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Old 7th June 2009, 02:58 PM   #1
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Default PP KT88 Mono blocks?

HI!

I'm pretty green at DIY, but have completed a ground up scratch build of a highly modified ST70 amp using ALL new parts. I now want to build up my Main System!

For the pre i'm going with Coffedj's 12B4 once he has the chokes and power supply ready to go. While waiting i'd like to research the power amps. I've done some reading here, but haven't come up with a schematic yet.

What I want is your basic PP Ultra or E linear class AB 4 power tube (KT88?) Mono blocks! Around 100W per block would be nice! I want something at least equal to the better commercial offerings in sound quality. If I can't equal them, i'd rather buy new. Any schematics floating around her fit the bill? Did I pick the right topology for higher powered block?
I mainly see alot of lower powered amps being built.
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Old 7th June 2009, 03:49 PM   #2
chrish is offline chrish  Australia
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If you are green to DIY, and green to tube amp building, a scratch built 100 watt amp is very ambitious, and the chance of failure of disappointment high. Such an amp is going to be expensive, and if it does not perform up to your desires you will feel like you have wasted a bunch of money for nothing. As well as the 'traps for young players' with these builds, there is also the danger with the high voltages involved. First question though, why do you need 100 watts? Every watt in a tube amp adds to the cost and weight of the project. I moved from a 50 watt chip amp to a 7 watt tube amp and enjoyed the experience! I do have reasonably efficient speakers though.

I would describe the music you listen to and the type and efficiency of your speakers and you will get some advice here on what a realistic power level would be for the amp. As a tube amp tends to clip much more benignly than a solid state amp, you require much less headroom with the power, hence the advice that tube watts are more powerful than solid state watts.

Good luck with the quest, you will find plenty of friendly helpful advice here!

Regards,

Chris
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Old 7th June 2009, 04:05 PM   #3
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I listen to mostly classic rock, but do have of interests as well. The goal is to have the option to drive many speaker types up to the difficult Maggie MGM! I'm green, but i found the ST70 build not to bad. I do have a nice solder station, and fluke, as well as a new scope, and freq generator for trouble shooting. A low powered SET is something i'd like to do for fun later, but for the main system I really had High powered monos in mind. For the price I see even used monos going for on Audiogon, it would seem that DIY would be much cheaper, even with quality parts. This would not be a budget build. However, I would like to be well below the cost of even used commercial offerings. The main requirement would be at least equaling the quality of these commercial amps. If thats not possible than i'd never be happy!
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Old 7th June 2009, 04:57 PM   #4
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TM,

The BALLSY PSU in a McShaned Cit. 2 allows you to get 60 WPC continuously and more than 120 WPC instantaneously out of a PP pair of KT88s. PSU quality is step 1 in executing any good project.

In spite of their hunger for power, "Maggies" are tube friendly, thanks to a reasonably flat impedance curve. As the owner of a "Duece", the only speakers I would not connect are big Thiels, with their NASTY dips down to 1 Ω. IMO, a superior PP KT88 design, as opposed to a PPP design, is all you need.

There is no doubt in my mind that you can get better performance out of a DIY project, rather than spending the cash on something like ARC, CJ, etc.
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Old 7th June 2009, 05:42 PM   #5
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Now were cookin! If you think a deuce will drive the MGM's to decent levels, i'm in!

Now for the schematic? something that could trounce a Conrad Johnson would be nice, but it woulld have to be doable by a newbie. Any proven tested designs around that fit the bill? One that someone here with a pic of the p2p layout would be a plus!

One option may be to use the DIYtube Posiden replacement board for the Mark 2 amps. Perhaps follow that path but with mods and way uprated iron? I'd still want a Conrad / ARC killer!
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Old 7th June 2009, 08:44 PM   #6
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Quote:
Originally posted by TubeMack
However, I would like to be well below the cost of even used commercial offerings.

Very few diy projects can surpass in cost/quality used (<1/3 retail cost) commercial stuff. Once you factor in a presentable chasis and high quality components diy does not look so attractive. Not to mention quite unpredictable sonic results and close to zero resale value. Of course you may turn out lucky and end up with a Reference basher for peanuts but a more realistic scenario is fine tuning a prototype over many months and buying lots more components then you will actually use.
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Old 7th June 2009, 09:50 PM   #7
athos56 is offline athos56  United States
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Default Or

As in my case, you may find that what you built sounds better then anything you've ever bought, for 1/2 to at cost... (plus the added fun of fine tuning, seriously). Take your time, do it right, and don't let the neigh sayers here keep you from building what you want. My only advice is to follow the good advice of the experienced builders that helps you accomplish your goal, keep one hand in the pocket when the amp is on, and find a good/complete schematic to work from, at least for the power section, the PSU can be knocked out with PSUD II and a bit of fiddling. All that being said, good luck and I look forward to seeing what you build.

Oh one more thing, find a good article on star grounding. I read lots of posts about ground issues causing hum and lots of DIY frustration people have spoken about, I've dealt with myself.
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Old 7th June 2009, 11:00 PM   #8
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Quote:
Originally posted by TubeMack
Now were cookin! If you think a deuce will drive the MGM's to decent levels, i'm in!

Now for the schematic? something that could trounce a Conrad Johnson would be nice, but it woulld have to be doable by a newbie. Any proven tested designs around that fit the bill? One that someone here with a pic of the p2p layout would be a plus!

One option may be to use the DIYtube Posiden replacement board for the Mark 2 amps. Perhaps follow that path but with mods and way uprated iron? I'd still want a Conrad / ARC killer!

The H/K Citation II is a stereoblock and its complex, 3 nested NFB loop, topology is not (IMO) something a comparative beginner should tackle. OTOH, the "classic" topology of the Mullard 5-20 IS straight forward. A 6GK5 voltage amplifier and 10M45 CCS loaded tail 12AT7 LTP will provide plenty of gain for KT88 "finals" and the global NFB loop. Pay attention to PSU quality and don't cheap out on the O/P trafos. Better sounding and more durable than either CJ or ARC? I think so. Their primary (sole?) object is profit.
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Old 7th June 2009, 11:11 PM   #9
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hey-Hey!!!,
I'd go for a 6AC7 E-Linear front end. B+ in the low 400's to drive PPP KT88 with a 3k a-a load. For output iron, Citation II iron, or Heathkit W6m( in either original or new Heyboer production ), would do quite well.

You'd be able to idle each tube at 80-90 mA if you keep to something like 420 Volts o the B+ to deliver ~80W or so. With a single pair of KT90 I get ~60W from my W6m. They're currently doing bass service in my bi-amp rig.
cheers,
Douglas
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Old 7th June 2009, 11:21 PM   #10
JLH69 is offline JLH69  United Kingdom
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It's well worth getting a copy of the book 'An Approach to Audio Frequency Amplifier Design' by GEC.

Loads of KT88 circuits in there.
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