Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: philcarson on February 15, 2016, 08:58:20 am

Title: Fender blues junior bias adjustment
Post by: philcarson on February 15, 2016, 08:58:20 am

Newcomer to the forum and looking for some help

Id like to install an adjustable bias pot to my 2015 FBJ. Ive seen pots in the catalogue and not sure what value is needed and where the pins are connected to the board.
While the board is out any tips on rectifying other issues?
Many thanks

Phil

Title: Re: Fender blues junior bias adjustment
Post by: sluckey on February 15, 2016, 09:17:06 am

Hoffman has the parts. Here's a proven bias mod...

Title: Re: Fender blues junior bias adjustment
Post by: Paul1453 on February 16, 2016, 12:56:04 pm

I was just wondering if maybe C33 shouldn't be changed to 50V as the TP is showing -26.5 which is over the 25V shown for C33?  :dontknow:

Title: Re: Fender blues junior bias adjustment
Post by: sluckey on February 16, 2016, 01:07:13 pm

yes

Title: Re: Fender blues junior bias adjustment
Post by: Fresh_Start on February 16, 2016, 10:08:13 pm

Adjustable bias is the first thing I fixed in my Blues Jr.  Power tubes were biased so hot before, they only lasted a few months.

Not trying to be a buzzkill, but opening up a Blues Jr. can be hazardous to your mental health. Yes, lots of guys have done tons mods to this amp. However, those ribbon cables are problems waiting to happen. Every time you move the board you risk breaking one of those fragile solder joints. There is a very good reason Doug Hoffman sells replacement circuit board for these amps.

Respectfully,
Chip

Title: Re: Fender blues junior bias adjustment
Post by: philcarson on February 17, 2016, 03:08:31 am

Hi
any details about how you installed your adjustable bias mod and what values you used?

Title: Re: Fender blues junior bias adjustment
Post by: Fresh_Start on February 17, 2016, 07:43:41 am

reply #2 shows a pic of how I added adjustable bias.


http://el34world.com/Forum/index.php?topic=10029.msg91414#msg91414 (http://el34world.com/Forum/index.php?topic=10029.msg91414#msg91414)

Chip