BIXONIC: Expandora

OK - so this is what happens when you agree to make a pedal, before knowing what someone might ask for.   I'm not sure that this is 100% my kind of thing, but here it is.

Yes it's a huge layout, but it is going in a 1590XX enclosure, so why not.  I started this layout using the Dirtbox Layouts Expandora vero layout (and basically made it wider), as the first version that I did had a few too many rows, for the space available in the enclosure - and I will probably only be making one of these.  So credit where credit is due...  

I may still add the guitar / bass switch - see how it sounds.  There is room on the board for it.  Not hard to work out if you want to add it.  

If you are unfamiliar with the signal flow on the Expandora - it's kind of a Rat with an extra gain stage up front, and an unusual compressor like gain control that's always on, which is combined with a couple of switches to help set the overall gain of that stage.

The fun doesn't start until "forbidden mode" is engaged - which opens the gain right up.  Talking big ugly square waves.  The Expandora moves from a fairly innocuous drive or mild distortion, to a full blown fuzz monster.     

BIXONIC EXPANDORA

This is it on the bench for some testing - just a bit of rough / temporary wiring to confirm all works as expected, before I wire it up properly.    I think the tin can LM308 opamps look cool, and they were all I had.

It's been a while since I worked on small vero, or should I say normal size.   It took some adjustment, and I did have a tiny little copper trace left on a cut that took a decent amount of time to track down.  It took a magnifying glass and a multimeter to track it down - the damn thing was about the size of a bee's dick (which is a unit of measurement in Australia). 

ENCLOSURE: Wiring rig

This was long overdue - I finally got around to making a half-decent wiring rig.  I’d been using cardboard from pizza boxes (yes really), which worked, but it was pretty ordinary to say the least.

Once the parts were laid out and the enclosure marked up, I drilled the pilot holes in the chassis, then cut and drilled a sheet of lightweight galvanised iron to match.  It turned out to be far easier than I expected, and I honestly can’t believe I waited so long to do something like this.

This one in particualr is for a Foxx Tone Machine, but as a generic layout for a 3 knob circuit in a 1590XX this will be fine.  

I quite often do most of my wiring in the enclosure, this makes things a little easier.  I'll have to make a few more, for different types of enclosures.

GIBSON: GA-30 Schematic

GIBSON GA-30 GUITAR AMPLIFIER SCHEMATICS

This is one of the GA-30 amps - there were a few by the sounds of it.

While they share a similar topology, there are clearly some differences from the one above.

Apart from changing the circuit, their appearance also varies over time - I'm not going to try and document them all, and I'm by no means an expert on the topic.  I'm just using these as a reference for a Bell & Howell conversion.  

Text below is for the first schematic 

6SJ7 MIC PRE

Grid leak 1m

Grid stopper (none)

Ra 470k

Rk 2k2 20u bypass 

Screen 2m2, 50n bypass

10n coupling cap to 500k volume pot.  100k mixing resistor to PI

6SJ7 INST PRE

Grid leak 470k

Grid stopper 47k (forms a voltage divider with the grid leak)

Ra 220k

Rk 2k2 (not bypassed) 

Screen 2m2, 50n bypass

20n coupling cap to 500k volume pot.  100k mixing resistor to PI

470k NFB from OT secondaries, 700pf in series with bypass switch 

6SN7 PHASE INVERTER

Ra 100k on each plate

Rk 1k shared (not bypassed)

50n coupling to 6V6

220k / 4k7 pair to PI grid

Tone 2m pot, mixing 250pf and 50n to grid of PI input 

6V6

220k grid leak

Rk 250 ohms

No grid stoppers or screen resistors 

GIBSON: BR6-F Schematic

6SJ7 preamp, 6SN7 paraphase inverter, 2 x 6V6 with local negative feedback.  

GIBSON BR6-F GUITAR AMPLIFIER SCHEMATIC

Clearly a bit of work has been done on this one - just using the pic as an example of circuit construction.

Low impedance input

6SJ7 PRE

Ra 220k

Rk 2k2 20u bypass 

Screen 2m2, 50n bypass

10n coupling cap to 500k volume pot

6SN7 PI

Ra 100k

Rk 3k3 (not bypassed)

50n coupling to 6V6

100k / 10k pair to PI grid

6V6

100k grid leak

Rk 200 ohms

470k NFB from 6V6 plate to 6SN7 plate

No grid stoppers or screen resistors 

GIBSON: GA-40 Schematic

5879 pentode premap, which is an improved version of the E8F6 - uses a Vox style cut control across the plates of the phase inverter.  Paraphase inverter, driving 2 x cathode biased 6V6.

GIBSON GA-40 GUITAR AMPLIFIER SCHEMATIC

VOX: Vintage AC15 Schematic

Ignoring the gigantic and overly complicated trem section - this is one of my favourite sounding AC amps, which is most likely the inspiration for the Matchless DC30 pentode channel.

VINTAGE VOX AC-15 SCHEMATIC

Version:  NO V-1-5 / OA/031

Apparently this schematic is a v3, redrawn by Thomas Organ, with some different component numbering.   I can understand why they redrew it - a lot of people, including me, struggle with some of the original Vox schematics.   It appears that they may have done the same thing for the AC10 (which practically shares the entire output stage as the AC15, with a couple of minor differences)

Below is the original, which isn't the most confusing Vox schematic that I've ever seen, but I still prefer the one above. 

SUMMARY

In the Normal channel, the AC15 is an EF86 voltage amplifier feeding a volume control, which then drives an ECC83 long-tail pair phase inverter and a cathode-biased EL84 push-pull output stage.

It's nice and direct, with limited tone shaping along the way - there is of course the bright switch, and the traditional Vox Top Cut, but apart from that, this is a fairly bare bones signal path.

I'm not going near the trem channel - just too complicated for me, and I'll never build one.  

Inputs: 

• Two jacks wired High / Low 

• Each jack feeds the EF86 grid through a 68k series resistor (R5, R6)

EF86:

• Grid leak: 1M to ground (R1)

• Plate load (Ra): 220k (R15)

• Plate supply / filtering: from +315V through 22k (R14) to the EF86 supply node, filtered by 8uF (C3). Plate voltage is marked ~90V on the schematic

• Screen supply: 1M (R20) feeding the screen, with 0.1uF (C8) bypass to ground

• Cathode: 2k2 (R13) bypassed by 25uF (C5)

• Coupling cap to volume: 0.01uF / 10nF (C12)

Volume, Brilliance, Top Cut

• Volume pot (Normal): 500k (R29)

• Brilliance switch: SW3 inserts a 250pF capacitor (C17) as a “bright” bypass around the signal feed, letting extra treble through when switched on

• Tone: Vox “Top Cut” placed after the phase inverter — a 250k pot (R49) with a 0.005uF / 5nF capacitor (C36) working across the two opposite-phase drive signals

Phase inverter (ECC83 / 12AX7)

• Type: Long-tail pair (LTP) using both triodes (V5A, V5B)

• Normal volume feeds V5A grid via 10nF (C27)

• Trem channel volume feeds V5B grid via 10nF (C28)

• Plate loads: 100k each (R44, R48)

• Supply to PI node: from +315V through 22k (R43), filtered by 8uF (C26). PI node is marked ~220V

• Shared cathode / tail: 1k2 (R42) in series with 47k (R38) to ground

• Coupling caps to power stage: 10nF each (C32, C33)

Power stage (2 x EL84 / 6BQ5, push-pull, cathode biased)

• Grid stoppers: 1k5 each (R56, R57)

• Grid leaks: 220k each (R53, R54)

• Screen resistors: 100 ohm each (R62, R63)

• Shared cathode resistor: 130 ohm, 5W (R60)

• Cathode bypass: 50uF (C39)

• Plate supply is marked ~310V

Output transformer

• Secondary taps: 15 ohm, 8 ohm, and common

• Primary: 8k p-p

Rectifier and power supply (main)

• Rectifier: EZ81 (6CA4)

• Filtering: 16uF reservoir (C10) → choke L1 (10–20H) → 16uF smoothing (C15) to the main +315V source

Speakers

• A few variations of single and twin speakers - all 12" 

• Goodmans Axiom, Fane and of course Celestian blues

EL84 OPERATING CONDITIONS

Operating conditions are similar to the EL84 datasheet. 

EF86 OPERATING CONDITIONS

The preamp circuit is almost identical to the EF86 datasheet