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1xKT88 Class A SE amp - build it or not ? What to do with these now ?
1xKT88 Class A SE amp - build it or not ? What to do with these now ?
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Old 17th September 2019, 05:02 PM   #1
Vortex is offline Vortex  Hungary
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Send a message via ICQ to Vortex 1xKT88 Class A SE amp - build it or not ? What to do with these now ?
Dear Forum members,

after having built many tube and transistor amps, my father designed me (on my request) a Class A SE tube amp.. or better said, 2 monoblocks for the 2 stereo channels (my idea too).

This was about ~10 yrs ago.

First we made a design inspired by the EAR-859 with EL509 tubes, but then after some feedback and specialities regarding tubes (including tube availability) we dropped this design and decided to go ahead with more common tubes and a new circuit design from scratch.

I still have the tubes but don't know what to do with them (1 monoblock here) :
- Input stage: 1x JJ E88CC (ECC88/6922/6DJ8 equiv.) (datasheet)
- Driver stage: 1x JJ ECC99 (datasheet)
- Output stage: 1x JJ KT88 (datasheet)

Tubes were bough directly from JJ at the time of thinking about building a tube amp, so .. about 10 years ago, all the same types are matched pairs.

Due to 2 moves since then, both of my output transformers damaged (my sister packed them wrongly, together with other "junk techie stuff" while I was living abroad) and recently when I wanted to revive the project I recognized damaged windings etc. so these hipersil EI core trafos have to be rewound again. No problem so far, father has just started winding in his free time, as a hobby, simple cores only (no toroid winding device).

What would you do if you were me ?

Option 1:
- use these tubes, design a good classic reliable Class A SE circuit with them and enjoy the tube sound
- with the unused E88CC double triode half you can drive the headphone output if you wish so
- allow plenty of headroom in the design to be able to replace these with KT150 tubes when the KT88-s are worn out

Option 2:
- sell them on eBay as NOS tubes (?) .. They're old (not THAT much, but still).. but never used yet. If they can be called "NEW" then it's fine too.
- make your dream come true and build those fully balanced pushpull-AB KT150 monoblock beasts with ~300W each

Option 3:
- buy 2 more of the KT88 tubes and make classic simple reliable pushpull Class AB monoblocks with the 4 KT88-s, enjoying sound and quite some more power (would the never used old pair be much different from the brand new pairs?)
- surprise your father with this amp for Xmas

Some other combination of above - or something else.

You might ask me now what I really need. I can only tell you: I don't need one more amp lying around. My real world vision could be an integrated stereo amp for my computer with 2 DIY larger bookshelf speakers (8" full-range with AMT tweeter, passive 2-way).

Any ideas are welcome (strange ones too)

Oh, I have 4x 6S33S-V (6C33C-B) also, ready to work. How about these ? Do they sound great in normal triode operation ? (First intended to be OTL output tubes based on Tim Mellow's article).
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Last edited by Vortex; 19th September 2019 at 08:40 AM. Reason: Little update and cosmetics..
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Old 18th September 2019, 01:49 PM   #2
kward is offline kward  United States
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My biggest amp is a 14 watt SE amp. I need a 60 watt push pull amp. Build baby build.

Now thatís what I would do. But I have no idea what you should do because Iím not you.
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Old 18th September 2019, 02:49 PM   #3
Bigun is offline Bigun  Canada
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1xKT88 Class A SE amp - build it or not ? What to do with these now ?
Your father will not live forever this is a great opportunity to build something with your father that you will treasure - it doesn’t matter what it is - you may find yourself listening to it 20 years from now so build it to last
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Old 18th September 2019, 03:46 PM   #4
OldHector is offline OldHector
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For those wondering what 'Hipersil' is ...
Hipersil, the myth and the truth
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Old 18th September 2019, 06:34 PM   #5
leadbelly is offline leadbelly  Canada
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I'm a bit suspicious that the transformers were damaged. May be just cosmetic damage.
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Old 19th September 2019, 08:41 AM   #6
Vortex is offline Vortex  Hungary
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Send a message via ICQ to Vortex 1xKT88 Class A SE amp - build it or not ? What to do with these now ?
Quote:
Originally Posted by kward View Post
My biggest amp is a 14 watt SE amp. I need a 60 watt push pull amp. Build baby build.

Now thatís what I would do. But I have no idea what you should do because Iím not you.
Great.. yeah.. got it
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Old 19th September 2019, 08:45 AM   #7
Vortex is offline Vortex  Hungary
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Send a message via ICQ to Vortex 1xKT88 Class A SE amp - build it or not ? What to do with these now ?
Quote:
Originally Posted by Bigun View Post
Your father will not live forever this is a great opportunity to build something with your father that you will treasure - it doesnít matter what it is - you may find yourself listening to it 20 years from now so build it to last
Omg, well said. Many-many thanks for pulling my mind out of the tech-"cloud" around it and opening up my eyes on this very important and valid aspect. Family comes first and if the amp is built well, kept well, it will serve way more than 20 yrs. Now this gave me a huge motivation, thank you
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Old 19th September 2019, 09:13 AM   #8
Vortex is offline Vortex  Hungary
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Send a message via ICQ to Vortex 1xKT88 Class A SE amp - build it or not ? What to do with these now ?
Quote:
Originally Posted by OldHector View Post
For those wondering what 'Hipersil' is ...
Hipersil, the myth and the truth
Interesting article (or comment).

Why is a Hipersil core touted to be better than an EI core?
Good question, because I think hipersil is meant for the material itself while EI is meant here to describe the basic structure of the transformer. So when somebody asks such a question I think it's weird already, the question itself.

Our cores are partly from good-old Russian heritage, originally some military transformers (now dismantled, etc. dont't ask me why, how.. probably damaged, etc).. and partly from old heavyweight computers (mainframes maybe - there were 1-phase and 3-phase PSU-s for them too). He just gathered these during the last 30-40 years "just for fun" and there're 2-3 different sizes of them. All "hipersil" or call it what you want, the material is the important thing.

Btw I was also keen on doing the pushpulls with toroids, have them wound by a company or maybe buy them from Transformatory toroidalne - Producent transformatorow Toroidy.pl or Plitron Manufacturing - Engineered to Perform Like No Others or any other similar quality manufacturer..

But then I stumbled upon some forum threads regarding the almost-always present DC current (unbalance) in pushpull designs when using toroid OT-s (despite all efforts to make it zero) and I came to the conclusion we might be better off with a so called pseudo-pushpull config. by using 2 SE trafos instead of 1 center tap toroid one. But this is a different topic, bit off here. Click here if interested

Nevertheless, thank you for all the suggestions and tips, I'm rerouting ourselves now towards the Class AB design.
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Last edited by Vortex; 19th September 2019 at 09:17 AM. Reason: Inserted link ..
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Old 19th September 2019, 11:01 AM   #9
pblix is offline pblix  United States
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I would use the KT88s and transformers in a Tubelab SSE and call it good.
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Old 19th September 2019, 02:44 PM   #10
Vortex is offline Vortex  Hungary
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Send a message via ICQ to Vortex 1xKT88 Class A SE amp - build it or not ? What to do with these now ?
Great link, thanks. Wow. Exactly what I need now for my very first standalone build.
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