Well, this month’s trophy for the most stupid design goes to this little treasure, an AMC CVT3030 mk2 hybrid valve amplifier.
On the surface, looks quite standard. 4 x EL34s in push pull for both channels. Phase splitting and drive is carried but by high voltage MOSFETs, unusual. Not the easiest of equipment to work on. Big dismantle to get at some of the boards.
This amplifier had a fault on one channel. Intermittent output and instability. The faults could be brought on with a gentle tapping on one of the EL34s. The output on this channel clipped and distorted before the good channel. This valve was also running cool, suggesting it wasn’t really conducting too much. Also, the bias control made no difference to the valve’s cathode current. Now in the “normal world”, one would remove the valve from its socket, pop it in the valve tester to prove a point, replace as required and off to the pub for a glass of sparkling water. But NO. These 4 valves are soldered into the circuit board! The two boards, left and right channels can be removed as they are fitted with Molex connectors that suffer in the heat and cause their own problems. The fibre boards become very stressed with a few years of heating and cooling. In this case, the board had started to burn, delaminate and the tracks coming away. Even with the most care, desoldering the big pins of these valves damages the tracks and through hole plating. Especially if the amplifier has serviced a few times with valve changes made. So, be ready to solder in lots of links.
With the faulty valve removed and a new one fitted and associated bias components, the amplifier worked well. The other valves had been resoldered in the past with the amplifier putting on considerable weight as a result. These were cleaned off and resoldered. Fitting valve base sockets did come to mind, but due to the design, these is no space to accommodate the added height. One also has to be careful which new valves are selected. Some makes are slightly longer than others and would not fit in the tight space given.
If anyone can tell me why soldering big hot heavy valves horizontally directly to a board is a good idea, I would love to hear from you.
Available in English for ignorant people like me?