testing a OT?

[iknowjohnny]

Any way i can check a OT (4/8/16) out of circuit to see if it may be bad?

[dorrisant]

Read here:
http://www.premierguitar.com/Magazine/I ... ester.aspx

Tony

[iknowjohnny]

Thanks, i'll give that a try

[dorrisant]

If the ot is in the amp, lift and isolate the primary leads. Make up the neon bulb tester circuit inside a 1/4" mono plug. Plug this into your main out jack and apply a 9v battery to one of the primary sets. In this configuration all other leads besides the two connected to the primary and the two connected to the bulb are still isolated.

Tony

[iknowjohnny]

If the ot is in the amp, lift and isolate the primary leads. Make up the neon bulb tester circuit inside a 1/4" mono plug. Plug this into your main out jack and apply a 9v battery to one of the primary sets. In this configuration all other leads besides the two connected to the primary and the two connected to the bulb are still isolated.

Tony

Let me see if i understand this. (this is a 2xel34 50w OT) Remove the 2 primary leads from the sockets. Plug a 5 volt bulb into the speaker out. Touch a 9 v battery's + to one of the primary wires and the - to amp ground. Do the same test for the other primary. Is this correct? If so, what am i looking for, the bulb to light, to NOT light, to flash? Thanks.

Oh, and is that one of those small Xmas lights? Does it have to be a certain type of bulb? Not sure how to tell a neon from others.

[dorrisant]

Not a christmas light bulb... a neon lamp... you know, the orange red glow from the old deep freeze. They strike an arc somewhere around 80v. Get them at Radio Shock if you have to.
You don't have to hook it up as I described but it is easier to test the ot while in the amp with this method.
Usually if you have two primaries, one will be taped off... leave it alone. If you make the bulb circuit as described above, plug it into the "main" speaker out. Impedance doesn't matter. Just make sure the bulb is plugged into the output jack that is grounded when nothing is connected if there is one. Disconnect the other primaries (yes from the sockets) and connect the 9v battery, b+ to one primary and b- to the other primary. None of the ot tx leads should be grounded. This will generate enough voltage for a shock, so be aware. If the bulb flashes you are golden... no shorted turns. No flash, sorry about your ot!
This works for pts too, but I will leave you to figure that one put.

Hope that helps.

Tony

[martin manning]

Why disconnect the primary leads from the sockets? They terminate in a vacuum. Actually, there may be no need to disconnect anything:

1) Put a 1/4" plug into the main speaker jack to open the shorting contact (or use a cable with the other end unconnected).
2) Connect the neon bulb and resistor across the plate leads at the tube sockets. I use a neon line voltage tester from the hardware store.
3) Momentarily connect a 9V battery across the plates and watch for the flash.

My Express is open on the bench and I just now tried the test this way. Standby on or off doesn't matter.

[iknowjohnny]

Are neon bulbs all 120v? I looked at radio shack online and it appears so unless they just don't carry different. If not, is 120 what i need or not?

[martin manning]

You are probably looking at neon indicator lamps. Per the article linked above the bulb is type NE-2, which you wire in series with a 100k resistor. Alternatively, just get a neon line voltage test light from a hardware store, and forget about the resistor.