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
Comments
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.
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.
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...
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!
I did order the alu sheet for a new base plate and it arrived this morning.
And yes, this is how I spend Christmas Eve while Mrs Keefy has fallen asleep in front of rubbish TV.