For the past several weeks it's been obvious that one of the valves in my cloned VOX AC30 amplifier had gone microphonic; the tell-tale sound (even before tapping them for confirmation) can be heard in the short video clip just below when the amp is first powered up:
Several other problems also began to occur at practice a couple of weeks ago, involving a 'tinkling' sound and general feedback wails which weren't being caused by interference from the guitar's pickups (or at least not by its usual means anyway). Thankfully both of these issues seemed to have been remedied by simply removing the valves and refitting them, so I'm assuming they stemmed from poor socket connections (?) ... You can see in the following photograph how the valves appear to be sitting slightly uneven, indicating likely bad connections with their associated sockets. I'm guessing vibration can cause them to shake loose over time.
The next photo shows the wonky 12AX7's all nicely re-aligned.
The microphonic valve was verified as being the EF86 at the front end of the amp (sitting far right in the above photograph). A simple test to find the valve responsible is to simply tap the valves individually (using a drum stick or similar implement) whilst the amp is on. The normal result of doing this should just be a gentle tapping sound on the glass of the valve itself. However, if a valve has gone microphonic you'll hear the tapping amplified through the speakers. This is demonstrated in the following clip:
The faulty valve has now been removed and I await in the post a pair of Telefunken EF86's (new old stock) which were purchased through an online auction last weekend, and only now beginning to learn about the inner workings of my amplifier, knowledge is minimal and therefore ongoing, my approach simply being to trial various components to see what works and what doesn't.
If nothing else, the old Telefunken tubes at least look the part ...
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