Debugging voltage problems in new amp build

[fingersoccer]

I just finished building an amp similar to a Fender BF Princeton but without the reverb and tremolo circuits. I tried powering it on and something doesn't seem right; nothing noticeably bad is happening (sparks, smoke, scary things, etc.), but the pilot light isn't turning on. The only tube installed right now is the rectifier (a SS copper cap rectifier), and I've checked to make sure the pilot lamp is installed as well as a 1A slo-blow fuse.

• 48V between ground and the power switch
• 7V across the filament wires
• 485V at point A on the schematic
• 490V at point B on the schematic
• 491V at point C on the schematic

Things that jump out at me is that there's only 48V at the mains and 491V at point C on the schematic. Also, I'm not sure why the pilot light wouldn't light up if there's 7V across its terminals. It would probably have been wise to run my schematic by another person first before turning it on...does anything in the schematic look amiss? What's the next thing I should check?

Thanks!

[labb]

How are you taking the measurement at the power switch? double check the wiring at the pilot lamp.

[sluckey]

48V between ground and the power switch

That's because you have the neutral connected to the power switch. Measure voltage between the line (live) and neutral and you'll see 120vac or whatever your line voltage is.

A better way to wire the mains to the transformer would be to connect the neutral directly to one of the PT primary leads. Put the power switch between the fuse and the other PT primary lead.

If you really have 7vac across the pilot light terminals and it's not lit then you have a defective bulb (or wrong bulb) or the lamp socket is defect (or the wrong type).

[fingersoccer]

labb:

How are you taking the measurement at the power switch?

I'm measuring from ground to one lug of the power switch

double check the wiring at the pilot lamp.

The wiring here is the green 6.3V leads from the Classictone power transformer to each lug of the pilot lamp.

sluckey:
Thanks for the tips—I'll try replacing the pilot bulb and measuring voltage between live and neutral when I get home from work and see what kind of reading I get. Stand by!

[92Volts]

Sorry if this seems obvious, but if your voltmeter has AC/DC volt settings make sure you're measuring AC voltage at the mains (and across the pilot light/heaters too).

48v is a weird number because house wiring "should" have one live and one neutral line... you could be measuring the wrong one but usually that gives options of 120v or 0v.

It suggests your ground might not be done properly, or there's something wrong with your house wiring... in other words your "ground" is hooked to the 3rd pin of the power cord but that part of the outlet is not equal to the Neutral line for some reason, or isn't connected to anything at all.

[tubeswell]

• 48V between ground and the power switch
• 7V across the filament wires
• 485V at point A on the schematic
• 490V at point B on the schematic
• 491V at point C on the schematic[/l][/l][/l][/l][/l][/l]

You're getting the right kind of voltages across the HT supply rail (assuming you measured VDC) and the Heater (assuming you measured VAC) for non-load condition (i.e. no tubes plugged in).
48V across the Primary seems weird. Did you measure that as VAC?[/list]

[fingersoccer]

Well, I had to skip town for the Thanksgiving holidays before I had a chance to remeasure stuff, but all this is very helpful. I'll check in again when I get back and post an update. Thanks for the suggestions, everyone!

[ckilgore]

Sounds to me like you have an open ground in your house wiring.  At the outlet measure between the ground (round hole) and the neutral (larger of the two slots).  If it is not zero volts AC then your ground is open.

[fingersoccer]

Ah ha! Everyone here had the right idea:

• Apparently my house isn't grounded properly (or at all). I'm getting 120V between the live and neutral leads and some non-zero number between ground and neutral.
• I replaced the bulb and now it's all shiny and bright, as it should be. I guess that goes to show that just because a bulb is brand new doesn't mean it will work!

I guess there was no problem at all, which is the best kind of problem. Thanks for helping me step through this, folks!

[tubeswell]

While you're thinking about it, you should get a sparky to wire the mains ground in your house system.

[PRR]

> some non-zero number between ground and neutral.

2? 57? 193?

We never have dead-Zero Volts N-G.

Much more important: is "G" actually connected to anything? Or is a connection missing? This can be hard to determine safely.

[fingersoccer]

IIRC, there was ~48V between Neutral-Ground and ~74V between Live-Ground. I just moved into a house built in 1959, and the inspector said the house isn't grounded.

I do need to get a sparky to ground this place, but I need to save up a few thousand first. It's expensive!