Output Transformer for EL86

Hope everyone is doing well out there! It is approaching 2 years since COVID19 outbreak started and yet, our country is experience the 3rd (or 4th, I don't really know!) full lock down to control the spread of the virus. Blame the government? Blame to foolish citizen for our own demise?

Life goes on, online, not in the physical world. Since we can't do much in the factory, we can focus our energy to generate some post to past time. Apologies for those that ordered - we are truly sorry for not being able to churn out the irons on time. 

Now let's talk about tubes --> EL86. Huh? Not EL84? Typo? No... you've read it correctly --> EL86. It is a low impedance output pentode suitable for audio use. It is very similar to EL84, just that it has a higher maximum current tolerance and lower Ra or Rp (plate impedance) compared to EL84. The output transformer designed for EL84 may be usable but is not that suitable or optimum.

Why we choose to bring up EL86 instead of EL84? NOS EL84 is quite expensive now and is being used in commercial amplifiers. EL86 on the other hand, is not, and NOS is cheap and plentiful. I myself have collected some for personal usages, such as Mullard, Telefunken or SIemens made NOS EL86, which are such beautiful and excellent sounding tubes. Due to such low price, fake EL86 are rare, unlike fake EL84, plentiful!!! 

I do not like Pentode sound personally, but luckily, EL86 can be triode connected. 

DHTs are pure blood

IDHTs are half blood

Pentodes are muggles

Just joking... Meanwhile... 

With the low impedance of 650 Ohm when triode connected, output transformers used for 6080 tubes, or 2A3 tubes, can used. Such 2A3 or 6080 SE OPTs usually have primary impedance of ~1.5K to 2.5K which suits EL86 perfectly. The triode connected EL86 calls for 170Vp with bias current of 74mA, which most SE OPTs happily fits. For connoisseurs, they can custom make SE OPTs for perfect fit to optimize the bandwidth and primary inductance. 

Primary impedance: 1.5K to 2.5K

Primary DC current: 70mA-80mA

Output power: ~2.5W (recommend a 10W OPT) 

For those that needs a bit more power, then can happily purchase more tubes and have PSE or Parallel SE. Exotic NOS EL86 are plentiful anyways, use as many as you like! Also, maybe you can run it near maximum to have even higher power. NOS are made to last and won't be as fragile as modern tubes. 

On top of being a power tube, EL86 is a decent driver tube too since it can churn out quite a lot of current for grid current hungry tubes, or large transmitting tubes. Due to the low plate voltage, one may need to use a step-up interstage transformer to get the required V-swing to drive the output tube to full power. 

It is not as warm and pure as true triode, or true DHT (direct heated triode), but the tonality is still good, and one gets to play with multiple types of exotic NOS brands such as Telefunken, Mullard, Siemens, and such. They are many, many times better than modern day new production tubes. In addition, we have to mention the jaw-dropped faces of the visitors when they get to know that you're using Telefunkens or Mullards or Siemens that they paid loads of hard earned cash for (I wonder how much ECC802s cost now for real NOS Telefunken) while you just paid peanuts for it, and have another dozens of different NOS pairs in a box as spares. 

I'd say, why not! Now, it's time to go back to my den and have a bunch loads of beers to endure the MCO while we can. Nuh, just kidding. We'll still try to catch up with work while we can. Also, we will find another tube to introduce in the next round. 

Stay safe everyone and enjoy life. 

J&K Audio Design

6/6/2021  

50 Tube

Type 50 Tube

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. 

Shuguang type 50 with mesh plate

Type 50 ST19 - old stock

If you look at the picture of the type 50 ST19 above, you will see that there is some darkened patch on top of the tube, quite near the vent to the filament/plate cavity. I've seen quite some type 50 tubes having the same black patches. Some say that it is some sign of use and/or abuse but I cannot confirm that yet. Maybe someone with more knowledge can enlighten all of us via comments tab below.  

Type 50 Globe - old stock

Tips: If you have (lucky you) and use a type 50 globe on your amp - consider lowering the actual operating voltage as compared to the datasheet to prolong the tube life. 

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! 

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

NOS Vacuum Tubes

We're always on the hunt for exotic NOS tubes. Our audio buddies and us have just scored a batch from UK and we'd like to share the pictures on our little treasure chest here. Some are mainstream and some are not.

One of our specialty is to use non-mainstream high quality NOS tubes in our designs. This way, our customers pay peanuts and yet they could get extremely high quality sound with those legendary NOS brands like Telefunken, GEC, Cossor, Mullard and etc vacuum tubes.

Cossor 41 MP triode

A box of NOS premium NOS tubes!

Cossor 45IU Rectifier

 This is my favorite! Antique shape rounded base old style rectifier, like the famous U52 rectifier. Yummy!!! Things do not get better than this!

Cossor Tube Boxes

NOS Cossor Valves

Treasures in old boxes!

Ain't this a beauty!

Cossor 41MP triode.

41MP triode. Modern tubes just don't come close!

A box full of premium NOS tubes! OSRAM, GEC, Mullard, Cossor, Telefunken, oh yes!!!

Match quad of Telefunken EL84

2nd match quad of Telefunken EL84, yummy!!!

Close up look of Telefunken EL84

STC 3A/108A triode

Osram 6J5G triode.

GEC Pentodes

Mullard GZ33, matched pair!

We can't wait to put these tubes into use!

J&K Audio Design
10/17/2015

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