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 Post subject: Re: KT120 Oddblocks
PostPosted: 11 Dec 2016, 06:37 
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Joined: 25 May 2016, 16:30
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Being new to KT120s I just want to double check what I'm getting is not some design feature of the KT120. What I get is a red vertical line on the back of the anode with the indent mounted on the left hand side. The red line is in pretty well the same position on all four KT120s I have


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 Post subject: Re: KT120 Oddblocks
PostPosted: 11 Dec 2016, 09:26 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, I have no way to tell if your tubes might be either "seconds" that were sold as full spec or perhaps even fakes. I have not personally seen any fakes yet but it is always possible. Both issues have cropped up from time to time with KT88s though. If you got them from a trusted source I would probably ask for a new set. If they came from Ebay or some essentially unknown source you may be stuck. In that case I would just use them and hope they don't do anything strange. As a precaution I would probably put a fuse in the B+ feed to the output transformers. A 500 ma slow blow or 750 ma normal one (one for each channel). If a tube should fail it would be likely to open before you cooked the LM317s. Going back to the KT88 issue though....I (and Oddwatt Audio that I am half owner of) have discovered that not all KT88s live up to specs. There were some early EH KT88s that would not stay balanced (I believe the newer one do but have not checked it) and several Asian KT88s including some premium ones fail. Often within minutes. Most were sold by reputable companies and could well work in other applications. Since a large majority of such tubes are used in push-pull class AB designs the stresses on the tubes are different than from class A designs. That may account for the problems. Tubes that are marketed on places like Ebay as NOS premium vintage ones are particularly suspect as many have been found to be re-marked lessor tubes. It is unfortunately a buyers beware situation.

EDIT: The tubes should not have any red on the anodes. A very tiny amount is probably of no consequence, more than about a square centimeter or so especially if concentrated in one spot is likely too much and I would reduce the current to get rid of it. BTW, my personal test set of KT120 demo amps is actually running at 450 B+ and 145 ma per tube. They have done so for over three years now and have well past 1000 hours on the tubes (probably close to twice that but I stopped listing the hours a while back) and still are perfect. I expect tube life to be between 3000 and 5000 hours but in all the 8 years I have been using variations of the amps none has ever worn out a set of tubes. The first amps using this design (actually from 2007) with EL84s had daily use (8-9 hours in my office) for 6 years and are still just fine (that BTW for the math folks is a huge number of hours) . This is by design as while the tubes in all class A designs run hot, if you keep the dissipation lower than 90% (preferably 85%) they last a long time.


Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 11 Dec 2016, 18:59 
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Interestingly Jan G is experiencing the same red lines on I think two of his valves rather than four I've tried (but on one amp). All our tubes came from reputable, trusted sources, mine from a well-known German supplier and his from a well-known English supplier, so that would make seconds or a bad batch unlikely. I will try lowering dissipation but, like you, feel inclined that I should ask for replacements


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 Post subject: Re: KT120 Oddblocks
PostPosted: 12 Dec 2016, 16:30 
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OK. Dissipation is now at 124ma and I'm still getting red spots both pairs. In all cases it's a red line on the back or front of the plates in relation to the indent of the valve base. Something is not right........


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 Post subject: Re: KT120 Oddblocks
PostPosted: 13 Dec 2016, 00:43 
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Maybe a photo will say more than description?


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 Post subject: Re: KT120 Oddblocks
PostPosted: 13 Dec 2016, 05:35 
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Location: Chilliwack, BC
Every manufacturer now and again will experience a "Made 4:30 on a Friday afternoon" lot of product. See if you can get replacements from another run?

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 Post subject: Re: KT120 Oddblocks
PostPosted: 13 Dec 2016, 21:09 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Everyone, The red anodes are not normal. I have been communicating off line with someone having that problem and it is presently ellusive in nature. With a combined anode and screen dissipation below 60 watts (they are supposed to handle 68) they should not behave this way. I checked my amps last night in the dark and there is no glow and mine are at 145ma per tube ....a bit higher than the ones posted above that are glowing and indeed more than I recommend. Mine are early "prototype / demo" amps and I have not bothered to alter them for the lower setting. I have seen photos of the glowing amps. So far the only explanation that fits the data is that the tubes are not full spec. They were obtained from reliable European sources so this is even a bigger puzzle. Has anyone else run into red anodes when operating at dissipations below the max? Perhaps we can identify why some apparently do. As a related topic on the commercial side I have had difficulties with two supposed to be reliable suppliers and have received tubes (KT88s) that (a) were supposed to be matched and were not and (b) ones that failed shortly after use. So it is possible to have a bad batch. The supplier might not even know if they were and if they were matched at the factory then they might not know they were not matched either. I give them the benefit of doubt.

I just hope that Tungsol has not decided to cut corners and is putting out sub standard tubes. The originals are really super.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 14 Dec 2016, 16:48 
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Joined: 25 May 2016, 16:30
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[img][IMG]http://i1380.photobucket.com/albums/ah169/ADAudio2014/image-3_zpsdjxkgvr2.jpeg[/img]/img]
Image
Image
http://s1380.photobucket.com/user/ADAudio2014/media/image-2_zpszjdxzojn.jpeg.htmlHere' some photos. In general the 'red lines' are at the back of the tube relative to the indent. Only on one tube is the red area at the front, and on that one there is no red at the rear


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 Post subject: Re: KT120 Oddblocks
PostPosted: 26 Feb 2017, 17:08 
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Joined: 18 Jun 2016, 06:52
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I would like to try the Oddwatt in triode mode, and if I like the result, I would install a triode/UL switch (in fact switch plus relay). Can I just disconnect the UL tap and connect the screen grid to the plate, or do I have to install an extra resistor or another component to avoid upsetting the circuit?


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 Post subject: Re: KT120 Oddblocks
PostPosted: 26 Feb 2017, 17:54 
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Hi, I would attach the screen via a resistor to the anode. Something in the 470 ohm range would seem appropriate. I recall trying triode mode a while back and didn't like it as much as the U/L mode. Plus the loss in power output was not to my liking.

Good listening
Bruce

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