Amplifier Bias and the Type 300B - Application Note 120423
A basic understanding of tube bias is required to engage with this application note. Furthermore, tube biasing involves risk of electric shock. It is strongly recommended that all amplifier users work with a qualified technician.
This note furthers our commitment to quality by communicating supplemental details and helpful insights about our product.
INTRODUCTION
The Western Electric type 300B is a power triode with operating conditions originally published in 1938, when the tube was first commissioned. Since then, manufacturers have developed a broad range of 300B enabled amplifiers with varying levels of performance. Western Electric cannot account for each amplifier variation and is limited to the service and support of its own products. Furthermore, many manufacturers design, build, test, and package their amplifier products with non-original, replica type 300Bs. When a user decides to upgrade from such substandard 300B types to the Western Electric 300B, they must use extra caution to avoid misuse or damage. We strongly recommend reviewing the contents of the End User guide included with your electron tube purchase. This guide contains helpful safety information, complete tube data, and warranty information.
A BRIEF EXPLANATION OF TUBE BIAS
Tube biasing is a complex and layered technical issue. For the purposes of this note, we will use the analogy of a car. A properly biased amplifier is like a properly idling car. When the driver presses the accelerator, the engine responds accordingly and the car speeds up. Power in, power out. The same is true when the car decelerates and the engine approaches an idle state. The air-fuel mix remains balanced and efficient throughout. To maintain a similar “ready” state, an amplifier often requires bias adjustments. A correctly biased amplifier has an efficient mix of voltage and current and is therefore able to reproduce the up-and-down dynamics of music with precision. Recordings sound linear, colorful, and free of distortion.
For technical information on electron tube bias and amplifier operation, we recommend Valve Amplifiers, 4th Edition.
EVALUATING YOUR AMPLIFIER’S COMPATIBILITY
It is the end user’s responsibility to operate the Western Electric 300B in conformance with the published parameters. In some cases, the Western Electric 300B may not be optimized for or compatible with your desired setup. Do not install Western Electric 300B(s) before taking time to evaluate and patiently consider your amplifier’s compatibility.
In general, there are three (3) distinct types of amplifier bias.* If you are unsure which of the following applies to your amplifier, contact the manufacturer.
Adjustable Fixed Bias - The bias voltage supplied to the grid is adjustable by trim pots within the amplifier or by controls on an exterior panel. With the help of a qualified technician, you may bias the amplifier in accordance with the 300B’s published parameters (see table below).
Non-Adjustable Fixed Bias - The bias voltage has been set by the manufacturer and is not adjustable. In this case, make sure the amplifier can be operated safely within the Western Electric 300B’s published parameters (see table below).
Cathode Bias - A low value resistor placed between the cathode and ground connections allows the amplifier’s bias to vary conditionally. Also known as self-bias. In this case, we strongly recommend you contact the manufacturer to learn your amplifier’s plate voltage and factory-set bias before tube installation.
If we can assist with the evaluation of your amplifier in accordance with the published parameters (see table below), contact us.
*The Western Electric 91E Integrated amplifier uses a unique, microprocessor assisted bias with each startup to keep 300Bs within their safe operating area. The tube’s intended linear region is maintained throughout its life.
| WESTERN ELECTRIC TYPE 300B OPERATING CONDITIONS | |||
|---|---|---|---|
| Plate Voltage | Grid Voltage | Nominal Plate Current Milliamperes | |
| Recommended Conditions |
200 | -33 | 77 |
| -45 | 20 | ||
| 250 | -46 | 74 | |
| -57 | 22 | ||
| 300 | -60 | 68 | |
| -70 | 23 | ||
| 400 | -86 | 68 | |
| -98 | 19 | ||
| Maximum Conditions | 450 | -100 | 60 |
POST EVALUATION
After evaluating your amplifier and taking any necessary action to ensure compatibility, you may move forward with tube installation. Please use caution when loading the tubes, ensuring all pins are correctly aligned with the tube socket.
For users with the ability to manually adjust their amplifier’s bias setting, we recommend checking it at least once a month for drift. Further instructions on how to safely self-test your amplifier may be provided by the manufacturer.
For some users, it may be necessary to continue operating their amplifier with tubes originally supplied by the amplifier manufacturer due to incompatibility with the Western Electric type 300B.
SUMMARY
To summarize, the Western Electric type 300B is a high performance electronic device that, especially when compared with substandard or replica 300Bs, may not be fully compatible with your amplifier, resulting in unexpected or harmful results during operation. The most common compatibility issue is caused by amplifier bias. We strongly recommend evaluating your amplifier’s bias type to ensure its operation is in conformance with the Western Electric 300B’s published parameters.
*See End User Guide. A printed copy is included in the product carton.
SAFETY WARNING
Electron tubes can be operated at hazardous voltages. Before attempting to insert or remove an electron tube from your equipment, it is recommended that you unplug your equipment, ground all circuits, check for exposed circuits, and ensure that all power switches are off. For further safety information, see the End User Guide included with your 300B purchase.