Can the 8417 Vacuum Tube Ever Be Resurrected?

Despite the return of the formerly extinct vacuum tubes – like the 7591A vacuum tube – during the late 1990s, can the 8417 vacuum tube be ever resurrected for the benefit of 21^st Century audiophiles? 

By: Ringo Bones 

If you are “fortunate” enough to have started do-it-yourself high fidelity audio during the 1980s, you might find it disconcerting – especially for American DIY hi-fi audio enthusiasts – that it was the “modern” vacuum tubes - like the 7591A power pentode and 8417 beam power tetrode vacuum tubes - that became extinct, especially during the Reagan Administration. But given that the 7591A power pentode eventually got reintroduced by Electro-Harmonix during the late 1990s thanks to Jazz electric guitarists in the former Warsaw Pact countries, is there any hope for the 8417 vacuum tube to be resurrected? 

Historically speaking, the 8417 beam power tetrode is for intents and purposes a “modern” vacuum tube because not only it was the last ones to be developed and marketed during the latter half of the Golden Age Of Stereo back in 1963 before solid state devices of similar capabilities became commercially viable years later but also like every solid state audio amplification the 8417 vacuum tube is designed to work with various degrees of negative feedback in its operation. During its early days, the 8417 vacuum tube is described as a “cavity anode” type specifically designed for the Fisher SA-1000 audio amplifier.

The 8417 vacuum tube has a rated anode dissipation of 35-watts RMS that results in a 100-watt RMS output in push-pull AB1 fixed bias configuration that results in an audio amplifier using a pair of the tubes. Even though it first gained popularity in the Dynaco Mark III amplifier during the early 1970s, it was Quicksilver Audio that brought the 8417 vacuum tube to worldwide fame back in 1981 when the former founder and owner of Quicksilver Audio Mike Sanders began selling the Quicksilver MS 190 stereo amplifier. 

The Philips 8417 Mono Amplifiers were later developed in 1984 to bring that great vacuum tube sound to the average consumer. Harry Pearson of The Absolute Sound magazine published a positive review of the amplifier during that time. Philips managed to manufacture the amplifier until 1998 before manufacturing their batches of the 8417 vacuum tube became no longer economically viable at the tail end of the 1980s. Quicksilver kept manufacturing their versions of the 8417 vacuum tube amplifier well into the 1990s to take advantage of the 1990s era Hi-Fi Renaissance despite of the output tube becoming virtually extinct at that time.      

Given that the 8417 beam power tetrode vacuum tube has no “modern” equivalent – i.e. post Soviet Russian and post Warsaw-pact manufactured equivalents – various replacement schemes for the output tubes of 8417 equipped audio amplifiers were proposed. Owners of Dynaco Mark III amps that use 8417 beam power tetrodes can be “rebiased” to use the more plentiful and still manufactured 6550 vacuum tube, but unfortunately the Dynaco Mark III’s output transformers are primarily optimized for the 8417. Too bad nobody manufactured electric guitar amplifiers that used the 8417 vacuum tubes during the 1960s because if one company did, Electro-Harmonix would be remanufacturing their very own 8417 beam power tetrode vacuum tube back in the late 1990s. 

When buying new old stock – or NOS – 8417 beam power tetrode vacuum tubes, Sylvania are considered the best brand and is relatively widely available, while General Electric and RCA manufactured ones too with varying quality and some batches are prone to grid overheating in long hi-fi listening sessions. It’s best to consider buying General Electric and RCA 8417 vacuum tubes only when they are those of “specifically manufactured for the U.S. Air Force types” – i.e. reinforced anode military specification  mil spec  types.