Month: January 2013

a C3g driver…

Sunday morning and my brain already started with a lot of activity early today. Should be the other way round. I spent all week working and weekends should be about relaxation, but can’t help it 🙂 Before going up to the workshop and continue with the 4-65a SE amplifier build, I put into work some of these ideas that are flying around.

The 46 filament bias driver with its hefty supply will be “the driver” for the 4-65a, yes, no doubt. I may want to try some other combinations such as 4P1L pentode in filament bias, or why not some other drivers as 6e5p, C3g and D3a. All these are brilliant candidates. But final judge should be my ears. We all know that what looks really good in paper not necessarily translates into a great sounding driver, but at least is the best start.

C3g driver with gyratorC3g (as well as D3a) a really linear and revered valves in triode mode. Huge gain, current capability and transconductance. Perfectly engineered valves. C3g can easily provide a great performance as a driver with a gyrator load. I found playing around that with a simple LED bias, a bias of -2V, 190V and 30mA provides an outstanding 0.15% @ 200Vpp at least on the simulations..

I may try this in my 4P1L/6C4C project as well…. (so much to do)

Ale

Edit

 

Hi Ale,
A few observations.
190V with 31mA provide dissipation of 5.9 W !! It is excessive.
Use Telefunken data. Pa max is 3,5W.
Also, look data : “G2 and G3 connected to A

Just as a long-time user C3g as driver :-)

Rajko

You’re right, interesting oversight which reflects that I shouldn’t be doing this early on a Sunday morning! Looking at the datasheet, we could get out 4.9W maximum if using Pa=3.5W + Pg2=0.7W + Pg3=0.7W. Well, a bit too much, perhaps safer running it below 4.5W.

So let’s see how it performs if we dial down the anode current. I ended up changing the LED to minimise distortion:
C3g driver gyrator load

The THD is 0.26%@ 200Vpp which is similar to my two stage 4P1L-46. I wonder how much distortion the 4P1L will give at 200Vpp?

 

 

4P1L Pentode Driver (Test 2)

Improving the driver with a gyrator load

After the early experiments with the 4P1L driver in pentode mode, I decided to look at improving it somehow given advice given. The gyrator load is not a good match for a pentode unless the reflected impedance is low enough to control the gain of the stage. Gary Pimm recommends:

“In the driver experiments the plate resistor was increased to a value larger than in traditional Pentode driver stages to get more gain.A CCS was placed in parallel with the plate resistor to add plate current to compensate for the high value plate resistor. This allows you to have independent controls of the gain and operating current. The resistor is chosen to set the gain and the CCS is used to set the Pentode operating current.
To maximize the circuit performance the resistance in the screen circuit is adjusted for minimum distortion. There are draw backs to this- The circuit has to be tweaked for each tube. As adjusting the screen voltage and resistance also effects the gain of the stage you have to compromise some to have the gain match between 2 channels. This is not a circuit where you can swap tubes around without “calibrating” the stage on the test bench.
Another interesting way of applying the circuit is to place the plate resistor in parallel with the Pentode and have the CCS supply all the current needed by the stage. This allows the Pentode driver stage to have PSRR similar to CCS loaded triode stages. It also makes the signal current loop very small including only the Pentode, cathode, and plate resistors. The noise and capacitor colorations of the power supply are quite effectively removed.”

So I opted for adding a resistor in parallel (RL) to adjust gain, minimise distortion and improve PSRR:

4P1L Pentode Driver TestThe load resistor is 68K. I optimised the operating point to reduce distortion at maximum swing (i.e. 200V peak to peak). The input impedance of the soundcard interface which is 100K didn’t produce a significant impact on the distortion when measuring from the anode output or in the mu output:

4P1L pentode filament bias RL=68K

Interesting to see that distortion is now nearly half of previous operating point and 0.27% for 200Vpp is very good.

The screen current is approximately 1.8mA at 81V bias.

 

 

C3g and D3a triode SPICE models

Using my C3g and D3a triode curves, I developed the following SPICE models:

D3a triode SPICE

 

** D3A TRIODE ************************************************************
* Created on Sat Jan 12 09:17:53 GMT 2013 using tube.model.finder.PaintKIT
* model URL: http://www.bartola.co.uk/valves/valve-curves/d3a-triode/
* Created by Ale Moglia [email protected]
*--------------------------------------------------
.SUBCKT TRIODE_D3A 1 2 3 ; P G K ; 
+ PARAMS: CCG=6.7P CGP=3.3P CCP=1P RGI=2000
+ MU=72.1 EX=1.456 KG1=65.625 KP=534.0 KVB=300.0 VCT=0.0557 ; Vp_MAX=500.0 Ip_MAX=0.07 Vg_step=0.5
*--------------------------------------------------
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))} 
RE1 7 0 1G 
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1} 
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID 
C2 2 1 {CGP} ; GRID=PLATE 
C3 1 3 {CCP} ; CATHODE-PLATE 
D3 5 3 DX ; FOR GRID CURRENT 
R1 2 5 {RGI} ; FOR GRID CURRENT 
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N) 
.ENDS 
*$

C3G triode SPICE modelC3g triode SPICE

 

** C3G TRIODE ************************************************************
* Created on Sat Jan 12 09:40:08 GMT 2013 using tube.model.finder.PaintKIT
* model URL: http://www.bartola.co.uk/valves/valve-curves/c3g-pentodetriode/
* Created by Ale Moglia [email protected]
*--------------------------------------------------
.SUBCKT TRIODE_C3G 1 2 3 ; P G K ; 
+ PARAMS: CCG=7P CGP=2.7P CCP=6P RGI=2000
+ MU=50.4 EX=1.428 KG1=199.6875 KP=426.0 KVB=204.0 VCT=0.5760 ; Vp_MAX=500.0 Ip_MAX=0.07 Vg_step=1.0
*--------------------------------------------------
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))} 
RE1 7 0 1G 
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1} 
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID 
C2 2 1 {CGP} ; GRID=PLATE 
C3 1 3 {CCP} ; CATHODE-PLATE 
D3 5 3 DX ; FOR GRID CURRENT 
R1 2 5 {RGI} ; FOR GRID CURRENT 
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N) 
.ENDS

4P1L SPICE model updated

Dmitry Nizh kindly worked out the 4P1L triode SPICE model using his great tool:

4P1L_triode_match

 

Here is Dimtry’s model:

** 4P1L_TRIODE ************************************************************

* Created on Mon Jan 07 07:31:48 PST 2013 using tube.model.finder.PaintKIT
* URL: http://www.bartola.co.uk/valves/valve-curves/4p1l/
*————————————————–
.SUBCKT TRIODE_4P1L_TRIODE 1 2 3 ; P G K ;
+ PARAMS: CCG=8P CGP=7P CCP=9P RGI=2000
+ MU=8.232 EX=1.3719 KG1=851.25 KP=108.0 KVB=528.0 VCT=-1.0 ; Vp_MAX=450.0 Ip_MAX=0.08 Vg_step=5.0
*————————————————–
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))}
RE1 7 0 1G
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}
RCP 1 3 1G ; TO AVOID FLOATING NODES
C1 2 3 {CCG} ; CATHODE-GRID
C2 2 1 {CGP} ; GRID=PLATE
C3 1 3 {CCP} ; CATHODE-PLATE
D3 5 3 DX ; FOR GRID CURRENT
R1 2 5 {RGI} ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS
*$

4P1L Pentode Spice Model

Inspired by Rajko’s model, I tried creating my own Spice model of the 4P1L pentode:

4P1L Pentode SPICE take 1Dmitry’s tool for pentode is difficult to fit to the pentode curves, not as the triode tool which is very accurate. Here’s Dmitry’s explanation:

“The pentode model equations, as defined by Koren, have one very, very important property: they reduce to a pretty accurate triode model of the same device if the screen moves together with the plate (I have a parameter UL for that and for ultra-linear arrangement, UL=1 is triode – try it! – then ul=0.4 or so is for UL connection, UL>1 is so called supertriode connection). That’s the good news, a really good one but that is the end of the good news. The bad news is that pentode and tetrode fitting can be at best very approximate – that is, the knee region and the slopes are not right for most tetrodes and pentodes, with the exception for some small-signal pentodes.”

Here is my model. Hopefully someone can try it and report some results:

** 4P1L PENTODE ************************************************************
* Created on Sun Jan 06 18:21:28 GMT 2013 using tube.model.finder.PaintKIP
* model URL:
*————————————————–
.SUBCKT PENT_4P1L PENTODE 1 2 3 4 ; P G K G2
+ PARAMS: CCG=9P CGP=0.1P CCP=9.5P RGI=2000
+ MU=9.27 EX=1.4139 KG1=1658.84 KG2=3528.0 KP=469.2 KVB=40.504 ; Vp_MAX=450.0 Ip_MAX=0.07 Vg_step=1.0
*————————————————–
RE1 7 0 1MEG ; DUMMY SO NODE 7 HAS 2 CONNECTIONS
E1 7 0 VALUE= ; E1 BREAKS UP LONG EQUATION FOR G1.
+{V(4,3)/KP*LOG(1+EXP((1/MU+V(2,3)/V(4,3))*KP))}
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1*ATAN(V(1,3)/KVB)}
G2 4 3 VALUE={(EXP(EX*(LOG((V(4,3)/MU)+V(2,3)))))/KG2}
RCP 1 3 1G ; FOR CONVERGENCE
C1 2 3 {CCG} ; CATHODE-GRID 1
C2 1 2 {CPG1} ; GRID 1-PLATE
C3 1 3 {CCP} ; CATHODE-PLATE
R1 2 5 {RGI} ; FOR GRID CURRENT
D3 5 3 DX ; FOR GRID CURRENT
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS

46 DHT driver final tests

20130105-125459.jpg

Having built the 4P1L filament bias driver stage in a breadboard, I now have the sufficient voltage swing to drive the 46 to maximum sweep. In my 4-65a SE amp, a maximum of 200Vpp is required to drive the amp into class A2.

The following tests conditions were used:

  • 4P1L first stage:
    • DN2540 gyrator in mu follower output
    • 220nF/450V Capacitor coupled into 46 driver
    • Filament bias: 15 ohms, Vgk=-10V
    • Vsupply=355V and Va0=210V
    • Output set to about 30-32Vpp to drive 46 at 200Vpp
  • 46 driver stage:
    • IXYS 01N100 gyrator in mu follower output
    • Load impedance is 100K (Pete Millett’s interface)
    • Filament bias: 10 ohm / 100W Vgk=-17V
    • Vsupply=355V and Va0=204-208V
    • Output set to 200Vpp

I tested 28 valves. Just a few of my lot are NOS. The average THD was about 0.4-0.5% but a good selection of 8 valves (mainly Sylvania NOS) provided a consistent 0.18% THD:

4P1L into 46 driver test2Happy now with the initial tests and selection of 46 pairs for the amplifier, I can now continue with the build…