For those that plays with DHT (direct heated triode), type 50 tube tube should not be a stranger to them - either a love or hate situation. They love it for the tone and character, but hate it for the scarcity of getting NOS (new old stock) tubes. They are getting rarer everyday, and price is quite hefty for such a low power tube - definitely a collector's item.
No doubt, it is an interesting tube though, with many variations of the type 50 tubes, that I can't even comprehend all. These are the main old types and there may be more: 450 globe, 350 globe, 250 globe, 50 ST19, and 50 ST16. It is said to have a type 50 globe with mesh plate that I have not seen of myself, with heavenly sound some claimed.
There are some new type 50 being manufactured due to the popularity of such tubes, such as Shuguang SG-50, Shuguang SG-60, EML 50, ER 50, Psvane globe mesh 50, Psvanwe WR50 and some that I may not even be aware of.
I would suggest new users to just go with new tubes. Old tubes are quite hard to match and those that comes within the same date code, matching Gm, matching gain, matching internal structure, are almost impossible to come by, unless one is willing to pay an arm + a leg for them.
The Shuguang SG-50 will be cheap as a test rig and casual listening tube to own. The EML 50 is ~$750/pair, oops. The EML 50 does have higher plate dissipation at 28W and can be pumped up to 6W output with a 4.1K OPT.
Below are some pictures of the type 50 tubes.
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| Shuguang type 50 with mesh plate |
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| Type 50 ST19 - old stock |
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| Type 50 Globe - old stock |
Below is the snippet of the type 50 operating conditions and characteristics. It's somewhere in between type 2A3 and 300B, with a maximum power of 4.6W when driven to the max. I wouldn't mind if I'm using some cheap type 50 clones, but definitely not for the NOS tubes.
Somewhere at 400Vp with 55mA plate current would be something I'm more comfortable with or maybe even slightly lower at 400Vp and 50mA to get 20W plate dissipation, for longevity of the type 50 tubes. Hey, they are very rare, mind you!
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| Type 50 tube datasheet |
One of the very important application note of this type 50 tube is the grid leak resistance value. It must not exceed 10K Ohm. That leads to either grid choke, or interstage coupling, where latter is what would prefer for the uncompressed sound. A 10K grid leak resistor is possible too but that is rather a waste considering the price and rarity of the tube, blasphemy!
Filament voltage is 7.5A 1.25A - for those that like AC heating, please stay away. For acceptable hum level, DC filament heating is a must to get it to acceptable level low enough for enjoyment. Exotic high frequency AC heating might work, but I've not tried that personally.
I did not personally built a full set yet for this type 50 tube, but just a test rig, built on a piece of wood (LOL!) with a level 1 5K output transformer, powered with some parts that I dug out from my stash of components. The pre/driver tube was a 5687 RC coupled 5687, with some cheapo diode bridge rectification and some FT caps. B+ is 470Vdc with cathode bias and -70V of grid voltage. With the few variations of type 50 tubes I have on hand, all seem to fall to the bias quite closely on the very same circuit.
Even with that, the body, harmonics and tonality stands out from all the other tubes I've built with.
A fully built version will come soon, when I have more time on hand, or when there is a order for it being placed. Well, in actual fact, there is one set to be built for a local audiophile, but the build date has yet to be confirmed due to busy schedule.
Meanwhile, enjoy life, and then get back to work again!
17/10/2018
J&K Audio Design