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PP-18 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. R18 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 one of the speaker output sockets
* Turn all of the amp's knobs to zero
* Check that both S1 and S2 are 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 should 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 (Note: this hum may be hard to hear in a noisy room)
* Set your multimeter to the low AC voltage range and check the voltage from the '0' lug to the '6.3V' lug on the 'Sec2' winding on the power transformer; you should get a reading of around 6.5-7V
* Now switch your multimeter to its high AC voltage range and read the voltage from A to B on the test-point diagram. You should get around 280-300V
* 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.
* Switch S2 on
* Set your multimeter to its high DC voltage range and check the voltage from D to C. You should get a reading of around 380-400V
* Switch S2 and S1 off. Keep measuring the D-C 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 is connected and plugged into the mains, there will ALWAYS be around 240V present on the lugs of the IEC socket and S1, the PP-18's power switch

Powering up with valves installed

Check that all of the D-C, D-G and M-L 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: the EL84s into the V3 and V4 sockets (use the clips to secure them) and the ECC83s into the V1 and V2 sockets (add the shield cover to hold them in place).
      Flip the chassis over, taking care that T1, T2, and all valves are 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 your speaker to the appropriate speaker output socket
* Turn the amp's knobs to zero
* Check that both S1 and S2 are 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
* Switch S2 on
* Set your multimeter to the high AC voltage range
* Measure mains voltage; around 240V
* Set your multimeter to the high DC voltage range
* Measure D-C; normally 340-350V
* Measure D-E (V3's plate voltage); normally 335-345V
* Measure D-F; (V4's plate voltage)normally 335-345V
* Measure D-G; normally 320-335V
* Measure D-H (the EL84s' screen voltage); normally 315-330V
* Set your multimeter to a low DC voltage range (e.g. 20V)
* Measure H-G; normally around 7V
* Measure D-I; this is the EL84s' cathode voltage, normally around 12V
* Set your multimeter to the high DC voltage range
* Measure M-L; normally 270-290V
* Switch S2 and S1 off
* Set your multimeter to its high DC voltage range and check the D-C 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:










_________V (AC)

_________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 - Combined EL84 cathode current: Divide your EL84s' cathode voltage (D-I) by the value of R16 (150 ohms). A typical result is around 75-85mA. Contact Amp Maker if it's more than 90mA.


2 - Combined EL84 screen current: Divide your H-G figure by the value of R17 (1000 ohms). A typical result is 7-9mA. Get in touch if it's more than 10mA.


3 - Average EL84 plate current: Subtract the EL84 screen current (2, above) from the EL84 cathode current (1, above) to get the current drawn through V3 and V4's plates. Divide this figure by two to get the average; a typical result is 35-38mA. Contact Amp Maker if your result is more than 45mA.


4 - EL84 cathode-to-plate voltage: Subtract your EL84s' cathode voltage (D-I) from the highest of the EL84s' plate voltages (D-E or D-F). A typical result is 325-345V. Contact Amp Maker if your result is more than 350V.


5 - EL84 plate dissipation: Multiply the EL84 cathode-to-plate result by the average EL84 plate calculated (3, above). For example, 335V * 0.035A = 11.7 Watts dissipated by the EL84s' plates. This is a typical result. A value under 10W or over 13W needs some consideration (see Dissipation explained, right); contact Amp Maker.


Dissipation explained
Every valve dissipates power as current passes through its electrodes. In V1 and V2, the ECC83 valves, the current is very low and the power dissipated is negligible, but in the power valves, V3 and V4, it's vital to check the power dissipated by the plate (the electrode that's directly connected to pin 7 in the EL84).
      The JJ EL84 is rated at a maximum dissipation of 12W, and has a good reputation for running beyond that. The classic 18W circuit (like the EL84-based AC-15 ands AC-30) runs EL84s close to or slightly beyond this rating. If plate dissipation is significantly higher, the plate overheats - frequently turning cherry red - and the valve wears out very fast.
      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 mains supply) and the cathode resistor. You can try different cathode resistor values to vary the EL84's bias and its dissipation. If your dissipation figure is very high, and you'd like to tame it, get in touch with Amp Maker with your figures - there is a very simple modification that can fix this.

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 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 both of the knobs to 5 and and plug in and play your guitar. The guitar tone should give clean to mild crunch tones depending on how hard you hit the strings. If you get unexpected effects from these knobs, it's possible that you've mis-wired the controls. Go back and check (remembering to unplug the amplifier and discharge the capacitors before going in to work on the circuit).
      Now crank the amplifier and check its overdrive tone. You can compare it to the MP3 snippets on the Amp Maker PP-18 kit page to confirm that it's working properly.
      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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