# How is tube bias measured?

## How is tube bias measured?

Measure Bias With the Output Transformer Resistance Method

1. With a powered and warm amp, measure the output transformer center tap voltage and both power tube plate voltages (usually pin 3).
2. Subtract the plate voltage from the center tap voltage to get the output transformer voltage drops for both tubes.

### How do you bias a tube amp with a multimeter?

Set your multimeter to DCV > 200m. Insert the black and red ends into the proper test points and note the reading on your multimeter. Locate the bias trim/knob labeled V1, V2, etc, (usually located on the top near the front of the amplifier) and with your screwdriver make very slight turns and watch the readout change.

How is tube plate resistance measured?

The plate resistance of a given tube, Rp, can be calculated by applying Ohm’s law to the values of Ep and Ip. Figure 1-14 is a typical diode Ep – Ip curve. The plate resistance has been figured for Rp under three different conditions, as follows: Figure 1-14.

Is biasing a tube amp necessary?

Thus, no biasing is required when replacing tubes — but using a matched set of replacement output tubes is, once again, highly recommended for obvious tonal reasons. Amps with cathode-biased power-tube circuits are lower-output ones — 30 watts or less.

## What is plate resistance?

: the ratio of the potential difference between plate and cathode of a vacuum tube to the resulting current.

### What is AC plate resistance?

The number by which the a-c grid voltage has to be multiplied to give the input voltage in the equivalent circuit, is called the amplification factor (or magnification factor) of the tube. The value of the fixed resistor in the equivalent circuit, is called the plate resistance or a-c resistance of the tube.

Are Fender amps self biasing?

1) Self biasing which is also known as cathode biased. These amps do not require rebiasing in most cases. Examples are most Fender Silver Face, Black face and Reissue amps such as Deluxe Reverb, Pro (Reverb), Super Reverb, Twin Reverb and Vibrolux Reverb. Most Marshall amps also fall into this category.

When to change bias on eurotube bias probe?

Leave the amp on in the play mode for at least five minutes because as the tubes heat up the bias will change a little. After the tubes are hot do a final bias adjustment if necessary and you’re done! If you have purchased your tubes as a matched set from a reputable source such as Eurotubes, it is only necessary to measure one of your tubes.

## Can a bias probe be used with a multimeter?

Since 2006 we have offered a single octal bias probe for use with a Multimeter. We will continue to offer these because in conjunction with a Multimeter this is the most cost effective tool for measuring true plate current allowing you to accurately bias your amp.

### How many milliamps is a 25 watt bias probe?

For example, if you were biasing a 25 watt tube and your DC plate voltage was 450VDC the calculation would look like this. 25 Watts divided by 450VDC = .0555 multiplied by .7 = .0388 and rounded off this would be .039 or 39 Milliamps. You can run the tube as high as 85% but you will wear them out quite a bit quicker.

Can a bias probe be used on a cathode biased AMP?

The ET “Pro One” Bias Probe can also be used on cathode biased amps. It uses pin eight as ground which will give the actual DC plate voltage your tube sees. Typically it’s necessary in a cathode biased circuit to subtract the voltage at the cathode away from the voltage at the plate, with the ET “Pro One” Bias Probe this step in unnecessary.

01/04/2020