This is what happens to the EL84 Power Valves in UK FBJ Amps after just 2 Hours use.
- Fender Blues Junior Valve Amp (UK ONLY) -
All Fender Blues Junior Valve Amps supplied in the UK are a Disaster waiting to happen. I have Modified/Serviced hundreds of these Amps and every single one of them has 'COOKED' Power Valves. The Bias Voltage setting on the Power Valves is too LOW compounded by Fender supplying all Amps in the UK with a Mains Voltage setting of 230vac. This is the EURO Mains Voltage Rating, NOT the Real Life UK Voltage - which is 240vac.
Here are the Problems - The 2x EL84 Power Valves are Grid Biased at about '-10.8vdc'. This causes the Valves to 'Idle' at DOUBLE their Maximum Rating - in simple language - OTT, they run WAY TOO HOT. The Stock UK FBJs are running an Idle Current of 50ma on each Valve (Should be around 24~28ma). Anode Dissipation is 17 Watts (Should be 12 watts MAX - usually set at around 70% for Class A/B - so 9 Watts would be normal). Because the UK Amps are supplied with their Mains Voltage set Low, at 230vac - this causes 'Over' voltage in the Amp. The actual B+ Voltage is 344vdc and the Heater Voltage is over 7vac - The expected HT (B+) Voltage should be 329vdc and the Heater Voltage should be 6.3vac. All of this needs to be corrected ASAP, Luckily it's easy to do and only involves swapping two wires and changing one small component (Details Below). The Mains Transformer runs HOT and the 2x EL84 Valves 'COOK' themselves to death. This happens very rapidly, any UK Blues Junior Amp that has been 'ON' for more than about 2 hours will have destroyed it's EL84 Valves. Not Possibly, or Perhaps, but DEFINITELY. Other problems caused by excessive Heat from the EL84 Valves include a Cooked PCB and Solder Joints (on the Valve Holder PCB, directly above the Power Valves). Thermionic Valves are Very robust and will put up with a lot of abuse, more than any other type of electronic device, and they will still work (for awhile), but not correctly. After a couple of hours the output of the Amp will fall from 15 watts RMS to somewhere between 10 or 12 Watts RMS and Sound Very Harsh. Prolonged use will cause an Internal failure of one or both EL84s, resulting in (Usual and Very Common) R35 or R36 (Fuseable 100R Resistor) going 'Open Circuit' and/or the Screen Grid Power Supply Resistor 'R47' Burning up. In older V2 Amps, 'R47' will actually burn a Hole in the PCB. On the Newer V3 Amps Fender have lifted 'R47' away from the PCB so that it can Burn up without damaging the PCB. Crazy..... When 'R47' Burns up, it Kills the High Voltage Supply to the rest of the Amp. If you have a 'Dead' Amp - Replace 'R47' (2K2 - 2w).
Don't take my word for it - Pull the rear panel off any Blues Junior Amp (UK Only) and take a look at the the 2x EL84 Power Valves - You will soon see that they are 'COOKED'.
Do a Resistance Test on R35 and R36, you should see around '100 Ohms' (<>10%), if Not, Replace the Part/s. Also, Take a look at 'R47' (2K2), it may need replacing. MAKE SURE C25 is FULLY DISCHARGED before doing any tests.
Fender (UK) Presents - Freshly Cooked EL84 Valves
This is what you can do -
Reset the Mains Voltage - V2+V3 FBJ Amps - UK Amps ONLY
1. - Unplug the Amp and leave it for 24 Hours.
2. - Pull the Rear Panel off the Amp.
3. - Locate the Back of the Mains On/Off Power Switch - Pull the 'WHITE/BLACK' wire (WHITE wire with a BLACK Stripe on it) off the Switch and Swap it with the 'BLACK' wire (Solid BLACK wire - NO Stripe) located at 'P6' on the PCB. This will convert the Amp to 240vac UK Mains Voltage.
Reset the Valve Bias Voltage - V2+V3 FBJ Amps Only - UK, Euro, USA
1. - Remove the 2x EL84 Power Valves and place them in the Bin.
2. - Change R52 (22k) to 27k. This will raise the Bias Voltage to an acceptable level - Approx '-12.5vdc'.
3. - Fit a New Matched set of JJ EL84 Valves - If you don't want to Use 'JJ' Valves then change the value of R52 to 33K.
4. - The Amp will now produce 15 to 18 Watts RMS without 'Cooking' it's Valves ever again.