An 812A Vacuum Tube is included with the purchase of a M.O.P.A. amplifier. If you are ONLY ordering a SPARE Vacuum Tube (or tubes) or even a replacement tube AFTER your initial M.O.P.A. order has shipped, a $15 shipping charge will be added because the vacuum tubes are SHIPPED DIRECTLY FROM OUR MANUFACTURER who stocks and tests the tubes at a different location. They charge for shipping a vacuum tube or any part. Call us at 385-220-5000 to order a spare or replacement vacuum tube(s)...we will give you the phone number of the manufacturer to order the tube(s) directly from them. They don't sell the GB-4000, SR-4, or M.O.P.A. insturments directly, but they do stock parts or tubes that you may need in the future.


We require that our international customers purchase a spare vacuum tube (812A) at the time of their M.O.P.A. amplifier order. One 812A vacuum tube is included with the M.O.P.A. amplifier, however, it's a good idea to have a spare on hand if you're involved with a research protocol and cannot afford a day off (or longer) if something were to happen to your tube. Usually the tube will last for a few years before needing replacment, but there are occasions when human error can inadvertently break a tube and another one is needed right away. If you need to order a tube at a later date, the cost of shipping an extra single vacuum tube by itself to an international destination is relatively high with expedited FedEx or USPS courier services. It can also take up to a week or longer to receive. Our experience with this technology is that it is wise to have a spare vacuum tube on hand at all times. The extra 812A tube will be shipped INSIDE your M.O.P.A. amplifier in bubble-wrap packing material. To add a spare vacuum tube to your order, click on the "Add to Cart" button. The cost is only $52 when purchased with a new M.O.P.A. amplifier. If you have any questions, please call us.

812A Vacuum Tube - Shipping cost will be added IF SOLD SEPARATELY.

SKU: 812A
  • NOTE: The 812A vacuum tube is not sold separately to the general public from our company. It is ONLY sold and shipped at the time of purchase with a new M.O.P.A. amplifier. Please call us if you need to order a fully tested 812A vacuum tube and we will put you in touch with a supplier. 

  • Back in 1904, British scientist John Ambrose Fleming first showed his device to convert an alternating current signal into direct current. The "Fleming diode" was based on an effect that Thomas Edison had first discovered in 1880, and had not put to useful work at the time. This diode essentially consisted of an incandescent light bulb with an extra electrode inside. When the bulb's filament is heated white-hot, electrons are boiled off its surface and into the vacuum inside the bulb. If the extra electrode (also called an "plate" or "anode") is made more positive than the hot filament, a direct current flows through the vacuum. And since the extra electrode is cold and the filament is hot, this current can only flow from the filament to the electrode, not the other way. So, AC signals can be converted into DC. Fleming's diode was first used as a sensitive detector of the weak signals produced by the new wireless telegraph. Later (and to this day), the diode vacuum tube was used to convert AC into DC in power supplies for electronic equipment.

    Many other inventors tried to improve the Fleming diode, most without success. The only one who succeeded was New York inventor Lee de Forest. In 1907 he patented a bulb with the same contents as the Fleming diode, except for an added electrode. This "grid" was a bent wire between the plate and filament. de Forest discovered that if he applied the signal from the wireless-telegraph antenna to the grid instead of the filament, he could obtain a much more sensitive detector of the signal. In fact, the grid was changing ("modulating") the current flowing from the filament to the plate. This device, the Audion, was the first successful electronic amplifier. It was the genesis of today's huge electronics industry.

    Between 1907 and the 1960s, a staggering array of different vacuum tube families was developed, most derived from de Forest's invention. With a very few exceptions, most of the vacuum tube types in use today were developed in the 1950s or 1960s. One obvious exception is the 300B triode, which was first introduced by Western Electric in 1935. Penta's PL300B version of the 812A, plus many other brands, continue to be very popular with audiophiles around the world. Various vacuum tubes were developed for radio, television, RF power, radar, computers, and specialized applications. The vast majority of these vacuum tubes have been replaced by semiconductors, leaving only a few types in regular manufacture and use. Before we discuss these remaining applications, let's talk about the structure of modern vacuum tubes. SEE ARTICLE... Click Here.