[R-390] B+ current

[2002tii]

Wei Li  wrote:

>So, if I am reading Don's post re B+ current correctly, one 
>important concept is that cathodic vacuum rectifiers, such as the 
>expensive 26Z5, does add a small delay until they pass current, thus 
>allowing the receiver tubes to warm up, and allows the choke input 
>filter to *see* a load.... unlike the behavior seen with silicon diodes.
>
>So the substitution of a cheaper 12BW4 for the expensive 26Z5 makes sense.

The 12BW4 is electrically a good substitute, but it requires 
substantial rewiring of the sockets.  The otherwise identical 12DF5 
has the same basing as the 26Z5, so if you use that one you just need 
to move the heater feed for the rectifiers to the center tap of the 
25.2 V transformer winding (from terminal 8 to terminal 9).

I wouldn't say I consider using a tube rectifier to avoid the 
~15-second high-voltage condition at turn-on important, however.  As 
I said, I generally use silicon diodes and make sure the filter caps 
are healthy, and have never had any problems even at my high line 
voltage (normally 126-128 V).  Nor, to my knowledge, has any 390A 
I've passed on or fixed for someone else had problems as a result of 
using silicon diodes (that's going on 200 R390As -- a pretty decent 
statistical sample).

Best regards,

Don


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