Wow what a treat. This 1966 AC100 needs a service.
Well after all these years yes I agree it’s about time.
One owner from new, original cover and used twice weekly in the band. The sort of thing vintage amp enthusiasts dream about.
This rarity is one from the erase when pop music was really developing. The Beatles had stopped touring and concentrating on recording Revolver, Jimmy Page was coming out of the studio. Pop was progressing and diversifying.
100 watts doesnt sound like much by todays standards. But don’t think this is a slouch though. This amp drives speakers very deeply indeed and even at low volumes is absolutely room filling.
Few gut shots below and keep your eyes open for one of these in your attic.
Great looking vintage amps powered by tiny chip amp like you might find in car stereo but don’t let that put you off. Inside you have a pair of quality Celestion speakers, twin channels clean and valve powered drive channel, tremolo and reverb depending on model.
Construction wise it’s pretty poor inside with a budget Hi Fi style PCB. Partly bad luck or design on korg ‘s part these amps have an amp failure disease named after them. Cambridgitus effects this series of amps by reducing the output level in number of ways. Either temporarily, or gradually according to temperature or permanently at some random level. The tremolo effect may or may not stop working also.
In the temperature related version of Cambridgitus the level will creep down over 15 minutes or half an hour making you think you are losing your hearing. With the permanent version the amp is sort of stuck on a lower level. It is all due to components in the tremolo circuit namely the optocoupler. This optocoupler controls the level of the signal through a smooth light controlled sensor. Only problem is that korg picked the wrong type in its design and they just wrong all the time some from new, a suitable replace is the increasingly hard to find device called VTL5C3.
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.
This AC30 came to me with a strong smell of burnt mains transformer. This is not unusual as old transformer windings are insulated with Paper and wax – sometimes you can see it leaking out of your vintage amp. But you know what 40 years without the occasional leak is not that bad.
Finding a replacement is possible as there are a number of companies offering replica transformers or rewound transformers which is possible if the metal core, or laminates are in good condition. Too much rust though and you have to replace. The modern ones are well made and relatively easy to fit one you have adapted the chassis due to differences between imperial and metric dimensions of the old and the new.
If the transformer has gone you may need to look at why it chose to blow at the precise time. In other words there might be another cause such as a shorting out power tube or rectifier. With vintage amps the benefits of repair are definitely worth it when you consider the high value of such amps.
Again with this AC30 the tone controls have a curiously counter intuitive where the boost comes from turning down? Weird but seems to be the original wiring. I like to reverse this and in fact on old amps like this I usually encourage clients to replace all the pots in one go as they only cost a few pounds and keep the same feel across all the controls.