[AMPS] Re: TL-922 transformers

Sun, 13 Jul 1997 20:12:16 +0000

> From:          Rich Measures <measures@vc.net>
> Date:          Sun, 13 Jul 97 13:37:25 +0000

> >I take your response as indicating the filament circuits are indeed  
> >different. The TL-922 places the filaments in series, ...snip...
> 
> The center-tap of the 922's 10v filament transformer winding connects via 
> an RFC to the common point of the series-connected filaments---so each 5v 
> filament is esssentially fed from a 5v source.  

That is ONLY true if the resistance of the entire path to the 
center tap is MANY times less than the cold resistance of the tube's 
filaments. 

Since the TL-922 does not meet this requirement, the 
filaments are effectively in series for inrush current like the 
lights in a Christmas tree, or the tubes in a ALL American Five 
radio.

It is bad enough to do this with 39 cent light bulbs or 35Z3's and 
50L6's, it is idiotic to connect 150 dollar tubes filaments in 
series. 

If I owned a TL-922, the FIRST thing I'd do would be to replace the 
filament transformer and put the filaments in parallel, or install a 
very good inrush protection circuit for the series connected 
filaments.

> The issue is current, not current limiting transformer design.  So far, 
> it appears that I am the only person who measured the filament inrush 
> currents with an oscilloscope. Who is the person who wants to talk about 
> the nitty-gritty of how current limiting transformers are designed?

I have measured dozens and dozens of PA's filament inrush.  It is a 
normal part of design procedures. Like any measurement, it must be 
done properly. The discussion was transformers, not measurement 
procedures.

HOW did you measure the filament inrush, can you give an example of 
the method and connection you use?? That would be both interesting 
and useful than claims the 220 transformer looks, to you, like a 
neon sign transformer.

73, Tom W8JI 

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