How about the 6080 military spec tube? It looks more like a pentode 6550 on the outside. But, its the same tube just not as tall. There are also 7236?. I have seen joint army navy 6080WC not sure what the WC stands for. And finally Sylvania 6AS7GA again in the 6550 shape. These are all new old stock. I understand the Atmasphere may not use these as they may not be available in quantity for continued production. Are the sonics build quality comparable?
I am not sure if my Triplet 3423 tube tester is testing the tubes as a diode for each section. The plate voltage is 70 volts and the bias at 54 on the chart which is what I used. However, I could put 100 volts on the plate. Don't know if that would be a better test. Or even altering the bias would produce a more accurate test. I do have one NOS GE 6AS7 tube. And it tested much better than any of the Russian tubes under the same test conditions.
These tubes sound fine. The problem is that we are using fixed bias, and under these conditions those tubes don't hold up. The 6080 is the best of the lot (some of them hold up fine); when the 7236 arcs, they usually develop a hard short inside (nasty); the GA has problems with the grid warping due to a small grid heatsink (they tend to die very quickly).
OK, now we are back to Russian tubes. There are some sellers in the US that will test these properly. Looking at these companies they have choices that may not be different. I see SED branded 6AS7G and then Winged C 6AS7G and then Sovtek 6AS7G. I got a feeling these are all made in the same factory? And not much different than each other? I will pass on Chinese tubes of this type?
If they are 6N13Cs (marked as 6H13C) then they are made by Svetlana, now known as Winged C. They can be spotted by the round getters in the base of the tube.
If you want adjustable bias, simply replace the 37.4K (bypassed by a 1M resistor) with a 25K pot, wired as a reostat.
I'm not sure what you are up to in the diagram you supplied. How were you putting it into the rest of the circuit?
Right. FWIW, you are better off using resistors in the bias network. As the line voltage drifts, so will the bias voltage. But the voltage is negative, not positive, so as line voltage increases, the bias voltage goes more negative and vice versa. This gives you a little more leeway so that changes in line voltage don't fry your power tubes.
The meter is used to see if there is any DC Offset current. The 5K pot is used to adjust it to zero.
OK, I finally found and bought 8 Russian 6AS7 that tested really well.
Since, I am only using 4 tubes per channel. I don't see the need to run in class A/B. I can run class A all the time. This will take out a switch and the 22.1K and 110K resistors(2x 49.9K).
Also, why are resistors 37.4K and 1M used in parallel? I calculated that they equal 36,101 ohms. And one 36K 2W resistor can replace these?
Since, I am only using 4 tubes per channel. I don't see the need to run in class A/B. I can run class A all the time. This will take out a switch and the 22.1K and 110K resistors(2x 49.9K).
Also, why are resistors 37.4K and 1M used in parallel? I calculated that they equal 36,101 ohms. And one 36K 2W resistor can replace these?