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Old 12th February 2008, 04:14 AM   #1
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Default KT88: EH vs. Winged C

I designed a KT88 PP amp and used EH KT88s. I love them. They seem to be able to take high voltages well and sound great. Recently, I have been looking at 'Winged C' Svetlana KT88s as well. I wanted to try them out, but wanted to get some feedback before I blow a bunch of money on them.

In particular, I was wondering if anyone had used these in higher voltage applications (>500V). I like the way they look, but they do cost more than the EHs, and I'd like to know if that additional cost will translate into additional reliability or at least the same reliability as the EHs.

I would be particularly interested if there is anyone out there who has dealt with a large number of these tubes and has a good idea of how statistically reliable they are. (e.g. we get 1 return for every 50 that we sell)

Oh yeah, and do they sound better?

Thanks in advance,

SS
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Old 12th February 2008, 05:13 AM   #2
OzMikeH is offline OzMikeH  Australia
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I stumbled over this last night. Mostly about old tubes but I think you will find some useful info in here.

http://www.jacmusic.com/KT88/kt88.htm
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Old 12th February 2008, 05:35 AM   #3
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Sofar have clocked up 4750 hrs on an event meter with winged C in an amp with quiescent current of 80mA per tube at 265V B+ (22W quies and 32W full load dissip ). This may seem lowish but I run most of my tubes with low B+ in parallel pairs and high quiescent current.
Nothing unusual with the quiescent side has drifted over time and I might have to wait a long MTBF.

However, on my other amp which runs at 530V B+, the KT88's "C" gm's can vary considerably if unmatched and I would double check the gm so they all track sim signal current. I find them well-made with thicker glass than NOS GEC and can accept a hefty tap test. Make sure not to strain the screens as I have lost a couple on hook-ups. >500V ballpoint does seem to be an itchy UL operating voltage and your post isn't unusual.
My constructional philosophy is to fit lower-rated screen resistors to act as fuses.


richj
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Old 13th February 2008, 06:04 AM   #4
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Thanks OzMikeH, there was some good information there.

Anyone else have any experience with these besides richwalters. I ask because I am considering doing my next amp as a 500V fixed bias triode strapped setup and want to know which tubes would be best up to the task.
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Old 15th December 2009, 12:16 PM   #5
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Quote:
Originally Posted by richwalters View Post
Sofar have clocked up 4750 hrs on an event meter with winged C in an amp with quiescent current of 80mA per tube at 265V B+ (22W quies and 32W full load dissip ). This may seem lowish but I run most of my tubes with low B+ in parallel pairs and high quiescent current.
Nothing unusual with the quiescent side has drifted over time and I might have to wait a long MTBF.
Hi rich,

just curious why the lower B+ voltage and higher bias current if you mind?

Cheers Michael
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Old 15th December 2009, 12:58 PM   #6
pointy is offline pointy  United Kingdom
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https://www.watfordvalves.com/products.asp?search=kt88

watford valves invented cryto treatment and a lot of test equipment.
if you read between the lines it may give some idea.........they charge more for cyrto treated.
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Old 16th December 2009, 12:22 AM   #7
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Hi Pointy,

I visited the site you suggested, I have read and surfed throught that site for a while and I can't really find what what you are pointing at. Where should I start read?
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Old 16th December 2009, 03:09 AM   #8
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Quote:
Originally Posted by Ultima Thule View Post
Hi rich,

just curious why the lower B+ voltage and higher bias current if you mind?

Cheers Michael
Biassing so Running near closer to class A operation gives much better HF response with lower THD at 10Khz. YOu will find alot of yesterday tubes where run with higher Iquies (they were designed for harder operation). I like pairs, it makes o/p tranny design easier.
Hope this helps.

BTW; I've haven't noticed any difference using cryo treated tubes compared to those produced by normal manufacturing processes. It' a gag.



richy
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Old 16th December 2009, 03:16 AM   #9
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Quote:
Originally Posted by SpreadSpectrum View Post
I designed a KT88 PP amp and used EH KT88s. I love them. They seem to be able to take high voltages well and sound great. Recently, I have been looking at 'Winged C' Svetlana KT88s as well. I wanted to try them out, but wanted to get some feedback before I blow a bunch of money on them.

In particular, I was wondering if anyone had used these in higher voltage applications (>500V). I like the way they look, but they do cost more than the EHs, and I'd like to know if that additional cost will translate into additional reliability or at least the same reliability as the EHs.

I would be particularly interested if there is anyone out there who has dealt with a large number of these tubes and has a good idea of how statistically reliable they are. (e.g. we get 1 return for every 50 that we sell)

Oh yeah, and do they sound better?

Thanks in advance,

SS
I know everyone has their own opinion on tone quality, but I find the SED (made in St. Petersburg) KT-88 unlistenable. It has no top end, it has muddy, one-note bass. I think it's awful. I don't even carry it for sale.

I like the KT-88 EH - I wish it had a tad more bass, but it's quite nice - especially for the price.
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Old 16th December 2009, 03:33 AM   #10
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+500V for some newish brands is unknown terroritory: vacuum isn't as hard as it used to be and various other cut short issues related to facilate production runs- biassing gets more critical esp as myself who like to run tubes fairly hard. It's easy to ruin the screens of a pair of Svetlana KT88 (black circle with C in middle) by pushing them too hard at 550V was exactly what happened by overdrive but thats just myself.; These BTW start to show reddish anodes at 530V @ 60mA Iquies and a tad more, the Iquies
seems to vary and unsettle as if a lower grid leak is required. Behave like 6550's ? they nearly all look the same construction.
JimMcShane mentions the EH versions seen a better bet. I'd agree with him.More robust.

r):
richy
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