I bought a Burgundy VOX AC10C1, during Black November 2020. My ex-wife commented it is a good-looking amplifier. Nice glass-topped coffee table base?
I bought it having no VOX-sound experience, and internet research indicated that it should be a fun amplifier. Initial impressions? It is LOUD. 10 Watts? It also has an abundance of low-end. I am currently using it in a small-ish bedroom (3 x 3 ½ meter), and cannot evaluate it at higher volumes. I normally play with the Gain at about 8 – 9 O’Clock, master Volume at 12 O’Clock, and have settled on Bass at 8 O’Clock, Treble at about 09:30, Reverb at about 08:30. (The faceplate is not numbered.) This is with DiMarzio PAF 36th Anniversary humbuckers and a DiMarzio Red Velvet single coil in my Ibanez Jem Jr, with guitar volume around 5/10, tone 3-7/10. Turning the Gain up to 3 O’Clock, I can barely crack open the master Volume, sounds OK for slightly fuzzy/distorty sound. Would be fun to evaluate at higher volume levels. With the Gain low, the master Volume does not really make the amp louder above about 2 O’Clock. The amp can get very Boomy as well as very Ice-picky. Easy to get flubby and incoherent bass.
Specs – Vox VX 10 (Celestion) speaker (16 Ohm), two 12AX7 valves in the pre-amp, two EL84 valves in the power amp. Gain, Bass and Treble controls, Reverb level, and master Volume, claimed 10 Watt output. Back panel says “220 – 230 V AC 50/60 Hz 100 W”. I have not yet measured wall Voltage here, but it would be the usual 240+ VAC, thus the amp will be running hot.
I refer you to TDPRI, where there is a 40+ pages thread on this amp, under VOX AC10C1 owners club. I did get some good information off other pages as well, where tech-minded posters gave information on the silicon stuff and circuit design. This amplifier is well documented.
Also visit “The VOX Showroon”, hosted by North Coast Music, for infornation and pictures: “VOX AC10C1 Amplifier Under the Hood”. (Copyrighted material.)
The owners posting on the internet are almost unanimous in praise for this low-cost (?) Chinese PCB semi-hybrid amp.
The construction is weird: The chassis is an “L” shaped plate, screwed to the cabinet on top (two screws) and to the back cover (nine screws) for support. It is solid. The power valves hang upside down right behind the speaker magnet, on a PCB separate from the main unit, attached to a bent-out section of the chasis plate. No ventilation holes. The main unit PCB holds the pots, pre-amp circuit and two 12AX7 valves, sitting in a horizontal position, one under the Gain pot, the other between the Reverb and Volume pot. No ventilation holes. The reverb module PCB is piggy-backed on the main PCB. Valves plug into sockets, the power valve sockets are screwed onto the chassis plate, but all the socket pins are soldered directly to the relevant PCB. Long-term ability of the solder joints to handle vibration has not been reported on yet.
Cooling seems to be a joke, a small slot at the bottom rear, a rather small grill vent on the top above the speaker. There is another slot in the front bottom, but the speaker frame covers a lot of that one. Care has been taken with insulation material between the chassis plate and the back cover. Cooling is very much a “smoke stack” affair. The chassis plate at the power valve end gets quite hot to the touch (at low volume playing!), and I cannot see this layout being beneficial to long term component bliss. Cabinet is said to be MDF. Neat covering all round, even the back panel is covered.
I shall post photos of the inside when I open it up again, to swop valves and fit a modified back panel.
The signal path is based on the VOX Top Boost circuit, and uses a LND150N3-G MOSFET for the first gain stage (gain about 100?), and another as signal boost (gain of 5?) prior to the phase inverter, after the digital reverb module. The first half of the first 12AX7 is used as an amplifier stage after the Gain pot, the second half of that 12AX7 is a cathode follower feeding the tone stack.
So – Input goes to the LND150, through the Gain pot, the first 12AX7Y, tone stack, from the treble pot the signal goes through an op-amp (NJN2115, running at 5V) to the digital reverb module, exits through yet another NJN2115, goes into the low-gain booster LND150N3-G, into a 12AX7 phase inverter. The master Volume pot is between the phase inverter outputs and the EL84 power valves. The amplifier is Cathode biased. Unfortunately, no Tone Cut control. This should be easy to implement on the connecting wires from the power amp module to the output transformer.
Interesting, the LND150N3-G MOSFET runs on the same voltage rail as the 12AX7, and is wired similar to a valve. Apparently this small signal transistor acts just like a 12AX7 valve in the first gain stage, and is good for adding gain to any valve amp. Problem being, one cannot swop out another spec/gain value valve to alter gain, and the VOX AC10C1 PCB uses surface mount components, so swopping out capacitors and resistors to change gain and voicing is out of my capability – initial gain is pretty much fixed for life. Sad.
It would have been easy to implement a “normal” channel around the Top Boost circuit, if the PCB allowed it. Should have been designed in, even if it would have been a solid state channel. Might have been beneficial to be able to switch out (bypass) the reverb PCB and anciliaries as well. A Tone Cut control would be easy to do, but there is no space for that pot on the control faceplate. “High / Low” inputs? Never used the “low” input on my other amps, so not missed. With the gain turned up high, the master Volume pot is a bit scratchy low down, with little effect just off zero, a sudden “switch on“ and a bit sensitive at low volumes. Yes, one can play at low volume, but it would be nice if the master Volume was more controllable at low settings.
I did remove the back cover to hear “open back” tone, I do think it is slightly better, but, of course, louder with two open sides on the speaker. (Phase cancellation? Never experienced that yet.) Did not do much to reduce the Bass response, just more “open” and slightly less “boom”. I shall at some stage make up a vented back panel to do a proper evaluation.
The internal depth is just enough to accommodate the speaker and the power valve bracket. A deeper speaker with bigger magnet will hit the power transformer, and press against the power valve bracket. No fitting Celestion Golds. Or Blues. It accepts Greenback, Creamback, V-Junior, and speakers with less than about 110 mm depth. Some owners have relocated the power valves and transformers, or spaced out the back panel, to allow the fitting of “better” speakers. But, many owners report that the stock VX 10 is fine. I cannot comment, being unable to push the amplifier, and I only have a few hours soft playing on it. Could be less boomy, and I did not look at the circuit close enough yet to see if this is speaker/cabinet derived, or just a capacitor/resistor choice somewhere in the signal chain. Not that I would know. There are some gripes about the speaker being 16 Ohm, though, with some owners threatening to fit 4/8/16 Ohm output transformers.
All this has lead to a silly desire for a wired, turret board VOX. The AC4HW, or the AC15HW? (Many owners say that the Class A AC4HW, even with the 12 inch Greenback, is “thin”, the push-pull AC15 having a fuller, stronger tone.) They apparently have better cabinet construction (Birch ply), and there is the ability to “easily” swop out components as required/desired. (The current AC15HW has no Reverb tank, or Tremelo effect, no effects loop. It has separate Normal/Top Boost inputs, both channels having a High/Low option - a switch between all the options, would be good).
I would enjoy to have a turret board version of this AC10C1, with both normal and top boost channel (on a switch), tone cut control, with valves all round, by-passable real spring reverb tank and effects loop, slightly deeper, open back cabinet allowing for speaker choices. Ten inch speaker is fine. But that is just a smaller (and much lighter) AC15C1? Ah well. Would be fun. I am open to offers to build me one, though.
