Amp Maker Home | WF-55 purchase page

Home > WF-55 Construction manual > WF-55 Initial testing

WF-55 Initial testing

Few - if any - amp builders are perfect, and it's vital that you test the work you've done on your amp before plugging it into the mains. The following steps cover both essential safety tests and other checks on your wiring. Have the schematic to hand to use as your guide, and have your multimeter ready (see Testing with a multimeter, right).
      Note: Because your amp has never been plugged in before this point, this series of tests is the ONE AND ONLY TIME that you can safely assume that the capacitors are not storing a potentially lethal charge. If you power up your amp, and then come back to re-do these tests, you MUST first safely discharge the amp's capacitors (see Discharging capacitors).
Testing with a multimeter
For these initial tests - with no power supplied to the amp - you use the resistance range(s) on your multimeter. Most tests are for continuity - which tells you whether two points in the amp are connected with (effectively) zero resistance between them. You can use the lowest range on your multimeter's resistance setting, or the audible continuity tester (if there's one included on your multimeter, shown below on a low-cost DMM).

Testing ground connections

Poor or absent ground connections are a potential safety hazard. And even in cases where poor grounding doesn't directly compromise safety, it can cause unwanted noise, low power output or even no sound output at all. Do all of the following tests, checking for continuity - a 'zero ohm' reading - in each case:
* Plug an IEC cable into the amp (but DON'T plug the other end into the mains), plug a guitar cable into the input socket and check that there's continuity from the sleeve of the guitar cable to the Earth pin on the mains plug
* Now check the speaker output jack socket in exactly the same way
* With one of your multimeter's probes still in contact with the mains plug's Earth pin, use the other probe to work through the whole schematic to check that all of the relevant component leads - starting from the bottom end of R4 and ending up at D5 - are grounded (see Check leads, not turrets! right)
* Check that lug 3 on VR1 is also connected to ground
Check leads, not turrets!
In these tests, by checking to see whether individual component leads are connected correctly you are checking three things at once:
1) Is there a good connection from the component lead to the turret?
2) Is the turret connected to a ground bus wire?
3) Is the ground wire connected to the chassis?
If you tested by touching the multimeter probe to the turret instead of the component lead, you'd only be testing the second two questions. You could get a good reading, but still face problems later on if you've overlooked a poor lead-to-turret solder joint.

Other continuity tests

In addition to the continuity-to-ground tests, you can do the same type of test for the connections from the turret board to the valve sockets and other components. Open the turret board wiring diagram for a reminder of where each connection ought to be, and use your multimeter to check for a good connection from the relevant turret board component.
      This may seem like an unnecessary step, but it doesn't take long and it pays off if you find a wrong or poor connection. If you were to power up the amp with a faulty connection, you may destroy expensive components, or worse.
      Finally, check for lack of continuity between adjacent pins on the valve sockets. So check pins 1 and 2 of V1, then pins 2 and 3, and so on. The only places where you should see continuity are between pins 4 and 5 on V1 (which you have directly connected) and pins 7 and 8 on V2 (where the very low resistance of the power transformer's heater winding makes these pins appear to be connected - this reading is normal).

By testing adjacent pins for continuity you can find shorts caused by wire 'whiskers' that are easily overlooked by the naked eye

Amp Maker Home | WF-55 purchase page
All text and graphics are copyright of Amp Maker and may not be used without prior agreement.