Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on December 08, 2014, 09:13:18 pm

Title: Cathode bias switching in plate-derived bias supply.
Post by: chocopower on December 08, 2014, 09:13:18 pm

Hi,



I´m gonna install a cathode/fixed bias switch, and after some research and read the Sluckey´s "Amp Scrapbook (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf)" it seems i can use a SPDT switch (Figure 2 at bottom of the 6th page).


In Kevin O´Connor web, he explain "In cases where there is both no tremolo and the bias supply is plate-derived (not from its own winding), then an SPDT can be implemented as in Fig. 3 (http://www.londonpower.com/graphics/pwr-fig3.gif). Here, a very large resistance - typically 150k - protects the bias supply diode and source winding from excessive dissipation when the bias supply is shunted to ground."

So, my question is if the position of that resistor (before or after the diode) matters at all. Will it do that "protection" work wherever it be?
I have seen schematics with both configurations and used to thought that there was no really a reason and you can just put it wherever you want, but right now i have my doubts.

Any information?

Thanks in advanve!

Title: Re: Cathode bias switching in plate-derived bias supply.
Post by: sluckey on December 08, 2014, 10:27:37 pm

Using this schematic for a reference...

     http://home.comcast.net/~seluckey/amps/november/november.pdf (http://home.comcast.net/~seluckey/amps/november/november.pdf)

It makes no difference if the 100K is placed immediately before or immediately after the diode.

Title: Re: Cathode bias switching in plate-derived bias supply.
Post by: chocopower on December 08, 2014, 10:32:42 pm

Thanks!!