Valve curve tracer gm tester

[mogliaa]

Hi Tom,
Good suggestions. I will look at the op amp options to see if I can get the 40vpp sweep. I will then add the amplified 40vpp signal to the gate bias supply of my current curve tracer which provides 0 to -100v bias to the gate.
Re the generator tracer, why not dividing the 100hz pulse obtained by conditioning the plate swing rectified voltage as you recommend but then dividing the clock signal by 2 so you can ensure that at least 2 plate swing cycles occur every step change in the gate? Also my oscilloscope has a function to retain 1s or 2s the trace so this could help? By doing I could avoid adding any signal sync as you suggest. Does it make sense?


For the plate swing voltage I was thinking of two 220/12v back to back transformers to produce a 220v 100ma ac signal isolated from the mains. I would then add a variac to the mains to allow a 0-250v plate signal. Have you done anything similar?

[mogliaa]

Hi there,
Finally used two transformers back to back plus a full wave rectifier to drive the plate. I've made a couple of tests (RCA Royce 45 and a Brimar 807 in tetrode mode) and manually saved each plate curve and combined them afterwards in my computer. Not too tedious, but definitely will build the gate step generator to avoid doing this! Will have to wait for the christmas break to do this

Only point to make is that the 807 curves actually are a combination of the screen and the gate currents since my circuit senses the cathode current only to avoid using a matching transformer in case of sensing the plate current on the anode - which will be way above the 0V ground and will fry my inverting circuit

Cheers,
Ale

[mogliaa]

Here I'm again with an interesting application of the valve curve tracer. I used the brilliant SPICE modelling application from James Lanier (K4BPM PSPICE Home Page) and managed to get very closed matching with my Royce 45 by playing with the model parameters. Not easy to do at all, you can follow the instructions from Norman Koren (Improved vacuum tube models for SPICE, Part 1) and I managed to match very closely the -40V and -30V plate curves which are the ones i'm interested in for my SET 45 valve amplifier.

See the attached results. here is the LT Spice model I used with the results above:

.SUBCKT 45 1 2 3 ; P G C; NEW MODEL
+ PARAMS: MU=3.5 EX=1.33 KG1=3615.57 KP=49.50 KVB=4.39 RGI=2000
+ CCG=3.5P CGP=7.5P CCP=3.5P ; ADD .7PF TO ADJACENT PINS; .5 TO OTHERS.
E1 7 0 VALUE=
+{V(1,3)/KP*LOG(1+EXP(KP*(1/MU+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 IN MU-FOLLOWER
C1 2 3 {CCG} ; CATHODE-GRID; WAS 1.6P
C2 2 1 {CGP} ; GRID-PLATE; WAS 1.5P
C3 1 3 {CCP} ; CATHODE-PLATE; WAS 0.5P
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


it's great to be able to plot your valve curves and then try to match as closely as posible a SPICE model to support your design work

Will keep working....
Cheers,
Ale

[Zibi]

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