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Hi -- This is my first amp build. I finished everything up and started through the startup procedure. The first two checks are fine -- the jewel light comes on when no tubes are connected, and the rectifier tube comes on when plugged in. However, when I tried to check the voltage as described (by placing the black probe on the chassis and the red probe on the turret lug connected to the 16 uf capacitor) it immediately arcs and blows the fuse. I've done this twice -- the second time I touched the positive lead from the capacitor instead of the lug -- same result. Since everything worked before touching it with the probes, I wondered if this indicated that my ground connection to the chassis is not good. I can send pictures if needed.

Update -- Checked continuity between chassis and ground plug and seems to be continuous, so grounding probably isn't the problem.

I don't mean to be condescending, but do you have your meter on the correct setting? A voltmeter should never pass current, i.e. allow an arc, but an amp meter certainly will.

Right. Are you setting your meter to DC voltage?

If you set your meter to the wrong setting, you can short that cap to ground, which can cause too much current to run through the fuse and blow it.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

Not the same circuit, but sometimes it helps to see someone doing it.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

The meter (Etek 10709) is set to DC voltage and the 600V setting. It has settings for 200mV, 2,20,200,600, and I figured 600 was the correct one to use in this case. Also the red lead can be either unfused with 10A max or fused with 200 mA max, so I used the unfused setting.

I went ahead and plugged the 12ax7 in and it comes on as well.

OK -- I think the breakdown was between my ears. I had been using one hand to hold the black lead to the chassis because I don't have alligator clips (wearing rubber soled shoes and leather work gloves). When I tape the probe to the chassis I get 450 V. Thanks for all your help guys!

Real quick on the isolation. The issue isn't the voltage going to ground via your feet. It's that you have one hand on the red lead to HV, and one hand on the black lead going to chassis ground. If by some chance both hands were to slip, you've got that 450V potential between both hands. Exactly in the center of both hands is your heart. You never want to be in this position. Never use both hands when working with high voltage or an unknown voltage. Sounds like you are aware of the potential dangers, but I just wanted to chime in to be extra cautious. I don't know what the breakdown voltage on leather is, but skin is conductive.

Carry on!

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If practice makes perfect, and nobody is perfect, then why practice?

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Looks like I've spoken too soon. When I install the 6V6 the problem re-occurs. Everything comes on and sound comes out of the speaker. However as soon as I touch the turret with the red probe, arcing and fuse blown. This time there was also some flickering from the jewel light, but i think that's just a poor connection with the light because everything else stays on. I did notice some weird little sparks within the 5Y3 as it was warming up, but those went away within a minute or so. I'll do some more investigation. and maybe go buy an alligator clip lead for my meter!

Please post clear photos of all of the wiring. It certainly sounds like a bad rectifier tube but we should check the wiring first.

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MasterDelayer/Reverbrador/Ampaholic/TopJacker

Here they are -- you can see I used red plastic insulated wire in two places instead of the cloth wire --result of mismanaging my cloth wire supply

One more

OK, made a chance observation that I think answers the question. As I said, my meter has two inputs for the red probe: one fused and rated at 200 mA, and one unfused for up to 10A. When I made the measurement that worked correctly, I had accidentally left the red probe in the fused input. Then when I moved it back to the "correct" input -- arced and blew fuse. So I tried moving it back to the "wrong" input -- worked again; 455 V. After I unplugged the amp, I tested the leftover voltage in the power cap with no problem (377 V or so). When I switched back to the "correct" input, arcing and dissipation to 97V (measured after going back to the "wrong" input). So I plugged the 6V6 and 12ax7 in, and measured the power cap at 370 V or so and the 12ax7 measurement at 1.5V.

So it looks like maybe it's my meter!!! Maybe they connected the probe inputs backwards????

You want to use the port on the right hand side. The right side is for use with: ohms, volts, temperature (thermocouple), and miliamps. The left port is for measuring amps only. If you were using the left, it would present a short to the power supply of the amp.

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If practice makes perfect, and nobody is perfect, then why practice?

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Yep, the breakdown was between my ears, as suspected. I guess the lesson is "read the manual". However, I still don't understand how these measurements don't exceed the 200 mA limit on that side.

The good news is that the amps sounds great and I'm still upright. Thanks for all your help!

It's only measuring what you have the dial set to. If the dial is set to volts, the 200mA limit of that port does not come in to play. When in Volts mode, the meter conducts 0mA (up to 600V when the meter shorts out, allows current to pass, then catches fire). Then there is also the fact you cannot measure current by probing the circuit. To measure current, you need to cut the circuit and insert the meter in series with the circuit. The meter then becomes an 'intelligent short', where as, it can tell how much current is passing through it. Setting your meter to any current setting and placing it across the high voltage power supply will short out the power supply and cause the fuse to blow, as you've observed. Hope I'm making sense. Sometimes I ramble on a bunch of words, and even I don't understand once I go back and read.

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If practice makes perfect, and nobody is perfect, then why practice?

It hurts me to say this.. but.. follow me on facebook and.. instagram

I'm no amp expert, but between my job and hobbies i do know meters. The only way for your problem to happen is a mismanaged meter lol. Glad you got it figured out. This is a killer little amp.