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Hoffman Amps Forum image Author Topic: Help biasing a Hoffman 5F6A?  (Read 2851 times)

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Offline frpax

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Help biasing a Hoffman 5F6A?
« on: November 17, 2018, 11:00:02 pm »
I searched the boards and really couldn't find what I am looking for, so here I am (again) asking probably silly questions!

OK, so I got my 2x12 Hoffman 5F6A all together and I want to bias the power tubes.
AND, I do not have one of those nifty biasing devices. Just a multimeter.

I had a friend help me with this on my first amp (Plexi 6V6), and I didn't have any of those biasing devices then, so I know it can be done.

I just need someone to walk me through it or point me to a thread that explains how to bias with just a multimeter.

Thanks for any help!!
“Whether you think you can, or you think you can't--you're right.”
~Henry Ford

Offline sluckey

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Re: Help biasing a Hoffman 5F6A?
« Reply #1 on: November 17, 2018, 11:35:13 pm »
Read this...

     http://www.duncanamps.com/technical/lvbias.html

All you need is a meter and a calculator. The 1Ω resistors are already installed on the Hoffman 5F6A board.
A schematic, layout, and hi-rez pics are very useful for troubleshooting your amp. Don't wait to be asked. JUST DO IT!

Offline frpax

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Re: Help biasing a Hoffman 5F6A?
« Reply #2 on: November 19, 2018, 10:15:37 am »
Read this...

     http://www.duncanamps.com/technical/lvbias.html

All you need is a meter and a calculator. The 1Ω resistors are already installed on the Hoffman 5F6A board.
Thank you!
“Whether you think you can, or you think you can't--you're right.”
~Henry Ford

Offline frpax

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Re: Help biasing a Hoffman 5F6A?
« Reply #3 on: November 26, 2018, 05:00:42 pm »
The 1Ω resistors are already installed on the Hoffman 5F6A board.
Where?
“Whether you think you can, or you think you can't--you're right.”
~Henry Ford

Offline Willabe

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Re: Help biasing a Hoffman 5F6A?
« Reply #4 on: November 26, 2018, 05:19:43 pm »
The 1Ω resistors are already installed on the Hoffman 5F6A board.
Where?

On Doug's layout drawing, 5F6A, their by the power tubes, black wire runs from pins 1 and 8 back to the turret board, 1ohm R's.
   

Offline frpax

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Re: Help biasing a Hoffman 5F6A?
« Reply #5 on: November 27, 2018, 10:40:02 am »
The 1Ω resistors are already installed on the Hoffman 5F6A board.
Where?

On Doug's layout drawing, 5F6A, their by the power tubes, black wire runs from pins 1 and 8 back to the turret board, 1ohm R's.
 
Ah yes! I see them now. Many thanks
“Whether you think you can, or you think you can't--you're right.”
~Henry Ford

Offline shooter

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Re: Help biasing a Hoffman 5F6A?
« Reply #6 on: November 27, 2018, 10:53:06 am »
Quote
Same voltage reading. 370-ish
fixed bias changes G1 vdc, pull PA tubes, put meter 'tween G1 and ground and measure -vdc, rotate pot ccw read, rotate pot cw read
Went Class C for efficiency

 


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