Is there any reason one cannot substitute decent 572B tubes for the 811A modulator tubes in the ART-13?   The form, fit, and specs are all OK.  I firmly believe they will work just fine but  are any different opinions?  The only reason I ask is that when I recently divested my large stock of tubes, I failed to retain any 811As as spares.  I do have spare 572Bs, however, as used in my AL-572 and SB-200 amplifiers.  TNX

Jeep K3HVG