[size=small]This is a wonderful preamp/headphone amp with the right tube. I originally started with the unit with the chinese 6N11 tube and gave that a try for 100+ hours. After some use, the bias was off - setting the bias equally to 16.5V helped, but the unit was overly bright and lacking low-end, especially at moderate to high volumes, and especially with headphones. A warmer JJ-E88CC tube went in - bass improved, but the sound was veiled, especially at moderately low volumes, and the headphones didn't improve much at all - I didn't give this tube more than about 30 hours.[/size][size=small][/size]
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[size=small]Re-setting the J3, J4, and J5 jumpers to take a 12AU7 tube and installing, in this case, the JJ european-number 12AU7 equivalent, the difference was immediately apparent.[/size][size=small][/size]
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[size=small]With the 12AU7, the PA-10 is stunning - it really opened up - solid and even through the frequency range and at all volume levels, very dynamic, excellent presence and separation both as a preamp AND with headphones (Grado 325i in this case). In this configuration, definitely a very good amp for the price.[/size][size=small][/size]
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[size=small]With a multimeter, setting the bias and changing the internal jumpers is easy - with the 4 top face plate screws the top lifts off to allow access to the test points, pots, and jumpers. There is a short technical page from the Trends site that details this, and there is also a version of this amp available with a 12AU7.[/size]
