Blowing fuses

[martin manning]

True the small (820R) FB resistor would make the ground, and that would increase the NFB to 100% of the output voltage. However, the PI tube is puled and the problem still occurs. I'm thinking the close proximity of the screen node eyelet to the V9/V10 grid feed is the most likely source.

[JerryFJA]

The only way I can see the PI ground affecting power tube bias is if the PI coupling caps are leaky. The anode voltage will rise if no current is flowing, and of course pulling the PI tube would have the same effect.

Try lifting the ground on that resistor and watch what happens. Seen it in my shop many times.

I can't think of any reason to suspect the OT is causing this. What's the logic there?

If nothing else in the circuit is causing the circuit to draw too much current what are you left with?

Doesn't blow the fuse when the tubes are pulled.
Bias voltage being low won't cause it to blow a fuse that fast IMO.
Jerry

[gui_tarzan]

I lifted the wires that Martin suggested, even pulled the screws out of the bias and main boards and lifted them a tad and it still blows the fuse.

In looking at the solder joints on the main board it is obvious to me that someone else has been in here (no surprise, of course) so I touched up a bunch of them and it still blows the fuse. I can't help but believe it's one or both of the boards being conductive that's doing this.

[martin manning]

Any difference in measured voltages at the power tube grids with V9 and V10 out? That would indicate some effect from taking the HV off that part of the main board.

This is one tough nut!

[gui_tarzan]

Can't tell, I had the meter on it when I took it off standby and I watched the voltage climb like it did before the fuse blows. I don't know if it changed at all or not.

[martin manning]

I still can't figure out why you are seeing very low negative bias voltage, and why that would blow the fuse rather than just shutting off the plate current.

[Stevem]

would you please do the PT test out I posted about over a week ago now and confirm that it's not just the PT ?
It takes all of 10 minutes to do this check!

[sluckey]

You may want to get several different current ratings of these circuit breakers. Put gator clip pig tails on them to use during bench testing.

http://www.tedweber.com/wcb-3

Have you unscrewed and lifted the board to see if there are any solder blobs or other boogers underneath?

[gui_tarzan]

oops

[gui_tarzan]

You can test just the PT itself by disconnecting all of the secondary wires and on the primary side after the fuse and before the power cable install a 1 ohm 5 to 10 watt resistor. Next hook up a AC voltmeter across that resistor and turn the amp on and if the meter reads over .4 volts then the transformer is toast!

My mistake, I apologize, I tested the bias tap with a fuse, not this suggestion. Is this what you mean?

[martin manning]

If it will run with no tubes installed without the PT overheating, then the PT doesn't have shorted turns.

You could still try to get bias from the HT as proposed a few pages ago, and that is easy to do. Just remove the red/blue wire from the rectifier board and put a resistor from the eyelet where the red/blue was to the nearest HV (red lead) eyelet.

To get the very low bias voltage the HV must be leaking into the bias tap, which could be happening inside the PT. Getting bias from the HT would get around that. Find a value that gets the needed bias voltage, maybe 33-50k(?) in 2W

[gui_tarzan]

.3v AC with everything disconnected on the PT and 1 ohm 5W inline after the power cord and before the fuse.

[gui_tarzan]

Something else I noticed, which may or may not make any difference, is how the white wire is routed to the bias pot instead of to the 33K. I broke the original line in the drawing and drew the red section to show how it's actually wired.

[martin manning]

That shouldn't make any difference.

[gui_tarzan]

I made a new bias board, put that in and I have the same results. About 15 seconds in after flipping the standby switch it blows the fuse.