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Old 21st March 2003, 08:59 AM   #1
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Location: Sweden
Question OPT for SEKT88

Next Tuesday (the 25th) is payday for me. I'm going to order my OPT's for my amp from Sowter. I just wanetd to know if the OPT I have specified is the "ultimate" for my amp.

Here are the specs:

> Type: SINGLE ENDED
>
> Package Style: "S" FULLY SHROUDED
>
> Secondary impedance: Single 8 ohms
>
> Valve type: Svetlana KT88
>
> Configuration: Other
>
> Primary impedance (ohms): 5000
>
> Internal rp (ohms): Seems to be 2k as pentode
>
> Screen taps: 43%
>
> DC mA: 91 mA ************UPDATED***********
>
> Power (Watts): 20
>
> Full power frequency (Hz): 45
>
> Size preference: SOWTER to optimise for bandwidth

Here is the schematics of the amp:

Click the image to open in full size.

The values of the components in the input stage is from Svetlana's tech. bulletin no. 43. I've had the inputstage running for about a week now connected to an oscilloscope, and it seems to be working beautifully. Assuming that the raster of the scope is correct I have almost 120V P-P from just a CD-player. That is measured with single sinus-curves at 0dB.

The current ratings for the transformer is from the pdf spec sheet of the KT88. On page 2 of the document it says that typical ratings at 400V is 87mA at the plate and 4mA at the screen at zero signal. Hence the 91mA.

I still haven't figured out the plate resistance of the KT88 in UL-mode. But from what I've read on the net it seems to be something near 2k. The reason I only specified the lowest freq. of the transformer at full power to 45Hz is quite simply that I don't need to go deeper.

The only thing I have to figure out now is the cathode resistor for the KT88. My 30W rheostat should arrive in the mail today or monday at the latest. I figured I could turn in the resistance with the rheostat and then measure it and put in equal fixed resistors.
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Old 30th March 2003, 05:00 PM   #2
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Join Date: Nov 2002
Location: hong kong
pls visit my site, hope I can help U.

anybody who had interest for tube audio parts pls visit my new build website www.diyaudiocraft.com.
I also a crazy diyer & part time hifi writer in hong kong.
Now I talk to some excellent parts factory & can provide thier product in good price to u.
I hope more people can enjoy diy interest.
many jensen oil cap, tango, tamura & james output, wooden horn all in good price.

thaks

Thomas
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Old 30th March 2003, 05:07 PM   #3
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Default IF I MAY SAY SO...

Hi,

Thomas, if you care to put your websites' URL in your profile here at the forum I think people would appreciate.

Cheers,
__________________
Frank
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Old 30th March 2003, 05:18 PM   #4
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thanks frank,

I need more time to build more & get more product that can in lower price to benefit to more diyer.
I will do my best.

thanks

Thomas
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File Type: jpg 2003-02-03 meeting 026.jpg (74.9 KB, 373 views)
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Old 30th March 2003, 10:55 PM   #5
SHiFTY is offline SHiFTY  New Zealand
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Location: New Zealand
You might want to experiment with a little loop feedback on that amp, I found it greatly cleans up the sound on a SE pentode.

In my SE pentode amp, I split the preamp cathode resistor into two sections (in your amp it would be 900ohm and 100ohm, then I wired a 4.7K resistor in series with a 10K pot and wired it from the output + to the junction of the two resistors. Then you can vary the feedback by adjusting the pot.

Try it, you may be surprised.

Heres an interesting link

BTW I am jealous at your transformers, they look damn impressive. Please tell us what it sounds like when you are finished!
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Old 4th April 2003, 04:47 PM   #6
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Talking Just got them!!!

I just opened the package with my trannies...

OH MY GOD!!!!

People talk about The Niagra Falls, Grand Canyon etc. They should take a look at an OPT for a tubeamp. The feeling is almost religious. Well... not almost. It IS religous.

I have seen the light.

First of all, I just can't believe how big and heavy they are. My arms are numm after carrying the package a few hundred meter from the busstation.

Second, I never could imagine that something made out of metall could hold such beaty. I'ts breathtaking. The guys at the Sowter factory are not just ordinary people. They are artists, profets doing the work of the lord. I never considered myself a religious man untill now.

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Old 4th April 2003, 06:46 PM   #7
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Join Date: Nov 2002
Location: hong kong
Hi everybody,

I had a news, may be better to anyone.
I talk to japan's factory about the despatch days.
They promise me that can supply their items to me & on my hand within 14days after I pay to them.
So I decide every month will send one item that was 100% brand new to benefit to all the diyers.
I promise the price will lower than ebay that U bidded.
Also I recent I got some telfon cable already that the factory only charge me USD 1.2 for 1M mult-core & solid core was same to me. I prepare to sell them 1.5 per metre.
telfon tubes with different colour will be same.
I hope more people will love them & enjoy diy experience.

thanks

Thomas
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Old 4th April 2003, 10:13 PM   #8
SHiFTY is offline SHiFTY  New Zealand
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Quote:
People talk about The Niagra Falls, Grand Canyon etc. They should take a look at an OPT for a tubeamp. The feeling is almost religious. Well... not almost. It IS religous.
Hahaha!!!

Pics???

I think transformers are pretty darn neat... Its like the massive heatsinks that are the best part of a solid state amp...
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Old 5th April 2003, 05:06 AM   #9
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I can't produce pictures if them right now since I don't have a digital camera. But I'll se what I can do. Maybe I can borrow a camera from a friend and share some pics with you.
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Old 5th April 2003, 01:57 PM   #10
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Unhappy It doesn't work

I've just got one channel up and running on my kitchen table. Something os horribly wrong. First of all, I seem to have the same voltage from both windings on the OPT, the anode AND the 43% UL-winding. Shouldn't I get a much lower DC-voltage at the UL-tap???

Also the signal I get at the output sounds and look like hell! Is it possible that one half och the 6n1p doesn't have the power to drive a KT88?

My whole world has just fallen apart. I really hope you can give some answers.
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