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What the hell? Tube Related

yungin's-dad

yungin's-dad

Last of the Baby Boomers
I am having a problem with one of my Quicksilver KT88 Mono's again. When I start one of them up I get a a quick flash in the rectifier tube, then it goes away. It's a single arc that I can see plainly when right before it energizes. It took out one fuse, and I'm pretty sure one rec. tube.

Any thoughts?

Thanks,

Ron
 
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I'd be looking at the bias circuit in that amp.

That, or maybe a bad supply cap, are the most likely causes...

8417s (if that's what's in them) are some pretty temperamental b***ards... if that bias circuit ain't perfect, they'll throw a s***-fit...

Regards,
Gordon.
 
I had a similar problem with my Mark IIIs. My problem was bad output tubes though. I had replaced the bias circuit and the filter caps. Finally traced it to bad output tubes.
 
A flash in the rectifier tube usually indicates a very heavy load. That could be caused by very leaky (AKA shorted) filter capacitors. A bad output may cause it as well. When the amp is working, do the filters get warm? They should not get warm unless they are being heated by something hot near them like a tube or power transformer. Filters should be cool.
 
GordonW said:
I'd be looking at the bias circuit in that amp.

That, or maybe a bad supply cap, are the most likely causes...

8417s (if that's what's in them) are some pretty temperamental b***ards... if that bias circuit ain't perfect, they'll throw a s***-fit...

Regards,
Gordon.
Click to expand...

No Gordon, these are the series II KT88 amps. They came out after the 8417's, due to those unobtainium tubes. This is getting kind of old! I guess I need to save my pennies and get them overhauled. If I knew more about tubes it might be different. It seems to be running fine, it's just right at start up. One arc on a post coming out of the top plate, I assume this is a heater, but I don't know for sure, it glows.

Thanks everyone!

Ron
 
byoung334 said:
Are you sure the rectifier tube socket is not "loose" where the pins insert?
Click to expand...

Very good question!!!!! I will check and report back. I have always been taught to check all the easy stuff first when troubleshooting and as usual my mind ventures off to all the tough things.

Thanks by334!

Ron

Oh yeah..... Thanks Mod's, for helping a thread posting challenged knucklehead!
 
I'm gonna lean towards a cap too..

I don't have that problem on my quickies, but one of my amps will every now and then cut out, and if I knock a few times on the top of one of the caps, it will come back and be fine..

I know the caps have been replaced, but I'm sure it was years ago, and I'm sure they've dried again.. It's one of the two large caps on the top of the amp.... I've checked and reflowd all the solder in the amp, but the problem didn't change.. Hit that one cap, and life is good. Rapping elsewhere doesn't fix it..

I don't have the flash problem you do, but I'm sure if I keep ignoring the problem I will eventually be like you...

Just something to consider..
 
Oh yeah..... Thanks Mod's, for helping a thread posting challenged knucklehead!
Click to expand...

I didnt even post on this thread until now.:D

Good luck with the fix...at some point I almost gave up tubes as I was tired of the "gremlins"..
 
Had a similar problem.........

I had a Cary/AES amp that would flash both the rectifiers on startup. I sold it and the buyer put in new KT88s and never had a flash problem again. So, it could be your output tubes. (Mine tested fine) Do you have another set to try out and see if there is flash with them?
Hope you figure it out. I don't like the feeling you get when you want to power up your amp when it acts up like that.

Larry
 
Replace the rectifier first, more then likely a simple rectifier gone bad, the usual.

While cleaning the sockets is always a good idea, it won't help a rectifier arcing.

And I haven't forgotten about you, just been busy around here, ;)
Keg
 
You can see in my avatar that I've used these amps before and I can say they are very very very hard on rectifiers. Chinese, russian ect tubes have no chance in them, only the GE 5AR4's. I modified my power supply because these amps eat rectifiers.
 
What rectifier does it use? If it's a Sovtek 5AR4 I'd lean toward that being the prob. Hey, why not ditch those old tech glass diodes with some state of the art silicon ones.:yes: Sorry LPD overpost, but it sounds like we were thinking the same thing about the sovteks.
 
haha jink you owe me a tube!!! If I recall.....this amp had ridiculous amounts of capacitance of of the rectifier, potentially a ticking time bomb. I blew up a few rectifiers before I began a quest to save time and money. I first tried a variac, ramping voltage slowly, worked fine but hell who wants to spend time ramping up an amp. I think I put a diode in front of the tube rectifier to take the current spike. Not my idea, I highjacked it from a fella on the diy site. Even with a good rectifier these amps have a ps that is designed to fail.
 
I wonder if a CL90 between the rectifier out and all that capacitance would help? They do make them in voltages that high I think.
 
I also put separate bias taps for each output tube as well. I believe there is 800+mf on the rectifier, its just plain overkill!!! Put a diode in series with the rectifier, and it will take the surge, and the tube will filter out the crap. You could also rewire smaller cap as first cap, which is what some people do. These are great sounding amps. What are your driver tubes?
 
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