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JCM800 2204 Ready to rock
Hello, I’ve finished my JCM 800 2204 build. It uses TRL transformers (made in Brazil). Sozo, NOS Philips, and Atom capacitors. Vishay resistors. For the electrolytic capacitors, I used 47µF F&T units. The amplifier sounds fantastic and has very low noise. I installed an LND150
effects loop from EvolveDIY, and also added a "bright" switch and a PPIMV (Post-Phase Inverter Master Volume). I included a few other modifications aimed at keeping noise to a minimum. It’s loaded with Tung-Sol EL34 power tubes and JJ E83CC "frame grid" preamp tubes. Thanks!
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Subject: 18W TMB build solved – possible corrections for layout rev 1.0.0
I have solved it, my 18W TMB build is now working and sounds great. Thank you for designing this board.
While troubleshooting I found a few points where the wiring layout (rev 1.0.0, 05/05/23) does not match the schematic (rev 2.0.0, 28/09/24). I'm sharing them so other builders don't run into the same problems.
1. Standby switch and EL84 cathode wires swapped on the PCB
In the layout, the wire from the standby switch S1 goes to the pad above C21 (cathode node: C21/R27 130R), and the wire from EL84 pin 3 goes to the pad next to R26/C20. Following the layout, B+ was connected straight to the 130R cathode resistor when standby was switched on: the voltage dropped from about 290V to about 12V. According to the schematic it should be the other way round:
- S1 output -> pad next to R26 (C20+, together with the OT centre tap)
- EL84 pin 3 -> pad above C21 (C21+/R27)
After swapping the two wires, everything measured correctly.
2. EZ81: D1 and C26 drawn on pin 2
The layout shows D1 (UF4007) and one leg of C26 on pin 2 of the EZ81. Pin 2 is an internal connection; the anodes are pins 1 and 7. The schematic correctly shows pin 1.
3. Input jacks have no ground connection
In the layout, the sleeves of J1/J2 are only linked to each other, and J3's sleeve only goes to pin 1 of the MV pot (P7). There is no wire to ground, so the layout assumes the jacks are grounded through the chassis. With insulated (plastic Cliff-style) jacks, J1/J2, J3 and P7 pin 1 are left floating. In my build this caused loud hum and almost no signal on the Normal channel, and a very weak TMB channel. Adding a ground wire from the jack sleeves to a ground point solved it. A note or an extra ground wire in the layout would help.
A small extra tip: the layout shows the output transformer centre tap as a black wire. On the Hammond 1750PA the centre tap is red (blue and brown are the plates). I had red and brown swapped at first, which unbalanced the output stage.
Final measurements (no limiter, EL84s in, speaker connected):
- B+ (C20): 326V
- EL84 cathode (R27): 10.1V
- EL84 plates: 314V / 315V
- EL84 screens: 305V / 305V
I'm happy to send photos if that's useful. Is there a newer revision of the layout?
Best regards,
Ronald