Category: valve curve tracer

2Ж27Л / 2SH27L / 2Z27L

2J27L photo1

I was looking at my valve stock when I found the 2SH27L/2Z27L Russian universal directly heated pentode and asked myself the question of whether this could be a good candidate for a DHT pre-amp.  This little fellow was developed alongside the 4P1L after the second world war and was intended to be operated by batteries.

IMG_3530If you remove the aluminium screening case, you can discover a lovely valve inside.

 

 

 

Looking at the key characteristics we can see that:

  • Heater voltage: 2.2V
  • Heater current: 57mA
  • Anode voltage: 120V (200V maximum)
  • Anode current 1.9mA (2.5mA maximum)
  • Transconductance: 1.9mA/V

The heater requirements are minimum (120mW) so this could be easily run from batteries. I was intrigued by this valve working in triode-mode. How will perform?

Let’s have a look at the  characteristic curves first:

2j27L triode LARGEThe anode resistance is high, about 12K. The gain is also high for a DHT, so 16 is attractive for a pre-amp depending on the specific use/need.

What about its linearity? Let’s look first at the distortion at low signal levels (1Vrms output):

2J27L THD Vo=1Vrms

The distortion is very low, which is great for the preamp, however looking at the triode performance at large signal levels we can find that the linearity is not that great: 0.2% @ 10Vrms output:2J27L THD2 Vo=10Vrms

Obviously the optimal bias point for low level signal is not good for large signal as grid current kicks in and distortion goes up. Limitations on anode voltage and current may impact our capability of pushing the bias point further right in the curve to avoid grid current al large signal.

If we look to reduce the anode voltage supply we can see that distortion increases but a good compromise point is Ia=2mA, Vgk=-2V and Va=72V in which THD is 0.026% @ 1V rms.

A 70V HT supply is more attractive, however if we think about using this valve in a preamp with a cathode follower output driver like this one, then the HT supply requirements will be enforced by the latter.

Finally, if anyone is interested in playing with this valve and simulate the preamp in SPICE, here is a quick model which I haven’t tested yet:

2j27L spice model2Z27L-Triode SPICE MODEL

 

 

 

 

4П1Л/4P1L triode curves 2

Previously I publish my 4P1L/4П1Л triode-strapped curves using my curve tracer. A forum friend who helped me in building my tracer as he was building his own, suggested using two tools for capturing curves (“Graph Grabber“) and plotting curves, loadlines, etc. (“Graph“).

Here is an example of the 4P1L/4П1Л triode-strapped curves using both applications to produce the safe operating area based on 7.5W anode dissipation. This can be extended to 9W if we consider the screen dissipation as well.

 

814/VT-154 triode mode curves

First set of characteristic curves in triode mode:

A first attempt to produce a SPICE model was quite successful:

The valve glowing in the dark, nice….

4-65a EIMAC

4-65a EIMAC NOS DHT

Just got a couple of NOS EIMAC which I will be using in my SE design

4-65a EIMAC NOS

So did some test on distortion, transconductance and driving them to +22.22dBu output to check the quality of these two ones.

I used similar test rig as before. At some point will be able to get a proper filament supply for this valve, but for the time being I will continue to use the hum pot and the big electrolytic cap across my old bench power supply which can gently provide the 3.5A for the hungry filaments!

I tested them at the limit of my CCS and bench HT supply which at the moment cannot provide more than 360V @ 100mA.

Transconductance is in the region of 3,800 – 4,000 μmhos.

So biasing the valve at -2.5V and over 90mA of anode current, the harmonic profile looks like this:

4-65A NOS EIMAC

Both EIMAC measured about 0.12% THD.

 

VT-25 / 10-Y triode curves

VT-25 triode curves 

 

 

 

Everyone loves this thoriated-tungsten DHT valve. I’ve only used it in a preamp and was hooked with its sound. Really warm and nice. Downside is, it’s very pricey these days and also is quite demanding from a filament perspective.  You can check the characteristics here.

For those who like testing their designs with LT SPICE, I produced a model which matches really well the traced curves. Would like anyone to use this one, to drop me a note with any feedback 🙂

 

 

 

RCA10Y VT25 spice model

Here is the VT-25 spice model. Let me know how it works for you!