07/31/2026
Today's repair - an AKG SolidTube from the late 90s! These were one of the first "affordable" tube mics for the home studio, coming in around $1500. They had some clever design ideas, as well as some baffling design choices that make the power supplies incredibly unreliable.
Let's start with the good - a clever electret-style cardioid capsule using very similar mechanics to the "nylon CK12", but with better chambering. the capsule actually sounds pretty nice and has a clever shockmounting arrangement.
The baffling - an all-plastic internal structure to the microphone. An AKG special, which is using proprietary components - in this case, the power transformer, low-cut inductor, and low-cut switch are all specially-made components. A "NATIONAL" brand Chinese 12AX7 is at the heart of the microphone, which makes for an easy upgrade path - although extracting the tube is a bit tricky.
The unreliable - the power transformer supplies an insanely unnecessary 23V heater winding. That means that after rectification and diode bridge voltage drops, there's 30V heading into the heater regulator. That means the regulator is sinking 18V! Additionally, the transformer is undersized for the amount of current and voltage its supplying - including a 400V(!) high voltage secondary after rectification, which is subsequently dropped to 205V for the tube plate. Long story short - it pulls a LOT of power and then has to dissipate that power as heat. Heat is NOT GOOD for components and reduces their usable lifespan.
The transformer is potted into an epoxy, making it even less able to dissipate its own heat load. The PSU transformers fail with regularity, and often take several other components with them when they do.
Unfortunately their $500ish used value makes repairing them almost not worth it, especially since AKG no longer supplies replacement transformers. Fortunately Electronic Music Services in the UK has manufactured replacement transformers, and I have exactly one of these transformers left on my bench. If your solidtube isn't working, I can fix exactly one more of these, so shoot me a DM.