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.