<!– Generated by hand for SushiDSP phase D6a. –>

`jcm_cf`

JCM800 2203 V2a gain stage direct-coupled into the V2b cathode follower. Modelled as one block because the CF’s grid conduction compressing into the tone stack depends on the coupling being implicit.

Status: simulated. Commit 2bdbf91 ran this block and committed its CSVs.

LTspice version: LTspice 24, the only detail 2bdbf91 records; the exact build is not recorded by that commit.

How the schematic is written

jcm_cf.asc carries the whole circuit as SPICE directive text on the sheet rather than as placed symbols with wires. Directive lines are netlisted verbatim by LTspice, so node names are explicit and the netlist cannot depend on symbol pin geometry that could not be checked without running the tool. Tube subcircuits are instantiated as X lines against ../models/12ax7_koren.sub, whose pin order is P G K.

Analyses

Both analysis lines live on the sheet as comments and tools/spice/export.py enables exactly one of them per run:

;ac dec 40 10 20k
;tran 0 40m 20m 1u (40 ms run, the last 20 ms saved as steady state)

.options plotwinsize=0 keeps the raw file uncompressed.

Probe and stimulus

Probed node: out (the cathode-follower cathode, which drives the tone stack).
Input node: g2a
Source: V1 g2a 0 SINE(0 {AMP} 1k) AC 1
Input level(s): AMP is stepped over 10 mV, 100 mV and 1 V.

Stepped control

.step param AMP list 10m 100m 1

This block has no control, so the input level is stepped instead. setting 0/1/2 corresponds to 10 mV / 100 mV / 1 V.

Components

Ref	Value	Nodes	Spec reference
V1	SINE(0 {AMP} 1k) AC 1	g2a - 0	D6d block 3 stimulus
XV2a	12AX7_KOREN	P=p2a G=g2a K=k2a	D6b.3 Triode12AX7
Rp2a	100 kΩ	B - p2a	D6d block 3: V2a 100 kΩ plate
Rk2a	820 Ω	k2a - 0	D6d block 3: V2a 820 Ω cathode
Ck2a	0.68 uF	k2a - 0	D6d block 3: V2a 0.68 uF bypass
XV2b	12AX7_KOREN	P=B G=p2a K=out	D6b.3 Triode12AX7
Rk2b	100 kΩ	out - 0	D6d block 3: CF 100 kΩ cathode load
Vplate	360 V	B - 0	B+ per the D6a brief

Judgment calls

V1 drives V2a’s grid directly rather than through a coupling cap and grid leak. In the amp that coupling cap belongs to block 2 (jcm_v1b_cold already ends with it), so including it here would double-count it. The source’s 0 V DC also supplies the grid reference.
V2b’s plate goes straight to B+ with no plate resistor, which is what a cathode follower is; its grid node is V2a’s plate node, which is what “direct-coupled” means.

Note on the AC runs

AMP scales only the SINE(...) stimulus; the AC 1 magnitude is fixed. Stepping AMP therefore produces three identical AC sweeps (setting 0, 1 and 2) in <block>_ac.csv. Only the transient CSV carries three genuinely different runs. That is intentional: the acceptance criterion for this block is a transient null, and keeping one .step line for both analyses keeps the export path uniform.