Author Topic: Single Ended Triode KT88  (Read 2926 times)

Offline Mars

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Single Ended Triode KT88
« on: March 12, 2017, 06:24:10 PM »
I am building a stereo SET KT88 power amplifier with XLR and RCA inputs.

So far sticking with the 6SN7 voltage amplifier/driver setup and cathode bias.

The first step was to manufacture the transformers. The OPT were 2500ohms:8ohms and should give a wide bandwidth at 8 watts.

Here is the result:

Mars SE88 by Karel Mars, on Flickr

I wanted a beefy choke, and settled on a 10Henry 250mA design.

Here you can see all the hardware bolted together:

IMG_20170311_182149 by Karel Mars, on Flickr
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Offline Mars

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Re: Single Ended Triode KT88
« Reply #1 on: March 12, 2017, 06:34:42 PM »
After many hours of soldering, the amp was ready for power up.

IMG_20170311_182312 by Karel Mars, on Flickr

Mars SE88 by Karel Mars, on Flickr
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Offline King_Julian_S

Re: Single Ended Triode KT88
« Reply #2 on: March 12, 2017, 10:30:13 PM »
Nice sire 👍🏿   
& whoever buys it I have a quad matched set of the highest end KT88 in the EAT golds here ... 😉
Non audiophile music lover...

Offline fredeb

Re: Single Ended Triode KT88
« Reply #3 on: March 12, 2017, 11:12:10 PM »
Looks good Karel , have you listened to it yet ?
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
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Offline Mars

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Re: Single Ended Triode KT88
« Reply #4 on: March 13, 2017, 04:53:35 AM »
I listened to her on Saturday and Sunday and took her over to Louis to check on the Tannoys. He has a McIntosh preamplifier, and the input sensitivity is spot on. At decent listening levels the Master volume is at 12 O'Clock.

The sound is big with excellent bass, crisp tops and wide sound stage. I am very happy with the design. I've been building these KT88 style single ended amplifiers since about 2004, and I think I have consolidated the design.
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Offline Mars

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Re: Single Ended Triode KT88
« Reply #5 on: March 13, 2017, 04:57:58 AM »
Here is the final schematic with voltages:

Mars SET88-kragversterker by Karel Mars, on Flickr

and a shot of the rear showing the connectivity:

IMG_20170311_182219 by Karel Mars, on Flickr
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Offline fredeb

Re: Single Ended Triode KT88
« Reply #6 on: March 13, 2017, 09:47:32 PM »
I love the industrial look with the naked transformers , and those transformers look beefy enough to deliver more than 8 watts if they were required to . Could they deliver more than 8 watts ? With a GU-50 maybe ?

http://www.r-type.org/pdfs/gu-50.pdf

http://www.r-type.org/exhib/aaa1539.htm

I also really love the simplicity of the design , I hope I get to hear it some day soon .  ;)
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline King_Julian_S

Re: Single Ended Triode KT88
« Reply #7 on: March 14, 2017, 08:39:44 AM »
So me freed and mars...
If I may ask yet another dumb question ...
Does the size of the transformer determine the output or is it the valves or ?

Non audiophile music lover...

Offline Mars

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Re: Single Ended Triode KT88
« Reply #8 on: March 14, 2017, 06:51:29 PM »
Always nice to chat, freed and King Julian :mates:

The size of the transformer question.....

To start with; a few things are interrelated:

power output <=> bass frequency reproducible <=> magnetic flux in the core

Let us keep one of the variables fixed (anyone could be chosen) namely max flux -> should not go beyond 1.1 Tesla for these laminations

Here is a screenshot of the OPT calculator

Screenshot from 2017-03-14 18:07:51 by Karel Mars, on Flickr

Notice it is an 8 watt design that can reproduce 40 Hz at full power with flux maxed at just below 1.1 Tesla.

Now if I want to squeeze 16 watts from the same OPT, this is the result:

Screenshot from 2017-03-14 18:09:11 by Karel Mars, on Flickr

As you can see, 40Hz would distort at 16 watts. (clue: the flux is too high and the iron is saturated).

If I compensate by seeing which lowest frequency I can reproduce at 16 watts keeping flux below 1.1 Tesla, I get this:

Screenshot from 2017-03-14 18:10:46 by Karel Mars, on Flickr

So at 16 watts the lowest frequency I can reproduce at full power, without distortion, is 55 Hz (not that bad at all; most speakers can barely reproduce 50 Hz)

If I crank up the volume even more, and squeeze out 32 watts my lowest frequency reproducible, without distortion,  is 78 Hz. (not so good for hifi; fantastic for guitar amplifier)

So to sum up: the size of the transformer affects the lowest frequency the OPT can reproduce at full power.

I other words: a small transformer can have the same lowest frequency reproducible as a big transformer, just at a lower power. Band width increases as you turn down the volume.
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Offline King_Julian_S

Re: Single Ended Triode KT88
« Reply #9 on: March 14, 2017, 07:04:59 PM »
Wow ... very interesting .. tx Karel...
I thought of a simple plan to make huge huge transformers for the 2a3 you made for me ...lol...
Guess not 🤔...
Non audiophile music lover...

Offline fredeb

Re: Single Ended Triode KT88
« Reply #10 on: March 14, 2017, 08:41:39 PM »
Thank you so much Karel ! Very interesting indeed !

So ... for a SE amp to start at 30hz , it would have to be about 4 or 5 watt ? Or is that only for the size iron core you choose ?

Also , if I may ask , what are the advantages/disadvantages between EI , C-core and amorphous  in SE output transformers ?







« Last Edit: March 14, 2017, 08:52:16 PM by fredeb »
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Mars

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Re: Single Ended Triode KT88
« Reply #11 on: March 15, 2017, 05:29:36 PM »
Wow ... very interesting .. tx Karel...
I thought of a simple plan to make huge huge transformers for the 2a3 you made for me ...lol...
Guess not 🤔...

I checked calculations around the 2A3 OPT. The way to make the 2A3 louder would be to improve the efficiency of the OPT. I usually aim for 5%. By using M6 laminations, I can increase the magnetic properties, and use less copper to get the same result. If I make EI96 (with a 32mm stack) transformers, like in the SE88 above, I can actually get the copper losses lower; down to 3.5%.
That would probably make your 2A3 a bit louder.

I do not know how much louder an amp with 1.5% less copper losses in the OPT will sound; maybe Ampdog can chip in.



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Offline King_Julian_S

Re: Single Ended Triode KT88
« Reply #12 on: March 15, 2017, 05:42:28 PM »
Interesting if that can be done without loosing the magic ...
One also has to think if costs will be worth the extra gain ...
But tx again ,, will see what Oom Johan say ... at least there is an option cause I'd like to a use it to drive my tesla's ( they sound really great with the 2a3 ) plenty of efficiency , but they can sure take a lot more and that makes one itch 😉
Non audiophile music lover...

Offline Ampdog

Re: Single Ended Triode KT88
« Reply #13 on: March 16, 2017, 05:47:49 AM »
It's actually quite straightforward; for small resistive changes the output power is related to the total resistance in the circuit.

Taking 5% copper loss (5% wire resistance - on the high side for valve transformer copper loss), the power loss will be 0,45 dB (rounded) by simple Ohm's Law. Coming down to 3,5% copper loss the power loss will be 0,31 dB - thus a difference of 0,14 dB. Not even the highest carat of golden ear will claim to hear that. (This excludes some slight phase angle involvement, resulting in <10% of that, etc.)

That is also why I consider copper loss low on the scale of importance in the design of an OPT. In valve design there are far more important factors to consider than e.g. equal primary resistance of push-pull OPTs and such.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Qualityten

Re: Single Ended Triode KT88
« Reply #14 on: March 17, 2017, 12:45:14 AM »
Thanks for sharing.  I love the look of your transformers.