Vox AC30 1964 restoration and repair

Recently had the pleasure to work on a couple of 1960s AC30. The earlier one is from 1964 and is a great example of how a complex fault of various un-related problems can really be difficult unless you stick to as well disciplined diagnosis routine.

In this case the amp is working but the sound is lacking top end. The amp itself has been restored externally and upon opening up the six screws on the rear panel and removing the amp chassis we find that internally it has also been extensively rejuvenated; most coupling or tone capacitors replaced with high w quality Sozo poly props and many of the resistors replaced thankfully with carbon composition half watt types the power supply itself had the decoupling capacities replaced with F and T modern equivalent values. All the work done to as pretty high standard so surprising to find the tone lacking.

With a problem amp like this my approach is to work up from the bottom.  So I start by inspecting the circuit connections under a powerful inspection lens then measure the voltage and scope out the amp stage by stage: Power supply, pre amp (stage by stage), Phase inverter and the power stage. By taking this methodical approach, i can usually spot the apparent and future  faults.

In this amp’s case  there were a couple of components shorted out to ground on the tag board screws – the impact was a subtle change in tone (nothing critical) and a simple one to  fix.  Also one to watch out for is the bad wire connections you might get some loud cracks coming through the speaker and on really old amps like this you can find that the valve sockets are gummed up with what appears to nicotine staining.  I have some great tools for dealing with this and re tensioning the pins so the sockets grips well and there is no movement or accompanying annoying crackles.

Having gained some experience with AC30 now I have build up a good database of accurate measurements around the amp.  Many faults in older amps is caused by a tendancy for manufacturers to use carbon composition resistors with valve amps.  There is good reason for using them but they do have a tendancy to drift high in value over time.   Sometimes this doesn’t matter but with certain resistors, there value is critical to gain.  So the results is that you amp’s gain lowers and the amp sounds a little weaker.  Unfortunately this gradual drift is difficult to notice, however repair and restoration is not expensive or difficult.

If you are having AC30 troubles please feel free to  get in touch.


8 thoughts on “Vox AC30 1964 restoration and repair”

  1. Hi amploft ,
    I have piggy back AC30 top boost ( top boost controls on the back) which needs a complete overhaul.
    Where are you situated?

  2. I have a early 60s AC 30 it needs a little love cosmetically but it’s pretty sound I think . It’s had a few minor alterations many years ago
    I kind of want to sell it
    Any interest ?
    I’m in Leicester

  3. Thanks for the article. I have a 63 AC30 and have noticed that the tone control makes no difference past about 1 o’clock on the dial. Is this normal?

  4. Hi

    not sure if you can advise, i have a 1965 AC30 TB that was serviced a while ago but I notice hum on all chanells when turned up half way or above. when playing you do not notice but when your not playing it is noticable. the higher the volume is turned up the louder the him. Turning up the cut to almost max eliminates it but also eliminates the tone.

    1. In general whenever horrible noise comes out of your amp like a hum and is effected by the controls like volume and tone controls is being introduced by some part of the amps circuit. This is as opposed to the case of hum that is unaffected by controls and is generally introduced by the amp’s power supply and most famously by tired old filter capacitors.

      If you know the circuit and you know which controls effect the noise and which dont then you can isolate the noise to a section of the amp and potentially which valve in question. If you don’t know much about circuits then you can still use the same logical process.

      So if all controls effect the noise then it must be introduced fairly early on. In the case of an AC30 I would suspect a problem with one of the input connectors first, why? Well the connectors that are used have springy contacts that wear out over time. When this happens it is a bit like having a guitar lead plugged into the amp but not a guitar, you get get a buzzy hummy noise as the lead picks up any old stray interference that is floating around in the air.

      You might be able to identify which jack connector it is by tapping each control with a wooden spoon whilst your guitar isnt even plugged in. If the noise is effected by the tapping, then you can be sure that you are getting close. Be aware that it is quite possible to be more than one connector that is at fault.

      Most amp techs use this kind of technique all the time and are able to quickly identify the cause. You may be able to save yourself a little bit of diagnosis time by working it out for yourself.

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