Fisher KX-100 crackles and smells, help

[Calvink100]

Hello Everyone,

New to the site, thought I would ask everyone here their opinion before I start hating my amp! I bought a restored Fisher kx-100 back in December. I had many months of problem-less use, until recently the right channel started making crackling noises. Then one day an output tube popped, but still worked. The next day the other out put popped also, but still worked. About this time I noticed it starting to smell sort of bad. Its not horrible but it the usual tube amp smell x10. Any opinion to whats going on? A local stereo shop told me my output transformer went and that i should just junk it. He suggested turn it on and feel for a temperature difference between the transformers. Any suggestions on what to do? Is the repair guy crazy? Are my vintage fisher tubes just dead? Or is it a much bigger problem?
Any thoughts suggestions would be a huge help! Thanks!

[gogofast]

kx-100 is a decent starter tube integrated with great fisher sound. no matter what, dumping it is a bad idea!!!:no: you can buy OP transformer off ebay for around $40. or you can sell various parts from your broken unit for maybe more than what you paid for the whole thing.:yes: (though not suggesting you do this).

running a tube amp while one tube fail is the worst thing you can do!! that's what probably caused the output transformer to go out also. those 7591 pairs must be matched and properly biased. the seller could've just used newer production tubes without adjusting the bias accordingly. if that's the case those tubes can die within a few weeks or months of use. if the OP transformer is really out, you should hear no sound at all from that channel.

try looking for a single OP transformer on ebay or ask someone here they have it. POST pictures of your amp's inside and we can tell what kinda restoration was done and whether it was properly done.

your stereo shop guy probably has no idea what he's talking about or not aware of vintage amp scene.

[dspear99ca]

Calvink100:

Welcome to AK.

If you're still getting sound, the output transformers (note plural) are probably NOT junk. What was done as far as "restoration"? What is your skill level? When you say "popped" what does that mean? Did it make a noise like a spark jumping a gap like the electronic igniter on a gas stove? Did it fail to light up afterwards? What happened to the volume level after all this "popping" subsided, if in fact it did subside? More information is required to correctly diagnose your problem. Depending on your skill level and ability to learn and the complexity of the design, most vintage tube amps are extremely simple to troubleshoot and repair. You'll need a voltmeter, do you have one? Are you comfortable working around high voltages (>400VDC, 750VAC)? If you are not, your best bet is to find a tech (I'd recommend avoiding the one who told you to junk it) familiar with tubed gear to help you out.

I don't mean to sound condescending, but does it not occur to you that when something smells like it's melting or burning, perhaps you should turn it off and not use it any more until you've gotten it fixed? Or even go real crazy and unplug it? I see more repair jobs come in which were compounded by a combination of stupidity, tenacity, and perseverance which cause a $0.50 failed capacitor turn into a $400 repair job. "The smoke was distracting, but the picture was really clear and oh, the sound, it was glorious!"

Dave

[Calvink100]

Wow the response time here is great! I did not do the restoration on this amp, I bought it off e-bay, which in hind sight was not a good idea but we live and learn. It was one of those "fisher restoration kit specials". Do not get me wrong, its clean and sounded fantastic. The info said it had an auto-bias on it. I will try and find the camera to take a picture of the underneath. As for the channel it is alive and well, just crackly. Trust me I did not play it long! It did crack, with a green flash, and the other tube went shortly after with similar results. Both tubes continued to glow, just more orange. Weeks before that, it sounded like an ocean in that channel from time to time. After the crack I turned it off, then just tried i t again that night. I had it on for nearly 5 min until i smelled "the smell" and turned it off. No smoke though. That was 2 weeks ago. Have not touched it since and though I am enjoying my Harmon-Karmen solid state's low end, its no Fisher mid and highs, thats for sure.

I am really a novice, at 21 I am a new young member to the tube world but I am in love with vintage amps and would like to learn the ropes of diagnosing tube amp problems, so thank-you for your time and any help is most welcome! I will post a picture of the amp, and my system if anyone is interested Thanks!

[dspear99ca]

Auto bias means the bias is set using a cathode resistor. Generally there's a cathode resistor for each pair of tubes... probably to save money. Some Magnavox designs and others even used a single cathode resistor. I guess you didn't happen to notice if the tubes were "red-plating" prior to the problem, did you? I'd check the cathode resistor, it'll be a 5W or 10W square power resistor and make sure the value checks out, then I'd FOR SURE suspect the bypass cap. This will be a 20-50uF cap (one for each PP pair) often found in one of the can caps. If the tubes arced they undoubtedly have sustained some damage, how much it's tough to say. I'd also check the coupling caps. Any restoration worth a damn would have included replacement of the coupling caps, though, as they're especially prone to leaking when they get that old.

Dave

[fsjonsey]

I also own A KX-100. One difference I noticed off the bat is that your amp does not have a bias control, yet the hole in the chassis for the bias pot seems to be there, just plugged. Mine has the bias control.

If the 7868's are original, the ones that haven't failed yet are probably very tired. Fisher tended to push their output tubes hard. Luckily, the tubes are being reproduced by Electro-Harmonix and are very affordable. Are the electrolytic can capacitors getting hot to the touch? If so, they are bad and need to be replaced. If the amp smells, something is getting hot, probably a resistor. I'd replace the bias capacitor, order a new quad of tubes, and check the output stage and power supply for any bad or drifting components.

[gogofast]

from the picture, they look like older stock OP tubes (which is good), but not sure. what tubes are they? if they're newer production tubes, i'm 90% sure it's the wrong bias issue. you'll have to increase the cathode bias resistor value to more than double probably - you'll have to try and adjust. it's like an automatic procedure when using newer production tubes in any amp.

[Calvink100]

Yes, I thought some one capped that! What the hell biasing can't be that hard, maybe I could change that back. Yes they are vintage tubes, 3 fishers and a Sylvania. Yea $96 for a quad is not to bad considering I am pretty sure the two fried in the lighting show they put on haha. So a bad bias capacitor would fry two out put tubes by over loading them? Would you suggest I return the biasing to the original setup?

[Dave C]

Don't give up on this one, you're just going through a learning curve. You need to remove the amp from your system and flip it over and look for discolored parts to see what got hot. If you lost a transformer it would probably be dripping wax out the bottom, the picture doesn't show this. I think you have an output tube problem that was caused by something close to them in the circuit. Keep the faith, it looks like you have a great amp!

[gogofast]

Yes, I thought some one capped that! What the hell biasing can't be that hard, maybe I could change that back. Yes they are vintage tubes, 3 fishers and a Sylvania. Yea $96 for a quad is not to bad considering I am pretty sure the two fried in the lighting show they put on haha. So a bad bias capacitor would fry two out put tubes by over loading them? Would you suggest I return the biasing to the original setup?

need to see under the chassis first.

that sylvania replacement could've been the mismatched tube that caused all this.

each channel should have a cathode resistor which is connected to one OP tube and then between two tubes with a wire. try measuring plate current by using a multimeter with ground to the chassis and + at the pin that the cathode resistor is connected to (set the reading to 200), divide the reading by the value of the resistor, divide that by two and times 1000, and then you have a plate current in milliamp for each tube. it should be around 35ma. if little higher, that's fine for older stock tubes, but shouldn't be too high. when you use newer stock tubes, these readings will go way up with existing resistor value - then you'll need to increase the value. if the problem was biasing, it'll be a simple fix, but it could've been one of many other things. so show us the under chassis first.

[gogofast]

by the way, your amp does look really nice. one of the cleanest fishers i've seen. that repairman suggesting you junk it is just CRAZY for sure.

[Calvink100]

Sorry about getting the pics so late, heres the chassis. I looked around for anything burn or suspicious looking, theres nothing. Thanks for the advice. I have a multimeter and over the weekend I will try what you suggested. Its finals week right now haha time is a little tight! Thanks for the help every one!

[Calvink100]

Thanks for the compliments on my amp!

[schwarcw]

Nice amp Calvin. Nothing wrong with that amp that a good technician can't fix. The guy you took the amp to initailly must be a real idiot. Most good technicians respect nice vintage gear as an art form. Good luck!

Carl

[fsjonsey]

Are you still using any of the bad output tubes? It would be much safer to invest in a new quad of EH's. I have a feeling the reason so many of these fishers fry their output transformers is due to the use of weak output tubes being pushed way past their service lives.