Now the 90s tube craze has gone full circle even budget brand are hitting the market hard with quality tube material. Many already went overboard when the Epiphone Valve Junior came out, but now thomann.de presents the Harley Benton GA-5 at 98 EUR, the ultimate beginner's amp mod kit has arrived. I don't think you could even buy a kit for a complete combo amp at this price -it retails at 98 EUR, the head version at 88 EUR
I've heard the GA5 is assembled in the same factory as the Epiphone and looking at the construction, that is no surprise. PCB layouts seems to match and the HB even has the pcb to host three output jack though it only has one external out. This really seems to be our punishment for not buying an Epiphone, it seems, but we'll come back to that later.
The power tube was a half decent Sovet EL84, even date coded 06 07 and made in Russia. The Epi, I heard had Sovtek for the preamp too, but the Harley has a cheapo made in China 12AX7 hidden in the tube retainer:
The first move would be to replace the stock tubes with decent ones of your liking. To get the preamp tube out, gently push down and twist the tube container and it will pop right off (thanks Rob!).
3. Biasing the EL84 power tube
This in fact is really easy and something you can do if you have a digital multimeter, an alligator clip and some caution. The most simple and basic rule is - NEVER PUT BOTH HANDS IN AN AMP THAT'S TURNED ON. When working on a life amp as you will need to to bias the tube, always keep one hand in your pocket.
Even unplugged a tube amp might kill you because the capacitors are still charged even when unplugged. So tap them with a screwdriver that has a rubber or plastic handle. Or: with a guitar plugged in, just unplug the power cord (don't use the on/off switch) and keep playing until you hear no more sound from the amp.
First, get the chassis out: remove the four plastic covers on the top of the amp, they hide the screws holding the chassis in the cabinet. Make sure you unplug the speaker jack and then put the amp on its head so it rests on the transformers and you can look at the component side of the pcb.
Now reconnect the speaker (you don't want to damage the output transformer) and turn the amp on, let it warm up for some time. Make a cup of tea in the meantime...
Ok, now let's check the bias!
We will be needing four values to calculate the bias:
maximum anode dissipation of the power tube, EL84/6BQ5, which is 12W
cathode biasing resistor, R14, which is 220 ohm
cathode voltage to ground - cathode is pin 3 on the EL84
anode voltage to ground - anode is pin 7 on the EL84
You can go by the value indicated by the resistor colours, but you might as well measure it -- it can be measured in circuit. Mine was 217 ohm. Now connect an alligator clip to the black lead of your multimeter and connect it to the ground point top right on the chassis, various ground point join togeter on the chassis here:
Now measure the cathode voltage on pin 3 with DMM in the 20V range:
And the anode voltage on pin 7 with DMM on the HV range:
With this value, we can calculate the plate to cathode voltage, by subtracting the cathode voltage from the anode voltage:
308V - 9,33V = 298,67V
Take the cathode voltage 9,33V and divide by resistor value 217 ohm to get the cathode current:
9,33V / 217 ohm = 0,043 A
Multiply the cathode currect by the plate to cathode voltage to get the total dissipation
0,043 A * 298,67 V = 12,84 W
If you want to take into account the screen dissipation, you can take it at 10% of the cathode current and deduct that from the final dissipation. Like this:
0,0043 A * 298,67 V = 1,3W 12,84W - 1,3W = 11,24 W
This could be improved by decreasing the value of the cathode baising resistor, to get total dissipation up, but is actually not so bad. If the dissipation is much more that 12W, you should increase the value of biasing resistor R14 to get it closer to 12W.
Hey -- was that easy or what?
Soon more on the Harley Benton, but it seems like we will be missing out on the Epiphone "kill the noise"-mods ;-)
