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Dynaco VTA-inspired KT88 PP AMP project
Dynaco VTA-inspired KT88 PP AMP project
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Old 7th February 2019, 02:09 AM   #11
peteki is offline peteki  Australia
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May I offer some suggestions, based on constructing KT88 amps. Put a balancing network in the plates of the phase splitter. When a sine wave is applied to your amplifier it can be adjusted to supply equal drive to the KT88's with the aid of a CRO. Also a balancing network in the cathodes of the KT88s al la Williamson is useful. Perhaps adjust the cathode resistor(s) to drop the current to 140 or so mA. It prolongs the life of your output tubes.
Lastly consider triode operation; it sounds more musical.
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Old 12th February 2019, 04:40 PM   #12
AnatolyAA is offline AnatolyAA  Russian Federation
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Dear Friends,

Any suggestions on the OT? I am planning to buy them in Europe. Could you recommend some models for >60 Wt output? Besides the 2 ones mentioned in this thread?

Are there any particular requirements to the choke? My father said that we could make ones by ourselves (and I am sure that he can do it)? How many Henries would you recommend?
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Old 12th February 2019, 06:08 PM   #13
multi is offline multi  Australia
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Toriody in Poland make excellent out put transformers.
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Old 12th February 2019, 06:20 PM   #14
cbueche is offline cbueche  Switzerland
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Location: PrÍles, Switzerland
Dynaco VTA-inspired KT88 PP AMP project
I like both Toroidy and Leszek : Ogonowski.eu - profesjonalne transformatry do wzmacniaczy lampowych
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Old 15th February 2019, 04:33 PM   #15
THIAGO LIMA is offline THIAGO LIMA  Brazil
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Accompanying this because I'm in Brazil and I want to start an amplifier with two KT88 per channel. Which scheme will you follow?
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Old 15th February 2019, 08:25 PM   #16
John Ess is offline John Ess
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Some of the answer depends on your power needs.
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Old 15th February 2019, 09:00 PM   #17
THIAGO LIMA is offline THIAGO LIMA  Brazil
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I do not need a lot of power with them, I'm thinking of keeping the + B around 450Vdc, so the source would be more economical besides being more secure with the current Kt's. I believe something between 35 and 45W, I want the least distortion possible.

Thank you for your help!
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Old 16th February 2019, 11:21 PM   #18
6A3sUMMER is offline 6A3sUMMER  United States
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peteki,

I believe that matched plate loads on a CCS cathode coupled splitter will by definition give equal amplitude and opposite phase output signals.

That is one of the characteristics of using a real CCS for a LTP phase splitter instead of using a "resistor pseudo current source".

As to being able to adjust the plate loads on the splitter, that would make up for the not so perfect balance of the Output Stage Tubes.
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Old 18th February 2019, 02:34 PM   #19
John Ess is offline John Ess
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Yes, that's what was used on the original VTA70 board...adjusted for 10.5 volts and using an LM334. The updated version uses an 8.2R resistor in place of the 10R that shows in the OP drawing.
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Old 18th February 2019, 02:39 PM   #20
John Ess is offline John Ess
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Quote:
Originally Posted by THIAGO LIMA View Post
I do not need a lot of power with them, I'm thinking of keeping the + B around 450Vdc, so the source would be more economical besides being more secure with the current Kt's. I believe something between 35 and 45W, I want the least distortion possible.

Thank you for your help!
You might look at their boards...the 6SN7 board is what's being discussed here and I'm taking a similar approach and will be using 450-460vdc. tubes4hifi home page
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