121v from end lug to chassis

[martinlb]

Hello again, new problem.

I've finished, once again I thought, a build.  This time on a MOD 5F1 tweed amp.

I've started going through their list of tests and got a "hit" right away on the power transformer testing.  I've got 121 volts between the end lug of the fuse holder and the chassis.

All the wiring looks right to me.  I'm going to attach a pic. in hopes of something standing out.

For some reason the pic is blurred but I don't know what to do about that.  I'm feeling pretty dumb at this point.

[mresistor]

Hopefully 121 VAC   so   what's the problem? You're taking a meter reading of the vac coming from the power cord to the chassis which is grounded.  Evidently the power cord is plugged into an AC outlet.

[mresistor]

I just read through the preliminary voltage checks that ascertain if the power trans is putting out proper voltages  and no where did it say to check if there is voltage on the fuse from the power cord.  It says first to do the checks on the rectifier pins and then you check to see if the 6.3vac filament voltage is present on the power tube pins. 
Just follow the instructions and you should be ok. Be careful because you are gong to be measuring some high voltage so make sure your meter has the correct voltage selected. Is your meter capable of reading 599VAC?

[martinlb]

The reason I checked for a reading between the end lug and the chassis is my finger got jolted as I was replacing the fuse, which blows after a few seconds of testing the rectifier 4 & 6 pins.

And yes, it's AC.  I was getting the correct reading - around 600v at pins 4 and 6 before the fuse blew.  This happened twice - not that I thought it would change its mind and not blow, but I guess I was hopeful.  Maybe my meter isn't good to over 600v because it was beeping.  I should have checked on that an I will.

I'm confused at the idea of voltage going from the power cord to the chassis though.  The idea of that just seems wrong.  I don't think there's supposed to be any sort of continuity or current between that lug and the chassis, is there?

[dogburn]

As MrResistor noted, all you have done is confirm that you have AC coming in via your power cable.

To put it another way: Putting a probe at the fuse means you are measuring what is coming through the hot lead of your power cable. The ground of the power cable is connected to the chassis, so by placing the other probe on the chassis, you are completing the circuit and are directly reading what is coming from the outlet in your wall. It's the same as just sticking your probes into the ground and hot of your wall outlet or of a power cable.

And getting shocked from the lug of the fuse holder is only because the amp is plugged in. You could prevent this by wiring the switch in front of the fuse (instead of after), and then you would only get zapped if you touched the wrong lug of the switch. 

[mresistor]

I looked through that MOD 5F1 kit instructions and there was no schematic. You'd think that MOD would be interested in providing novice amp builders that schematic of their kit so a person could a least start looking at a schematic to try to understand what's going on in the amplifier.  If there is a schematic link I sure missed it and looked for one for a while. Of course it's a tweed Champ  but   still    ...


Bro  read the warnings  about having one hand in your pocket at all times wihen you are probing voltages in a live (hot) amplifier chassis.  Even without a schematic I saw in the instructions that the power cord hot wire was connected to the fuse and it should have dawned on you that it was and so...    if you think AC voltages hurt then you definitely want to avoid HIGH VOLTAGE DC.  Don't touch anything with your fingers inside a hot amp.

Pins 2 and 8 are the filament of the rectifier and should read 5.4 VAC.   Did you have your meter set on AC volts?  You have to understand that when the rectifier tube is inserted into the amp and the amp is power on then there will be HVDC on pin 8. 

[mresistor]

Here's a 5F1 schematic for you  you'll have to look at vacuum tube pinouts to determing what the pin numbers are..   

[mresistor]

unplug the power cord when changing a fuse .. 

[mresistor]

I know you've completed a paint by numbers MOD 5F1 single ended guitar amplifier and have done well by what I've seen so far in assembling it.  However, it seems you have no understanding of what is happening electronically inside the amplifier.  Please read through this  link and ask questions if you don't understand there are many people here that can help you to stay alive and making more amps the right way.
https://robrobinette.com/How_Amps_Work.htm

[Willabe]

martinlb, you don't need to start a new thread when you find a new problem or want to ask a question about the same 1 build.

Please keep it all together in the same thread.

[Rontone]

unplug the power cord when changing a fuse ..

I will second this, I got a zap from a Champ years ago, the fuse fell back into the holder so I reached in to grab it and finger and thumb touched the chassis, completely forgot as the amp was off, but the power lead was still in the wall...

[scstill]

Sorry if this was already caught but the picture shows the switch on the white neutral line
2nd forum thread this week for a kit amp from MOD
That does not seem safe.
And doesn't the National Electrical Code prohibit switching the neutral wire?