Author Topic: KT88 push-pull amplifier with integrated phono stage  (Read 2927 times)

Online Mars

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #30 on: December 12, 2016, 01:02:25 PM »
Setting the bias has been an interesting exercise too.

The B+ is stable at 470VDC and does not budge as you bias the tubes hotter. I started to bias them at about 35 watts dissipation, but there was quite a bit of heat radiating from the tubes. That was with a cathode current of about 75mA.

Dropping it to 60mA (28 watts per tube) things felt a bit calmer. No excessive heat, and the sound is rock solid. Setting the bias is easy, and can be done with a multimeter from the top. Just set the meter to mV and adjust tube to read 60mV at the test points.

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Offline Ampdog

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #31 on: December 12, 2016, 02:59:31 PM »
As a side note, I also looked up the TT21/TT22; new to me.

Reports are that the innards are the same as KT88 as Kubusi intimated. Higher dissipation ratings mainly in the quoted applications. Folks reckon that one cannot do much more with them than with KT88 while the price is notably higher. (No criticism intended, Kubusi!) Yes, higher anode voltage possible, but then one ends with the predicament Steinhart landed in, regarding protection. 

In general, for several reasons it becomes desirable to go to two valves in parallel rather than the super-power behemoths. I do the same with parallelled 6L6GCs as already mentioned elsewhere (e.g. ending with 70W equivalent dissipation possibility and wider spread of heat source at not much higher cost).  I would also cite the use of two x EL84 in parallel rather than one 25W valve, as used in the Manley Stingray, there taking advantage of the sensitivity of EL84 for easy drive.

But going OT; sorry Karel! Thanks for your good work. 
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Offline charles

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #32 on: December 12, 2016, 03:32:47 PM »
I have some GEC TT21 valves.

I keep them in order to someday replace the KT88/6550 in my Sansui BA303 amplifier.

I am always looking for some more TT21 valves.

I do not trust the Chinese made ones.

TT 21 and TT 22 were used in radio transmitters.

The innards are not exatly the same as the KT88 (please refer to its specification sheet).

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #33 on: December 12, 2016, 05:30:34 PM »
Here is an informative article on biasing a fixed bias amplifier. Unfortunately only available as pdf.

https://www.tube-town.net/cms/userfiles/media/bias/bias-setup-e.pdf
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Offline King_Julian_S

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #34 on: December 12, 2016, 05:43:15 PM »
If it sounds anything close to the 2a3 set you build for me then Kudo's ...
Non audiophile music lover...

Offline Ampdog

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #35 on: December 13, 2016, 07:12:10 PM »
I do not trust the Chinese made ones.

Rather have to agree here.  I recently sat for a few hours perusing internet articles on experienced valve life. Most comments were from guitarists who have their own standards, but was valuable to the extent that many dozens of a type were used.

The emerging picture was not encouraging. A well-known Chinese manufacturer even unashamedly offers bulk purchasers a choice of brand logos to be put on the glass/box, including a JAN one! The Chinese policy* seems that if you pay a lower price, you must expect lower quality (some ironic sense in that) and they market accordingly, rather than to withhold sub-standard issues. The fake logos are accurately reproduced and only with experience can one detect which type of valve is inferior. What a world ....
_________________________________________________________________
*I do not wish to disparage Chinese products in general. My words here are relevant solely to the products under discussion. 
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline fredeb

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #36 on: December 14, 2016, 12:17:48 AM »
Here is an informative article on biasing a fixed bias amplifier. Unfortunately only available as pdf.

https://www.tube-town.net/cms/userfiles/media/bias/bias-setup-e.pdf

Good , straight-forward article for those that are learning , like me - thank you Karel .
“And those who were seen dancing were thought to be insane by those who could not hear the music.”
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Offline Ampdog

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #37 on: December 17, 2016, 03:43:56 PM »
... or elderly guys like me, as a check-up.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline fredeb

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Re: KT88 push-pull amplifier with integrated phono stage
« Reply #38 on: December 17, 2016, 04:35:58 PM »
... or elderly guys like me, as a check-up.

 :ROFLMAO: Funny Ampdog !
“And those who were seen dancing were thought to be insane by those who could not hear the music.”
― Friedrich Nietzsche