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, 4 ohms. 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.  

Of course, I had to change it...   I picked up a vintage G.B. Equipment alnico speaker, and it sounds pretty darn good.  It's likely to be made by Goodmans by the look of it.  

It has slightly lower impedance, probaly 3.5ohms or thereabouts (it measured about 2.5 DCR), which drops the reflected impedance down a little.  

As part of my rebuild, I removed the low input (cause who needs it), and added a switch to bypass the second triode with a 1u cap (explanation below).  I also added a proper light, as the plastic jewel kind of looks OK, but it's junk compared to the real deal.

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 as the board.   Transformers seem OK - the output transformer measures at 10k @ 4 ohms, which is a fair bit higher than your standard Champ.  It sounded OK to me, and I may 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.

I made some adjustments to the layout as I was building - mainly moving things around to make sure parts aren't too hard to access and that the coupling and bypass caps are easy to change. 

 

BUILD NOTES

B+ ~360 V DC

Rectifier / power

UF4007 diodes

22u,  10k / 8u,  22k / 8u

Heater elevated via 6V6 cathode.  2 x 100ohm resistors 

6V6:

Plate:    330 V DC

Screen: 311 V DC

Cathode: 18.7 V DC

Ik​ = 470/18.7 ​= 39.8 mA

$$\text{P ≈ 311.3 × 0.0398 = 12.4 W plate dissipation }$$

$$\text{But that overstates true plate dissipation, because some of the 39.8 mA is screen current.Assuming about 3–5 mA screen current:Plate current ≈ 34.8 – 36.8 mAPlate dissipation ≈ 10.8 – 11.5 WScreen dissipation ≈ 0.9 – 1.5 WSo the 6V6 plate is probably running at roughly:77 – 82\% of a 14 W 6V6 GTA rating or 90 – 96\% of a 12 W older 6V6 GT rating}$$

Output transformer

The original Champion 600 output transformer measured:

10 V AC on the primary

0.2 V AC on the secondary

50:1 turns ratio

10 k reflected primary impedance with a 4 ohm speaker

It has since been replaced with a vintage A&R single-ended transformer rated at 7 k into 3.5 ohms.  

Coupling caps

The original 22nf coupling capacitors were reduced to:

8.2 nF after the first triode

15 nF feeding the 6V6 grid

These changes raise the 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 networks rather than the complete amplifier response.

The reduced values retain the normal guitar range while limiting unnecessary low frequencies, helping to tighten the sound when the amp is overdriven.

Switchable second-stage bypass

A bit rough around the edges, more gain and a less lows:  A 1uf capacitor can be switched across the second triode’s 1k5 cathode resistor - increasing gain.

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. Negative feedback is reduced as a result of this.

FENDER CHAMPION 600 REISSUE SCHEMATIC

FENDER AA764 CHAMP SCHEMATIC

 

FENDER: Deluxe 5E3 Amplifier

Some reference notes on the Fender 5E3 Deluxe amplifier, possibly the most famous and widely copied variation of the Fender Deluxe, produced from about 1955 - 1960.

TWEED ERA FENDER DELUXE 5E3 SCHEMATIC

POWER

Transformer:  Triad 6452, 380–0–380 V

Rectifier:    5Y3GT

B+ ~360V  

Filtering:    B1 - 16uf,    B2 - 5k / 16uf,    B3 - 22k / 16uf

OUTPUT

OT:   8k P-P,  8ohm secondary.   Triad 108 (pre 60's).   Schumacher 125A1A.   Note the option to add another 8ohm extension speaker, which implies that the amp can handle a 4 ohm load

2 x 6V6

Va ~340 - 350

Cathode biased.  Rk 250hm, 50uf bypass capacitor

Grid stop: 1k5.  Grid leak:  220k

CATHODYNE PHASE INVERTER - 12AX7

First stage, boost:

Ra 100k,  Rk 1k5 ohms / 25uf bypass

22n coupling to grid of inverter

Second stage, invert:   

Ra 56k,   Rk 1k5 in series with 56k 

Rg 1meg to 1k5 / 56k resistor node

100nf coupling to 6V6 from cathode and anode of 12AX7

PREAMP - 12AY7

Ra 100k,  Rk 820 ohms shared / 25uf bypass.   (1,640ohms per cathode)

Grid stoppers 68k (in parallel, for high).   Standard Fender input wiring

Rg 1meg.  100nf coupling caps to volume

Normal and Bright channels

A1M VOLUME & TONE

A1M volume and tone pots.   

In tweed circuits, the volume pots are wired in an older style: the wiper (pin 2) is the input, while the outer lug (pin 3) serves as the output.  Do not change it to the modern pin 3 in, pin 2 out convention, as it will not work the same way.  

In terms of tone control, it is essential to note its position in the circuit.  On the Bright channel, treble can be increased via what is essentially a variable bright cap on the volume pot.  This does not occur on the Normal channel.

On both the Normal and Bright inputs, the treble can be turned down.

A 500 pF capacitor from pin 3 connects back to the preamp’s coupling capacitor (on the bright inputs). This acts like a variable bright cap—its effect diminishes as the volume is turned up.

A 4.7 nF capacitor from pin 1 goes to ground, providing a simple treble cut by shunting high frequencies away.

Pin 2 (the wiper) carries the combined signal forward to the grid of the phase inverter (also connected to the output of the volume pot).

One hallmark of these amps is the interaction between the two volume controls and the tone. Both volume pots connect directly to the grid of the first phase inverter stage, so even if only one channel is in use, adjusting the unused channel’s volume will still alter the response of the active channel.  There are tonal variations all over the place - be sure to experiment with extreme settings.  Mixing resistors were introduced in later models to remove this interaction.

12AY7 x 12AX7

Parameter	12AY7	12AX7
Amplification Factor (µ)	40	100
Plate Resistance (rp)	22.8 kΩ	62.5 kΩ
Transconductance (gm)	1750 µmhos	1600 µmhos
Typical Plate Current	≈ 3 mA @ Va=250 V.  Vg = –2 V	≈ 1.2 mA @ Va=250 V.    Vg = –2 V
Max Plate Voltage	300 V	330 V
Max Plate Dissipation (per triode)	1.5 W	1.2 W
Heater Current	0.3 A @ 6.3 V	0.3 A @ 6.3 V
Typical Bias for ~1 mA	–2 V to –4 V	–1.5 V to –2 V

6V6

Parameter	6V6
Type	Beam power tetrode
Plate Resistance (rp)	~62 kΩ (effective in Class A)
Transconductance (gm)	~4100 µmhos
Typical Plate Current	34–45 mA (idle, Class A)
Max Plate Voltage	350 V (design centre)
Max Plate Dissipation	14 W
Heater Voltage	6.3 V
Heater Current	0.45 A @ 6.3 V
Typical Bias	–12 V to –15 V (Class A)
Typical Load Impedance	8 - 10 kΩ  P-P
Typical Output Power	≈ 14 W (push-pull, Class AB)

5Y3

Parameter	5Y3GT
Type	Full-wave rectifier
Maximum DC Output Current	125 mA
Peak Inverse Voltage	1400 V
Typical DC Output Voltage	~350 V DC (at 100 mA load).
Max Input Capacitance	20 µF (first filter capacitor)
Heater Voltage	5 V
Heater Current	2.0 A
Typical Voltage Drop	≈ 60 V at 125 mA load

FENDER: Tweed Princetons

I'll start with the Fender 5F2A Princeton, as this is generally considered to be "the one" for a lot of people.

Basically take a tweed Champ, and add the classic Fender one knob tone control, which has no insertion loss, and you're in business.  It's hard to go wrong with this formula, which is why it appears in so many amps.  

FENDER TWEED PRINCETON - 5F2A, SCHEMATIC & LAYOUT

• 12AX7, 6V6, 5Y3

• Classic Fender hi / lo input stage

• Standard Fender preamp component values / biasing 

• Preamp is cathode biased

• Volume 1M, Tone 1M

• 22n coupling caps

• 22k negative feedback from OT secondary to second triodes cathode 

• Simple heater wiring - one side grounded

FENDER PRINCETON - 5B2 SCHEMATIC

• Preamp is grid leak biased

• Simple heater wiring - one side grounded

• 6SL7, 6V6, 5Y3

• Jensen P8T

• Volume & tone 1M

• OT 7765

• PT 6500

FENDER PRINCETON - 5C2 SCHEMATIC

• 6SC7, 6V6, 5Y3

• 2 x triodes in preamp - the first feeds the second via a voltage divider to reduce gain

• Shared cathode resistor

• Volume & Tone 1M

• Volume & Tone control is sitting on the grid of the 6V6

• Simple heater wiring - one side grounded

FENDER PRINCETON - 5D2 SCHEMATIC

• 12AX7, 6V6, 5Y3

• Preamp grid leak biased

• Volume & tone 1M - 1meg resistor in series before the volume pot

• No negative feedback

• Simple heater wiring - one side grounded

FENDER PRINCETON - 5E2 SCHEMATIC

• 12AX7, 6V6, 5Y

• Classic Fender hi / lo input stage

• Preamp is cathode biased

• Volume 1M, tone 250k

• Slightly more complicated tone control - tone control sits on the first triode's plate, before the coupling cap

• Simple heater wiring - one side grounded

• Choke on power supply

• 22k negative feedback

FENDER PRINCETON - 5F2 SCHEMATIC

Almost identical to the 5E2 - no bypass cap on the first triode

• 12AX7, 6V6, 5Y3

• Classic Fender hi / lo input stage

• Preamp is cathode biased

• Volume 1M, tone 250k

• Slightly more complicated tone control - tone control sits on the first triode's plate, before the coupling cap

• Simple heater wiring - one side grounded

• Choke on power supply

• 22k negative feedback

FENDER: Tweed Champ Schematics

I know these are everywhere, but humour me - this is mainly for my own reference.

FENDER CHAMP-AMP,  5C1 (WIDE PANEL TWEED)

Note: The Fender Champion 600 and 800 are said to use the same schematic (or at least extremely similar).

Preamp: 6SJ7 pentode, grid-leak biased (no cathode resistor)

• Inputs: two jacks, each 75k in series with 0.02uF (20nF) into the grid

• Grid leak: 5M to ground

• Plate resistor (Ra): 250k

• Screen feed: 2M, bypassed with 0.05uF (50nF)

• Coupling cap to volume: 0.02uF (20nF)

• Volume pot: 1M

Power stage: 6V6 single-ended, cathode biased

• Cathode resistor (Rk): 500 ohms, bypassed with 25uF

• Grid leak resistor = volume pot

Output transformer:  Triad 70189 

Note: I can't find a definitive source of original Champ OT measurements.

• Small single-ended output transformer

• OT Secondary 3.2 ohms 

• OT Primary, usually quoted as ~5k-7k  (some modern replacements offer 5k & 8k primaries). 

Turns ratios

• 46.8:1 = 7k @ 3.2 Ω    15VAC on primary = 0.3205 on secondary

• 39.5:1 = 5k @ 3.2 Ω   15VAC on primary = 0.3797 on secondary

Rectifier: 5Y3

Power supply filtering:

• 8uF reservoir → 500 ohms → 8uF (plate/OT node) → 25k → 8uF (screen + preamp node)

Power transformer:  Triad 6500

Typical Voltages:  Actual voltages vary a lot with wall voltage, rectifier condition, and transformer, but the commonly quoted targets for a 5C1 are roughly:

Rectifier DC output (first B+): 340V

B+ after the 500 ohm dropper / main supply node: 320V

6V6 plate: 300V

6V6 screen: 280V (lower because it’s fed through the 25k drop)

6V6 cathode: 16V across the 500 ohm resistor (so ~32mA cathode current as a rough check)

6SJ7 plate: 130V

6SJ7 screen: 21V

6SJ7 grid: -5V

Heaters:  Simple wiring, one side grounded (not twisted pair, no centre tap)

Speaker (stock): 1x6" alnico - commonly Permaflux 6-L (EIA 395), though other 6" period speakers exist due to Fender’s supplier variability and decades of replacements.

FENDER CHAMP 5D1

FENDER CHAMP 5E1

The classic Fender 12AX7 input stage and preamp values are used.  Negative feedback appears.  Choke on the DC power supply.  Changed to a regular component board; controls now on the top of the amp.

Preamp: 12AX7

Inputs: two jacks in the classic Fender input network.  When using input #1, the effective grid-stopper is about 34k because the two 68k resistors end up in parallel

• Each jack feeds the grid through 68k

• Grid leak: 1M to ground

First gain stage (V1A):

• Plate resistor (Ra): 100k

• Cathode resistor (Rk): 1k5, bypassed with 25uF

• Coupling cap to volume: 0.02uF (22nF)

Volume control:

• Volume pot: 1M (between stages)

Second gain / driver stage (V1B):

• Plate resistor (Ra): 100k

• Cathode resistor (Rk): 1k5, not bypassed

• Negative feedback: from OT secondary back to this cathode via a 22k resistor

• Coupling cap to 6V6 grid: 0.02uF (22nF)

Power stage: 6V6 single-ended, cathode biased

• 6V6 grid leak: 220k

• Cathode resistor (Rk): 470 ohms, bypassed with 25uF

• Screen supply: taken from the main B+ node, after the choke

Rectifier: 5Y3GT

Power transformer: Triad 6500

Power supply filtering:

• On 5E1, the choke sits where the 5F1 uses a 10k dropping resistor

• 8uF reservoir → choke → 8uF (main plate/screen/OT node) → 22k → 8uF (preamp node)

Typical voltages (ballpark):

• Main B+ node often lands around ~305V in many examples (varies with wall voltage and rectifier condition)

• 6V6 cathode: ~19V across 470Ω (≈40mA cathode current as a rough check)

• Preamp supply in many references is around ~260V

Speaker (stock):

Commonly a small 6" alnico in early narrow-panel Champs, often a Permaflux (though other 6" period speakers exist due to Fender’s supplier variability and decades of replacements)

FENDER CHAMP 5F1

Almost identical to the previous model, with a 10k resistor replacing the choke.  While there is no bypass on the first triode, this is considered to be a mistake on the schematic.  Apparently lots of vintage amps have the bypass installed from the factory (visible in the photos below).