4. Restoration of an AC30 integral top boost (1964)
An integral top boost AC30 from around 1964 that was in need of some restoration.
The cabinet was in pretty poor shape and the baffle and both rear panels had been replaced at some point with particle board ones. The amp had been recovered (badly!), with many staples holding the covering in place on the inside edges. Also steel brackets had been fitted throughout, and a steel strip on the centre of the baffle. (The baffle had actually split along the line of the gold trim strip).
The control panel had also been repainted bright blue for some reason!
The amp chassis had also been "got at", with all of the original carbon composition resistors having been replaced with film ones. There were also some bad examples of dodgy wiring, and components mounted with silicon sealant. The original vitreous cathode resistor in the output stage had been replaced with two ceramic ones, placed nice and close to the bypass capacitor (not a good idea), and fixed in place with the dreaded silicon.
The chassis itself was in pretty dirty condition, and needed to be stripped for cleaning,
...the amp seemed to have been dropped at some point too as the preamp chassis was bent and needed straightening.
I stripped the chassis of components and gave it a clean.
I rebuilt the output stages, replacing all of the valve sockets, components and heater wiring, which was up-rated to 8 amp. The screen resistors were up-rated too to 3 watt.
Chassis looking a bit cleaner and tidier now!....(no more silicon sealant!).
The preamp sections needed some work as all of the original carbon composition resistors had been replaced, and there were missing and out of specification capacitors that needed to be replaced.
The preamp after the work had been completed.
The customer provided several sets of valves for testing and for trying out in the amplifier. A full set of Mullards were eventually used in the amp. The usual AC30 hum issues were addressed with some modifications and improvements. (Heater hum modification and screening). The output bias resistor was changed from 47 ohms to 80 ohms to help the amp to run a little cooler, and allow a greater choice of output valves.
The cabinet needed stripping completely and recovering, as the previous recovering was very poor. (Staples everywhere holding the covering in place). Also as the front baffle had actually cracked along the line of the gold trim strip, I made a new baffle form 3/4" birch plywood (as original), also two new rear panels were made form 3/8" birch plywood.
All of the cabinet hardware was replaced (handles, corners, feet etc), but I was able to re-use the grill cloth, scrim, badge, trim and piping.
Not looking too bad now, and certainly an improvement on how it was when it came into the workshop. The amp sounds great!
