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Old 28th December 2007, 12:32 AM   #1
WE91 is offline WE91  Norway
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Question Three PP KT88 designs. Which one ... (please;-)

Hi good folks!

I need help to choose!

I have a question that have been posed a lot of times in here!
The difference is that this question isn't a general one, it's more specific than "the usual" ...
So please show pity on a "new-bee" - and excuse yet another question on "which to choose". I know that I probably would have been happy with any of these. At least I have narrowed my choice down to three designs.

First a short description of "my needs":
I need a good PP amp of approx 30 to 50W power for an active bi-amped loudspeaker.
I allready have a Lectron JH50 that I plan on using for the low end - up to 1000Hz. To make a long story (very) short: This is one of the best PP amps in the bass and mids that I know of. To match it, and possibly better it in the mids/highs, I have found that a very sound PP KT88 may just be right ...

I have looked around, and found these fine designs:

KaT88 from World Audio Designs
- But does World Audio Design still exist?
- Where to find more info.?

Ella Signature with KT88 from DIY Hi Fi, used as a pure poweramp.
This amp. with these modifications?

Or Mono Bills, designed by "Triode Dick"
Here I can get a transformer/choke-kit from AE at a good price also ...

Please give me feedback! All is welcome! I really need your opinion and experience to find wich one of these that will give me the best result!

Looking very much forward to all thoughts, advice and general comments you may have!

All the Best for the New Year to all of you!

Aril.
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Old 28th December 2007, 01:45 AM   #2
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Default Re: Three PP KT88 designs. Which one ... (please;-)

Quote:
Originally posted by WE91
KaT88 from World Audio Designs
Looks like the KT88's are being run in pentode mode. More power but more output impedance and more NFB needed to run it stably. Not that NFB is bad...

Can't find a schematic!

Personally it all depends on your taste, the type of music you like, your source, your speakers, and the way all these things interact, before you start thinking about mods. Try to "demo" different ones if you can, and if you can't, be prepared to take a hit to your wallet distributing on A-gon... sorry I can't be more specific.
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Old 28th December 2007, 02:05 AM   #3
SY is offline SY  United States
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No schematic for the Ella, so hard to say.

The WAD is based on a classic Mullard 5-20 circuit (probably less linear), but their choice of an ECC82 for a long-tail phase splitter is particularly unfortunate.

The Bill looks somewhat better, but will need careful choice of speakers- there's no feedback, the source impedance will be pretty high, and the frequency response with most speaker loads will look like a roller coaster.

Just MHO.
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Old 28th December 2007, 03:31 AM   #4
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Quote:
The WAD is based on a classic Mullard 5-20 circuit (probably less linear), but their choice of an ECC82 for a long-tail phase splitter is particularly unfortunate.

SY,

IMO, the Mullard circuit is not without virtue. If nothing else, pesky phase shifts are reasonably well controlled. Do you think a 6AB4 as the voltage gain tube and an ECC99 as the LTP have merit driving PP KT88s?
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Old 28th December 2007, 04:54 AM   #5
SY is offline SY  United States
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I hope I didn't imply anything different- the 5-20 is a classic for a reason.

I haven't used the ECC99, but it sure looks good on paper. For an input tube, I might consider the 6AB4 (it's half a 12AT7 if memory serves). A 6AQ4 might be close to optimum; the higher mu will result in higher feedback, and the linearity is excellent. Trioded D3a would be overkill, but if cost were no object...
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Old 28th December 2007, 07:21 AM   #6
Svein_B is offline Svein_B  Norway
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Some ELLA technical information:
The circuit, at least for the older version (=Conconance M100), is also based on the Mullard 5-20, but uses a triode SRPP input stage. This is DC coupled to LTP phase splitter with resistor tail, and EL34/KT88 output in UL at 430V.

The Ella signature with remote volume control may be somewhat wasted if it is to be run as a pure power amp?

Svein.
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Old 28th December 2007, 01:39 PM   #7
SY is offline SY  United States
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What's the point of using an SRPP there? That circuit sacrifices linearity for swing, and in the first hole, the required swing is only a few volts.

That would make me wonder about the competence of the designer.
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Old 28th December 2007, 02:48 PM   #8
WE91 is offline WE91  Norway
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Thank you for all your input on this sorenj07, SY, Eli Duttman and Svein B.!

There exist a schematic for the Ella on the net, but it is of an older version. The difference to the one offered today I believe is "just" a dual-mono psu. Here

I thougt I had chosen wisely, when asking for your advice of one of these three designs ... I guess I have to add at least one other construction that originally was "skipped" because of the 20W output. I thought I would need more, but this is not so, since I'm going to use it from 1000Hz and up. The loudspeaker-drivers are configured in MTM with ribbon mids and air motion transducer tweeter. These will give a stable impedance of approx. 7 to 8 ohms from 1KHz to approx. 3KHz and falling to 4 ohms from this point and staying there further up ... Sensitivity is 92dB/1m. and the energy needed to drive mids/higs are less than what will be needed below 1000Hz (where I have 50W).
Since I see that phase-splitting, sufficient drive for the output and the need for NFB with pentode-mode KT88's is issues with the three designs, I thought this design might be better:
Andrea Cuiffoli's PP KT88

And, guys, - thank you very much for your input and sharing of thoughts and knowhow! It is very much appreciated!

Aril
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Old 28th December 2007, 03:21 PM   #9
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hi,
I've had an ella amp for a number of years and did a lot of modifications to it for my own enjoyment. Its a good platform to work with.

I'd agree with using a SRPP at the input... not really necessary. I think it was just fashionable to use that topology wherever you could put it a few years back. Still, it peforms reasonably in that role. I have changed to an active loaded 6fq7 for the input as I don't need much gain. I also modded the 6sn7 LTP to swing 200Vpp for driving DHT outputs, which I use.

not sure what the considerations for your amp choice are now. If its fully DIY, there are many design choices. The Cuiffoli design looks good if you get the lundahl IT. Another option is using a center-tapped choke for splitting. The ella is easily modified for triode, no-feedback operation if thats of interest.

-z
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Old 28th December 2007, 05:53 PM   #10
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How about this one ??

Help modifiying schematic

It's a modification over the Byrith design. Should bee quite good.
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