Hint: Due to the high gm of the D3a, a conventional measurement with a tube tester is not possible. At least not meaningful.
Only a measurement in autobias mode is reliable
1 x pair D3a Siemens tubes
3-mica quality
Can be used for any: 7721 5A/185K
Tested with RoeTest in triode mode:
Test result: gm = 25 + 26mA/V which is good
Random samples were taken for noise. No abnormalities