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13th November 2011, 06:05 AM  #1 
diyAudio Member

Closed loop stability help needed.
I'm still working on my 6P41S amp with a servo bias amp, but have run into a stability problem with the servo. I'm breaking the servo out as a separate issue from the 6P41S SE amp as it is my main issue and the 6P41S amp seems to be coming along fine.
My goal is to make an amp with a MuStage for the input driving the 6P41S in SE mode. I will continue my observations on the 6P1S in the original thread. I am hoping to elicit discussion on servo here. The schematics are the full amp, and the servo. The servo is sufficient for discussion (help?). I vaguely remember stability criterion from 30 years ago in college. Regrettably I have forgotten much. The issue is that if I increase the input capacitor (C1) above 0.047uF the system oscillates (instability). This was demonstrated in the strange plot in the original thread on the 6P41S (another SE amp)" when I changed the input cap from a 0.022 Teflon to a 0.068 PIO. Another SE amp design. Intuitively I see that increasing C6 should make matters better rather than worse. That is, increasing it sould increase the filter capability and improve the response of the system. In reality increasing it increases lag in the system making the system less stable rather than better. So, how do I improve the stability of the system? 
13th November 2011, 06:32 AM  #2 
diyAudio Member

It looks like it oscillates through the bias control loop. Have you tried to slow down this loop (it should be slow anyway) like putting a cap on Q1 bc?
What's the oscillation frequency? jan didden
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13th November 2011, 10:15 AM  #3  
diyAudio Member
Join Date: Feb 2010

Quote:
I have plenty of these tubes but not Spice models. Also I am looking for Spice models of soviet tubes in general. 

13th November 2011, 05:24 PM  #4 
diyAudio Member
Join Date: May 2007

You need to calculate the loop gain. This is A (amp forward gain) times b (feedback circuit response). Ab, when plotted as a complex number as a function of frequency, must not loop around the (1,0) point and ideally not get too close to it either.

13th November 2011, 10:17 PM  #5 
diyAudio Member

Artsalo, I made the model for the 6P41S using "Paint_KIP.jar" from this site:
Model Paint Tools: Trace Tube Parameters over Plate Curves, Interactively Both models suffer from g2 not accuratly depicting screen current at low current levels. I have seen the same problem with Koren models for 6V6, so this seems to be a common problem. ** 6P41S ************************************************** ********** * Created on Wed Jun 15 21:32:10 EDT 2011 using tube.model.finder.PaintKIP * model URL: * .SUBCKT PENT_6P41S 1 2 3 4 ; P G K G2 + PARAMS: CCG=3P CGP=1.4P CCP=1.9P RGI=2000 + MU=91.184 EX=1.3719 KG1=275.0 KG2=4500.0 KP=10.65 KVB=45.0 ; Vp_MAX=420.0 Ip_M AX=0.256 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} ; CATHODEGRID 1 C2 1 2 {CPG1} ; GRID 1PLATE C3 1 3 {CCP} ; CATHODEPLATE 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 The 6J9P is a hodgepodge of bits and peices I stitched together and follows: * * * Filename: 6J9P.inc 6142011 * Simulator: LTSpice * Device type: Pentode * Device model: 6J9P * * Author: Steven Parfitt * Date: 6142011 * Copyright: (C) * Pins A Anode * S Screen * G Grid * K Cathode * * * * The following parameters are not modelled: * * (1) Heater * * Please note that this model is bits and pieces taken from several sources and as such * is provided "as is" with no warranty or other guarantee of its suitability * for any application. * * .SUBCKT 6J9P A S G K .PARAM VA0=100 .PARAM VS0=150 .PARAM VG0=1 .PARAM IA0=43m .PARAM IS0=5m .PARAM MU=72 .PARAM STEPEN=1.5 .PARAM KSA=0.1 .PARAM NORM=PWR((VS0/MU+VG0),STEPEN) .PARAM PEREGIB=1000 .PARAM NORM2=(2*PWR(VA0,2)+VA0)/(2*PWR(VA0,2)+VA0+PEREGIB) Bgs gs2 0 V=PWR((URAMP(V(S,K)/MU+V(G,K))),STEPEN)/(NORM) Bn n 0 V=V(A,K)/(V(gs2)+1u) Bat WIN 0 V=(2*PWR(V(n),2)+V(n))/(2*PWR(V(n),2)+V(n)+PEREGIB)/(NORM2) Banode cc 0 V=V(gs2)*V(WIN) * * Calculate anode current * Ba A K I=(IA0+IS0)*V(cc)*(1KSA) * * Calculate screen current * Bscrn sc 0 V=V(gs2)*(1V(WIN)) Bs S K I=(IA0+IS0)*V(sc)*(1KSA)+KSA*((IA0+IS0)*V(cc)) * * Grid current (approximation  does not model low va/vs) * Bg G K I=PWR((URAMP(V(G,K)),1.5))*50E6+V(G,K)/30Meg * * Capacitances * Cg1 G K 13.5p Cak A S1 1.8p Cg1a G A 0.045p R1 S1 K 0.01 Cs1 S1 G 1p Cgs G S 3p .ENDS 
14th November 2011, 07:00 AM  #6 
diyAudio Member
Join Date: Feb 2010

Thank You.

14th November 2011, 06:08 PM  #7 
diyAudio Member

Oscillation frequency is 0.25Hz with the 0.068uF cap at C1. With the 0.022uF cap it does not oscillate.

14th November 2011, 06:26 PM  #8 
diyAudio Member

Are you sure base of Q1 does not have resistor to minus rail?
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14th November 2011, 07:03 PM  #9 
diyAudio Member

I checked PCB layout. Q1 base through 33K then wire to grid resistor of 6P41S. The bias point can be adjusted by (1) adjusting R5 (acatually a pot on pwb), or (2) adjusting R11 to change current through upper pentode. The servo works and I can set the bias on the 6P41S from 50mA to 150mA, depending on the 6J9P. They seem to vary a good bit.

14th November 2011, 07:35 PM  #10 
diyAudio Member
Join Date: Feb 2006
Location: Ashland,Ky U.S

Your servo circuit is basically the same thing as a series regulator for power supplies. I would look into how they are made stable. My first thought would be to remove C4 and C10 and apply C10 from collector to base of Q1.

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