Hi redeyedjim,
This is with MB output tubes, and can be different with other output tubes depending on their rating and specs. The Plate voltage in the TA-15 is about 415 VDC, and the Cathode current is about 30 mA, in the 25 watt, fixed bias mode. This means that the stock tubes idle at just over their max rating of 12 watts. This Plate voltage far exceeds the specs for many EL84 tubes, and you need a very robust tube to be able to handle the high voltage and idle wattage. With the TA-15 in the 15 watt mode, Cathode biased, the Plate voltage is a more reasonable 315 VDC, but now the Cathode current is closer to 39 mA, so the tubes are still idling close to their max wattage rating of 12 watts.
I tried a pair of Mullard reissue, and the Cathode current went up to about 36 mA in the 25 watt mode, so I didn't even need to check the Plate voltage to know that these tubes were idling even hotter than the stock tubes, so I pulled the Mullards and went back to the stock tubes. Always check the tube specs, and as the site that you linked to mentioned about pin 1, I don't know what is tied to pin 1 on the TA-15.
Another thing to consider besides the tube specs, is the tube rating. Some tubes will run hotter than others at the same fixed bias setting, and the only way to know for sure is to measure the voltage and current with the tubes in the amp. Frankly, I don't care if Bob Tubeseller or Joe Blow says any particular tubes will work in this amp, I would still check the bias and tube specs myself and heed the fact that the Mesa warranty states that you will void your warranty using anything but Mesa tubes. Just like with a car salesman, take what many tube sellers say with a grain of salt.
In other words, I wouldn't want to risk using expensive tubes in this amp unless you are sure that the specs are good enough, and you are ready to adjust the bias if necessary, and you are willing to risk your warranty. In fact, I'm not willing to use any tubes in this amp other than Mesa tubes, unless I actually measure the parameters and check the specs myself. Preamp/PI tubes are a different story, as just about any will work fine.
But hey, a new amp is only about $900, NOS tubes only about $100 a piece, so if you have plenty of money and if your insured against fire, go for it. :shock:
