Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tomahawk on January 07, 2020, 10:31:13 am

Title: 5F1 Champ Bias Variable Resistor Sourcing
Post by: Tomahawk on January 07, 2020, 10:31:13 am

I am trying to replace the 470/5 resistor (R11) in the Fender Champ 5F1, link below. I have included Uncle Doug's video where he discusses the trim pot as well. The goal is to be able to bias the tube without having to discharge the caps and remove the amplifier from the cabinet. Any leads on where I can find a quality 1k/5 trim pot or variable resistor that uses a screw head or similar to avoid accidental adjustments?

https://el34world.com/Hoffman/files/Hoffman_5F1.pdf

=578

Title: Re: 5F1 Champ Bias Variable Resistor Sourcing
Post by: Williamblake on January 07, 2020, 10:46:27 am

you can get those resistors cheaply on ebay, they have a screwed on sleeve and they are not pots.

Title: Re: 5F1 Champ Bias Variable Resistor Sourcing
Post by: sluckey on January 07, 2020, 11:34:35 am

I'm shocked to see u. doug using a 1/4 or 1/2 watt 10 turn trim pot for that cathode resistor!  :huh:

Title: Re: 5F1 Champ Bias Variable Resistor Sourcing
Post by: PRR on January 07, 2020, 01:12:57 pm

Good 5W rheostats are rare and expensive. They also beg to be turned to zero and burn-up the tube.

You can trim the bias by leverage. Take a fraction of the voltage on an over-large cathode resistor to the 6V6 grid.

BUT..... what is the point?? What is "optimum" bias in this circuit? A correctly designed self-bias stage should not need manual trims. And up to a point it does not matter much. The "up to a point" is clipping. Setting bias really should be done with normal signal on normal load. Play LOUD into your loudspeaker. Monitor current. For smoothest playing the current should be steady from silent to LOUD, or maybe rise 10%.

However I suspect that most Champs are so over-volted that the idle current had to be set much lower than the full-roar current to avoid melting the 6V6. The real fix is to add resistance in the B+. Since most Champs are also buzzy, this is an opportunity to add a stage of R-C filtering. A trail values for the B+ resistor is about like the cathode resistor.

Title: Re: 5F1 Champ Bias Variable Resistor Sourcing
Post by: DummyLoad on January 07, 2020, 01:48:00 pm

https://www.mouser.com/ProductDetail/Ohmite/REL250E?qs=sGAEpiMZZMtC25l1F4XBU9nYwyv%252B3FBF21l1e3AIxA4%3D (https://www.mouser.com/ProductDetail/Ohmite/REL250E?qs=sGAEpiMZZMtC25l1F4XBU9nYwyv%252B3FBF21l1e3AIxA4%3D)

ouch! not cheap. 73bux + freight + gov't cut
 
--pete

Title: Re: 5F1 Champ Bias Variable Resistor Sourcing
Post by: Williamblake on January 10, 2020, 09:50:22 am

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.de%2Fulk%2Fitm%2F202593229703