UPDATE: Finished my Express build last night > it's working!

[prairie]

Here are the voltages at the pins with the amp off standby (hooked up to limiter)

V1
pin1: 260v
pin2: 0v
pin3: 1.4v
pin4: 2.4vac
pin5: 2.4vac
pin6: 197v
pin7: 0v
pin8: 1v
pin:9: 2.4vac

V2
pin1: 293v
pin2: 0v
pin3: 2.2v
pin4: 2.4vac
pin5: 2.4vac
pin6: not used
pin7: not used
pin8: not used
pin:9: 2.4vac

V3
pin1: 200v
pin2: 20v
pin3: 32v
pin4: 2.4vac
pin5: 2.4vac
pin6: 208v
pin7: 18v
pin8: 32v
pin:9: 2.4vac

V4 (power tube)
pin1: 0v (ground)
pin2: 2.8vac
pin3: 384v
pin4: 380v
pin5: -27v
pin6: 380v
pin7: 2.8vac
pin8: 0v (ground)

v5 (power tube)
pin1: 0v (ground)
pin2: 2.8vac
pin3: 384v
pin4: 380v
pin5: -27v
pin6: 380v
pin7: 2.8vac
pin8: 0v (ground)

[sluckey]

Pull the output tubes and plug the AC cord straight into the wall outlet. Recheck the voltages on the power tubes.

[prairie]

Pull the output tubes and plug the AC cord straight into the wall outlet. Recheck the voltages on the power tubes.

Those readings are with no power tubes. And if I plug straight to the wall, I blow a fuse.

[romberg]

Pull the output tubes and plug the AC cord straight into the wall outlet. Recheck the voltages on the power tubes.

Those readings are with no power tubes. And if I plug straight to the wall, I blow a fuse.

Just to clarify the situation. Does the fuse blow when plugged straight into the wall with *only* the preamp tubes installed (no power tubes installed)?

Also, have you adjusted the bias pot to give the maximum negative voltage on pin 5 of the power tubes? It may be that the bias circuit is not producing enough negative voltage to control the power tubes.

Pulling the two power tubes should allow you to test the voltage range your bias circuit can produce without risking red plating your power tubes. Since a current limiter will lower all the voltages the best way to test this is to not have the power tubes installed.

It is possible that a resistor or two in the bias circuit may need to be adjusted to create more negative voltage. General rule of thumb is that you will need about 10% of the plate voltage (but negative) on the power tube grids. So, if you have 400V on the plates you need around -40v bias (probably a bit less).

Mike

[prairie]

Pull the output tubes and plug the AC cord straight into the wall outlet. Recheck the voltages on the power tubes.

Those readings are with no power tubes. And if I plug straight to the wall, I blow a fuse.

Just to clarify the situation. Does the fuse blow when plugged straight into the wall with *only* the preamp tubes installed (no power tubes installed)?

Also, have you adjusted the bias pot to give the maximum negative voltage on pin 5 of the power tubes? It may be that the bias circuit is not producing enough negative voltage to control the power tubes.

Pulling the two power tubes should allow you to test the voltage range your bias circuit can produce without risking red plating your power tubes. Since a current limiter will lower all the voltages the best way to test this is to not have the power tubes installed.

It is possible that a resistor or two in the bias circuit may need to be adjusted to create more negative voltage. General rule of thumb is that you will need about 10% of the plate voltage (but negative) on the power tube grids. So, if you have 400V on the plates you need around -40v bias (probably a bit less).

Mike

Hi Mike,

On your first question: correct. Fuse blows without power tubes installed and plugged straight into the wall.

Interesting about the negative voltage thing. I’ll see what I can fanagle.

[romberg]

Unless I missed something (which is always possible , it looks like your power supply has no shorts to ground all the way to V1. But something is drawing excessive current and taking out the mains fuse when you go off the limiter. How bright is the bulb when on the limiter and no power tubes? It should glow bright for maybe a few seconds tops and then be very very dim with no power tubes. Is your bulb glowing more than faintly?

Mike

[sluckey]

Disconnect white wire from rectifier diodes. Still blow fuse when plugged straight into the wall?

[prairie]

Unless I missed something (which is always possible , it looks like your power supply has no shorts to ground all the way to V1. But something is drawing excessive current and taking out the mains fuse when you go off the limiter. How bright is the bulb when on the limiter and no power tubes? It should glow bright for maybe a few seconds tops and then be very very dim with no power tubes. Is your bulb glowing more than faintly?

Mike

Barely glows. Bright for a half second on startup, and then almost no glow.

[prairie]

Disconnect white wire from rectifier diodes. Still blow fuse when plugged straight into the wall?

Will try this morning.

[prairie]

Disconnect white wire from rectifier diodes. Still blow fuse when plugged straight into the wall?

I disconnected the above wires (300v windings) and the fuse still blows when plugged straight into the wall.

[martin manning]

If the fuse won’t hold with all secondaries disconnected, the PT is toast. It could have been damaged when you initially powered it up with tubes in and positive bias voltage. That would cause excessive current draw from the HV winding.

[prairie]

If the fuse won’t hold with all secondaries disconnected, the PT is toast. It could have been damaged when you initially powered it up with tubes in and positive bias voltage. That would cause excessive current draw from the HV winding.

Well that’s worrying. I’ll check the primaries in a bit here (checking continuity, impedance).

[martin manning]

In any case I'd try it with all secondaries disconnected. If there is an internal short it would start to smoke even on the limiter. Another possibility is that the fuse isn't big enough to stand the initial current surge. A 2A works for me, though.

[prairie]

I disconnected the primary leads and checked the impedance. Got 5 ohms, so that’s something (I think?). I was afraid of having an open lead or no reading at all.

Is there any other way I should be testing it?

[martin manning]

The primary DC resistance will be very low, a few ohms. Does the fuse blow with all secondaries disconnected?