The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    KT88 poor service life

    Share
    avatar
    LeGrace

    Posts : 218
    Join date : 2016-08-07
    Location : Ontario, Canada

    KT88 poor service life

    Post by LeGrace on Thu Aug 31, 2017 7:41 pm

    Lost a second Genalex KT88 today, second one to blow in under a year. Getter was consumed same as with previous one. Don't think I'll be buying anymore of those puppies. Evil or Very Mad
    avatar
    Tubes4ever

    Posts : 96
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: KT88 poor service life

    Post by Tubes4ever on Thu Aug 31, 2017 10:51 pm

    Sounds like you lost the vacuum all at once. Any visible cracks?
    avatar
    jfine

    Posts : 82
    Join date : 2017-06-19

    Re: KT88 poor service life

    Post by jfine on Thu Aug 31, 2017 10:58 pm

    LeGrace wrote:Lost a second Genalex  KT88 today, second one to blow in under a year. Getter was consumed same as with previous one. Don't think I'll be buying anymore of those puppies.  Evil or Very Mad    

    Weird how you are going thru those, I use those too and so far no issues.
    avatar
    peterh

    Posts : 837
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: KT88 poor service life

    Post by peterh on Fri Sep 01, 2017 2:58 am

    LeGrace wrote:Lost a second Genalex  KT88 today, second one to blow in under a year. Getter was consumed same as with previous one. Don't think I'll be buying anymore of those puppies.  :evil:    
    Do you "roll tubes"? Tensions at the socket may result in cracks when removing tubes.
    avatar
    LeGrace

    Posts : 218
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: KT88 poor service life

    Post by LeGrace on Fri Sep 01, 2017 9:34 am

    I cannot discount the possibility that this could to a certain degree be self inflicted from rough tube handling. Starting out as a total novice I was not knowledgeable of proper tube handling technique. Good news is MTBF is steadily improving as I have gradually adopted helpful tips shared in this forum. Genalex may deserve an apology.
    avatar
    Peter W.

    Posts : 458
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: KT88 poor service life

    Post by Peter W. on Fri Sep 01, 2017 11:43 am

    LeGrace wrote:I cannot discount the possibility that this could to a certain degree be self inflicted from rough tube handling. Starting out as a total novice I was not knowledgeable of proper tube handling technique. Good news is MTBF is steadily improving as I have gradually adopted helpful tips shared in this forum. Genalex may deserve an apology.    

    Tubes, like any other commodity shipped long distances, should be able to withstand a protected 2-meter drop (bit over 6' in the old currency). Protected, being in proper packaging to reduce the possibility of glass breakage. I doubt anything you may have done at home approaches this level of stress. Accordingly, I suspect something else other than 'rough handling'.
    avatar
    LeGrace

    Posts : 218
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: KT88 poor service life

    Post by LeGrace on Fri Sep 01, 2017 12:35 pm

    Kinda anticipating some push-back on your reply above. What I've read is that rocking the tubes back and forth to remove them is a no-no. Creating stress on the pins which can in turn induce micro leaks. Both failures were clearly loss of vacuum as opposed to ageing related?
    avatar
    Peter W.

    Posts : 458
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: KT88 poor service life

    Post by Peter W. on Fri Sep 01, 2017 12:47 pm

    LeGrace wrote:Kinda anticipating some push-back on your reply above. What I've read is that rocking the tubes back and forth to remove them is a no-no. Creating stress on the pins which can in turn induce micro leaks. Both failures were clearly loss of vacuum as opposed to ageing related?

    OK let 'em push.

    I have a set of GE pre-blight 6550s that have been around the block a few times. I have had occasion to do a forensic dissection of such a tube, and the base is set up such that unless one pries using the glass envelope, there will be no stress on the tube base. I have experienced broken leads, or leads that have pulled loose from the pin, or grid-caps that have pulled loose - all of which have responded to soldering. And I have seen leads broken off right at the glass from stress. But (at least in pre-blight US-origin tubes), I have not yet seen an actual vacuum leak from this sort of behavior.

    The recent group of Tung-Sol KT88s I tested for a neighbor did seem solidly-built enough, and although of the group, only a very few tested 'as new' per my tester, they did not set off my overly-fragile meter. I still don't think you are necessarily to blame, unless you used a full rocking-horse motion when removing and installing the tubes.
    avatar
    deepee99

    Posts : 1693
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: KT88 poor service life

    Post by deepee99 on Sat Sep 02, 2017 11:20 am

    LeGrace,
    How are you grabbing your tubes when you pull them?
    There's kind of a knack with pulling big octals. First, never do it while they're hot or even warm. They won't like it and you'll get 3rd-degree burns.
    You should be putting equal torque on both the glass and the base, which means wrapping your hand around the whole smash - or at least the bottom third of it.
    If the tube doesn't readily come out, which is actually a good thing, instead of rocking it, consider sliding a little jeweler's screwdriver (power disconnected) between the base and the socket, and prying it loose. Go around the tube like you would with head-bolts on a car until it's up a bit, then grab it and gently pull.
    avatar
    bluemeanies

    Posts : 193
    Join date : 2015-02-09
    Age : 67
    Location : Folsom Pa.

    Re: KT88 poor service life

    Post by bluemeanies on Sat Sep 02, 2017 10:19 pm

    Have not had a problem with the Sovtek KT88's in almost 2years
    avatar
    LeGrace

    Posts : 218
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: KT88 poor service life

    Post by LeGrace on Sun Sep 03, 2017 8:14 am

    Could a marginal tube be behind the wonkiness I've been experiencing with this amp? Two rectifier red plates on same amp, both with this tube on board. The second red plate (July 16) I noticed my left side bias (yup pair w/failed tube) had dropped really low which is what I believe triggered the red plate. The bias reading on this side has always been erratic. With new tubes in there now appearing more stable.
    avatar
    deepee99

    Posts : 1693
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: KT88 poor service life

    Post by deepee99 on Sun Sep 03, 2017 9:35 am

    LeGrace wrote:Could a marginal tube be behind the wonkiness I've been experiencing with this amp? Two rectifier red plates on same amp, both with this tube on board. The second red plate (July 16) I noticed my left side bias (yup pair w/failed tube) had dropped really low which is what I believe triggered the red plate. The bias reading on this side has always been erratic. With new tubes in there now appearing more stable.

    LeGrace, just spit-balling here, but I'm still thinking you've got a problem on that driver board somewhere. Odds of multiple tube failures on the same side are pretty long.
    avatar
    LeGrace

    Posts : 218
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: KT88 poor service life

    Post by LeGrace on Sun Sep 03, 2017 9:48 am

    Darn gremlins. Next time I go to open up the amp (new RCA's to install) I'll take another close look at the driver board. What is recommended procedure for cleaning the bias pot?
    avatar
    deepee99

    Posts : 1693
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: KT88 poor service life

    Post by deepee99 on Sun Sep 03, 2017 9:55 am

    LeGrace wrote: Darn gremlins. Next time I go to open up the amp (new RCA's to install) I'll take another close look at the driver board. What is recommended procedure for cleaning the bias pot?

    Wouldn't know and wouldn't try. Just buy a new one and solder it in. They cost 99 cents at Radio Shack - make sure you get one of the same resistance values, though. Not sure what they are but Bob or Roy would be able to tell you. That'll give you a chance to re-inspect your solder connexions, too.
    avatar
    jfine

    Posts : 82
    Join date : 2017-06-19

    Re: KT88 poor service life

    Post by jfine on Sun Sep 03, 2017 6:26 pm

    deepee99 wrote:

    Wouldn't know and wouldn't try. Just buy a new one and solder it in. They cost 99 cents at Radio Shack - make sure you get one of the same resistance values, though. Not sure what they are but Bob or Roy would be able to tell you. That'll give you a chance to re-inspect your solder connexions, too.

    All the radio shacks around here have vanished.
    avatar
    jfine

    Posts : 82
    Join date : 2017-06-19

    Re: KT88 poor service life

    Post by jfine on Sun Sep 03, 2017 6:32 pm

    LeGrace wrote: Darn gremlins. Next time I go to open up the amp (new RCA's to install) I'll take another close look at the driver board. What is recommended procedure for cleaning the bias pot?

    Couldn't you deoxit the bias pot? After all this I would probably try to reflow any solder connection I could access. I had a cold solder on a dynavector phono stage that was intermittent and drove me %!$@ batty. One channel would get louder than the other channel, and the weaker channel would partially distort. Just randomly would act up, no pattern, and wouldn't do it for long. The only way I found out where it was is using a toothpick and pushing on every solder joint while spinning a record. Factory solder job that was!
    avatar
    deepee99

    Posts : 1693
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: KT88 poor service life

    Post by deepee99 on Sun Sep 03, 2017 6:45 pm

    jfine wrote:
    LeGrace wrote: Darn gremlins. Next time I go to open up the amp (new RCA's to install) I'll take another close look at the driver board. What is recommended procedure for cleaning the bias pot?

    Couldn't you deoxit the bias pot? After all this I would probably try to reflow any solder connection I could access. I had a cold solder on a dynavector phono stage that was intermittent and drove me %!$@ batty. One channel would get louder than the other channel, and the weaker channel would partially distort. Just randomly would act up, no pattern, and wouldn't do it for long. The only way I found out where it was is using a toothpick and pushing on every solder joint while spinning a record. Factory solder job that was!

    Been there, done that, got the T-shirt. When I put the screwdriver to the bias pot, all was well on the Fluke because it forced a solid connexion. Then, when I pulled the screwdriver off, the pot's connexion with the board would go pear-shaped. Cold-solder joint. Off and on, off and on. Good way to fry the outputs without prior notice. Took months to track that down.
    Yeah, what jffine said: pull that driver board and re-flow everything. Unless you've got a bad jumped connexion to a pair of tube sockets, this is likely where you'll find it.
    Otherwise, get one of these: https://www.adafruit.com/product/356?gclid=EAIaIQobChMImcfMzYyK1gIVUpR-Ch1v3QFVEAQYAiABEgJZ2vD_BwE
    but again, make sure you're getting the right values.

    wildiowa

    Posts : 167
    Join date : 2012-03-19

    Re: KT88 poor service life

    Post by wildiowa on Mon Sep 04, 2017 10:54 am

    I'm staying out of this one....
    avatar
    LeGrace

    Posts : 218
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: KT88 poor service life

    Post by LeGrace on Mon Sep 04, 2017 11:37 am

    Thanks for advice guys. Since swapping in a new pair of 88's I'm encouraged by something I'm not used to seeing, stable and highly repeatable bias values. For past couple months have been checking and logging readings daily. Quite bouncy before. But since the tube swap no longer jumping around, albeit only 4 days so far...
    avatar
    deepee99

    Posts : 1693
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: KT88 poor service life

    Post by deepee99 on Mon Sep 04, 2017 11:54 am

    wildiowa wrote:I'm staying out of this one....
    Why?
    avatar
    Bob Latino
    Admin

    Posts : 2555
    Join date : 2008-11-26
    Location : Massachusetts

    Re: KT88 poor service life

    Post by Bob Latino on Mon Sep 04, 2017 12:12 pm

    LeGrace wrote:Since swapping in a new pair of 88's I'm encouraged by something I'm not used to seeing, stable and highly repeatable bias values.

    1. Remember that your bias goes UP and DOWN as your line voltage goes up and down. This is characterized by ALL the tubes being slightly high or slightly low from the previous day. Even if your line voltage goes up or down 1/2 volt, this will make a noticeable difference in what you read for the bias.

    2. Also bias will rise slightly from a cold amp over the first 1/2 hour or so. If you know that your bias is "good", there is no sense in even checking it when the amp is cold. Make your final bias setting when the amp is fully warmed up.

    3. If your amp has ONE TUBE that has a wandering bias (up or down) either that tube is bad or the socket that tube is in has an issue with socket pin contact with the tube. Try the tube in another socket. If the bias on that tube wanders in another socket, then most likely that tube is bad. If another tube has bias issues in that same socket, then probably the issue is with the socket. Try retensioning the socket pins in that socket.

    Bob

    wildiowa

    Posts : 167
    Join date : 2012-03-19

    Re: KT88 poor service life

    Post by wildiowa on Mon Sep 04, 2017 2:26 pm

    I jumped in on a similar post a few weeks ago with comments on sources and brands of KT88's and 6550's and kinda got taken to the woodshed...so you guys can take it from here! In the past I had a couple of Genalex KT88's take a dump but it could have been a flaw in the amp...still using that brand, no problems recently.
    avatar
    LeGrace

    Posts : 218
    Join date : 2016-08-07
    Location : Ontario, Canada

    Re: KT88 poor service life

    Post by LeGrace on Mon Sep 04, 2017 4:25 pm

    wildiowa wrote:I jumped in on a similar post a few weeks ago with comments on sources and brands of KT88's and 6550's and kinda got taken to the woodshed...so you guys can take it from here! In the past I had a couple of Genalex KT88's take a dump but it could have been a flaw in the amp...still using that brand, no problems recently.

    Nice to know I'm not the only one. I love you
    avatar
    jfine

    Posts : 82
    Join date : 2017-06-19

    Re: KT88 poor service life

    Post by jfine on Mon Sep 04, 2017 7:34 pm

    wildiowa wrote:I jumped in on a similar post a few weeks ago with comments on sources and brands of KT88's and 6550's and kinda got taken to the woodshed...so you guys can take it from here! In the past I had a couple of Genalex KT88's take a dump but it could have been a flaw in the amp...still using that brand, no problems recently.

    Well I don't know about anyone else but I can always use someone's genuine experience, put em on your blocked user list like I did, problem solved.

    Sponsored content

    Re: KT88 poor service life

    Post by Sponsored content


      Current date/time is Thu Sep 21, 2017 7:12 pm