{"id":1947,"date":"2013-09-29T08:50:54","date_gmt":"2013-09-29T07:50:54","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=1947"},"modified":"2013-09-29T09:40:30","modified_gmt":"2013-09-29T08:40:30","slug":"4p1l-model-improved","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2013\/09\/29\/4p1l-model-improved\/","title":{"rendered":"4P1L model improved"},"content":{"rendered":"

\u00a0Improving the DHT model in A2<\/h1>\n

After playing for some time with the uTracer<\/a>, I found that the tracer wasn’t measuring accurately A2 curves. Ronald clearly advised me (as explained on his site<\/a>) that uTracer wasn’t designed for this purpose although there was a great trick to use the screen driver to generate the A2 curves and also measure grid current. Measuring grid current is key in A2 mode so a better grid current model can be derived to better simulate the non-linear and low impedance behavior of the grid in positive bias.<\/p>\n

My incarnation of the uTracer is not neat. I’m using my existing analogue curve tracer. As shown in the pictures below, my current tracer have a plethora of sockets and just adding right jumper cables for anode, cathode, screen and grid connectors will still give me the flexibility I had with my analogue tracer<\/p>\n

\"Additional <\/a>
Additional 3,300uF capacitor across C13 to help A2 traces <\/div> <\/div> <\/div>
\"My <\/a>
My version of the uTracer <\/div> <\/div> <\/div> <\/div>
\"uTracer <\/a>
uTracer using my analogue tracer sockets \ud83d\ude42 <\/div> <\/div> <\/div> <\/div> <\/div>\n

The process of generating the A2 curves (and SPICE model) starts by plotting the normal curves. The uTracer is great for this. Then you have to overlay the A2 curves in Excel (or whatever tool you prefer) to combine both set of curves.<\/p>\n

<\/p>\n

4P1L is not a power transmitting valve and wasn’t designed for positive grid current operation. Despite this, it can withstand some abuse:<\/p>\n

\"4P1L<\/a><\/p>\n

4P1L has a fantastic set of curves (I said it already like a dozen times). it’s extremely linear and interestingly when filaments are connected in series and with G3 at the filament center point, the mu is slightly higher (above 9) than when in wired parallel (about 8).<\/p>\n

The grid current curves are shown below:<\/p>\n

\"4P1L<\/a>To use 4P1L in A2 a nice cathode follower or mu-follower grid driver is needed to provide the 10-20mA of grid current as shown above. This way we could get 5W out of a SE stage.<\/p>\n

Ronald explained me why an additional 1,000uF is needed across C13 to improve the accuracy of the A2 grid and anode current tracing:<\/p>\n

“The problem is in the way the boost converter work.\u00a0The boost converter pulses charge which is in the inductor\u00a0into the capacitor. After every 4 pulses the voltage of the\u00a0capacitor is checked, and if the voltage of the capacitor is\u00a0higher than the set point, the pulsing stops.\u00a0When the voltage is very low, the increments can be large compared\u00a0to the voltage itself (from energy conservation CVnew^2 = CVold^2 + Li^2).\u00a0By increasing the capacitance, the increments become smaller.\u00a03,300 uF is perhaps an overkill, 1,000uF will be good enough.\u00a0I think and makes the discharging faster.\u00a0An idea could be to add an additional discharge transistor\u00a0which only discharges the 3300 uF cap with a much higher current.” (Ronald Dekker)<\/em><\/address>\n

\u00a0I had a 3,300uF\/63V at hand so simply hacked the uTracer by adding a switch for A2 mode and the 3,300uF capacitor with a 56V zener to protect it across it. This addition is only used during A2 mode.<\/p>\n

Now let’s look at how well we can approximate the grid current. I played with the model for some minutes and quickly got a nice result:<\/p>\n

\"4P1L<\/a><\/p>\n

The A2 curves also fit very well as shown below:\"4P1L<\/a><\/p>\n

 <\/p>\n

The model generated:<\/p>\n

**** 4P1L TRIODE version 4 ** Composite DHT with Advanced Grid Current **************\r\n* Created on 09\/29\/2013 08:35 using paint_kit.jar Version 2.4 Beta. May 2013\r\n* www.bartola.co.uk\/valves\r\n* Created by Ale Moglia valves@bartola.co.uk using uTracer\r\n* Plate Curves image file: 4P1L TRIODE 4\r\n* Plate Data source link: www.bartola.co.uk\/valves\r\n*----------------------------------------------------------------------------------\r\n.SUBCKT DHT_4P1L 4_A2 1 2 3 4 ; P G K1 K2\r\n+ PARAMS: CCG=6P CGP=8P CCP=4P RFIL=13.5\r\n+ MU=9.5 KG1=855 KP=150 KVB=745.3 VCT=0.407 EX=1.4 \r\n+ VGOFF=-3.94 IGA=0.01 IGB=0.432 IGC=11.8 IGEX=1.92 \r\n* Vp_MAX=350 Ip_MAX=140 Vg_step=5 Vg_start=10 Vg_count=9\r\n* Rp=4000 Vg_ac=55 P_max=9 Vg_qui=-48\r\n* X_MIN=77 Y_MIN=89 X_SIZE=492 Y_SIZE=526 FSZ_X=1157 FSZ_Y=724 XYGrid=false\r\n*----------------------------------------------------------------------------------\r\nRFIL_LEFT 3 31 {RFIL\/4}\r\nRFIL_RIGHT 4 41 {RFIL\/4}\r\nRFIL_MIDDLE1 31 34 {RFIL\/4}\r\nRFIL_MIDDLE2 34 41 {RFIL\/4}\r\nE11 32 0 VALUE={V(1,31)\/KP*LOG(1+EXP(KP*(1\/MU+V(2,31)\/SQRT(KVB+V(1,31)*V(1,31)))))}\r\nE12 42 0 VALUE={V(1,41)\/KP*LOG(1+EXP(KP*(1\/MU+V(2,41)\/SQRT(KVB+V(1,41)*V(1,41)))))}\r\nRE11 34 0 1G\r\nG11 1 31 VALUE={(PWR(V(32),EX)+PWRS(V(32),EX))\/(2*KG1)}\r\nG12 1 41 VALUE={(PWR(V(42),EX)+PWRS(V(42),EX))\/(2*KG1)}\r\nRCP1 1 34 1G\r\nC1 2 34 {CCG} ; CATHODE-GRID\r\nC2 2 1 {CGP} ; GRID=PLATE\r\nC3 1 34 {CCP} ; CATHODE-PLATE\r\nRE2 2 0 1G\r\nEGC1 81 0 VALUE={V(2,31)-VGOFF} ; POSITIVE GRID THRESHOLD\r\nGG1 2 31 VALUE={0.5*(IGA+IGB\/(IGC+V(1,31)))*(MU\/KG1)*(PWR(V(81),IGEX)+PWRS(V(81),IGEX))}\r\nEGC2 82 0 VALUE={V(2,41)-VGOFF} ; POSITIVE GRID THRESHOLD\r\nGG2 2 41 VALUE={0.5*(IGA+IGB\/(IGC+V(1,41)))*(MU\/KG1)*(PWR(V(82),IGEX)+PWRS(V(82),IGEX))}\r\n.ENDS \r\n*$<\/pre>\n
I’m very happy now with the performance of the uTracer and I recommend you highly to get one after tracing valves for many years with various tracers. In my view:<\/p>\n