Tweed Pro 5e5a cathode/fixed bias switch

[TubeGeek]

I would like to install a switch to go between fixed bias and cathode bias on a tweed pro 5e5a build.  I figure using the 5e5 cathode bias circuit would work well. Correct me if I am wrong here please.

Does anyone have a handy diagram of how to accomplish this?

[sluckey]

here's one...

[TubeGeek]

Thanks Steve!

I knew it would be in your scrapbook…I looked before I posted but missed it the first time.

[HotBluePlates]

Add 1Ω resistors between pin 8 and the top of the cathode resistor and bypass cap for easy current measurement in both modes.

EDIT: I know you know that, but it might not be obvious to onlookers. 

[TubeGeek]

 

[sluckey]

Actually, it's not in the scrapbook on the website. Guess I need to update.

[TubeGeek]

Just finished up the Pro build today and it sounds great.  I like having the feature of fixed or cathode bias.

Quick question… I wouldn't think it is recommended to switch between the bias config's while the amp is on.  Best procedure is to power down and then switch?  Does it matter?

Opinions please

[HotBluePlates]

My concern would be a massive thump sent to the speaker on switching. Either due to differing d.c. bias levels, or simply because of the break-make action of the switch (however, your particular switch could be make-before-break, I don't know...).

[TubeGeek]

My concern would be a massive thump sent to the speaker on switching. Either due to differing d.c. bias levels, or simply because of the break-make action of the switch (however, your particular switch could be make-before-break, I don't know...).

You echo my thoughts too.  I'll recommend powering down before switching just to be on the safe side.