Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mageerc on June 24, 2010, 07:24:23 am

Title: can't bias tubes hot enough RESOLVED
Post by: mageerc on June 24, 2010, 07:24:23 am

I suppose I just don't understand an adjustable fixed bias circuit... I have a Fender Twin Reverb with the AB 763 circuit and can't bias my tubes more than 45% maximum plate dissipation.  The typical circuit that I have attached a layout for has a 470 ohm  resistor on the board with the diode and a 27K resistor soldered to ground on the back of the bias pot.  Which of these resistors do I need to raise or lower the value of to obtain a hotter bias for the tubes?

Title: Re: can't bias tubes hot enough
Post by: chocopower on June 24, 2010, 07:45:59 am

Hi!

before change anything check all the componets involved in the bias circuit.

You can have burn resistors, bypased caps....etc...

Title: Re: can't bias tubes hot enough
Post by: sluckey on June 24, 2010, 08:57:42 am

Quote

The typical circuit that I have attached a layout for has a 470 ohm  resistor on the board with the diode and a 27K resistor soldered to ground on the back of the bias pot.  Which of these resistors do I need to raise or lower the value of to obtain a hotter bias for the tubes?

After following choco's advice....

Increase the 470Ω up to about 1KΩ or decrease the 27KΩ to about 15-22KΩ. Doing either will decrease the negative bias voltage on the grid which is what you must do to run the tubes hotter.

Title: Re: can't bias tubes hot enough RESOLVED
Post by: mageerc on June 24, 2010, 01:48:17 pm

Eureka!  I have found it!  A previous tech had worked on this amp and installed 22K instead of 220K bias resistors!  It's all warm and fuzzy now...  Thanks for the lessons guys!

Title: Re: can't bias tubes hot enough RESOLVED
Post by: bigdaddy on June 24, 2010, 02:04:45 pm

That's why I always say before you work on any amp, even though it's a pain and time consuming check to see what someone did that should not have been done. The older the amp and the more beat up the more chances of someone doing something not correct to out right dangerous.

I have opened up amps and have seen others open the amps for me from other people and was amazed at the things I saw. Some times from supposed reputable well know shops and people. One guy in California comes to mind with some mind bogglingly bad work and ideas. Another from Texas who you would send your vintage pristine amp to and get it back with different transformers and shoddy work.