Tweed ‘Champ Amp’

As diminutive and simple as valve amps come (not even a tone control, but hey there’ll be one on the guitar…)

Rated highly as studio amps and reckoned to be all over many classic recordings over the years since; to check it out I put together this replica of a 1957 Tweed Fender Champ (5E1 spec circuit, with power supply choke) as I couldn’t believe how much was being asked for a Fender re-issue one. Once I’d collected up all the parts (pre-bent and screen printed chassis from Hong Kong, about £20), I built it in an afternoon. I’d decided to go further than Fender’s reissue in the authenticity stakes, and I got an output transformer from an old ’50s tape recorder that also used a 6V6 output valve with a 4ohm speaker, and a Jensen alnico speaker also from the era. I even found some pine and plywood of the correct thickness in my loft that must be forty years old so I can build a well-seasoned tweed cab for it at some point. At the moment though it’s quite capable of roaring through the two 12″ speakers of my Fender Twin Reverb amp, even though it’s only 5 watts (those speakers being rated for 50 watts each). Inevitably in comparison it’ll sound much ‘boxier’ when it’s in its own 8″ speaker cab.

I stayed true to the original Fender design for circuit layout, so everything is grounded to the chassis, including one leg of the heater filament supply, and I expected it to be somewhat noisy, with audible hum, but in fact it actually turned out very quiet considering it’s a single ended amp (which means there’s no hum cancellation in the output stage). One departure from the original circuit spec was made – both preamp stages’ coupling caps have been reduced in value from the stock 0.02uF to 0.002uF. With such a small enclosure  – only 13 by 12 by 8 inches – and only an 8-inch speaker, this helps limit the low frequency response to prevent excessive bass signals overwhelming the cab (ie causing ‘farting out’).

As the power cord I used (recycled from a Dyson vacuum cleaner) is only 2-wire and the amp has no earth, I put an RCD plug on it for safety over vintage authenticity.

Cab is now finished (though yet to be ‘tweeded’), and it is a great low volume combo. Overall good tone for such a small package, the altered coupling cap values keep it composed, and it’s very responsive to picking dynamics as single ended amps should be. The overdrive (above half volume for buckers, three-quarters for single coils) has all those glorious Layla tones, and no bleeding ears!

I tried two vintage (ie 1950s) 8″ speakers with this, both Jensens with alnico magnets. One measured 5.2ohm dc resistance on the voice coil, and the other 6.2ohm. The latter clearly was an 8ohm impedance speaker and so a load mismatch of x2 too high for the output transformer on this amp. That mismatch should significantly reduce the acoustical power. However, it was much, much louder than the 5.2ohm. I think it had been reconed, so had more piston effect from the rigid new cone and suspension, and it anyway had a much more massive magnet assembly for greater efficiency. Using it overwhelmed the cab at anything over a quarter up the volume control, with all kinds of rattles, buzzes and uncontrolled baffle resonance, even with the rear panels removed.

These old Tweed-style construction cabinets do ‘sing’ in their way (all of the panels resonate when rapped, unlike ‘dead’ MDF), and this is part of the sound, so careful speaker matching is a necessary part of the deal. The 5.2ohm speaker was a much better performance. The smaller magnet results in less cone excursion, and so fits better with what the cab and floating baffle can deal with. The distortion arising from the preamp stages and power amp valve clipping, together with output transformer saturation and speaker cone breakup, all coming in at the same time at a relatively low volume in a lively cab is what made the likes of Eric Clapton, Jimmy Page and George Harrison swoon over these amps, Getting all the synergies to work together for a classic rock or blues tone is perhaps much the same at 5W in a combo as it is with a 100W stack…

Luckily Leo Fender took brand loyalty seriously enough to make even his little ‘student’ amp a proper, well-built deal (and it was, a Champ cost US$55 in 1957, just $350 in today’s money).

Tweedy Champ:

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There’s an excellent analysis of the Champ circuit (amongst other amps’) by Richard Kuehnel:

1. Preamp

2. Power amp

3. Negative Feedback

4. Power Supply 

Oh – mine cost about half the Fender re-issue one.