[HBR] Need advise: protection for my only 1600 kcs xfmr

[Ron Barlow]

Hi Cal,
 I did radio repair work for many years, and I don't recall ever seeing an IF trans winding that was "cooked", as the result of a short circuit, so I don't believe that you need be concerned. The built in caps, in the base of the low cost xfmrs, that were used in BC receivers, would frequently fail, and that failure mode is repairable.
 If I understand correctly, you are using an 1800 ohm resistor, for B+ decoupling. My suggestion is to use a carbon film resistor, in this application. If a short circuit did occur, on the plate side of the IFT, the film resistor would fail, "open", like a fuse. Do not use a composition resistor, as they don't always work well, as fuses.
                       73 & GL de Ron N4GJV

--- On Mon, 5/17/10, C Eus <catman351 at gmail.com> wrote:

> From: C Eus <catman351 at gmail.com>
> Subject: [HBR] Need advise: protection for my only 1600 kcs xfmr
> To: hbr at mailman.qth.net
> Date: Monday, May 17, 2010, 5:03 AM
> All:
> I reviewed just about all the HBR circuits.  One of my
> concerns is that B+
> to the mixer tubes and IF amps runs directly through my
> precious set of IF
> transformers. I wouldn't want some unfortunate short to
> take out any of
> these transformers.
> 
> Currently (pardon the pun) the 1600 Kcs IF amp is fed
> through the IF
> transformer through the primary winding and then through an
> 1800 ohm
> resistor, which is filtered by a .02 uF cap.  Ditto
> with the 100 kcs
> stages.
> 
> Is it possible to feed signal output from the plate of the
> mixer/IF amp
> tubes to the next stage using a plate blocking capacitor
> and then feeding,
> directly, the plates of the mixer/amp tubes using the 1800
> ohm resistor and
> placing the .02 uF bypass cap on the bottom end of the
> resistor nearer to
> the B+ source?
> 
> If this is a "yes,"  what would be the value for such
> a cap?  As to the 100
> kcs side of the house, I suspect the value for such a plate
> blocking
> capacitor will be much lower.
> 
> I don't have the background to determine these values
> (other than good old
> X=1/(2*pi*F*C) and then try to chose the value showing the
> lowest impedence
> so the tube "sees" the impedence of the transformers
> instead.
> 
> Any advice will be most helpful. Thanks. Cal, n6KYR/4.
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