# How to model a single-ended output transformer in LTspice?

#### sealand1

Hi,
I'm very new to LTspice and I'm trying to model a 300b amplifier, the 6sn7 stage looks good but the output looks it has some errors.
Here is the circuit,

I'm not sure how can I identify the parameters including inductance and series resistance for a 3.5k:8 output transformer.

Thanks.

#### euro21

Paid Member
TangoS2531_3.5k:8 parameters:

L1 23H
R1 115
L2 0.16H
R2 0.98
K1 L1 L2 0.99996
C1 5p
C2 20p
C3 200p

#### 6A3sUMMER

Where is the leakage inductance term?
K1 L1 L2 0.99996 at midband (1kHz).

But given leakage inductance from primary to secondary, the model may fall apart at 20kHz.
Where is that term in the model?

I am probably totally incorrect, I am not a LTspice user.

#### euro21

Paid Member
K = SQRT( (Lp-Lsp)/Lp )

#### sealand1

Hi euro21,
Thank you for the information. May I ask how can I use c1, c2, c3?

#### 50AE

Inductance ratio follows impedance ratio.

So if for example, a 3.5k to 8R impedance ratio transformer you get 20H of primary inductance, you'd get a reciprocal 45.7mH of secondary inductance.

#### euro21

Paid Member
Hi euro21,
Thank you for the information. May I ask how can I use c1, c2, c3?
C1 ; primary side capacitance
C2 ; secondary side capacitance
C3 ; primary to secondary capacitance

Otherwise, if you don't want to deal with capacitances, these files helps a lot.

#### Attachments

• Transformer Models Rev4.zip
68.9 KB · Views: 130
• ltspice_transformers.pdf
202.7 KB · Views: 140

#### Potentiallyincorrect

I often find that spice models for transformers are over trusted. Would love to see a model as good as , transistors, tubes, resistors and capacitors. Much is not modeled. Saturation, inductance vs frequency, and inductance vs excitation. These are not small shortcomings. The existing models are still useful, just expect that the sym may miss reality in some cases.

#### wg_ski

Nonlinearities in transformers cannot be modeled with the usual self and mutual inductance models since each of those terms just represents a linear not signal level dependent v=Ldi/dt. Nonlinear models require specific coding. The code already exists for transistors and even pentodes/triodes (canned models with limits of course, but a lot of work went into them). Basic nonlinear constructs for voltage dependent resistances and capacitances too - because those are the BASIS for the transistor models and are used internally to them. But for nonlinear inductors, you’re on your own.