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Old 3rd January 2012, 09:32 PM   #1
Dave S is offline Dave S  United Kingdom
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Default Which KT88 PP and which Transformers?

I want to build an ultralinear KT88 amp and have narrowed my choice down to 2 designs:
http://www.drtube.com/schematics/grant/g60ams.gif
or
Integrated5050

The reason for choosing the Grant is that a friend has a similar amp (same topology, KT88s and different part values + both small valves are ECC82) that sounds really excellent, so other than the output iron it's a known entity.
However Patrick Turner is held in high regard and seems to be very straight talking and really know his stuff. So maybe his amp could be even better?

Why is the gain only ~11 though? Seems a strange choice!

My other question is about transformers...
Research indicates that Sowter and Lundahl are my best local(ish) options. I can get Hammond for about half the price but these seems to be less well regarded. Any other suggestions and how to choose between these two?
The Lundahls seem to be very small physically for the power ratings. Is this because they are C-cores? Is the C-core really an advantage?

Sorry if these Qs have been asked before but I've sifted a lot of threads and not really found what I'm looking for.

General: looking for 35 to 60W to drive 87db/W speakers with ~6 Ohm impedance.

Thanks,
Dave
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Old 3rd January 2012, 11:18 PM   #2
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Default well, I can't offer any comments on the circuits you posted....but...

Here are a couple of additional designs from this forum (in case you haven't seen them):

"Mullard 5-20 KT88 PP blocks!

OPUS 5.0 A Modern Mullard

Both are Mullard based PP KT88 UL circuits with a few modern improvements, and lots of commentary about the designs. Putting a CCS in the tail of the LTP makes a lot of sense. Looks like the Turner 5050 has one, as well as the two Mullard designs I just posted. The Opus uses mosfet followers for additional drive capability.

Last edited by boywonder; 3rd January 2012 at 11:22 PM.
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Old 4th January 2012, 12:47 AM   #3
taj is offline taj
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I recently priced a set of James output transformers since I travel to Taiwan occasionally and could buy them direct and roll them home in my suitcase, saving the $100+ shipping cost. But the 100W P-P models suitable for the KT88 P-P Mullard amp mentioned in boywonder's post that I'm building, would end up costing a whopping US $375 each.

O'Netics transformers are much more reasonable, though of course they don't have the snazzy James box covers, but would otherwise be just as suitable.

I wish I could buy snazzy box covers for transformers...

..Todd
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Old 4th January 2012, 01:06 AM   #4
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Taj, The Opus is built with Electraprints. No snazzy covers but the sound good.

Dave S. Both of the suggested amps are winners. The KT-88 monoblocks need some tweeking on the input tube in its design the OPUS have been playing for a year and a half without issue.
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Old 4th January 2012, 01:21 AM   #5
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Quote:
Originally Posted by taj View Post

I wish I could buy snazzy box covers for transformers...

..Todd
You can, but you have to ship them from Hong Kong

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Old 4th January 2012, 01:31 AM   #6
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Quote:
Originally Posted by taj View Post
I recently priced a set of James output transformers since I travel to Taiwan occasionally and could buy them direct and roll them home in my suitcase, saving the $100+ shipping cost. But the 100W P-P models suitable for the KT88 P-P Mullard amp mentioned in boywonder's post that I'm building, would end up costing a whopping US $375 each.

O'Netics transformers are much more reasonable, though of course they don't have the snazzy James box covers, but would otherwise be just as suitable.

I wish I could buy snazzy box covers for transformers...

..Todd
Are the O'netics a made to order product? I have not seen them featured many places and I cannot find any sort of website.

Also Diyhifisupply had some covers in the clearance section.
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Old 4th January 2012, 01:54 AM   #7
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Dave,

The Grant employs Williamson topology, with the unusual selection of a 12AX7/ECC83 as the voltage amplifier/phase splitter. "Normal" for Williamson is a 6SN7 or an electrically equivalent type in both small signal "holes". The 12AU7/ECC82 is decidedly inferior to the 6SN7. The "Achilles Heel" of Williamson topology is low freq. instability. Absolutely top notch "iron" is a must in a Williamson implementation. Mullard style circuitry has far fewer stability issues.

Shipping costs for "iron" can eat you alive. If good stuff can be sourced close to home, that's where you buy. The Sowter Type U070 is close to perfect, in an "off the shelf" item. Contact Brian Sowter (support@sowter.co.uk) about ordering along the U070 lines, with a 3.3 KOhm primary impedance. The 4.5 KOhm primary of the U070 is certainly usable, but you'll need about 525 V. of B+. A 3.3 KOhm primary allows a 475 V. B+ rail to work well. It's a matter of step down ratio.

You should have little trouble in executing either Williamson or Mullard style circuitry with Sowter's excellent product, provided reasonable care is taken.
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Old 4th January 2012, 11:51 AM   #8
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If you do not care for fixed bias, I 'd recommend you stay tuned. I am having a go at designing such an amplifier these days.
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Old 4th January 2012, 06:44 PM   #9
taj is offline taj
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Quote:
Originally Posted by nic6paul View Post
Are the O'netics a made to order product? I have not seen them featured many places and I cannot find any sort of website.
Hi nic6paul,

O'Netics is Bud Purvine's company. (username BudP here.) He does high-end custom transformers for DIY'ers, but his bread and butter is supplying amplifier manufacturers with transformers. He's in Seattle I believe. And he does not have a website for his business, so I guess his hands are plenty full without a website.

And thanks for the tips about snazzy transformer covers. Love it!

..Todd

Last edited by taj; 4th January 2012 at 06:56 PM.
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Old 4th January 2012, 06:52 PM   #10
taj is offline taj
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Quote:
Originally Posted by SGregory View Post
Taj, The Opus is built with Electraprints. No snazzy covers but the sound good.
Yep, I've heard good things about Electraprint OPT's too. Though I understand he (Jack?) does not provide more than one secondary winding. I would want 4 + 8 ohm taps for any project I build. I have a multitude of various (DIY) speaker designs that I would use the amp with. Is he perhaps more accommodating than his reputation portrays?

..Todd

Last edited by taj; 4th January 2012 at 06:56 PM.
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