An illustrated guide to building an F5

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#1,162
I successfully fried one channel of my amp !

Series of events:

1. Amplifier had been left on for a few days.
2. Turns on preamp
3. Heard a quite pop in speakers and no noise from source when turned it on.
4. Pull out amp, checked the fuse, fuse was blown.
5. Replaced fuse, turned amp back on and it went BZZZZZZZZZ, sparked a bit inside at the terminal strip, at the live in and CL-60, then blew the fuse.
6. Said Oh Sh@@@@
7. Plugged in bulb limiter, replaced fuse and the bulb lit up like it was gonna explode
8. Unplugged left and right channel from PSU, tested PSU - good
9. Tested right channel fine, no short and audio worked.
10. Tested left channel - build lights up hot so there is something wrong
11. Look at left channel and it's FRIED.

Here is a picture.

Not sure what happened but the good news is that DIY store has all the parts I need.

I have some Toshiba FETS and my one working channel are from LS so it's probably best to redo both channels so they are all matched?
 

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#1,166
It is the Nutube B1 preamp and it doesn't have a thump. I did test the preamp on a two other amps afterwards and no pop or thump but it could have been from it.

I'd been using the amp for months with no issues, preamp for a few weeks no issues. Biased corrrectly, temps, offset should have all been fine as I checked it about a month ago.

Here is a picture of where it sparked.

Now I pulled the wires out and I originally soldered the Live In to the CL-60 leg for some reason then screwed it down into the terminal where i drew a circle. Maybe that had something to do with it because it wasn't a super pretty joint but was strong. Maybe it just didn't have good contact with the strip, the wire in and the CL60 leg.
 

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#1,169
I'm finally getting around to fixing my F5. It's one of those things where I wasn't too mad something went wrong because now I get to build a new one 🙂

One board is just fine so the JFETs I know are ok. Should I assume the JFETs on the fried board are no good?

Visually they look good while other components are clearly fried. I already acquired replacement JFETs but it would be nice to save these if they are still good.

Here is a pic
 

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#1,171
Mrshazbot said:
I successfully fried one channel of my amp !

Series of events:

1. Amplifier had been left on for a few days.
2. Turns on preamp
3. Heard a quite pop in speakers and no noise from source when turned it on.
4. Pull out amp, checked the fuse, fuse was blown.
5. Replaced fuse, turned amp back on and it went BZZZZZZZZZ, sparked a bit inside at the terminal strip, at the live in and CL-60, then blew the fuse.
6. Said Oh Sh@@@@
7. Plugged in bulb limiter, replaced fuse and the bulb lit up like it was gonna explode
8. Unplugged left and right channel from PSU, tested PSU - good
9. Tested right channel fine, no short and audio worked.
10. Tested left channel - build lights up hot so there is something wrong
11. Look at left channel and it's FRIED.

Here is a picture.

Not sure what happened but the good news is that DIY store has all the parts I need.

I have some Toshiba FETS and my one working channel are from LS so it's probably best to redo both channels so they are all matched?
Click to expand...

Your soldering looks pretty rough from what i can see.What kind did you use? I would resolder using flux and 63/37 solder.It may save you a failure down the road.
 
#1,173
Mrshazbot said:
It is the Nutube B1 preamp and it doesn't have a thump. I did test the preamp on a two other amps afterwards and no pop or thump but it could have been from it.

I'd been using the amp for months with no issues, preamp for a few weeks no issues. Biased corrrectly, temps, offset should have all been fine as I checked it about a month ago.

Here is a picture of where it sparked.

Now I pulled the wires out and I originally soldered the Live In to the CL-60 leg for some reason then screwed it down into the terminal where i drew a circle. Maybe that had something to do with it because it wasn't a super pretty joint but was strong. Maybe it just didn't have good contact with the strip, the wire in and the CL60 leg.
Click to expand...

You should never solder a wire then put it under a screw.The solder relaxes after a period of time and you get a loose joint.
 
#1,176
Here is a mockup of a barrier strip I used for one of my amps. I couldn't get a trustworthy airtight crimp on the thin leads, so I soldered the connection after crimping. In subsequent amps I used a different type of connector and also insulating "marking strips" under the barrier strip. I suppose some flame-resistant material on the exposed CL60 leads is also good practice. These critical PS connections need to be high-reliability, which is not something always achieved by screw-down on stranded wire.

BK
 

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#1,177
bk856er said:
Here is a mockup of a barrier strip I used for one of my amps. I couldn't get a trustworthy airtight crimp on the thin leads, so I soldered the connection after crimping. In subsequent amps I used a different type of connector and also insulating "marking strips" under the barrier strip. I suppose some flame-resistant material on the exposed CL60 leads is also good practice. These critical PS connections need to be high-reliability, which is not something always achieved by screw-down on stranded wire.

BK
Click to expand...


You also need a quality crimping tool, I got intermittent connections using a cheap consumer one. I changed over to soldering wherever possible.
 
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