Here is a WEM ER 15 …
15 Watts, 2 x 12AX7, 2 x EL84 and one EZ81 as double diode.
This is a mid-60′s valve amp from Watkins / WEM.
You could also see this site … vintage hofner !
OK, one fuse dead and the EZ81 died with a very cool blue flashlight after a few minutes (unfortunately for a mojo Valvo). New fuses, new JJ EZ81.
Voltage check without tubes, then with tubes. Everything seems to be OK. EL84 bias current at 46mA (cathode resistor is 130Ω on schematic, my amp has a 100Ω … quite hot).
Then, I plugged a cab instead of 16Ω resistor used for testing stuff.
Wow, this thing sounds !!! With a strat, very nice clean tone, and crunch at maximum volume. Pots are a little scratchy … to be fixed.
Some say that the Marshall 18W is based on this amp (in fact, I’m wrong here : Marshall 18W is based on Watkins Dominator). The WEM has two channels, why not mod one to exact spec of Marshall 18W ?
Power filter capacitors replacement :
Modification of one channel to Marshall 18W :
It works well, more crunchy at max gain, but the 2nd genuine Wem channel stays still so lovely. Gonna try to mix both with a special Y cable plugged into both input jacks …
Bias adjustment :
Before filter capacitors change, EL84 cathode resistor was 100Ω, and Ik was 46mA. After filter capacitor change, all voltages has increased so I changed Rk to 120Ω, for a new Ik = 44mA (Va=320V)… I could try 150Ω to preserve EL84 life … one day.
Coupling capacitors replacement :
On channel WEM, quite every capacitors leak DC current (up to 12V DC measured at Bass pot …) so I replace these capacitor with MKP :
Also replaced Ra resistors and all Ck (noise hunting) :
Two improvements this morning for the Marshall 18W channel : shielded input lines + a grid stopper (68k) on the tube socket (to prevent radio and mobile phone frequencies I noticed from time to time) :
Many thanks to forum ProjetG5 members
Ready to rock !
Some more pix and measurements for Jonathan. Hope this will help !