AMP-HEAD Amplifier Biasing Test Probes and Products
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1. Can you provide more information about your tube matching?

We purchase our power tubes in matched sets. All tubes are burned in, then tested to insure accurate matching for current draw and transconductance for maximum performance. All matched sets will have a label that identifies the plate current (Ip) and amplification gain factor transconductance (Gm).
Before we ship tubes to our customers, we re-check them on a state-of-the-art MAXI-MATCHER Digital Tube Tester. The MAXI-MATCHER is fast becoming an industry standard in power tube testing and matching. This unit has the capability to measure current draw and tranconductance. Pre-tested, burned in, and matched from the distributor and then rechecked on the Maxi-Matcher insures the best possible tube matching- "ULTRA MATCHED" power tubes from AMP-HEAD.

As tubes age they tend to drift and a matched set of tubes will likely become unmatched after a while. These changes will likely occur after the first several hours of operation. For this reason, tubes go through a burn-in before the tube matching process. This will minimize the amount of drift later in the tube life-span.

Most tube sellers will match for equal current draw but not transconductance. This parameter is important for good audio signal balance in the output stage. Most amplifiers use a push-pull configuration utilizing symmetry in the power output stage circuitry. This balanced output provides two important benefits - power supply noise cancellation and the elimination of undesirable harmonics. If one side of the power output stage has a different amplification factor then the symmetry will be lost.

Please visit our Terms & Condition page for warranty information on our tubes.

2. You sell two types of probes, BiasTester (mA) and BiasTester (mV) version. What is the difference between the two probes?

Note: The BiasTester mA version is sold only on request.

This can be confusing because both probes look identical. Both probes measure the cathode current on pin 8 of the tube. Bias current measurements is expressed in terms of mA (milliamps). The block diagram shows the BiasTester (mA) Version in the open cathode connection to allow the multimeter to become part of the electrical path. Current flowing through the tube is measured directly by the multimeter providing a direct measurement of the tube cathode current.

The BiasTester (mV) Version provides an in-direct measurment of the tube cathode current. It has a 1-Ohm shunt resistor which works as a simple current to voltage converter. A 40 mV multimeter reading is equal to 40 mA of tube current flowing through the 1-Ohm shunt resistor. Ohms Law states that the current I = V/R. Where V is the measured voltage and R equals the value of the shunt resistor: 1-Ohm.

A 1% tolerance power shunt resistor is mounted inside the probe. For the Dual Tester, resistors are matched so that the measurement difference between probes is less than 0.3 mV. This is ideal for tube matching.

While it is true that the accuracy of the direct current measuring probe (mA Ver.) is solely dependent on the accuracy of your multimeter, the benefits are insignificant. Since most amps are built with 5 - 10 % tolerance components, for all practical purposes the measurement error from the 1% shunt resistor is so small that it can be neglected.

Need help selecting the right probe? If you're uncertain about which probe to select, you can't go wrong with the mV Version with internal 1-Ohm shunt resistor. This is a very safe, trouble free, simple to use, low voltage probe. And among Amp Guru's this is considered the preferred configuration.

The mA Version can be problematic, particularly if you're new to amp biasing measurements. Two problems can arise:

1. The current input on most multimeters is fused protected. The fuse will sometimes blow when making current measurement or through improper measurements. Few people realize its there because its inside the meter. The meter will read voltage okay, but it won't measure current. Blown fuse results in an open circuit unconnected cathode and loss of tube current (tube will be cold to the touch).

2. Some amps (especially old Marshalls) will react to having a length of cable in series with the power tube cathode. This has been documented.

The mV Version with internal 1-Ohm shunt resistor completely eliminates these problems. In our view, the benefits of the mV Version outweigh those of the direct current measuring probe.

Please contact us if you have any questions, we will be more than happy to help.

3. Do you provide any instructions with your BiasTester?

We provide step-by-step instructions on how to measure the bias idle current for any amp. And if you buy a multimeter and probe from us, our instructions are tailored specifically for that meter, with settings and where the cables plug in. We try to make it as simple as possible.

Our instructions include:

  • How to install the probe correctly.
  • Initial amp setting before powering the amp on.
  • How to set up your multimeter for the type probe you purchased.
  • Bias current tables.

There are several tables provided that show typical bias current range for most common power tubes and information as to how these values are derived. You must be able to identify your power tubes and be knowledgeable about removal and installation of tubes. If you plan to adjust the bias then you must be able to identify the bias control location for your amp. We only include general information about its location and what to look for. On some amps, the bias control is accessible without having to remove the chassis. When working on any tube amp EXTREME CAUTION should be taken because tube amps run at very HIGH VOLTAGE.

Our probes measure the tube cathode current, which is the safest procedure. We recommend getting comfortable with measuring the bias on your tubes before making any adjustments. If you are changing tubes, we highly recommend practicing on a used set of tubes. This way you get an idea of the initial settings on your amp and if your measurements fall within the range that we provide in our tables. This will also help to insure your multimeter is working properly. You should always record your reading as well as label the tube to identify the socket from which it was pulled from.

Even if you never plan to adjust the bias on your amp, knowing how to measure the bias will be beneficial. As tubes age the bias set point will change. Usually this change will be dependent on how much you play the amp. Periodically checking the bias range will indicate whether or not your tubes are operating within an acceptable range.

There is plenty of information on the web as well as forums. Learn as much as you can about your amp. Find a good tech or get with a friend who has gone through the process. Once you have it down, you will be able to explore and discover the different tonal effects that various brands of tubes present. Remembering that your goal is to be able to change your own tubes and know they are biased correctly.

4. I just purchased your Dual BiasTester and Plate Voltage Probe. Can you piggy back the plate voltage and BiasTester together and check both at the same time or do I need to do them separately?

Yes, this is the preferred procedure - measuring the plate voltage and tube current at the same time! For simplicity a single socket tester is shown but this works the same with our Dual Tester.

With this configuration you have the plate voltage and current at the given operating temperature of the tube. Ideal for determining the the bias range based on the Maximum Plate Dissipation of the tube.

Our MPD probes provide measurements of both tube cathode current and plate voltage:

Plate Voltage Output: 1mV/V DC
Cathode Current Output: 1mV/mA DC
The plate voltage output has an attenuation factor of 1000X

Our Plate Voltage Probe measures only plate voltage:

Plate Voltage Output: 1mV/V DC
The plate voltage output has an attenuation factor of 1000X



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