Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mook on May 05, 2015, 11:29:43 am

Title: 6v6 Bias Switch -- between Fixed and cathod -- check my thoughts?
Post by: Mook on May 05, 2015, 11:29:43 am

So.....currently I have a mini-plexi amp that is 6v6 cathod biased.

I'm thinking of installing a DPDT (on-on) mini switch (the one that Doug sells) to go from cathode to fixed bias (I would take a neg. bias supply off the B+).

Do you see any problems in doing this?     Could bias be switched on the fly?   Or should the amp be off when switching?


Thanks!


Mook

Title: Re: 6v6 Bias Switch -- between Fixed and cathod -- check my thoughts?
Post by: sluckey on May 05, 2015, 11:44:58 am

No problems. I would not switch "on the fly". I'd put the amp in standby or turn it off. Why would you want to switch while playing?

Since you will source the AC for the bias circuit from the HT winding and will have to use a large value dropping resistor, you can use a SPDT switch. DPDT works fine too. See page 7 of this pdf...

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

Title: Re: 6v6 Bias Switch -- between Fixed and cathod -- check my thoughts?
Post by: shooter on May 05, 2015, 11:55:09 am

I asked the same question awhile back

http://el34world.com/Forum/index.php?topic=18462.0 (http://el34world.com/Forum/index.php?topic=18462.0)

From all I could find, it's probably not worth the effort, most of the "reviews" say there is no magic tonal or power benefit, and from a engineering standpoint, it's just one more thing to break :laugh:

Title: Re: 6v6 Bias Switch -- between Fixed and cathod -- check my thoughts?
Post by: VMS on May 05, 2015, 01:30:49 pm

Here's  KOC article, I think fig. 3 would be suitable for you:

http://www.londonpower.com/tube-power-amp-switching (http://www.londonpower.com/tube-power-amp-switching)

Title: Re: 6v6 Bias Switch -- between Fixed and cathod -- check my thoughts?
Post by: Mook on May 05, 2015, 02:38:30 pm

Thanks all, I'm going to go ahead and install one.    So, i have extra parts and pieces than can break!   ;-)


BTW, when I meant "on the fly", I didn't mean while playing.    Yes, put the amp in standby and switch bias schemes, without turning amp off.


Mook

Title: Re: 6v6 Bias Switch -- between Fixed and cathod -- check my thoughts?
Post by: tubeswell on May 05, 2015, 07:33:09 pm

Schematic attached