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Old 12th September 2007, 06:10 AM   #1
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Default Help needed with KT88 Amp design

Hi Folks,

I am relatively green to the tube amp world but I have successfully completed one project and I am looking towards building my second power amplifier.

I am keen to build a PP amp with KT88 output tubes and I have already searched the board but I would greatly value some specific advice.

I would like to build a stereo amp, not two monoblocks. I am having difficulty finding a decent schematic and parts list for a PP stereo KT88. Can anyone help with a recommendation?

I would like something that has

1. Tube rectification
2.50-60 w/channel output (I think that this should be very achievable with the KT88s in PP configuration)
3.Relative availability of parts, esp. with respect to the output transformers

I will build my own chassis so this is not an issue.

I looked at the Dynaco MK III schematic and I like the look of this amp (a lot) but I am keen for a stereo amp from the outset and not two monoblocks (just a personal thing). I was toying with the idea of just building the two monoblocks into one chassis. My main concern with this option was the power transformer. I dont want to use two weight and size start to become a real issue. I have looked at the Sunn P782 S transformer (Rated 400-0-400V [800VCT] at 200 ma maybe up to 300ma continuous if pushed) but this transformer is specifically for the MK III monoblock and it is not clear to me that it has enough grunt to use in a stereo build. I am not sure if there is another transformer around that might be suitable to use instead?

Does anyone have a solution? Either a transformer/power option for a 'stereo' version of the MK III (??MK IV) or another design altogether taking into account the requirements mentioned above.

Thanks in advance for the advice.

Rob
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Old 12th September 2007, 10:29 AM   #2
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No reason in the world not to use the Dyna design in stereo; there are better designs out there, but at the cost of complication. The Dyna is still excellent.

Given the total mass of such an undertaking, I think that you're not going to notice whether you have one larger power transformer or two smaller ones. Doing a quick look-see at the Hammond range, the P-T278CX seems like it would work fine for two channels. For more $$$, the One Electron BFT-1B looks even better.
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Old 12th September 2007, 10:33 AM   #3
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You could easily modify this to use 6550's or KT88's.

You could use Hammond output transformers if you wanted to keep the costs down.

I don't think however that your power transformer is big enough to do the job as I believe it calls for 200mA per channel to run it.
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Old 12th September 2007, 11:25 AM   #4
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Hi Sy,

The hammond P-T278CX is not going to work for me as it only has a 110v primary. The 240v equivalent does not seem to have a 5v winding. HOLY SMOKE, the One Electron BFT-1B looks like the mother of all transformers - I could really do some serious damage with that thing. Maybe two smaller ones is a better idea....however if you know where to buy them I might just have a quick look. Hmm.
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Old 12th September 2007, 03:48 PM   #5
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Toroidal power transformers are considerably lighter for a given rating - worth looking into. Cost should be similar. Search eBay for ANTEK transformers for an example. I used a Plitron toroid in a small amp and wound my own heater winding on it... of course, it can't have a potted center in this case!
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Old 12th September 2007, 06:45 PM   #6
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Here are some nice designs with the KT88,
this one uses an interstage transformer for the fase splitting:
http://www.diyparadiso.com/boully1.htm
A similar design:
http://www.audiodesignguide.com/my/pp3.html

Monoblocks but with tube rectifier:
http://www.diyparadiso.com/ppkt88mono1.htm

Another design:
http://wduk.worldomain.net/acatalog/AmpKits.html
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Old 13th September 2007, 01:52 AM   #7
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The One Electron BFT-1B is probably your best bet for a cheap, one transformer stereo setup. It's $160 at tubesandmore.com in the US now.

If it were me though, I'd take that $ and buy two slightly smaller power transformers, hammond or edcor and make a dual mono amp on a single chassis. You'll get a better sound that way for almost the same price...

Take a look at this page, the 110 is nice and only $60
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Old 13th September 2007, 03:04 AM   #8
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Hey thanks Cycline3,

My difficulty with some of the transformers that i have seen (including the 110) is the 110v primary winding.

I appreciate the advice re: the quality of sound and the single transformer option. I think after reading the comments above I will go for the two transformer option as you suggest. The triode electronics guys have a 240V version design for the MKIII and that may be my best option.

Cheers,

Rob
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Old 13th September 2007, 03:16 AM   #9
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Quote:
Originally posted by Rob11966
My difficulty with some of the transformers that i have seen (including the 110) is the 110v primary winding.
Oh yeah.. sorry... sometimes I forget we aren't all in the states... the hammond 3 series has world primary support you could look there...


Here's a link to some..
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Old 13th September 2007, 07:48 AM   #10
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You could always put two 110v primaries in series - actually, that might not be as daft as it sounds

Seriously, the Amplimo toroid transformers are pretty good, I think, (all for 230v mains, being of Dutch manufacture), as long as you don't worry about the noise from the mains. Trouble is, they seem all to be intended for cap input filters, which you may not like.

I use an Amplimo 7N607 power transformer, which has 340v @700mA, 50v @100mA and 6.3v @6.8A windings. I'm using it to power an EL34 triode-strapped PP stereo amp. Amplimo does have a higher current model but it's quite a bit more expensive.
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