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100 watt KT88/6550 Designs
100 watt KT88/6550 Designs
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Old 12th July 2007, 11:11 AM   #1
Fiat1 is offline Fiat1
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Default 100 watt KT88/6550 Designs

Can any one direct me to a good simple KT88/6550 design? Looking for push pull with ultralinear, probably operating in class AB1. Want around 100 watts, which the KT88 should easily do.

Has anyone got a design for some output transformers that could suit such a design? I want to get some prices on having some OPT wound, otherwise my only option would be one of the Hamonds such as the 1650P.
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Old 12th July 2007, 01:44 PM   #2
Sherman is offline Sherman  United States
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Quite a few PP KT88 schematics here-

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Old 12th July 2007, 04:49 PM   #3
kevinkr is offline kevinkr  United States
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100 watt KT88/6550 Designs
While 100W is quite achievable with a single pair of KT88, with current production tubes that is really pushing it. For good long term reliability I would recommend a quartet of them in UL PP with plate voltages not significantly higher than 500V.

About 60Wrms is the most I would ask from a pair.

I designed and built a few 120Wrms per channel stereo amps commercially using 6550 and KT88 and achieved good reliability.

Incidentally the current JJ KT88 seems to have build quality issues, and I have had reliability problems with them at >525V. (Catastrophic failure is an understatement.)

I would check with Jim McShane for his take on the current best bets sonically and reliability wise.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 12th July 2007, 11:51 PM   #4
Bandersnatch is offline Bandersnatch  United States
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I'd question the good sense in building a max-power/AB1 design. Or a parallel PP design for that matter. If you need that kind of power, get tubes with bigger plate dissipation ratings.
the Tnuctipun will return
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Old 13th July 2007, 12:13 AM   #5
gingertube is offline gingertube  Australia
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I built a prototype mono-block several years ago using a quad of Ultralinear connected KT88 driving a Hammond 1650T. It was very good but other projects took priority and the second channel never got built.
I used a CAE Dynaco MK4 front-end board to drive 6SN7 cathode followers to drive 2 pairs of KT88. I would probably do it a bit differently if designing it today BUT it may give you some ideas.

Schematics for the Amp and Power Supply and a "brag" document can be found on the Projects pages of Evatco website. Its the 4th project down at this link.
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Old 13th July 2007, 11:39 PM   #6
240z4u is offline 240z4u  United States
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Jim McShane reccomends several tubes KT88 wise, he sold me the gold lion reissues.

Yeah... it seemed like a good idea at the time.
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Old 16th July 2007, 07:32 PM   #7
richwalters is offline richwalters
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Get your tube vendor to tap test'em on burn in. I've had Svet 88's and JJ 88'S making rude noises when pushed at 500V+. Worse with KT90 when internal shorts occurred so regulary that I had to fit a fuse in the centre tap o/p tranny.
Running 6550C at 460V max in parallel pairs UL p-p should get one close to 100W around 2K a-a. Don't go and ruin the performance by using a crappy driver stage. The 6550C new version quotes 100K grid leak in fixed bias. I use half this but demands a slam driver stage capable of 50+50V rms at low Z at low thd. Once I get the software to run I will post suitable circuits.

The drawback is power supply issues...For "full pull" in a stereo pair in AB1/2 one is looking at a psu capability of 8x 0.15Ax470= 565 Watts with some reserve headroom for transients. I use a 1kW beast for min B+ volt drop between min and max sig. Amp weight becomes a measurable issue. A skimpy psu will ruin performance.

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Old 31st March 2008, 12:40 AM   #8
itsmejto is offline itsmejto  United States
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I just completed modifying an ELLA kt88 amp to run 100watts rms per channel.
Running the plates at 600vdc @ 140ma each and a pair of hammond 1650R output transformers.
B+ using Cree schottky diodes (bridge) and a power transformer from ebay China that supplies 550vac @ 750ma (and 3x6.3@5a and a 70v bias tapping)several Rubycon 450uf @400vdc in series parallel.
I need a choke that can handle 500ma and still fit under a 2" tall cassis if anyone can help.

Quad of re-issue gold lion kt88's.
Not expecting the tubes to last any where as long but it's holding up so far and makes a lot of smooth noise.
Just to see if it can be done easily.
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Old 31st March 2008, 01:15 AM   #9
hotbottle is offline hotbottle  Australia
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Just to chime in on power available from a pair of tubes, and the need for good quality tubes.

I fixed up a bass guitar amp that put out over 80 watts from a pair of EL34 tubes !! It achieved this with a voltage doubled power supply mof 700+ volts. It blew fuses regularly, and when I looked, it had a pair of el-cheapo chinese noname EL34s in it. I put in a pair of Sovteks that were said to be rugged, and no more problems were had.

Now if it can get 80 watts reliably from 2 EL34s, then I am sure 100 watts can be done with KT88s as well, but as others have said, you might need to choose good quality tubes.
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Old 31st March 2008, 05:49 AM   #10
Brett is offline Brett
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Originally posted by hotbottle

Now if it can get 80 watts reliably from 2 EL34s, then I am sure 100 watts can be done with KT88s as well, but as others have said, you might need to choose good quality tubes.
MI watts are usually rated at much higher THD than hifi watts, even for the same basic design. I'm not confident of a good sounding 100W from a pair of KT88 in AB1 fixed bias. Last one I tried was awful. At 70W (lower B=, higher Ia, different OPT config) it was quite nice, but was never worked on further; ran out of clipleads.
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