<!– Generated by hand for SushiDSP phase D6a. –>
jcm_v1a
JCM800 2203 input network and the V1a gain stage, through the coupling cap into the preamp pot with its bright cap.
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_v1a.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 preamp pot wiper). - Input node:
src - Source:
V1 src 0 SINE(0 {AMP} 1k) AC 1 - Input level(s):
AMP = 100 mVfixed (this block steps Preamp instead).
Stepped control
.step param PREAMP list 0.1 0.5 0.9
PREAMP is the normalised 1 MΩ preamp-pot fraction; 0 is fully down, 1 is fully up. The 470 pF bright cap bridges the pot’s upper section, so its effect vanishes as PREAMP approaches 1, exactly as in the amp.
Components
| Ref | Value | Nodes | Spec reference |
|---|---|---|---|
| V1 | SINE(0 {AMP} 1k) AC 1 | src - 0 | D6d block 1 stimulus |
| Rgleak | 1 MΩ | src - 0 | D6d block 1: 1 MΩ grid leak |
| Rgs | 68 kΩ | src - g | D6d block 1: 68 kΩ grid stopper |
| XV1a | 12AX7_KOREN | P=p G=g K=k | D6b.3 Triode12AX7 |
| Rp | 100 kΩ | B - p | D6d block 1: 100 kΩ plate |
| Vplate | 360 V | B - 0 | B+ per the D6a brief |
| Rk | 2.7 kΩ | k - 0 | D6d block 1: 2.7 kΩ cathode |
| Ck | 0.68 uF | k - 0 | D6d block 1: 0.68 uF cathode bypass |
| Cc | 0.022 uF | p - pin | D6d block 1: 0.022 uF coupling |
| Rpa | {1Meg*(1-PREAMP)} | pin - out | 1 MΩ preamp pot, upper section |
| Cbr | 470 pF | pin - out | D6d block 1: 470 pF bright cap |
| Rpb | {1Meg*PREAMP} | out - 0 | 1 MΩ preamp pot, lower section |
Judgment calls
- The bright cap is placed across the pot’s upper section (input to wiper), which is what makes it level- and setting-dependent as D6d describes.
- The stepped values never reach 0 or 1, so neither pot section is ever a 0 Ω resistor.