It is a real privilege to see some truly great vintage amps from time to time. Here's one that makes a particularly good case study -
A customer brought in a Vox AC15 manufactured in the early 60's perhaps someone reading this can decipher the serial number (5067N) and come up with a more accurate date of manufacture. Vox was a brand name of a company called Jennings Musical Industries (JMI) in the late 50's early 60's.
The Vox AC15 is a two channel combo with a power of about 12 Watts and twin 12" speakers. It has a great tremolo/vibrato effect and the definitive Vox tone. This amp is a real piece of history and I think it is of particular interest because Vox have recently re-issued the model in various guises.
The amp had not been used for many years and the customer told me that when he switched it on it started smoking and packed in very quickly.
When I opened it up I found a pile of wax under the power transformer indicating that the windings had got very hot due to a short circuit within the amp. Further inspection showed this to be the first smoothing cap after the rectifier and the current overload had taken the EZ81 rectifier out as well. Interestingly there is no fuse in the power supply on this amp other than the fuse in the mains plug which in this case was 13amp and was still intact, having given the transformer no protection at all in its time of need.
Some tests indicated that the power transformer was still perfectly functional, and after replacing the faulty cap and rectifier then testing and re-healing the rest of the capacitors for a few hours the power supply was back in working order.
I decided to replace the mains cord (not original) and wire in a fuse holder with a 250mA slow blow fuse to avoid mishaps in future.
This Vox has a full compliment of 60s era valves the pre-amp and tremolo section are Mullard, and the power amp are Pinnacle. The valves tested out fine and I cleaned the contacts on the input jacks and the pots.
The vibrato/trem was not functioning, I replaced several components in the oscillator and it burst into life. The vibrato/trem can be switched in and out with a footswitch. In this case I found the switch missing and fitted a new one into the original switch housing and replaced the cord.
The amp sounds absolutely beautiful, with no signal the background hiss is very low, at medium volumes the sound is clear and bell like - this amp really has a "voice", the response in the upper mid range is pronounced and the cab and speaker combination tends to act like an acouistic guitar sound box. At high volumes the break up is very musical and chimey.
The vibrato/trem sounds great this is an interesting design, tremolo modulates the amplitude of the signal whilst vibrato modulates the pitch . In this case the circuit attempts to do both together and the tremolo part of the effect can be switched on or off which changes the tone noticeably.
Have a listen to a few recordings (Strat used on each recording)
Vox AC15 Vibrato
Vox AC15 Tremelo
Vox AC15 Channel II
Vox AC15 Channel II
From left to right on the control panel
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Two jacks per channel Channel I is the Trem/Vibrato channel, Channel II is normal
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Speed a three position rotary switch for the Vib / Trem
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Vib/ Trem two position rotary switch, one setting corresponding to Vib gives a shimmer to the sound and retains the treble frequencies, the other setting corresponding to Trem gives a very impressive smooth tremolo.
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Channel I Volume
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Channel II Volume
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Brilliance two position rotary switch , this works on the channel II signal. The first position takes out all the bass from the sound, the second position restores the bass.
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Top cut this works on both channels, it affects the treble end of the tone and has a very wide range of attenuation
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On Off switch and red pilot light
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Five position mains voltage selector.
