Fried Power Tranny

[DFMoore]

Guys,
I'm building a simple Single Ended amp, a copy of the Eagle Kit I worked on earlier this year. When I powered it up the power transformer started smoking. I am only getting 14 volts AC from the Secondary side. I have attached a few pics. Can anyone see what mistakes I might have made? Any ideas are greatly appreciated; I'm still somewhat new at this.
Thanks, David

[xtian]

Sorry, man, tough to read the photos. Disconnect the PT's heater pair from your terminal strip and pull the tubes. Then use voltmeter to test heater legs to ground and make sure you don't have a short to ground.

[DFMoore]

Pulled transformer. Tube voltage is showing good, it's the AC side that's showing only 14 volts AC.

[Jack Hester]

When I powered it up the power transformer started smoking.

David -

You may have heard others ask, in other threads, but I'll ask it here: Do you have a lamp limiter that you can bring the amp up on?

Saves a whole lot of headache, when troubleshooting a new build. If you don't have one, make that your next project to build. Get 25W, 40, 60, and 100W bulbs. When you start out, have the 25W bulb in. I believe that you would have seen it very bright, with your problem. But, you'd still have a good PT. When everything downstream of the PT secondaries is in good order, the lamps will not burn very bright.

From what you've stated, go ahead and get a new PT, and start again on limited current. While it's coming, study your circuit with the schematic and/or layout. Sometimes the obvious is not so obvious.

My best lamp limiter is built in a 4" electrical handybox. Here's a picture of it:

[IMG:428:640]http://i132.photobucket.com/albums/q32/ ... B12N04.jpg[/img]

I have a fuse holder mounted in it, also. I put the same fuse size that the amp I'm working on is rated for.

Jack

[Jack Hester]

Looking at the photos, I see quite a bit of cleanup and lead dress needs to be done, before going too much farther. The signal ground end of your decoupling caps have individual green leads coming from them. Jumper each of these together (top or bottom of the board) and carry only one lead to wherever the other end needs to attach.

Also, twist all of your tube filament pairs. You have them twisted, coming out of the PT and going to a terminal strip. Twist all that leave this terminal strip. Source of much hum, if not done so.

What are the loose blue wires? Do you have a schematic that you can provide?

Jack

[DFMoore]

Just googled the light limiter. I have all the items needed laying around in my shop. I'll build one before I go any further. Like I said I'm somewhat new to this so any ideas like this is appreciated.

[DFMoore]

I have attached a layout of the amp. Also, the picture of the bottom of the board was snapped before I completed the soldering. The blue leads are soldered to the board from pins 1 and 6 of the 6H2N tube to the cap/resisters and from pin 5 of the 6N6C to the cap.

[ampbldr2]

Pull the rectifier diodes and check them one by one to see if one of them are shorted.

[ampbldr2]

It is always a good idea to measure the rectifier diodes before you insert them in your board to ensure they are not shorted. I built a HO and had smoke from my PT because of a shorted diode.

[surfsup]

I always make sure to touch metal when I pick up a diode or mosfet or something electronic to eliminate a static charge.

It looks like your B+ red lead goes directly to the power tube socket not the OT? On the pic that is not soldered it looks like that red lead from the socket got soldered to the diodes? hard to tell from those photos. Sry if i have that wrong.

[DFMoore]

Found a little time in my shop yesterday and completed the dim light that was recommended. Plugged up the Tranny with a 25 Watt lamp and sure enough it light up brightly. Lesson Learned the hard way.

Also pulled the diodes and found the problem.

Thanks to all that responded.
David

[Jack Hester]

Also pulled the diodes and found the problem.

David -

What kind of problem did you find with the diodes?

I was looking over the schematic. Nice, simple little amp. Do you already have the tubes that it calls for? If not, that configuration is very easy to convert over to use a 12AX7 and 6V6. The layout is very much like a Champ 5F1, and you can do the same easily.

If you already have the tubes listed, this will be a good amp to experiment. Like adding a tone stack. Or maybe an FX loop. Good learning ground for future builds.

Jack

[DFMoore]

Jack, I found that two of the diodes were shorted. I'm guessing their voltage threshold was too low. I'll mount 1000 volt diodes this next go around.

As far as the tubes, I do have them. As I understand the 6N6C is wired the same as a 6V6. I'm thinking the heaters on my 6H2N-EB are wired differently than the 12AX7.

I would like to hear your ideas concerning a tone pot and an effects loop.

[Jack Hester]

David -

Your pre-amp tube appears to be just like a 12AX7, except your 6.3v filament wiring attaches to pins 4 & 5. On the 12AX7, 12AT7, 12AU7, 12DW7, etc., pins 4 & 5 are connected together, and one lead of the filament wiring goes to one of these pins. The other filament lead goes to pin 9. Other than that, your tube has the same pinout as these.

I made a quick schematic of your pre-amp, though I couldn't tell what values of resistors that you had, by the colors.

Also, take a look at this fellow's site, for some ideas on tone stacks:

http://amps.zugster.net/articles/tone-stacks

You would want to start out with something simple, and go from there. The 18 Watt looks simple enough that you could build it right on the tone pot. Just know that this will load up the signal a bit. Maybe not enough to hurt, in your amp.

Jack

[DFMoore]

Ok, new power transformer and new diodes and she is singing. It, however, is only clean at 1 anything more is very distorted. Any ideas?