Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on January 11, 2014, 02:02:14 pm

Title: Tweed Pro 5e5a cathode/fixed bias switch
Post by: TubeGeek on January 11, 2014, 02:02:14 pm

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?

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: sluckey on January 11, 2014, 02:10:48 pm

here's one...

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: TubeGeek on January 11, 2014, 02:19:02 pm

Thanks Steve!

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

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: HotBluePlates on January 11, 2014, 08:43:25 pm

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.  :icon_biggrin:

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: TubeGeek on January 11, 2014, 09:33:25 pm

 :thumbsup:

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: sluckey on January 11, 2014, 09:49:25 pm

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

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: TubeGeek on January 18, 2014, 09:31:32 pm

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 :w2:

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: HotBluePlates on January 18, 2014, 09:42:23 pm

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...).

Title: Re: Tweed Pro 5e5a cathode/fixed bias switch
Post by: TubeGeek on January 19, 2014, 12:22:54 am

Quote from: HotBluePlates on January 18, 2014, 09:42:23 pm

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.