Vox AC4 build

Keefy

Inspired by @dazzajl's enthusiasm for his AC4HW1, and my own leaning toward Vox-style amps, I have embarked on an AC4 build. This will be a practice room amp as an alternative to my Princeton Reverb. I have decided to leave out the Cool/Hot switch as I don't care for the rude, mid-forward sound created by disconnecting the tone stack ground.

I am building it into a 1997 Laney GC80. It is a decent sized 1x12 combo with a chassis that breaks down into U-shaped front and rear panels and flat bottom panel. The power amp section works fine so I have offered it for sale in the Classifieds. The whole amp was utterly filthy when I got it and required two applications of Amberclens and some vigorous scrubbing to get it half decent looking. There is a wooden 'shelf' under the chassis which I will need to (mostly) remove to make room for the transformers and valves.

I've sourced some Vox-style grille cloth and ordered a load of parts form Modulus, transformers from Primary Windings, and some polypropylene sheet for making new face plates.

I re-drew the circuit diagram in a form that's easier for me to read, and drew out a board layout based on a photo I found online. Once the transformers arrive iI will draw a full-size layout diagram.


Donor amp - it's dirty!


Chassis innards - now all removed


Parts gathered together from my stash


Board layout of a current production AC4HW1


Re-drawn schematic and layout


Grille cloth

Supportact

Nice project! I will look forward to updates. 

Winny_Pooh

How cool. What chassis are you using? And more importantly what speaker?

Keefy

Winny_Pooh said:

How cool. What chassis are you using? And more importantly what speaker?

The chassis is the one from the GC80.

• I will mainly re-use the front panel holes, and any unused ones will be covered by a plastic face plate.
• The rear panel will also require a plate to cover the hole where the heatsink was.
• There will be extra holes to cut in the base plate. I have hole cutters for the valve sockets, and I plan to cut the rectangular hole for the drop-through transformer by drilling a hole at each corner and then using a hand nibbler tool (no I didn't just make that up!) to cut between them. It is 1.15mm thick (or US 22 gauge if you prefer) steel so I think it's doable.

As for the speaker, I'm going to try the 50W HH Invader that it came with, a late 70s square-magnet Eminence, and a Peavey Sheffield (because I already have them). If none of those do it I might try the Neo Creamback from one of my other amp builds.

Dave_Mc

Excellent! 

Keefy

It's been a while - I've been busy with other stuff!

I sawed out the internal 'shelf" and back panel to allow room for valves and transformers, and painted the cut surfaces black so that they don't stick out like a sore thumb. Unfortunately I used some 'vintage' Texas Homecare matt black paint from my loft, which predates the whole low-VOC thing - it still stinks! I also re-fitted the speaker mounting hardware so that it can be loaded from inside the cab.

I found a Johan Segeborn video praising the old square-magnet Eminence speakers, and I have one, so I've fitted it rather than drop a ton on a greenback.

I marked and drilled the board and fitted the eyelets and turrets. The reason for using both will become clear in later posts.

Looks like the transformers are due for delivery on Monday, so I'll then be able to make more progress on the overall layout.

Keefy

The iron has arrived! (And a free pen.)



Also I'm contemplating replacing the steel bottom plate of the chassis with some 1.2mm aluminium. It would be much easier to work, there would be no unnecessary holes to fill, and the c-section front and rear panels would give it plenty of rigidity. Hmm...

Keefy

Transformer test tonight. I connected the two 120V PT primaries in series for 240V operation and confirmed that the voltages on the secondaries corresponded to the label on the box - these things aren't always correct!

Then I tack-soldered the 0 and 16ohm secondaries of the OT to a stereo jack socket and plugged in some headphones. I placed the PT in its proposed position on the layout and experimented with positioning the OT for zero hum. I was able to have them quite close, with the OT right next to the speaker sockets. One benefit of a drop-through PT is that you have a head start in preventing inductive coupling. If I turned the OT on its side (equivalent to both transformers being stand-ups) I got very noticeable hum, which I measured at up to 34mA of AC off-load, unless they were much further apart.

NB - Don't try this at home folks, exposed high voltage conductors in the house!

Keefy

Progress on the layout. I moved the PT further towards the front of the chassis and checked that it still didn't cause hum in the OT (as in my previous post).

I did order the alu sheet for a new base plate and it arrived this morning.

Keefy

Layout completed, but I'll get a good night's sleep and come back to it before I start drilling or soldering!

Keefy

I finished drilling and cutting the new chassis base plate, cut the plastic face plates and glued them to the front and rear panels. Then I cut out the plastic in the right places, re-attached the panels to the base, and fitted all the front and rear panel hardware except for the input jack - I'll wire that up on its own before I fit it. There was one little boo-boo in that I had a couple of fuse sockets in my bits box for the HT fuse, and measured the larger one for the panel cutout. That one turned out to be for a 25mm fuse which apparently is not a common length in glass fuses. I used the rather thinner 20mm fuse holder instead and disguised my error by judicious use of washers - just don't tell anyone!

And yes, this is how I spend Christmas Eve while Mrs Keefy has fallen asleep in front of rubbish TV.

ElectricXII

Looks good. Way above my skill levels, so I’m impressed. 

Keefy

This evening I had recovered enough from Christmas lunch to fit the hardware to the chassis base. Good news - nothing's going to foul the speaker 

Keefy

I did some work on the circuit board today. I made a start with the ground bus, carried on, and ended up fully populating it. Not all the eyelets are soldered as they are awaiting off-board links. All the big resistors are raised off the board to allow air flow around them.

paulnb57

Impressive skills! There is something really pleasing about point to point components and wiring, PCBs seem so sterile….

Keefy

paulnb57 said:

Impressive skills! There is something really pleasing about point to point components and wiring, PCBs seem so sterile….

Thanks! It’s not strictly point-to-point as it’s on eyelet board, but yes, very satisfying

Chris.B

That's a nice bit of wiring and soldering  - looking forward to seeing how this develops.

Keefy

Leads fitted to one side of the board.

Keefy

Most board leads now in place. I realised I'd fitted a 1M plate load resistor in place of a 100k because it had an extra band in the colour code - d'oh! Also started to wire up some front panel stuff and decided to move the board about 1cm back to make some room. 

Keefy

More wiring progress - first the connections at the front of the amp (controls and ground links), then those at the rear (valve sockets).

Supportact

Enjoying seeing the progress of this. Very impressive.