With the stock tube, the sound was too high gain for me, with a mid-scoop sound that verged on being too metal. I replaced the 12AX7 with a 12AT7 (ECC81) made by Mullard in the UK. Now it has a little less gain, but still enough for a decent amount of drive when maxed and a wider range of settings with lower gain for more subdued sounds. The change in tubes also added some mids and gave the pedal more of a classic rock type sound.
There are knobs for gain, volume, hi, mid, and lo. The EQ knobs have a lot of range in them and you have a huge range of EQ settings available, thanks to the 3-bands. The pedal goes from no volume at it's lowest settings up to a bluesy/early rock sound, upwards to varying degrees of crunch all the way up to a fairly high gain sound, even though I swapped the stock 12AX7 for a lesser gain 12AT7/ECC81. The sound is fairly smooth and sounds like a nice tube distortion should, with the overall amp-like feel that is alot like playing directly into the gain channel of a high quality tube amp.
These pedals are usually fairly expensive for a used pedal on eBay, but are still alot cheaper than the Chandler Tubes Overdrives (new and used). If you want a nice vintage real tube overdrive, these are worth checking out.
