Half dead F6. Some help needed

[dyrevennen]

Hi Forum,

One of the channels on my F6 seems to be dead. It only gets lukewarm at startup, and no sound. I suspect that one of the output mosfets has died, as I have made no adjustment to bias settings.

Looking through my drawers with old stuff I couldn't find any 240, but I did find a perfectly matched quad of Semisouth E120R125. Can I use them instead? Just replace and rebias? Or do I have to alter the circuit (in which case I'll buy some new 240)?

I appreciate any feedback.

Thanks,
Dyrevennen

 

[6L6]

Save the SemiSouth magical unobtanium for a different amp.

What makes you suspect a dead mosfet? Are both Fets cold/barely warm? What is your measured bias current and offset (if any)? Is there any bad smell? Do you have any burnt components?

I would suspect a cold/intermittent solder joint first, that seems to be the culprit very often.

 

[dyrevennen]

Thanks for your reply (and excellent build guide, by the way, so much appreciated!). Poor solder indeed, up and running again now. Very happy, because I *love* that amp.

My suspicion towards dead Fets comes from my general pessimism. Getting away with fixing a solder, and I feel I cheated the universe.

Any good ideas for a project where I can put my R125s to good use? I only use full range speakers, and have been considering an F2 for a while. What would you suggest?

Thanks again,
Dyrevennen

 

[6L6]

There's not ideal project for them (yet), because the J2 is still in production and not For DIY yet. I'd wait for it. That's what I'm going to do with mine.

 

[dyrevennen]

There's not ideal project for them (yet), because the J2 is still in production and not For DIY yet. I'd wait for it. That's what I'm going to do with mine.

Thanks, I just re-read the review of the J2 at 6moons. Sounds like a good plan to wait for the J2 to become available for DIY. Meanwhile I'll listen to some jazz over my F6

 

[PRR]

...Getting away with fixing a solder, and I feel I cheated....

THE #1 fault in most electronics is JOINTS.

Friction joints (plugs, jacks) are popular trouble-makers, but easily wiggled into life or replaced.

My dad worked in mainframe computers, and even with the crappy transistors of the day, reliability came down to obsessive soldering. Trained workers, tested tools and materials, careful inspection.

Bad joints come two ways. Bad from the start, and goes-bad over time. With shades of gray: I had an amplifier catch fire, and on autopsy I discovered one joint had never seen solder, just lug-wrapped well enough to "contact" until tarnish grew thick. I've seen large joints with just a hair-line of actual contact, which held in until a few good slams broke the solder-hair.

Solder trouble led AT&T to wire-wrap, more reliable than soldering on jack-frames.

Reduction of soldered joints was a main motivation behind "microcircuits". Complex assemblies assembled on ceramic wafer and semi-automatically welded (not soldered). The result was Integrated Circuits, specifically mask-grown Silicon wafers where internal joint integrity was defined by process, not individual joinery.

While a "blown transistor" is possible in power amps or hot-rodded small audio, in DIY without whip-cracking Inspectors, bad or marginal solder joints are far more likely.