Please read this first. I'm not sure I made my point in the video. What I am doing here is measuring the transconductance of my 5881's and picking the strongest tubes and trying to match them within 5-7% in the circuit. Matching tubes is not absolutely necessary but it is supposed to make the amp work more efficiently. In the days of this 1959 Bandmaster, I assume they just inserted the 5881's and shipped it. It does not have an adjustable bias. That is probably why each amp sounded a tad different depending on how the power tubes matched up. I just bought some nice 1953 Tung-Sol 5881's but they seemed a little on the weak side. That is why a tube tester is a must if you play with tubes as much as I do. They tested a 530 on my Hickok 752 (great tester I bought on ebay for about $340). I was happy with them but I wanted to try some of my other tubes. I found a pair that were measuring around 550 and they were matched within 6%. They sounded better. What I am trying to show here is that you need to play around with your tube selection to "squeeze" every last bit of tone you can out of the amp. You need to have a digital voltage meter, a tube tester, and a dual bias probe at least to do all of this. It is worth the investment. If you have to take your amps to a tech all of the time, you could easily buy these with the money you would pay him/her. Use the Weber Bias Chart online to get a bias range. Don't run them over the max or they won't last very long. The max on mine with a 388v DC plate voltage is about 47 mA so I am running these at 37.5 and 40 mA. Look at the EL34 chart to get close ranges (hot to cool) for the 5881's.