18W TMB PCB – Possible Wiring Errors in Layout (S1, EL84 and EZ81)
Hi everyone,
I’m building the AA Electronics 18W TMB amplifier and have encountered a serious voltage drop at EZ81 pin 3:
Standby OFF: approximately 290V DC
Standby ON: approximately 12V DC
After checking the wiring against the supplied layout, I noticed two possible issues:
1. Standby and EL84 cathode connections
The layout appears to connect:
S1 (standby) to C21 (+), the 100µF cathode bypass capacitor, through which I measure 130Ω to ground via R27.
EL84 pin 3 to C20 (+), the 33µF/500V filter capacitor.
Shouldn’t these connections be reversed, with S1 connected to B+ and EL84 pin 3 connected to C21 (+)/R27?
2. EZ81 connection
The layout appears to show a connection to EZ81 pin 2 instead of pin 1. Is this a drawing error?
Could someone familiar with this PCB confirm whether these are layout errors or whether I’m interpreting the connections incorrectly?
Thanks!
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