Princeton clone build: blowing fuses

[timrobertson100]

Hi all,

I've just finished wiring up a princeton reverb clone.

Powering it up with no tubes for the first time and the pilot light went on for a couple secs, and then I powered down. I put in the rectifier and powered up... got a hissing sound and then blew the fuse after a second or so (pilot light was on).

I've disconnected the secondaries from the PT and confirmed where they would connect to are all open circuit.

I connected the only PT low voltage secondary to the heaters and powered up - now the fuse blows instantly...

Have I hosed my PT in that first 2 secs do you think?

Anyone fancy helping a newbie get through his first debugging? I am a bit at a loss of what to do now...

Thanks for any pointers!

[mark enger]

Pix will help, mark

[timrobertson100]

Can anyone tell me what the resistance across the secondaries of the PT when disconnected should be?

I have about 400R on the reds when disconnected and short circuit on the yellow.

I assume that is bad right?

[timrobertson100]

Some pics attached of it complete and when I started trying to disconnect secondaries.

Thanks for taking the time to look at this. Really appreciated.

[mark enger]

Your build looks excellent timrobertson100, are you sure the light is good?
Does the PT have a CT for the heaters?

[timrobertson100]

Thanks Mark! Kind words, but alas I am just following a tubeampdoctor build so I just follow the instructions (or not as it appears today)

The circuit and layout I am following are http://www.tubeampdoctor.com/images/Fil ... 200912.pdf

The light is good - verified in my other amp.

There is no center tap for heaters on the PT.

Any idea on the PT output resistance when disconnected? I am guessing 0 ohm is bad for that...

[mark enger]

I see the the 100 ohm R under the light now, but the way the heater wires are grounded looks odd to me ala old style did you use a modern layout? im not sure that's the right way to run heaters

[mark enger]

Take that grounded heater wire off chassis and connect to light and you should be good

[tubeswell]

The cause of mains-fuse-blowing is too much current. There must be a short to ground somewhere on one of the PT's secondary taps.

If the amp looked okay, e.g.; if the lamp came on and there was high VAC on the two red secondaries (pins 4 and 6 of the rectifier socket) and 5VAC on the rectifier winding (pins 2 and 8 of the rectifier socket) with no tubes in, but it sorted with the rectifier plugged in, then the short must be somewhere either in the rectifier tube, or in the B+ line after the rectifier tube.

That's where I'd start looking

Edit: that is to say, this is likely to be the problem after you fix the heater winding ground reference. Your instant-blowing-fuse problem was caused by you hooking up the heater CT after you had the initial problem. (You only should have one type of ground reference system for the heater circuit - see Mark's post below)

[tubeswell]

Take that grounded heater wire off chassis and connect to light and you should be good

Ah yes - good spotting Mark. Yes the orange heater winding CT should not be used if you are using the 2 x 100R as a ground reference. Its either one or the other, but not both.

[timrobertson100]

Thanks guys. Quick update:

I connected everything up again, and removed my drain resistor I put on the cap.

With no tubes, I have 700+ and 5VAC on the rectifier pins, so I now assume my PT is not hosed (phew).

Sorry I am a bit new on this. Could you please confirm that the orange wire should be removed and connected to either side of the lamp ? (I've verified it is as on the layout from tubeampdoctor - linked before)

Thanks!

[tubeswell]

Thanks guys. Quick update:

I connected everything up again, and removed my drain resistor I put on the cap.

With no tubes, I have 700+ and 5VAC on the rectifier pins, so I now assume my PT is not hosed (phew).

If you're getting that VAC on those windings, they should be okay (touch wood). Now, before you plug another rectifier in, you have to find the short-to-ground that was causing the fuse to blow in the first place. (see my earlier post)

Sorry I am a bit new on this. Could you please confirm that the orange wire should be removed and connected to either side of the lamp ? (I've verified it is as on the layout from tubeampdoctor - linked before)

Thanks!

If you are using the pair of 100R ground-referencing resistors, then you don't use the centre tap, and vice versa. You only should have one type of ground reference for the heater winding.

And if you're using the 100R pair, then you certainly should not connect the orange wire to any side of the heater winding. Instead you should snip the bare end off the orange wire and use heat shrink over the end of it to ensure that it's insulated from anything, and tuck in benignly into a corner somewhere where it won't do any harm.

[timrobertson100]

Thanks tubeswell, mark. Orange wire now disconnected, and confirmed no rect. tube seems ok, but with the rectifier it hisses quickly and blows or blows fuse immediately.

I've checked wiring and don't spot anything immediately wrong, but now will buzz everything through to see. I guess this is how we learn right...

Cheers

[timrobertson100]

I feel stupid asking this, but can someone please confirm the diode and the cap look oriented correctly at the back? I assume the "-" arrow points to negative and the solid line on the diode casing means the bar.

[timrobertson100]

Now it's back to intermittently blowing fuses with no rectifier in. Maybe that's enough for 1 day.