FENDER: Champion 600 reissue

I picked up this Fender Champion 600 reissue recently, and as it had already been modded to some extent and was sounding pretty bad, I thought, why not strip it back and start fresh.

Firstly, is it a Champion 600?  No, not even close.  It's based on a AA764 Champ, with bass and treble controls replaced with a fixed circuit in the amp, and of course it has a silicon rectifier (note schematics below).

FENDER CHAMPION 600 REISSUE

The only useful mod that had been previously done (in my opinion) was changing the grill cloth - apparently the original cloth is notorious for killing high frequencies. 

The stock speaker is interesting - 6" ceramic. Kind of looks like a little hi-fi speaker, as it has the foam rubber surround.  It doesn't sound terrible, but it certainly can't handle any bass when the amp is running hot.  I have an alnico Goodmans arriving as a replacement.  See how that goes...  

As part of my rebuild, I removed the low input, and added a switch to bypass the second triode with a 4u7 cap, which incidentally removes negative feedback from the output.  So a bit more gain from the preamp, a tad tighter on the bass, with no negative feedback.    

Apart from the modded bits, the original board is OK.  Nearly all of the hardware, apart from the power switch and output jack was junk.  Ceramic valve sockets, but at least the 6V6 was attached to the chassis as well.   Transformers seem OK - the output transformer measures at 10k @ 4 ohms, which is a fair but higher than your standard Champ.  It sounded OK to me, and I would have left it in if it wasn't for the fact that I had a couple of suitable transformers handy.

Here’s the rebuilt circuit, now converted to a 5F1-style Champ with silicon rectification. I would have preferred to use a 5Y3, but the original power transformer does not have a 5 V heater winding.

BUILD NOTES

B+ / output-transformer supply: approximately 360 V DC

6V6:

Plate: approximately 330 V DC

Screen: approximately 311 V DC

Cathode: approximately 18.7 V DC

Output transformer

The original Champion 600 output transformer measured:

10 V AC on the primary

0.2 V AC on the secondary

Approximately 50:1 turns ratio

Approximately 10 kΩ reflected primary impedance with a 4 Ω speaker

It has since been replaced with a vintage A&R single-ended transformer rated at 7 kΩ into 3.5 Ω. With a 4 Ω speaker, it reflects approximately 8 kΩ to the 6V6.

Coupling capacitors

The original 0.02 µF coupling capacitors were reduced to:

8.2 nF after the first triode

15 nF feeding the 6V6 grid

These changes raise the approximate low-frequency corners from around 7–8 Hz and 28–31 Hz to about 18–19 Hz and 37–42 Hz. These are estimates for the individual coupling networks rather than the complete amplifier response.

The reduced values retain the normal guitar range while limiting unnecessary deep-bass drive, helping to tighten the sound when the amplifier is overdriven.

Switchable second-stage bypass

A 4.7 µF capacitor can be switched across the second triode’s 1.5 kΩ cathode resistor. This increases stage gain by reducing cathode degeneration.

Because the 5F1’s global negative-feedback signal is returned to the same cathode node, the capacitor also shunts part of the feedback signal to ground. This effect is frequency-dependent: feedback is reduced strongly through the mids and treble, while more degeneration and feedback remain at low frequencies.

The result is a louder, more aggressive response with better bass control than would normally be obtained with a fully bypassing 22 µF capacitor.

FENDER CHAMPION 600 REISSUE SCHEMATIC

FENDER AA764 CHAMP SCHEMATIC