newbie help

[tubescent]

I built this -

Took me ages and got excited when I fired her up and the pilot light came on. But it blows after 2-3 seconds. I'm a newbie with a multi-meter - could some of the kind knowledgable folks round these parts talk me thru where i go from here?

[Gibsonman63]

There are far better troubleshooters here, but I have a couple of general suggestions to get you started.

The fact that it is blowing fuses means you have a short. Do a search and find the light bulb current limiter and build one, so you can power it up and not have to change out a bunch of fuses.

Double check the polarity of all of your electrolytic caps

Pull all of the tubes and plug it in and power it up through the light bulb current limiter. If it glows brightly you still have a short. With no tubes, you should have little or no glow.

Cycle power install one preamp tube at a time and repeat. (divide and conquer)

If you record all of your voltages, some of the guys that are smarter than me can help you. If you have a schematic with voltages on it, it would be helpful to compare.

Make sure you have working power tubes. Shorted power tubes will take out the fuse.

Hope it helps.

[Chevy]

First thing I'd suggest is disconnect the amp from the power supply completely, and see if the power supply works on it's own as a stand-alone unit. This includes the heater supply leads as well. See if all the voltages are right at A,B,C, and if the fuse holds up. If not, at least you know to start at the power supply.

Are you really running at 220V mains? (just double checking) Not sure what the polarity of the 25v/5W diode is, but you might double check that as well.

The schematic is kinda tough for me to follow, not sure if I'm seeing this right, but what is a CT winding on the power transformer doing connected to the cathode of the 6v6 ouput tube?

[JHow]

...what is a CT winding on the power transformer doing connected to the cathode of the 6v6 ouput tube?

Isn't that to provide DC-elevated heater circuit?

[Enzo]

I agree, that detail looked OK to me.

[tubeswell]

Hi tubescent,

Fuse blowing like that is a likely sign that you have B+ shorting to ground somewhere. Check the power rail for zero DC resistance to ground with your R meter (with the power off)

Wondering why want a 24V zener on your bridge rectifier? Surely you will get about 390V B+ without any zener, and that is good enough to run a cathode biased 6V6 (if you get the tube current right)?

[Chevy]

Isn't that to provide DC-elevated heater circuit?

Right... realized the DC elevated heater supply after posting...

[madkatb]

Maybe I'm being picky and maybe it doesn't make any difference but if that is a 6.3V center tap on the tranny shouldn't it be 3.15V-CT-3.15V? If that's not the case, could it be a problem source?

[52 Bill]

Maybe I'm being picky and maybe it doesn't make any difference but if that is a 6.3V center tap on the tranny shouldn't it be 3.15V-CT-3.15V? If that's not the case, could it be a problem source?

The two ends of the transformer winding are 180 degrees out of phase. When one is at 6v the other is at 0v. When the cycle continues the side that was 6 becomes 0 and the side that was 0 becomes 6. Or as you suggest in the middle of the waveform, both sides are at 3v.

[Enzo]

Agreed. Either way the winding is 6v with a center tap. Instead of 3-0-3, this draftsman chose to label it 0-6 and showing a CT. Note also on the mains side they labelled the primary as 0-220/230/240.

The relatively recent convention of writing a 6v CT winding as 3-0-3 has an implied grounding of the CT which would not be the case here. And in any event, the 3-0-3 presumes the center tap as the frame of reference, which much of the time would be the case, but not necesarily. Someone could use the same transformer and not use the CT - then 3-0-3 has little meaning. (They might want to install a hum balance control instead.)