friends are all healthy. Take care of yourself and take every precaution against the epidemic because it is very serious.
Regards
Oscar

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How to transform a Fender Champ 12 amp into a Champ Deluxe with a few mods