Voltage Swing

[Lost'n Found]

Hello.

I am using PSPICE to simulate the output stage of a tube amplifier. I am
using a 420v DC and an output transformer.

The voltage on the plate of the tube swings between 840v and 420v. I know
that the output transformer opposses the current change, but I thought that
the voltage should swing up and below the 420v, not between 840v and 420v.

Is it operating correctly? I mean can the voltage swing between VDC Max and
higher?

 

[Jim Thompson]

Hello.

I am using PSPICE to simulate the output stage of a tube amplifier. I am
using a 420v DC and an output transformer.

The voltage on the plate of the tube swings between 840v and 420v. I know
that the output transformer opposses the current change, but I thought that
the voltage should swing up and below the 420v, not between 840v and 420v.

Is it operating correctly? I mean can the voltage swing between VDC Max and
higher?

It's probably a simulator anomaly. Run more cycles of the simulation
to allow the inductor (transformer) to reach current equilibrium.

And post your schematic, in case you have mis-wired.

...Jim Thompson

 

[Eeyore]

Hello.

I am using PSPICE to simulate the output stage of a tube amplifier. I am
using a 420v DC and an output transformer.

The voltage on the plate of the tube swings between 840v and 420v. I know
that the output transformer opposses the current change, but I thought that
the voltage should swing up and below the 420v, not between 840v and 420v.

Is it operating correctly? I mean can the voltage swing between VDC Max and
higher?

No.

Something's certainly amiss. Check your DC conditions first.

Graham

 

[Jon Elson]

Hello.

I am using PSPICE to simulate the output stage of a tube amplifier. I am
using a 420v DC and an output transformer.

The voltage on the plate of the tube swings between 840v and 420v. I know
that the output transformer opposses the current change, but I thought that
the voltage should swing up and below the 420v, not between 840v and 420v.

Is it operating correctly? I mean can the voltage swing between VDC Max and
higher?

Is this a push-pull stage or single-ended? If push-pull, then one plate
should
go up as the other goes down, and the average of the two plates should
be close
to 420 V.

If the tube models are crappy, then they may be modeled as self-powered
current sources, and all sorts of insane anomalies are possible unless the
models are used in exactly the operating mode they were designed for. Since
SPICE doesn't have intrinsic tube models, I've seen all sorts of derived
models,
some pretty good, and some just optimized to fit curve tracer charts in one
specific case.

Yes, specifically, the voltage at the plate certainly can go above the
plate supply.
Any time the plate current is decreasing, then the voltage HAS to go up.
But. for the
plate voltage to ALWAYS be above the DC supply tells me SOMETHING has to
be wrong. Assuming the transformer model includes winding resistance,
which is
usually substantial in these transformers, then even the steady state
voltage should
be less than 420 V, and the average over an AC cycle should be just
below 420 V.
I'd take a close look at the tube model. It sounds like it might be one
of those
current-source type models, and is driving current the WRONG way, too!

Jon

 

[John Woodgate]

It sounds like it might be one of those
current-source type models, and is driving current the WRONG way, too!

Excellent; we've always wanted complementary tubes! What do they use to
emit the positrons?

 

[martin griffith]

Excellent; we've always wanted complementary tubes! What do they use to
emit the positrons?

Cross coupled black holes seem to work quite well


martin

 

[John Popelish]

Hello.

I am using PSPICE to simulate the output stage of a tube amplifier. I am
using a 420v DC and an output transformer.

The voltage on the plate of the tube swings between 840v and 420v. I know
that the output transformer opposses the current change, but I thought that
the voltage should swing up and below the 420v, not between 840v and 420v.

Is it operating correctly? I mean can the voltage swing between VDC Max and
higher?

To a first approximation, the average voltage across a transformer
winding must be zero. So I agree that the plate voltage for a
transformer winding whose other end is tied to 420 volts, should swing
above and below 420 volts.

 

[Wakarusa]

Hello.

I am using PSPICE to simulate the output stage of a tube
amplifier. I am using a 420v DC and an output transformer.

The voltage on the plate of the tube swings between 840v and
420v. I know that the output transformer opposses the
current change, but I thought that the voltage should swing
up and below the 420v, not between 840v and 420v.

Is it operating correctly? I mean can the voltage swing
between VDC Max and higher?

If you've modeled the transformer with coupled inductors (K
directive) the results may not end up acting the way you want 'em
to. I use the sub circuit below for simple OT modeling.

P1 and P2 are the two ends of the primary with Pc the primary
center tap. OUT is the secondary (the other end of the secondary
is grounded internally in the subckt). The parameters Zp and Zs
are primary and secondary impedance respectively. Hope it helps.

..SUBCKT OTRX P1 Pc P2 OUT
+PARAMS: Zp=5000 Zs=16
R1 N002 P2 1
G1 Pc N004 N002 Pc 1
G2 0 N004 OUT 0 {sqrt(Zp)/sqrt(Zs)}
G3 N002 Pc N004 0 1
G4 OUT 0 N004 0 {sqrt(Zp)/sqrt(Zs)}
R2 N004 0 1G
R3 OUT 0 {Zs}
R4 N001 P1 1
G5 Pc N003 N001 Pc 1
G6 0 N003 0 OUT {sqrt(Zp)/sqrt(Zs)}
G7 N001 Pc N003 0 1
G8 0 OUT N003 0 {sqrt(Zp)/sqrt(Zs)}
R5 N003 0 1G
..ENDS