Welcome To the Hoffman Amplifiers Forum

February 28, 2026, 03:08:32 am
guest image
Welcome, Guest. Please login or register.
-User Name
-Password



Hoffman Amps Forum image Author Topic: Another Biasing a 50watt fixed bias amp question  (Read 440 times)

0 Members and 1 Guest are viewing this topic.

Offline Yosser

  • Level 1
  • *
  • Posts: 10
  • I love Tube amps
Hoffman Amps Forum image
Another Biasing a 50watt fixed bias amp question
« on: February 02, 2026, 06:45:41 am »
I'm replacing the bias circuit on a Carlsboro 60par amplifier with a better marshall bias circuit and will be using the Duncan lord valve website one ohm cathode resistor procedure to set the output valve bias of the amplifier.
The first stage of the method states this: "Turn your amp on, but leave it on STANDBY. Set your DMM to the highest DCV scale, ground the black probe to the chassis, and take a reading from pin FIVE of any power tube socket. You should see a negative voltage in the -35 to -50 volt range if the amp has EL34s, or in the -45 to -60 volt range if the amp uses 5881s, 6L6s, or KT66s. KT88s, 6550s, KT90s, and KT100s can have bias ranges that go as high as -100 volts. Amps which use 6V6s will usually have bias supplies which produce voltages that are similar to EL34 amps...but not always. Note that you should *not* have any power tubes installed in your amp yet."
The Carlsboro 60par amplifier does not have a standby switch, if the amplifier does not have any tubes installed at all, would that be the same state as being in standby?

Offline SEL49

  • Level 2
  • **
  • Posts: 463
  • I love Tube amps
Hoffman Amps Forum image
Re: Another Biasing a 50watt fixed bias amp question
« Reply #1 on: February 02, 2026, 07:07:33 am »
Just remove all power tubes. The idea is to not damage the power tubes if the bias voltage on pin 5 is not correct. Also, while you're monitoring the bias voltage on pin 5 adjust the bias pot for maximum negative voltage.

Offline Yosser

  • Level 1
  • *
  • Posts: 10
  • I love Tube amps
Hoffman Amps Forum image
Re: Another Biasing a 50watt fixed bias amp question
« Reply #2 on: February 02, 2026, 09:07:16 am »
Thanks!

 


Choose a link from the
Hoffman Amplifiers parts catalog
Mobile Device
Catalog Link
Yard Sale
Discontinued
Misc. Hardware
What's New Board Building
 Parts
Amp trim
Handles
Lamps
Diodes
Hoffman Turret
 Boards
Channel
Switching
Resistors Fender Eyelet
 Boards
Screws/Nuts
Washers
Jacks/Plugs
Connectors
Misc Eyelet
Boards
Tools
Capacitors Custom Boards
Tubes
Valves
Pots
Knobs
Fuses/Cords Chassis
Tube
Sockets
Switches Wire
Cable


Handy Links
Tube Amp Library
Tube Amp
Schematics library
Design a custom Eyelet or
Turret Board
DIY Layout Creator
File analyzer program
DIY Layout Creator
File library
Transformer Wiring
Diagrams
Hoffmanamps
Facebook page
Hoffman Amplifiers
Discount Program