The following is a simple checklist to follow for your first power up of a new amp. The goal here is to protect your valuable circuitry rather than have it go "poof" due to a simple wiring mistake. Unfortunately, we've all given into the temptation to "fire it up" as soon as the last solder joint is cold. That's a mistake. I lost a brand new Hammond 269EX. Talk about bursting your balloon of excitement!! Following this checklist should help get rid of some pins lurking next to your balloon. This debug procedure actually starts prior to completing the final solder joints...

The starting point is to do some basic circuit checkout PRIOR to soldering the transformers into the rest of the circuit. If you've already soldered in your trannies, take a minute to desolder the secondaries from the rest of the amp. Iron is expensive and it's worth some effort to protect it. Trust me!

Start with no tubes installed, MAINS UNPLUGGED, trannies NOT soldered in and get out your ohmmeter. Also remove the pilot lamp if you have one. Use your ohmmeter to verify that the connection points for the PT secondaries are open circuits. With no tubes and no pilot lamp, the 6.3VAC and 5VAC secondaries should be open circuits, no continuity. If you used a tube recto, then the HV secondary of the PT should also read as an open circuit. With solid-state diodes, there is continuity to the filter caps. So, you will see an initial low resistance that increases with time as the filter caps charge. Actually, some meters use a pulsed current to measure resistance, so use the continuity setting on your meter for this. Also, check your speaker jack with and without a speaker plugged in to make sure it isn't shorted.

If all is well, solder in your tranny's low voltage secondaries. Don't yet solder in the HV secondary. You can also solder in your OT secondary.

With no tubes installed and switched to standby, install your pilot lamp and use a 250mA fast acting mains fuse. Click on the power for 1 second just to see the pilot come on nice and bright. This is primarily checking the 6.3VAC supply line is not shorted and is properly connected to the lamp. If the lamp did not come on, check to see if the fuse blew. If not, try another lamp and do the 1-second power thing again. If the fast acting fuse blows, there is a short on the PT or mains. If the fuse survives, then it's likely that the 6.3VAC is not properly connected to the pilot lamp. Use your AC voltmeter to check for 6.3VAC (actually more like 7VAC with no tubes installed) at the lamp and all the tube sockets. If you have a recto tube powered by 5VAC, then also check that you have 5VAC at the recto tube socket now. This will be up around 6VAC without the recto tube installed as yet.

In the following steps, B+ is going to go high. If your amp circuit does not include a bleeder resistor, you should install one at least for the debug phase. Solder a 100K, 2W resistor across the first filter cap (from B+ to ground). You can use four 470K, 1/2W resistors in parallel if you don't have a 100K 2W resistor.

Assuming you now have a nice bright pilot light, hook your AC voltmeter to the tranny HV secondary (which is still not soldered into the circuit). Turn the power on just long enough to get a reading to verify it is correct. You should get a value 10 to 20% higher than the rated output voltage. If you get a value less than the rating, shut down the amp and check the fuse. If you get a proper value from the HV secondary, power down and solder the secondary to the recto tube or diodes. Install the recto tube (if you have one). Now replace the 250mA fast acting mains fuse with a 1A slow blow (or whatever your amp calls for). Hook your VOM to the HV secondary again. BE VERY CAREFUL at this point, your B+ will charge up for this power up.

Before powering up LOOK AT YOUR RECTO DIODES AND FILTER CAPS!!! You ABSOLUTELY MUST have the diode and cap polarities correct. This is critical yet is a very common error. If either the diodes or caps are wired in reverse, you WILL destroy the caps, diodes and PT!!

Did you check the recto diodes and caps? Now walk away, watch some TV, come back in 20 minutes and LOOK AT THEM AGAIN! :) Sorry, but this is important.

Power up the amp and watch the following things as quickly as possible and roughly in this order: pilot lamp comes on brightly; HV secondary goes to nearly the same value as it did with the lines unsoldered; recto tube filament lights up right away; Recto tube plates do NOT glow red (overheat). If any of these three does not happen, shut off the amp immediately and find the problem by looping back to the beginning of this checklist. If these check out, power down. B+ MUST be discharged to safely continue messing with the amp guts. Your bleeder resistor will take at least a minute to bring B+ down to safe levels.

Now hook your DC voltmeter to B+ and ground. Power up again and check the B+ voltage. With no tubes installed, all the filter caps will charge up to the same voltage. If you do not have a choke, the voltage should be very close to 40% higher than the raw AC. Assuming your measured, say, 600VAC across the full secondary in the above steps, then each half is delivering 300VAC. B+ will be ~40% above this, which is ~420VDC. If you have a choke, the DC voltage depends on the size of capacitance prior to the choke. With no cap prior to the choke, the first filter cap should char to about 90% of the AC volts. With the same 300VAC on each half, the first filter cap should have ~270VDC. If there is a cap prior to the first filter, then the DC volts out will depend on the load current. With no load (as there is now without tubes installed), even a small capacitance will allow B+ to charge up all the way to the same as the no-choke case. As an aside, you can use this pre-choke cap to adjust B+ under load to some point between the 90% and 140% of the VAC.

KEEP IN MIND that every time you power up from now on that B+ will be high. In all the following stuff, allowing B+ to bleed is implied at each power down.

If all is well, check that B+ is at the OT on all the primary taps. Without the power tubes installed, the OT primary should be at B+. If not, something is wrong at the OT. Power down immediately and check for shorts of the OT primary. This should not be the case, however. An OT short should have been caught by now by checking B+ levels in the previous steps. This is really just a final sanity check to really make sure the trannies aren't going to be killed by any mistakes. Leave the amp on for a few minutes and make sure neither tranny is getting warm. The OT should stay dead cold and the PT should get just a little warm supplying the pilot lamp and tube recto. If your amp is fixed bias, make sure the negative voltage is being generated and check that it gets to the power tube grids and is adjustable with the bias pot.

OK, finally ready to put some tubes in. Power down and install all the signal tubes. You also need to hook up a speaker or dummy load for the OT. I actually suggest you hook up speaker rather than a dummy load, but preferably an old or less valuable speaker to get started. Turn volume and gain pots all the way down and tone controls to center. If your amp is fixed bias, turn it all the way down to start (most negative grid voltage possible. Power up and, again, watch for the following signs: pilot lamp comes on brightly; all tube filaments light up right away; tube plates do NOT glow red (overheat) this time paying attention to power tubes. If your amp is fixed bias, check power tube current and bias appropriately. Do this soon after power up to make sure all is well. You should be able to hear a little hiss or hum from the speaker. Of course, hopefully this is low level requiring your ear up next to the speaker to tell. If hiss and hum is loud at this point, there are problems. If there is dead silence, something is likely wrong, too. But, let's hold off on the AC signal debug until later.

Now is time for DC bias assessment. With one hand behind your back, measure and write down the B+ levels at each filter cap. Also write down cathode and plate voltages at all stages and also the screen grid voltage at the power tubes (if applicable). Compare all the DC voltages to those expected. In general, the triode gain stages should have ~1V on the cathode, 0V on the grids and ~1/2 B+ on the plates. If the DC voltages are not in the ballpark (within 50% of the general rule just stated), take some time to check the circuitry of the offending stage. If you have a cathode follower in the signal path, the grid should be at the plate voltage of the previous stage and the cathode should be about a volt higher.

The moment of truth arrives! Plug in your guitar and turn up the gain and volume knobs slowly and see how it goes. With a lot of luck, you'll hear the guitar sounding great with no hum or squeal. However, such a case is the exception and even then is typically reserved for experienced amp builders. To err is human and problems at this point are to be expected. Don't get discouraged. Jump down to the debugging stuff below...