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

 

AWA: Model PA-806, 5w Valve PA Amplifier

Such an awesome looking little amp from 1954.  Photos from an online listing, I am yet to track one down in person.

For a simple single-ended 6V6 based amp, it's an interesting circuit.

AWA MODEL PA-806 PA AMPLIFIER

Standard inputs and controls for a small Australian PA amp from AWA.  Inputs for a mic, turntable input (PU) with a simple tone control.  No power switch, or fuse.

6AV6 microphone input stage (single triode)

12AX7 mixing / gain stage.  Grid leak biased, negative feedback from OT.

6V6 single-ended output stage, 5 watt

6X4 full-wave valve rectifier

Power Transformer: TS2525823

Power for the rectifier and 6.3v heaters

B+ ~360v

Output Transformer:  1T357100

600, 300 and 150ohm windings on the secondary (plus winding for negative feedback)

This model has also been well documented here

AWA MODEL PA-806 SCHEMATIC

VOX: Vintage AC6 Schematic

The Vox AC6 is a departure from the Vox AC2 & AC4, in that it has a 12AX7 preamp, negative feedback, no trem, and for some reason, the tone control is before the first triode - it's a bit of an odd-ball in the Vox lineup from the time. 

The 12AX7 preamp arrangement would give the amp a different flavour to the EF86 versions, but it's still an EL84 driving an alnico speaker in about the same size cab, so similar to its friends in that sense.

Further reading:  VoxAC30.org.uk   

VINTAGE VOX AC6, GUITAR AMPLIFIER SCHEMATIC

SUMMARY

In simple terms, the circuit uses two 12AX7 triodes feeding a volume control, which then drives the EL84. The first triode is cathode-bypassed, the second is unbypassed and receives negative feedback from the output transformer, and the tone control sits ahead of the first triode.

Some of the schematic values are hard to read, so hopefully this is close.  That said, apart from the unusual placement of the tone control, the 12AX7 stages are biased cooler than a typical Fender-style preamp.  

This cooler operating point doesn’t so much reduce gain as it shifts the clipping behaviour, with the stage tending to run out of headroom on one half-cycle first, giving asymmetrical clipping when driven.

The 220 kΩ input resistors are also a little unusual.  As drawn, they don’t just act as isolation resistors — they form a voltage divider with the 1 MΩ grid leak on the first triode.  The result is a small amount of input attenuation, so some signal level is lost before the first gain stage even begins to amplify.

The 12AX7 Preamp

Inputs:  

• Two jacks.  

• Each jack feeds the first triode's grid through a 220k series resistor

First Triode:

• Grid leak: 1M to ground

• 12AX7 plate load (Ra): 100k

• Cathode resistor (Rk): 4k7

• Bypass cap: 25uF, 50v

• Coupling cap to next triode: 0.01uF (10nF)

Second Triode:

• Grid leak: 470k to ground

• 12AX7 plate load (Ra): 100k

• Cathode resistor (Rk): 4k7

• Negative feedback from the output transformer to the cathode, 47k series resistor

• Coupling cap to volume pot: 0.05uF (50nF)

Volume & Tone control:

• Volume pot: 500k (wiper feeds the EL84 grid via a 4k3 grid stopper)

• Tone: 250k pot with a 0.005uF (5nF) cap in series to ground from the grid of the first triode

Power stage: EL84 (6BQ5) single-ended, cathode biased

• Grid stopper: 4k3

• Grid leak: via the volume pot

• Cathode resistor (Rk): 150 ohms (2W or 3W)

• Cathode bypass:  25uF, 50v

Output transformer:

• Secondary: 3 ohms

• Primary: Probably 5k or 5.2k, which was common with Vox at the time

Rectifier: EZ80 (6V4)

Power transformer:

• HT secondary: 250V-0-250V (as shown)

• Heater winding: 6.3V

• Primary shown with 250v, 230v and 85v

Power supply filtering:

• 32uF reservoir (C1) → 1k (8W) series resistor (R3) → 32uF (C2) main B+ node - likely to be 270v

• From +270V:  47k dropper (R5) → 8uf feeding 12AX7

Speaker: 

• Elac 8" alnico, 3 ohm

VOX: Vintage AC2 Schematic

Another early Vox model is the AC2.  It’s often described as being the same as an AC4, but there is one key difference — the tone control.  

Instead of sitting in the preamp like the AC4, the AC2’s tone control is wired in the power stage, between the EL84 plate and the B+ supply (effectively across the output transformer primary).  

It obviously works, but no one does this anymore - the tone control is sitting at just short of 300 volts DC, which isn’t ideal for a number of reasons.  Curiously, they chose the opposite direction for the AC6, with the tone control before the first triode of the preamp.    

Further AC2 reading:  AC2 on VoxAC30.org.uk 

VINTAGE VOX AC2, GUITAR AMPLIFIER SCHEMATIC

VOX: Vintage AC4 Schematic

Despite the name - and even the familiar look - the original Vox AC4 has very little in common with the modern “AC4” amps wearing the same badge.  The cabinet and the single EL84 output stage are about all they share.

The AC4 started life as the AC2 in 1958.  Fair to say that the AC4 is probably best known due to its modern counterparts, despite being a different beast.

The circuit, layout and feel are very much of their era — the preamp and power stage are almost text box examples from valve datasheets - and if you want the deeper backstory, the Vox Showroom & AC30.org both have an excellent history of this little amp.  

VINTAGE VOX AC-4 GUITAR AMPLIFIER SCHEMATIC

Minor error on the schematic.  The on / off swich for vibrato is the wrong way around.

SUMMARY

It's a classic single-ended amp design, point-to-point construction, valve rectifier.  High gain from the EF86 firing pretty much straight through to the EL84, into an open back cab with an alnico speaker.   

There's very little loss in terms of low-end or with almost no frequency shaping along the way (at least before it hits the output transformer and small alnico speaker).  The tone control just rolls off some treble; no other shaping takes place. 

There is one unusual thing about the preamp - a huge 5.6meg screen resistor is taming the EF86: it starves the screen to drop gm and gain, improves linearity, and when you push it the screen current dynamics produce that soft ‘screen compression’ feel pentodes are famous for.  By comparison, 1meg is the datasheet value when paired with Ra 200k.

The trem wiggles the bias of the EF86 (cathode coupled), which is a fairly typical design from the era, not unlike a Vibro Champ.

The EF86 Preamp

Inputs:  Two jacks.  Each jack feeds the EF86 grid through a 100k series resistor

Grid leak: 1M to ground

EF86 plate load (Ra): 220k

EF86 screen supply: 

• 5.6M (Rg2) from the preamp B+ node

• Screen bypass: 0.1uF (100n) to ground

EF86 cathode:

• Cathode resistor (Rk): 1k5

• Bypass cap: 25uF

• Cathode voltage: ~2.7V

• Trem oscillator connects to the cathode

Coupling cap to volume/tone network: 0.047uF (47nF)

Volume & Tone control:

• Volume pot: 1M (wiper feeds the EL84 grid via a 6.8k grid stopper)

• Tone: 1M pot with a 0.001uF (1nF) cap in series to ground from the signal node (treble-cut)

• Power switch is shown as part of the tone control assembly

Power stage: EL84 (6BQ5) single-ended, cathode biased

• Grid stopper: 6.8k

• Grid leak: via the volume pot

• Cathode resistor (Rk): 150 ohms (2W)

• Cathode bypass:  25uF

• Cathode voltage: ~8.5V

Output transformer:

• Secondary: 3 ohms

• Primary: 5k / 5.2k as stamped on output transformers 

Rectifier: EZ80 (6V4)

Power transformer:

• HT secondary: 250V-0-250V (as shown)

• Heater winding: 6.3V, 2A (as shown)

• Primary shown with 230V / 115V options via selector plug

Power supply filtering:

• 32uF reservoir (C1) → 1k (5W) series resistor (R3) → 32uF (C2) main B+ node labelled +270V

• From +270V:  22k dropper (R5) → 8uF cap, ~260V node feeding the EF86 & 12AX7

Vibrato oscillator: ECC83 (12AX7)

• Cathode resistor: 3.3k with 25uF bypass (cathode marked ~1.7V)

• Speed pot: 1M

• Coupling caps shown: 0.02uF and 0.01uF, plus 0.01uF to ground in the network

• Footswitch shown to switch vibrato on/off

Speaker: 

• Elac 8" alnico, 3 ohm, sometimes Goodmans

EL84 OPERATING CONDITIONS

Operating conditions are similar to the EL84 datasheet.  I included both the 5.2k and 7k primary impedance conditions, as some modern transformers offer both options (or at least 5k and 8k).   

Many AC4s had transformers stamped with 5000, or 5200, indicating their primary impedance.  Given how common EL84 based amps would have been at the time, a stock transformer from any number of manufacturers would have been readily available.

EF86 OPERATING CONDITIONS

The circuit is very similar to the EF86 datasheet, but that massive screen resistor is hardly textbook.

GOLDENTONE: Model 1776 Guitar Amplifier

This is a Goldentone 1776, which is essentially a Goldentone 1774 with trem.   It had been reshoused in a 2 x 10 cab with Oxford speakers (one ceramic, one alnico), with the amp chassis mounted on the bottom, with the control panel up the top.  The amp would have been mounted the same way in its original 1 x 12" cab.

Based on the fact that it's made by Rose Morris, this would have been made before 1965.  Serial number 1375.  The 1776 models are listed as being made in 1961 - 1962

Apart from a DIY cab, I think it has had a little bit of work done on it.  There appeared to be an extra resistor in the power section - the rest looks stock, and I'd say there's a good chance that these are the original valves.  

Like its sibling, this amp has volume and tone controls before the first triode - which is kind of pointless.  It's low impedance, and it only really sounds good turned all the way up.  It basically puts another set of volume and tone controls before the amp, which you already have on your guitar.

GOLDENTONE MODEL 1776 GUITAR AMPLIFIER

• Single-ended 10w valve amp

• Volume 250k, Tone 250k, Depth 250k, Speed 2meg

• Silicon rectifier - looks like a voltage doubler

• 1 x 12AX7 preamp

• NFB to the first triode

• 1 x 6AU6 pentode driven tre, with Bakelite trem foot switch

• 1 x EL34 for the output

• Ducon caps

• OT  A&R 2584.  2k5 @ 16ohm

• PT A&R type 1971

Trem oscillator

POWER SUPPLY

• A&R type 1971 power transformer.

• Silicon rectifier, voltage doubler. 

• B+ 285v

SINGLE-ENDED OUTPUT TRANSFORMER

• A&R Type 2584

• Turns ratio 12.4

• 16 Ω speaker reflects 2.5 kΩ to the primary

COUPLING CAPS

These wax coupling caps had to come out - they didn't measure well at all - they never do.  When I tested the amp, it was sounding weak and thin.  Caps have been replaced.

GOLDENTONE 1776 - DRAFT SCHEMATIC

I've roughed out the main amplifier stage, and I'm nearly finished with the tremolo circuit. 

TREMOLO CIRCUIT

The 1776 uses a small 6AU6 pentode for the tremolo - the circuit is similar to the Silvertone 1482. 

I had to replace all of the caps, as they were well out of spec and a little leaky.

MODS

Modding a vintage amp is a little controversial, but this one was barely working when I got it, and it can easily be changed back. 

• Volume and tone moved to after the first triode

• Leaky wax paper coupling caps were replaced with slightly larger values, as while the amp sounded great when cranked with the smaller caps, they didn't do it any favours running clean.

• The tremolo had to be rebuilt to get it working, and as I had changed the volume and tone controls, I had to move the point where it was inserted into amp.  The trem still isn't perfect, slight tick when the amp is cranked.

• A couple of filter caps were replaced.

SUPRO: Super Six, Model S6606 Valve Amplifier

Valco made, Supro single-ended valve amp, very much the same as the Gretsch 6150 / 6151

SUPRO SUPER SIX, MODEL S6606

12AX7, 6V6, 5Y3

8" Jensen speaker, 3.2 ohm

OT 2640

PT 3036

B+ 300v

4.5w

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SELMER LITTLE GIANT MKI & MKIII

SELMER MERCURY 5