Atma-Sphere MA-1 amps

@twodolphins  If you have older Sound Labs, prior to the 'toroidal 2' backplate, there are some simple modifications that can be done to improve their impedance and efficiency. This makes them sound better too! This involves removing a resistor in the crossover. Contact me offline for more information (or look into getting the backplate updated by Sound Lab).

Spencer, we've not used 12SX7s at any time, but at one time it was a popular field modification. The 12SX7 has similar specs to the 6SN7 and it is easy to set op the filaments for 12V, so it was a way to get access to an unused tube stock.

The American 6AS7G and it variants can be problematic due to early arc-over failure. We have had some reports that the tubes hold up better in the MkIIIs and in fact what little testing we have done seems to confirm that. If American tubes are tried in the amp though, it must be regarded as experimental. One recommendation I can make is that the tubes, if NOS, should be preconditioned (tubes placed in amp and run in Standby only) for 72 hours before being used. That seems to reduce arc-over failure with them.