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WF-55 Final testing


With the initial tests completed - and any wiring mistakes corrected - you can now proceed with connecting mains power to the amp to carry out the final tests. It's vital that you do all of the following steps in the order shown, and that you take no shortcuts. Once your amplifier (or any valve amp) has been powered up, you must always assume that the chassis still contains lethal voltages - a few simple tests and checks to prove that it doesn't and you can safely work on the circuit.
      First, print this page so that you can write down the measurements from your amplifier. Then clear the space around your amplifier and make sure you're working in very good light. If you wear loose jewellery, I recommend taking it off. I also recommend that you turn off all sources of background noise - radio, TV, etc. Open the test-point diagram - it shows where you will measure the voltages inside the amplifier for the following process.


Danger - even when switched off
Even when your amplifier's Power switch is off, the inside of the chassis is potentially lethal. First, there's always 240V of mains supply present at the IEC socket, fuseholder and On/Off switch whenever there's an IEC cable attaching the amp to a switched-on wall socket. Some people just use the switch by the wall socket to disconnect the mains, but I prefer to physically unplug the IEC cable's plug from the wall or trailing socket: it's then a very quick, easy and (more importantly) 100% reliable visual check before going into the chassis.

      Secondly, the large capacitors store their charge for several seconds after the power is switched off. R13 helps to accelerate the rate at which these capacitors discharge when you switch off, but always measure the voltages before putting your hands into the chassis.

First power up

Prepare as follows:
* If you have plugged in the valves, remove them
* If you have a guitar plugged in, unplug it
* Connect a speaker to the speaker output socket
* Turn the amp's Volume knob to zero
* Check that S1 is switched off
* Connect the IEC cable to the wall socket
* Push the cable's IEC plug into the amp's IEC socket, being careful to keep your fingers out of the chassis
* Use your multimeter to check that you have 230-240V (your mains voltage supply) across the centre pair of lugs on S1
* Switch S1 on
* The neon indicator will light up
* You will hear a very low background hum from the speaker; this is normal, and the hum confirms that the wiring from T2 to the speaker is correct
* Set your multimeter to the low AC voltage range and check the voltage from pin 2 to pin 7 on V2 (the power valve socket); you should get a reading of 6-7V
* Now switch your multimeter to its high AC voltage range and read the voltage from A to B. You should get around 280-290V
* If either of your values are considerably different to these figures (more than 10% above or below), get in touch with Amp Maker before proceeding.
* Set your multimeter to its high DC voltage range and check the voltage from C to D. You should get a reading of around 380-390V
* Switch S1 off. Keep measuring the C-D voltage and you will see it fall as the capacitors slowly discharge. After several seconds it should fall to under 50V
* Unplug the IEC cable from the mains


Support the chassis ends so that it's stable for you to work on. T1 and T2 should both hang in free air (as must the valves when you add them for the next set of tests, below left)


Beware: whenever the IEC cable connected and plugged in, there will ALWAYS be around 240V present on the lugs of the IEC socket and S1, the WF-55's power switch

Powering up with valves installed

Check that all of the C-D, E-D, F-D and G-H voltages have fallen to under 10V - if not, wait until they have. Then it's safe to flip the chassis over and insert the valves: 6V6 into the V2 socket and the ECC83 into the V1 socket.
      Flip the chassis over, taking care that T1, T2, V1 and V2 are all hanging in free air. Get a pen so that you can record the voltages you measure (in the blank table on the right). Then follow these steps:
* If you have a guitar plugged in, unplug it
* Connect a speaker to the speaker output socket
* Turn the amp's Volume knob to zero
* Check that S1 is switched off
* Connect the IEC mains cable to the amp's IEC socket
* Switch S1 on
* Wait 30 seconds for the valves to warm up
* Set your multimeter to the high AC voltage range
* Measure mains voltage at the Live and Neutral lugs on the IEC socket; around 240V
* Set your multimeter to the high DC voltage range
* Measure C-D; normally around 345V
* Measure E-D; normally around 340V
* Measure F-D; normally around 300V
* Measure G-H; normally around 260V
* Measure the voltage between pin 3 of V2 (the 6V6) and point D on the turret board, being extremely careful to touch the multimeter's probe to pin 3 alone. This measures the 6V6 plate voltage; normally around 330V
* Set your multimeter to a low DC voltage range (e.g. 20V)
* Measure C-E; normally around 4V
* Measure E-F; normally around 40V
* Measure G-F; normally around 40V
* Measure I-D (the 6V6 cathode voltage); normally around 18V
* Switch S1 off
* Set your multimeter to its high DC voltage range and check the C-D voltage as the capacitors discharge. With the valves fitted, the voltage will drop a lot more quickly than in the first power-up test
* Unplug the IEC cable from the mains

Print this page and record the measurements from your amplifier:

Mains:

C-D:

E-D:

F-D:

G-H:

V2/pin3-D:

C-E:

E-F:

G-F:

I-D:



_________V (AC)

_________V (DC)

_________V (DC)

_________V (DC)

_________V (DC)

_________V (DC)

_________V (DC)

_________V (DC)

_________V (DC)

_________V (DC)


Some important calculations

With the voltages you have recorded, you can check a few important aspects of how your amp is operating.

1 - Total current consumption: Take the C-E figure and divide it by the value of R12 (100 ohms). You should get a value of around 0.035-0.045 (i.e. 35-45mA). If it's any greater than 55mA, get in touch with Amp Maker before powering the amp up again.

_________mA

2 - 6V6 cathode current: Divide the 6V6 cathode voltage (I-D) by the value of R9 (470 ohms). A typical result is around 38mA. Contact Amp Maker if it's more than 45mA.

_________mA

3 - Preamp current: Divide the G-F figure by the value of R10 (22k) to get the current drawn by the preamp part of the circuit. Typically this will be 1.5-2.5mA. Contact Amp Maker if it's higher than 4mA.

_________mA

4 - Screen current: First, divide the E-F figure by the value of R11 (10k) to get a combined screen and preamp current (typically 4mA or so). From this subtract the figure you just worked out for the preamp current (above) to get a figure for the screen current alone. Typically this will be about 2-4mA. Get in touch if it's more than 6mA.

_________mA

5 - 6V6 plate current: Subtract the 6V6 screen current from the 6V6 cathode current to get the valve's plate current. A typical result is 34-38mA. Contact Amp Maker if your result is more than 45mA.

_________mA

6 - 6V6 plate dissipation: First, subtract the 6V6 cathode voltage (I-D) from the 6V6 plate voltage (V2/pin3-D) to get the voltage between the plate and cathode. Now multiply this result by the 6V6 plate current to get the power dissipated in Watts by the 6V6's plate. A typical result is around 11-12W - a value under 10W or over 13W needs some consideration (see Dissipation explained, right); contact Amp Maker.

_________W

Dissipation explained
Every valve dissipates power as current passes through its electrodes. In V1, the preamp valve, the current is very low and the power dissipated is negligible, but in the power valve, V2, it's vital to check the power dissipated by the plate (the electrode that's directly connected to pin 3 in the 6V6).
      If plate dissipation is too high, the plate overheats - frequently turning cherry red - and it wears out very fast. The JJ 6V6 is rated at a maximum dissipation of 14W, and most valve gurus and text books recommend aiming a little under this (i.e. 12W or so). This is the range where the WF-55 fits (some vintage British amps and their modern-day clones run their valves a lot harder).
      On the other hand, if plate dissipation is low, the valve is unlikely to ever overheat, and it will last a long time. However, you may not be getting the best possible tone out of it. The question is one of balancing tone against economy.
      You can see from the calculations on the left that plate dissipation is a factor of plate voltage and plate current, and these depend largely on two other factors: the power supply (pretty much fixed by the power transformer) and the cathode resistor. You can try different cathode resistor values to vary the 6V6's bias and its dissipation.

Time to plug in

With the above checks and calculations done and any mistakes rectified, you can finally listen to your amplifier. First, check it with no guitar plugged in, crank all of the amplifier knobs and check that there's no oscillation (see Sound problems, right). You should only hear the usual low-level background hum.
      Now turn the amp's Volume knob to 5 and and plug in and play your guitar. The guitar tone should be fairly clean, with some compression and mild breakup if you hit the guitar strings hard. Now crank the amplifier and check its overdrive tone.
      Congratulations! :) With all of your testing completed, switch off and unplug the amplifier. Check the capacitors have discharged themselves and then build your working chassis into your enclosure (head, combo, etc). If you haven't decided yet, check out the Customer amp gallery (combos) and Customer amp gallery (heads) for ideas and inspiration.


Sound problems
If you've followed the Amp Maker wiring diagrams and photos, the layout should ensure that there are no sound problems. If you hear a high-pitched squeal, or if you get low or no output at all, suspect a wiring mistake.
      A high-pitched squeal is caused by oscillation. One part of the circuit is affecting an earlier part of the circuit and causing the a feedback loop (the electronic equivalent of the feedback you get between a guitar and a loud amplifier). Usually, oscillation only occurs at higher levels of the Volume settings. The solution is to find the wiring problem and fix it. Safely discharge the capacitors and unplug the amp. Then start by tracing and checking all of the connections shown in this construction guide - it's possible that what seemed a minor departure from the suggested layout has an unexpected result.
      Low or no output can be caused by bad valves, or wiring mistakes. Swapping in known-good valves is an obvious check of the first cause. If that doesn't fix it, retrace the wiring and check for dry solder joints. The continuity tests you carried out before should have found the most common problems, but it's possible that other turrets or wire connections have poorly made solder joints. Once again, disconnect the amplifier and discharge the capacitors. Then start checking: there's no substitute for working across the amplifier from input to output and checking that each connection is sound. Reflowing the solder is often all you need to do to fix a dry solder joint. If all else fails, get in touch with Amp Maker. I will do my utmost to help you to get your amp up and running.

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