All tubes are suitable for audio! 
(Even CRTs, for which without, how would most of us be here? )
Let's see.. video amp so not particularly powerful. Relatively high Vsat, especially at high Vs. Might be able to find a use in PSE, pentode or UL mode. I wouldn't bother with triode mode given the low power. Can probably get 10W in PP class AB1 into a 6V6 sized OPT (that is, 10ka-a).
Tim
(Even CRTs, for which without, how would most of us be here?
)Let's see.. video amp so not particularly powerful. Relatively high Vsat, especially at high Vs. Might be able to find a use in PSE, pentode or UL mode. I wouldn't bother with triode mode given the low power. Can probably get 10W in PP class AB1 into a 6V6 sized OPT (that is, 10ka-a).
Tim
It's too late for this answer, but this tube is Russian equivalent 6P9 tube in metallic balloon is great for audio, especially if you do not want too much power.
I think a stereo 2x4W sound amplifier in Class A SE auditions quality is sufficient for a normal apartment, of course, are necessary and high quality speakers with SPL.
Two tubes 6J4 and two tubes 6P9 can make a great amp but with a vintage look.
Even though it's late response I hope that this project can be useful for other DIY audio enthusiasts.
I posted two projects: one with 6J4 and one attack with attack ECC85
tube 6AG7 equals tube 6P9
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I also like the look and features of 6P9.
I once built a 15 W PP-UL prototype with Soviet metal tubes.
Below some info.
I once built a 15 W PP-UL prototype with Soviet metal tubes.
Below some info.
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An externally hosted image should be here but it was not working when we last tested it.
All tubes are suitable for audio!
(Even CRTs, for which without, how would most of us be here?
Tim
Mm, I don't believe you. I ca't imagine a 5U4, a Klystron, a Magnetron, a Vidicon, or a Thyratron amplifiying audio
I think there is actually a SE operating point for the 6AG7 in one of the datasheets for this tube. About 3W, 3% distortion (not unusual for video pentodes, they are quite linear).
It also works great as a driver, can be used as a class B PP amp (it's linearity actually requires a low bias current in PP or operation in class A), also excellent if somewhat low gain in triode. Problems: microphony, it's an RF tube so don't even think of using it without grid stoppers (both G1 and G2), can be prone to hum injection from the filament when used in small signal stages. As a preamp/driver it works best at relatively high currents. It also has rather high gain compared to most 'power tubes' so it's possible to build a single tube pentode amp with it. Due to relatively high gain it would be interesting to see a PP pair with a LTP pentode driver (6SJ7 for instance) and plate to plate feedback... seems to me it could work well.
It also works great as a driver, can be used as a class B PP amp (it's linearity actually requires a low bias current in PP or operation in class A), also excellent if somewhat low gain in triode. Problems: microphony, it's an RF tube so don't even think of using it without grid stoppers (both G1 and G2), can be prone to hum injection from the filament when used in small signal stages. As a preamp/driver it works best at relatively high currents. It also has rather high gain compared to most 'power tubes' so it's possible to build a single tube pentode amp with it. Due to relatively high gain it would be interesting to see a PP pair with a LTP pentode driver (6SJ7 for instance) and plate to plate feedback... seems to me it could work well.
6AG7 / 6p9 will be used as cathode follower to drive the triode connected 6550.
Therefore no problems due to microphonic can exist.
In general I can understand the tube can be microphonic when used as a voltage amplifier but not when used as output stage with hundreds volts of anode swing.
I once built 15 W UL PP amplifier with 6p9 and no sign of microphony due to output tubes.
Therefore no problems due to microphonic can exist.
In general I can understand the tube can be microphonic when used as a voltage amplifier but not when used as output stage with hundreds volts of anode swing.
I once built 15 W UL PP amplifier with 6p9 and no sign of microphony due to output tubes.
I just simulated one triode connected 6550.
In my example the +Ub = 380 V, Ubias = -38 V, Ia = 82 mA, Ug1 = 96 Vpp, Pout = 10,7 W, Rl = 3k5.
BUT, the grid current is 9 mApeak / 3 mA rms. Therefore the driver stage should be biased for at least 20 mA if real A2 mode and higher power is required.
In my example the +Ub = 380 V, Ubias = -38 V, Ia = 82 mA, Ug1 = 96 Vpp, Pout = 10,7 W, Rl = 3k5.
BUT, the grid current is 9 mApeak / 3 mA rms. Therefore the driver stage should be biased for at least 20 mA if real A2 mode and higher power is required.
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