Hi,
I would like to learn how can I plot the curves of a tube and see how they change by applying feedback, in order to understand how tubes are working in those specific configuration.
The purpose is optimize the configuration for an existing system, or optimize most parameters before starting a new project.
For example, I've read many times that applying local shunt feedback to a pentode transforms the curves into triode-like ones. But how to they will be and how much they will "triodize" based on the amount of feedback?
I attach the LtSpice file of my EL84 Baby Huey, but I would like to extend this possibility to all tubes (pre and power) I have.
This is the 12ax7 model I use:
and this the EL84 model I use:
Thank you all for your help.
I would like to learn how can I plot the curves of a tube and see how they change by applying feedback, in order to understand how tubes are working in those specific configuration.
The purpose is optimize the configuration for an existing system, or optimize most parameters before starting a new project.
For example, I've read many times that applying local shunt feedback to a pentode transforms the curves into triode-like ones. But how to they will be and how much they will "triodize" based on the amount of feedback?
I attach the LtSpice file of my EL84 Baby Huey, but I would like to extend this possibility to all tubes (pre and power) I have.
This is the 12ax7 model I use:
Code:
*
* Generic triode model: 12AX7
* Copyright 2003--2008 by Ayumi Nakabayashi, All rights reserved.
* Version 3.10, Generated on Sat Mar 8 22:41:09 2008
* Plate
* | Grid
* | | Cathode
* | | |
.SUBCKT 12AX7 A G K
BGG GG 0 V=V(G,K)+0.59836683
BM1 M1 0 V=(0.0017172334*(URAMP(V(A,K))+1e-10))**-0.2685074
BM2 M2 0 V=(0.84817287*(URAMP(V(GG)+URAMP(V(A,K))/88.413802)+1e-10))**1.7685074
BP P 0 V=0.001130216*(URAMP(V(GG)+URAMP(V(A,K))/104.24031)+1e-10)**1.5
BIK IK 0 V=U(V(GG))*V(P)+(1-U(V(GG)))*0.00071211506*V(M1)*V(M2)
BIG IG 0 V=0.000565108*URAMP(V(G,K))**1.5*(URAMP(V(G,K))/(URAMP(V(A,K))+URAMP(V(G,K)))*1.2+0.4)
BIAK A K I=URAMP(V(IK,IG)-URAMP(V(IK,IG)-(0.00058141055*URAMP(V(A,K))**1.5)))+1e-10*V(A,K)
BIGK G K I=V(IG)
* CAPS
CGA G A 1.7p
CGK G K 1.6p
CAK A K 0.5p
.ENDSand this the EL84 model I use:
Code:
*-----------------------------------------------------------------------
* Filename: 6bq5.inc 23/01/2004 V4
* Simulator: PSpice
* Device type: Power pentode
* Device model: 6BQ5/EL84
*
* Author: Duncan Munro
* Date: 12/5/97
* Copyright: (C)1997-2004 Duncan Amplification
*
*
* V3 [12/10/97]: Screen current limited to prevent screen current
* draw at Vs = 0.
*
* V4 [23/01/04]: LIMIT{x,y,z} statements changed to LIMIT(x,y,z)
*
* The following parameters are not modelled:
*
* (1) Heater
* (2) Grid current is an approximation
*
* Please note that this model is provided "as is" and
* no warranty is provided in respect of its suitability
* for any application.
*
* This model is provided for educational and non-profit use.
*
* Queries via the forum at [url=http://www.duncanamps.com/]Duncan's Amp Pages[/url]
*
* Pins A Anode
* S Screen
* G Grid
* K Cathode
*
*-----------------------------------------------------------------------
.SUBCKT 6BQ5 A S G K
*
* Calculate contribution to cathode current
*
Eat at 0 VALUE={0.636*ATAN(V(A,K)/15)}
Egs gs 0 VALUE={LIMIT(V(S,K)/19+V(G,K)+V(A,K)/1400,0,1E6)}
Egs2 gs2 0 VALUE={PWRS(V(gs),1.5)}
Ecath cc 0 VALUE={V(gs2)*V(at)}
*
* Calculate anode current
*
Ga A K VALUE={3.2E-3*V(cc)}
*
* Calculate screen current
*
Escrn sc 0 VALUE={V(gs2)*(1.1-V(at))}
Gs S K VALUE={2.0E-3*V(sc)*LIMIT(V(S,K),0,10)/10}
*
* Grid current (approximation - does not model low va/vs)
*
Gg G K VALUE={PWR(LIMIT(V(G,K)+1,0,1E6),1.5)*50E-6}
*
* Capacitances
*
Cg1 G K 10.8p
Cak A K 6.5p
Cg1a G A 0.5p
.ENDSThank you all for your help.
The basic way to do this is to set up a basic circuit of an amplifier stage.
You can 'sweep' things like Vgk using a piece-wise linear (PWL) voltage source in LTspice. Then plot things like Va and Ia if that is what you are interested in.
To get a 'family' of curves you use the .step command in LTspice to step a parameter, like B+, and that will plot the family of curves you are after.
define B+ as {B+} ; the curly braces tell LTspice that the actual value is determined with a .param statement. Then:
.param B+ = 300 ; sets the basic B+ to 300V
.step param B+ 250 350 50 ; does three sims, with B+ going from 250 to 350 in 50V steps.
Jan
You can 'sweep' things like Vgk using a piece-wise linear (PWL) voltage source in LTspice. Then plot things like Va and Ia if that is what you are interested in.
To get a 'family' of curves you use the .step command in LTspice to step a parameter, like B+, and that will plot the family of curves you are after.
define B+ as {B+} ; the curly braces tell LTspice that the actual value is determined with a .param statement. Then:
.param B+ = 300 ; sets the basic B+ to 300V
.step param B+ 250 350 50 ; does three sims, with B+ going from 250 to 350 in 50V steps.
Jan
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Thanks for the suggestion,
so the attached circuit cannot be used, but it is needed to set up a new one?
so the attached circuit cannot be used, but it is needed to set up a new one?
That circuit is a model/subcircuit. You must use a 'symbol' in an LTspice schematic that uses this model/subcircuit as its internal schematic.
I would recommend starting with setting up a schematic in LTspice, there probably is a tube in its library, and get that into a circuit and start simulating and graphing. There is a thread here on getting started with LTspice.
When that goes OK you can replace the tube with the one you want to investigate.
Jan
I would recommend starting with setting up a schematic in LTspice, there probably is a tube in its library, and get that into a circuit and start simulating and graphing. There is a thread here on getting started with LTspice.
When that goes OK you can replace the tube with the one you want to investigate.
Jan
Thanks jan,
I've already done that, and simulating a triode or pentode is fine with me on LTSpice.
What I don't get is how to see the curves of the EL84 in that specific schematic, where different local feedback can be applied.
I've already done that, and simulating a triode or pentode is fine with me on LTSpice.
What I don't get is how to see the curves of the EL84 in that specific schematic, where different local feedback can be applied.
OK. Does post # 2 make sense to you?
There is nothing special in the family curves. If you can plot say Ia while driving the grid with a DC sweep, you should get the
Vg versus Ia curve for a specific B+. To turn that into a family of curves use .step.
If you want to vary local feedback, you could do a .step on the cathode resistor, or the feedback resistor.
Possibly I don't understand the question though.
Jan
There is nothing special in the family curves. If you can plot say Ia while driving the grid with a DC sweep, you should get the
Vg versus Ia curve for a specific B+. To turn that into a family of curves use .step.
If you want to vary local feedback, you could do a .step on the cathode resistor, or the feedback resistor.
Possibly I don't understand the question though.
Jan
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