211 tube loadline

开工大吉!

Born of 16K primary output transformer for 211 / 845 use! 

In Chinese believe, the 1st day to start work for the Lunar year has significant impact on the following days in the year. 5th day of Lunar 2016 is supposed to be an auspicious day to work and here we are, our 1st post of Monkey year! 

What could be better than to talk about tubes, or tube amplifiers? In this auspicious day, we would like to share with you some insights we have on tube amplifier design. Although title says that it is about 211 tube loadline, it can be applied to many other tubes as well, especially for tubes that are capable to go to class A2 operation, where the grid can work at positive voltage versus cathode. 

Common sense says that the higher the load impedance, the better the linearity, and the lower the output power. This is generally true for other tubes but it does not apply for all tubes.  The most commonly used example are for 300B tubes, between 3.5K and 2.5K load. The output power variations are not much though in this case. 

Let's look at 211 as an example. Before we start, take a look at the 2 figures below:

Figure 1: 211 with 10K load

Figure 2: 211 with 16K load

Figure 1 shows 10K load for 211, and figure 2 shows 16K load for 211. The load line for 10K load is much steeper and for those sharp eyes readers, they will notice that the LINEARITY for steep curve is lesser compared to the 16K load line with less steeper slope. 

On figure 2 with 16K load, you can see that the grid can actually swing more negative & positive compared to figure 1 with 10K load, and that translate to more power, on top of better linearity. For 10K load on 211, significant compression starts at -120V grid, and would be very noticeable when measured or listened too.

Let's do some calculation here, with both tubes at same plate dissipation, with operating point adjusted & optimized for both cases.

For 10K load,

• Class A1 output power = 13.8W
• Class A2 output power = 22.8W

 For 16K load,

• Class A1 output power = 14.3W
• Class A2 output power = 31.3W 

Not only 16K primary yield higher output power in pure class A1, in class A2, the increase of power is significant! Imagine what additional 10W of reserve can do for your power hungry speakers. 

Therefore, for better sound and higher output power, 16K load line for 211 is the only way to go. One can go for 20K but the return will be lesser due to diminishing returns or tube characteristics limitations. 

211 PSE at 8K primary perhaps? Ya, but for purist design and best sonic results, single is better than parallel. NOS 211 are not cheap nowadays, and replicas are expensive, not to mention matching issues (more expensive, limited and not dynamic matching, ages differently with use and many cons, etc) Why waste on precious NOS tubes? Output transformers last forever! Invest in good OPTs and prolong your NOS tube enjoyment.

If 16K is that wonderful as 211 load, why aren't there many 16K output transformers in the market? It should have been a hot-cake, right? Although 16K is good for 211, not all output transformers are built the same. The worst enemy for 16K output transformer is the high frequency response! To achieve such high primary impedance, much more primary winding/turns are needed, and therefore the additional capacitance will bring the high frequency response down. 

Try searching for them, and find out their frequency response, with the -3dB point listed, if it is ever stated in the datasheet or site. I tried Googling for it, so far I've seen none. The best so far is none other than ours ->  

J&K 801A output transformer (Z11)
Audible bandwidth 20Hz (-0.26dB) – 1kHz(0dB) – 20kHz(-0.5dB)  
Full bandwidth 10Hz (-0.5dB)   – 1kHz(0dB)  – 55kHz(-3dB)
 
J&K Western Electric Output transformer (WE 1038A) 
Full bandwidth:7Hz – 60kHz at -3dB 
Audible bandwidth: 20Hz (-0.76dB) — 1 KHz (0db) — 20 KHz (-0.41dB)


, with -1dB and -3dB measured, in-circuit instead of in laboratory environment with unreal conditions using signal generator as source, and not in a real working conditions.

So, if you want to build the absolute best 801, 211, 845 or any other high Rp (plate impedance) vacuum tubes, you know who to look for, for high impedance output transformers. 

Postscripts:
- We can make very decent 10K, 12K, 15K (sample 1, sample 2)/ 16K / 20K OPTs for pre or power output.
- Due to the large grid swing and grid current needed, 211 drive 845 or 211 drive 211 might be needed to fully utilize the high primary impedance. Ordinary drivers will not be able to manage such swings. 

Happy Lunar New Year! 
J&K Audio Design
2/12/2016

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