Bias Master™ - TAD, tube tester, with 4 probes

Tube Amp Doctor (TAD)
Pictured: Octal
Pictured: Octal
Pictured: Noval
Pictured: Noval
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Pictured: OctalPictured: Noval
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Bias Master™ - TAD, tube tester, with 4 probes

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Bias Masterâ„¢ system BM4 Measures Bias current of the vacuum tubes in your amp. Includes 4 probes and english/german operation manual.

The Bias Masterâ„¢ allows you to monitor the bias current safely and easily. While adjusting the Bias control pot of your amp, the Bias Masterâ„¢ shows the actual result. This lets you set and evaluate the full reasonable biasing range your amp and tubes offer to find and set your personal perfect choice.

The Bias setting has a dramatic impact on the amp's response, break-up and dynamic character.

Probe Type Measures
Octal 6L6GC, 6550, EL34, 6V6GT, 5881, KT66, KT88, KT90, KT100, KT120, 7027A
Noval EL84 types

For additional probes (for testing other types of tubes), see S-TBMA2-X

Item ID:
Tube Amp Doctor (TAD)
Part Numbers
Probe TypeOur Part NumberUPC/EAN
Product Measurements by Type
Item Height2.1 in.
Item Length5.76 in.
Item Width3.6 in.
Packaging Information
Packaging Dimensions8 in. × 5 in. × 3.75 in.
Weight (Packaging)Noval versionS-TBM4-EL840.922 lbs.
Octal versionS-TBM41.048 lbs.
Product Measurements by Item
S-TBM4 - Octal versionItem Height2.1 in.
Item Length5.76 in.
Item Width3.6 in.
S-TBM4-EL84 - Noval versionItem Height2.1 in.
Item Length5.76 in.
Item Width3.6 in.
PDF: Operation ManualAll Models

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Specifications, Files, and Documents

PDF: Operation Manual Operation Manual67.08 KB

Questions and Answers

Click each question to see its answers.
Asked by Anonymous on January 15th, 2019.
January 16th, 2019
Staff Member
No, such a list is not included with this meter, but we have provided a bias calculator that may be of assistance:
January 23rd, 2020
I was looking at the formula to calculate bias. It's states "Let’s say we found it (PLATE VOLTAGE) measured 420VDC..." How does one measure that ? Does the Bias Master have that capability ? I notice for the 6L6GT in the equation 420V is used but that value differs a lot from both what is on the pdf file and the sheet that shown on the video of 300 -390 V for that tube.
Asked by Anonymous on November 8th, 2017.
Matt H
November 8th, 2017
Yes you can but the bias meter simplifies the process and does the mathematical conversions for you.
Asked by Anonymous on April 12th, 2018.
Matt H
April 12th, 2018
According to the TAD site this measures KT120 tubes. You may want to contact them for confirmation.
Asked by Anonymous on January 23rd, 2020.
January 23rd, 2020
Staff Member
A multimeter would be required to measure voltage. For questions on how to properly bias your specific amplifier, we advise consulting a professional repair tech.

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