blowing fuses

[UNCLEDUB]

Hello everyone, I'm new to the forum and fairly new to trainwreck amps. A friend of mine came to me and asked me if I could build him one of Niks expressions. I said sure mostly b/c I wanted to hear one for myself. So the amp came in and I got started. I studied the francesca files and wired it up as close as i could to the original.

Here is where the problems start. I googled niks tranie wire colors and the result I found said to use the orange wire for 120V not the blue. after emailing Nik I changed the wires and turned it on. I tested the voltages and this is what I came up with:

Mains: 120.6V
Sec: 623V
B+1: 429V
B+2: 429V
B+3: 420V
B+4: 419V
B+5: 414V
Bias: -29V
Heaters: 6.0V

This was with no tubes installed.

I emailed these to Nik and he said they should be ok and I could put the tubes in. So I did and a fuse blew. I tried removing the tubes and turning it on like before and now it blows a fuse with out tubes when the standby switch is flipped on. I have been emailing Nik and I sent him some pics. He is still working with me but I thought that maybe the collective wisdom from you guys would be a great help. This is my second build. My first being a vibro champ that went together with almost no issues. I apprieciate any help you guys can give. I have attached a few pics and have a few more.

Walt

[Gibsonman63]

A blown fuse means you have a short somewhere. Build a light bulb current limiter and you will save a few bucks in fuses and some time and frustration in the long run. Once you have the current limiter, try sticking in just the preamp tubes (divide and conquer). If the bulb doesn't glow excessively, try with the power tubes.

Make sure you have power tubes that are known to be good. Shorted power tubes will take out the fuse.

Make sure you have a slo-blo fuse, otherwise the inrush can take it out.

[Gibsonman63]

I couldn't see them clearly in the pictures, but also check the orientation of your cathode capacitors. If they are backwards, it will take you straight to ground.

[gearhead]

+1 on the slow blow fuses and the light buld limiter.

Otherwise you probably have a short somewhere (and maybe two; the original with tubes and one that now causes it without the tubes).

Look at the schematic for paths to ground with no tubes inserted. First up are the power caps and diodes on the EL-34 plates, Can't tell the polarity of those diodes from the pics.

[UNCLEDUB]

So Nik recomended that I disconect the OT CT and see if it is still blowing fuses and it stopped it from blowing the fuse.

Gibsonman: I do need to build a bulb limiter and i'll probably go pick up the parts to do that today. I checked polarity on all caps and they are correct. I have slow blo fuses.

gearhead: The polarity of the diodes is with the strip towards pin 3. could the diodes have gone out or over heated when I soldered them in and cause the fuse to blow? I took extra care to be quick with the soldering so they didn't over heat.

Thanks for the help guys.

[KellyBass]

could the diodes have gone out or over heated when I soldered them in and cause the fuse to blow?

Absolutely! Don't ask me how I know this....

Now, whenever I solder a diode of any kind, I always use a tiny alligator clamp between the diode and whatever to act as a "heatsink". works great.

+1 on the bulb limiter. Make it your first priority as it will save you $$$ in the long run. Don't ask me how I know this...

[gearhead]

Check the diodes.

I assume you had a load (speaker) connected through all the above?

[UNCLEDUB]

So I tried the amp with the diodes unsoldered just to see if they were shorting out and it didn't blow a fuse. I'm going to pick up some new diodes while i'm out picking up the parts for the light bulb limiter. good call guys and thanks for the help. Hopefully she will be making sweet music soon.

[gearhead]

It's still possible you have another problem and the diodes did their job.

Good read: https://tubeamparchive.com/viewtopic.ph ... ate+diodes

Meaning they reached breakdown voltage.

[UNCLEDUB]

I'm aware that I may still have a problem. I had the wrond wire for the 120V from the PT and i'm thinking that was what caused problems. I also found a possible bad solder joint early on in the testing that could have been the original problem to begin with. So i'm hoping that I have fixed whatever caused the diodes to fail already. If not I'll be seeking more help. You guys have been more than helpful and I apprieciate the hospitality.

[UNCLEDUB]

OK guys, After searching for the diodes I needed for a few days, I found them and replaced all 6. I built a light bulb limiter and used a second hand socket for the bulb and it didn't work. Living in the middle of no where makes this a little difficult. I had a few extra fuses so I tried it out and it came on with no problems....so I thought...Turns out the volume and tone stack don't affect the signal at all. I have gone over the preamp section and everything checks out. The pots are all wired according to the layout. Do you guys have any idea what may cause this?

Thanks guys

Oh the presence knob does effect the signal but it only adds hiss into signal.

[gearhead]

You need to describe the exact conditions for each try. Were tubes in? Light bulb limiter didn't work? Any volume at all? Did you check B+ voltages?

Although it makes for a very neat build, trying to trace where what wire goes where is not possible for under-board runs.

[Gibsonman63]

Maybe some clarification on the volume and tone not affecting the signal would help us figure out what is going on.

Do you have sound coming from the amp, but can't turn it down or change tone?

I would start by checking the grounding of the pots.

Also, if you have already checked your layout against the layout drawing, it is time to start tracing through the schematic against your build if you haven't done so already. I learned a lot on my build by going through this exercise. Some things on the layout are much more clear when you correlate it to the schematic.

[UNCLEDUB]

OK here is the low dow.

I replaced the diodes and tried with the light bulb limiter to start it up without tubes. The lightbulb never flashed or anything and the voltages were the same as without the limiter. So I checked and rechecked that I constructed it correct and everything was right. I tried a different bulb and nothing so I decided to move on with out it sense I wasn't blowing anymore fuses.

Here are the voltages I got without tubes:
Mains: 119.7V
Sec: 621V
B+1: 423V
B+2: 423V
B+3: 416V
B+4: 412V
B+5: 409V
Bais: -24V
Heaters: 5.9V

These looked to be about right so I put the tubes in and started it up. I got sound but none of the pots did anything with the exception of the presence pot. It just added more hiss. And here are the voltages with the tubes in.

Mains: 119.8
Sec: 608
B1: 378
B2: 353
B3: 277
B4: 260
B5: 245
Bias: -24V
Heater: 5.5

V1:
pin 1 plate: 173
pin 6 plate: 155
V2:
pin 1 plate: 220
V3:
pin 1 plate: 164
pin 6 plate: 174
V4:
pin 3 plate: 374
pin 4 screen: 343
V5:
pin 3 plate: 374
pin 4 screen: 343

[KellyBass]

With the amp off and the filter caps bled, start at the input jack (double check these connections!) and slowly work your way back through the circuit. Check periodically for continuity. Solder joints can be deceiving.