8x Push-pull KT88 class A in triode mode - diyAudio
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Old 14th May 2012, 12:52 PM   #1
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Default 8x Push-pull KT88 class A in triode mode

Hello,
I am looking forward to build my second tube amplifier after assembling Scott's kit. I have never heard before a tube amplifier, never ever, so you can imagine the emotion I felt while assembling it. How will it sound after all the raving heard from tube fans? Before that, I have built several LM3886 based gainclones, a BPA-300 beast (6x LM3886 per channel), a Nelson's F5 class A, several discrete/opamps preamplifiers....

Now I understand it. It's not about measurements, or technology advances. It just sound VERY pleasing to my ears. As with having sex, you cannot measure pleasure. And you cannot neither compare opinions and likes to each others, and quantify it.

Those soooooooo sweet 5W per channel, KT88 class A in SE triode mode, are the best "food" I have tried. But I am still hungry, and I would like to have more of the same food but in a bigger dish. that means, more power.
I have been thinking in what would be my next project. Making amplifiers is based on making compromises, so more power will means less of other property.
But still, I would like to preserve all that is possible from the original SE amp. That means, I am thinking in building a KT88 based push-pull class A operating in triode mode. Being a big beast, it will be assembled on monoblocks, so I would like to put 4x or better 8x KT88 tubes per channel.

What would be a recommended design? Be aware that I am a tube noob, so I need a complete circuit and description. In the tube world I have seen that it is too much common for someone to post a design but leaving many factors up the the builder. Like saying something like "this is the driver section for this amplifier design, you just have to hook up the tubes and you're done". OK, but how to connect the tubes? What resistor values to use? Etc.
As an example, the said Nelson Pass F5 design is a pretty explanatory design. You don't need to understand at all how does it work (despite being advisable), you just get the schematic and assemble it as it is drawn. You want to tune it, or make modifications? OK. But if built as conceived, it will work at its best.
I would like the same, but for a tube amplifier with 8x KT88 in triode mode. I think I am done with solid state for a looooooooong time.

I have seen Peter's drivers board here. It may be possible for it to drive 4x/8x KT88, but it is an incomplete project as is. I am an idiot, there will be a moment that I may be able to easily conceive a tube design in minutes (like I am now able to do with opamps, for example), but for this moment to come I need some guidance.
Push-pull driver board
I just don't know how to hook up the tubes, set the bias, calculate resistor values, etc.

So I would like to ask you, is there any project out there that can fit my requirements (4x/8x KT88 Triode class A)?.

Best regards,
Regi
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Old 14th May 2012, 01:54 PM   #2
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I forgot to add some more info. I will not be using tube rectifiers, this time I will go with solid state rectifiers. I am no longer afraid of using SS in depending what parts of the circuit. I have read enough arguments to convince me of their advantages versus tube rectification.

I have already ordered a pair of these boards, seems that lot of people like this kind of shunt regulators for preamp and gain stages:
GB for Salas SSHV2 regulator

So I have think in heatsinking the shunts in a Modushop enclosure with the heasinks on the sides.

I will use rectification and regulation for the filaments of the valves too, so if the enclosure has 1 heatsink per side, one of them can be used for the shunt PSU while the other one may go with a stack of, say, LM317 regulators.

I like this one for example, 3mm aluminium panels and available from 2U up to 5U rack size:
modushop.biz

Without the heatsinks, a Galaxy maggioratto seems like a good option too with 2mm top/bottom covers to mount so many tubes in a stable manner:
modushop.biz

For some money they will custom drill me the enclosures, something that will simplify the most (by far!) difficult part of the project:
modushop.biz

Front panel drilling, text engraving with ink filling...

They even provide top covers without slotting:
modushop.biz


Sorry for the OT! I may have gone far from the real issue of the topic.
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Old 14th May 2012, 01:55 PM   #3
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I forgot to add some more info. I will not be using tube rectifiers, this time I will go with solid state rectifiers. I am no longer afraid of using SS in depending what parts of the circuit. I have read enough arguments to convince me of their advantages versus tube rectification.

I have already ordered a pair of these boards, seems that lot of people like this kind of shunt regulators for preamp and gain stages:
GB for Salas SSHV2 regulator

So I have think in heatsinking the shunts in a Modushop enclosure with the heasinks on the sides.

I will use rectification and regulation for the filaments of the valves too, so if the enclosure has 1 heatsink per side, one of them can be used for the shunt PSU while the other one may go with a stack of, say, LM317 regulators.

I like this one for example, 3mm aluminium panels and available from 2U up to 5U rack size:
modushop.biz

Without the heatsinks, a Galaxy maggioratto seems like a good option too with 2mm top/bottom covers to mount so many tubes in a stable manner:
modushop.biz

For some money they will custom drill me the enclosures, something that will simplify the most (by far!) difficult part of the project:
modushop.biz

Front panel drilling, text engraving with ink filling...

They even provide top covers without slotting:
modushop.biz


Sorry for the OT! I may have gone far from the real issue of the topic.
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Old 16th May 2012, 05:29 PM   #4
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Can anybody recommend a KT88 triode design?
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Old 16th May 2012, 06:03 PM   #5
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8x output tubes is plain crazy. What is it for?
You can get around 60W with 4x KT88 in triode mode, or just with a pair 60-80W in CFB, which is quite close to triode.
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Old 16th May 2012, 06:18 PM   #6
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Quote:
Originally Posted by regiregi22 View Post
Those soooooooo sweet 5W per channel, KT88 class A in SE triode mode, are the best "food" I have tried. But I am still hungry, and I would like to have more of the same food but in a bigger dish.
Also, PP is not a bigger dish of SE! If you like the SE dish, you have to get the same dish again, but bigger. As an example

the new EL34 Parallel Single Ended
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Old 17th May 2012, 11:17 AM   #7
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This monoblock parallel 300b also looks good.

http://www.audionotekits.com/PDF/Man...wChassisv2.pdf

Regards
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Old 17th May 2012, 11:53 AM   #8
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Quote:
Originally Posted by LinuksGuru View Post
8x output tubes is plain crazy. What is it for?
You can get around 60W with 4x KT88 in triode mode, or just with a pair 60-80W in CFB, which is quite close to triode.
Because I play on 87db/W speakers (B&W CM-5). And to have the ability of producing concert-like levels if I carry it to a friend's place where we have A) Space and B) Big speakers. Open air, barbecue, beer... music is the fourth element of the equation

You are sure, 8x may seem a bit crazy. But there's no real reason for so much power, just want to do it because its uncommon. Doing it as big as we can, just because we can
I will go with 4x tubes per channel and 60W in triode, seems enough for my needed power.
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Old 17th May 2012, 12:12 PM   #9
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Quote:
Originally Posted by costis_n View Post
Also, PP is not a bigger dish of SE! If you like the SE dish, you have to get the same dish again, but bigger. As an example

the new EL34 Parallel Single Ended
Yeah, I know it is a very different beast (From what I've read. I haven't heard any PP yet), but I wanted to keep it as close as possible regarding:
-Triode mode
-Class A1 operation
-No feedback
-Conservative rating: Many tubes means same power, but long life for them.

I like your idea of more tubes in a SE setup, I didn't know it was possible to use more than 1 tube in SE to have more power. As it seems, I am a totally newbie in the tube world. Still much to read and learn, on the way
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Old 17th May 2012, 01:49 PM   #10
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Although those speakers are 88db sensitivity, they are a nominal 8ohm load, but by the specs drop as low as 3.7ohms.

Just a thought...

Nice speakers BTW

Edit:scratch my idea, I was dreaming with the output

Last edited by data; 17th May 2012 at 01:58 PM.
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