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Tuesday, February 24, 2015

KR T1610 Triode

We were researching some high power tubes to be used as our next project to obtain single ended class A amplifier that is capable of driving most speakers and we looked into tubes such as GM70, 211, 845, 212, 300B XLS, 813, 833 and this tube pops up on our radar - KR T1610 Triode.

It is such a huge tube with a whopping of 150W of plate dissipation! It is 2x of a RCA 211 tube! Rules of thumb is that for class A1 amplifier, we would get ~25% of efficiency at reasonable distortion output. I like use 20% personally to arrive a real output power that normally people would operate at. 

Therefore, a single KR T1610 will get us about 37.5W (25% of 150W), and having 2 in parallel will get us 75W. We saw some people claimed 100W per channel using 2 KR T1610 in parallel. Is that possible? Well, let's try and see using theoretical calculation.

Rp (plate resistance) of KR T1610 is 370 Ohm. The primary impedance of the output transformer can range from 2.5x to 7x, depending on whether someone is looking for lesser distortion, more finese, or more power. When KR T1610 is used in parallel, the primary impedance of the output transformer should be halved to ~1.25x to 3.5x. This translates to 463 Ohm to 1.3K Ohm. Let's take the plate characteristics and draw out the loadline for maximum power.

Let's assume we are working with single tube first. The above loadline would be ~1.5K Ohm. That is about 4x of Rp. It is biased for maximum output power. Since the maximum plate swing voltage is not specified, I just ignore them for calculation sake to make things simpler. Vplate is 490V and biased at around 270mA, which is about 30mA above the absolute maximum allowable plate current (not recommended!). Yet, this operating point is still below the plate dissipation.

Let's just go with this first. In actual design, the Iplate should be lower, resulting a lower output power. Also, below is assuming full swing, meaning that to get such theoretical output power, the distortion figures will be very high since full plate swing is considered and operates at non-linear, saturation or cut-off regions. In real world, the output power should be reduced further to quote at a power rating with reasonable distortion, far away from the cut-off, non-linear and saturation region.

Using the a simple formula of output power = (Vp swing * Ip swing) / 8, that leads us to:

Power (single tube)  = (820 - 140) * (0.52 - 0.02) / 8 = 680 * 0.5 / 8 = 42.5W
Power (2 tubes) = 42.5W x 2 = 85W

This theoretical bloated output power is still 15% away from the quoted 100W figures. I have already exceeded the maximum Iplate to even get this result! This still has not factor in the discount for reasonable distortion figure. I wonder how the 100W is achieved. Bias super OVER the maximum specifications? Super bloated figures? Figure with super high distortion? Go figure yourself.


Let's try with 220mA per tube to be more reasonable, adhering to go below the maximum Ip, using a new operating point but with the same impedance.

Power (single tube) = (790 - 140) * (0.48 - 0) / 8 = 39W
Power (2 tubes) = 39W x 2 = 78W

Even lower output power! Still, we are operating at near cut-off or saturation at both ends. It should be much lower when we are at reasonable distortion figures.



If I'm the one designing it, I will choose a 2.2K Ohm primary impedance, and operate it at a more linear region, far away from cut-off or sturation, with everything within the manufacturer specifications. With the above operating point, that's what I will get, with reasonable distortion figures since I do not calculate the output power at maximum swing but stopping at reasonable region (green line).

P single = (690 - 150) * (0.38 - 0.1) / 8 = 18.9W.
P double = 37.8W.


This will be a more comfortable (or honest) figure I would specified for a Parallel Single Ended KR T1610! The tube operates in a more relax mode (more relax sound too!) as it is still under the maximum dissipation. Usually tubes that pushed to the maximum will sound tense.

What if I operate it at higher Vplate? That's not recommended since the positive swing will be compressed as it is close to the saturation region near the far right. Using a higher primary impedance OPT will help but it is still not optimal.

Well, that's it for today. This is just a rough calculation that I did without much thoughts just to see if what is quoted by some websites are reliable or trustworthy or not. I did not fully check the figures, so error might be present in the above calculation. I'm not diving further since I'm not going to design or commercialize this design due to the price of these tubes. I'd probably choose GM70 that is 125W that is much cheaper but just slightly less in power.

Conclusion is - do not always trust what you read online! It is better if you can prove it yourself. It is sad that nowadays we see such con jobs are being performed daily everywhere. There is just no free lunch!


J&K Audio Design
24/2/2015

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* Finished amplifiers, DACs, audio gadgets, upgrades and repairs - this is not our core business and we do it out of passion. We do not have fixed models, fixed price and we customize for each individuals. The sky is limit of creativity.

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* Please email for volume discounts, distributor and OEM pricing.

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