The GZ34 and the Bendix 6106 are indirectly heated tubes, heater and cathode are parallel connected. Both tubes have the same base diagrams and both are bi-phase halve-wave rectifiers or full wave rectifiers.
"Slow start" means the warm-up "rise time" (time taken to change from 10% to 90%) of their output voltage when fully loaded, which reduces the inrush current of other electronic components.
I gathered this information from some books, I´m not a technician.
The question is, is it the same thing "indirectly heated" and "slow start", or in other words, all the indirectly heated tubes have slow start?.
When I asked Mick if I could use the U54, a tube with the same base diagram as GZ34 and that is also an indirectly heated rectifier, his answer was: NO!!! It is not slow start.
In some substitution book I found that the U54 can be used instead of GZ34...Could someone help and explain this technical issue?.
Slowhand here you have a very complete data for the Bendix 6106: http://www.mif.pg.gda.pl/homepages/frank/sheets/131/6/6106.pdf
(cut and paste in your browser).