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

The Bias Master™ allows you to monitor the bias current safely and easily. While adjusting the Bias control pot of your amp, the Bias Master™ 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)1.048 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.
January 5th, 2021
If you know your plate voltage, you calculate bias current by multiplying the percentage of max wattage you want, typically 70%-80%, and divine that result by the plate current. For example, if your plate voltage is 408 VDC, 75% of 19 watts max (this is for 7591A tubes) is 14.25 watts, divided by plate voltage = .035 amps or 35mA. Set your bias current at 35mA.
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.
Asked by Anonymous on July 12th, 2021.
July 13th, 2021
Staff Member
The Bias Master simply installs into the tube sockets of the amp and then the tubes are inserted into the probe sockets for any amp. It reads the current flowing from the cathode of the compatible tube types. If your amp disables certain tubes on different settings the meter will show no bias current for the disabled tube.

Product Reviews

2.00 out of 5 based on 1 reviews
- April 12th, 2021
2 out of 5

One of the Novel biased probes had a continuity issue, I called and they provided me a return label for exchange. I was not willing to wait for the return process and new one to arrive. I took it apart and did the repair myself using a solder iron, they failed to pre inspect before shipment