Vox AC4 build

Keefy

Supportact said:

Enjoying seeing the progress of this. Very impressive. 

Ha! It's only impressive if it works!

Supportact

Thanks said:

Supportact said:

Enjoying seeing the progress of this. Very impressive. 

Ha! It's only impressive if it works!

True, but if I attempted it it would probably not work, and definitely look like a total mess. 

Dave_Mc

Supportact said:

Thanks said:

Supportact said:

Enjoying seeing the progress of this. Very impressive. 

Ha! It's only impressive if it works!

True, but if I attempted it it would probably not work, and definitely look like a total mess. 

LOL same here!

Keefy

Wiring is now complete*. I found that there wasn't good continuity from the input jack ground (front panel) to the mains safety ground (bottom panel) so I had to scrape some paint off in a couple of places. I checked the build against the schematic and everything looks good. I won't fire it up until tomorrow as if there is anything wrong it will keep me wake.

I've bought some hardboard for the false baffle, which I'll probably cut tomorrow (weather permitting). I've also ordered some 3D letters for the name plate - there may be a cunning plan afoot!

*After I'd tidied all my stuff away I realised I hadn't made up the speaker lead - D'oh!

ElectricXII

It looks great Graeme. I’m looking forward to hearing about the next stage of the build. 

Keefy

Yesterday I cut and painted the false baffle. Its sole purpose is to be home to the grille cloth, which I have asked Mrs Keefy to iron. Half-way through cutting the circular hole it occurred to me that my pad saw was about as sharp as the average spoon - I carried on regardless.



After wiring up the speaker jack I tested and fitted the valves, which happen to be all Tung Sols.



Then I connected the amp to the mains via a Variac and dim bulb tester and wound up the voltage, keeping an eye out for (a) the bulb lighting, and (b) smoke - no sign of either. And it works!

I checked all the DC voltages and they're good. With the 220ohm cathode resistor as per the Vox schematic, the EL84 is biased at 75%. I'd prefer it to be closer to 100% but I don't have any smaller resistors rated 3W or higher, so that will have to wait until I have ordered some appropriate values. The cathode of V2a (cathode follower) is at 95V which I think should be ok.

The sound is very much in the lovely smokey Vox zone that I wanted, with a lot of great combinations on tap from the gain, master, and highly interactive tone controls. It really opens up with a bit of reverb added on the way in, but it doesn't like the Boss switched-mode power supply that I was using (neither do I), especially if the treble is turned up. There is a little background hum so I will experiment with lead dress in an effort to reduce this when I next have the chassis out.

There are a couple of other issues to resolve:
(1) There is a DC offset of -100mV at the input. I expect that's coming from the cathode bias voltage of V1a. It manifests itself as scratchy guitar volume pots.
(2) If I turn the gain above about 2 o'clock there is an audible whistling oscillation unless I touch earth on the guitar or the amp. I may still have to improve the earth continuity through the chassis, and/or lead dress may help.

I'm going to hold off from external photos until all the cosmetic work is done. Stay tuned!

Keefy

All done! I settled on a 150ohm cathode resistor which gives 93% dissipation in the EL84 and doesn't pull the B+ down too much. I haven't done anything about the DC offset but I am plugging in via a reverb pedal which I presume has a coupling cap at its output so it's not really an issue. The apparent oscillation turned out to be guitar-dependent and was actually feedback from single coil pickups when the guitar was right in front of the speaker - there's no issue with the amp itself. Also the background hum is barely audible in the room, and perfectly normal for a single-ended amp.

I also found an issue where the Bass control was behaving oddly. Turns out I had correctly followed Vox's own schematic, but either it was wrong or they had deliberately deviated from the top boost tone stack circuit. All it took was a snipped connection and one piece of wire and it was fixed.

I'm really enjoying playing through this amp, it does exactly what I wanted in that I can dial in the Gain control to add some harmonic interest that I don't get from my Princeton Reverb unless it is antisocially loud. I can get power amp distortion at reasonable levels, but it's not something I would do after the neighbours' kids are in bed.

So here it is in its completed glory. I'm pretty happy with my first attempt at fitting grille cloth (although it's not that taut), and the AC4 logo makes me think of Ford Sierras!

Supportact

Looks great!