| Fiat1 |
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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| kevinkr |
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. |
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| Bandersnatch |
hey-Hey!!!,
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.
cheers,
Douglas |
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| gingertube |
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.
http://www.evatco.com.au/project.htm |
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| 240z4u |
Jim McShane reccomends several tubes KT88 wise, he sold me the gold lion reissues.
Evan |
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| richwalters |
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.
richj |
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| itsmejto |
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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| hotbottle |
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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| Brett |
| quote: | 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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| kevinkr |
| quote: | Originally posted by Brett
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. |
Not to mention that with most currently available output tubes life will be short under the conditions required for 100Wrms output. There is a good reason why most 100Wrms+ hifi amplifiers use 2 or more pairs of output tubes..
Brett is right about the sound quality issue as well, my findings as well.
I built a few 120Wrms per channel amplifiers for clients using quartets of Svetlana 6550C - 8 yrs later the originals are still running fine. (B+ 500V and idle current of 50mA per tube.) |
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| SpreadSpectrum |
| quote: | | ...otherwise my only option would be one of the Hamonds such as the 1650P |
Wouldn't the 1650R be more appropriate if you are shooting for 100W?
Edcors are another option. |
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| Driller99 |
Fiat1,
There is a schematic and instructions to build a 125W per channel KT88 amp in AudioXpress magazine of June and July 2006. The amp is calle MAD KATY. Good luck and keep us informed.
:drink: |
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| kstagger |
just an odd idea I'll throw out there - check out the 50W plate EL156 tube. The Shuguang copies aren't bad - I've abused one in a SE amplifier test setup and it appeared to be quite tough. Eventually planning to make a pair of monoblock SE amplifiers (once I'm done moving to my next house).
http://www.mif.pg.gda.pl/homepages/...128/e/EL156.pdf |
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| richwalters |
I'm tempted to have a relook at the EL156.....after being dissapointed with KT90's this could be a better avenue. There are some similiarities.
Anyone using them in p-p ? Getting the sockets could be awkward or not ?.
rich |
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| Bandersnatch |
| quote: | Originally posted by richwalters
I'm tempted to have a relook at the EL156.....after being dissapointed with KT90's this could be a better avenue. There are some similiarities.
Anyone using them in p-p ? Getting the sockets could be awkward or not ?.
rich |
The Chinese EL156 is an octal base compatible with 6550/KT88/6L6 tubes. No problem there, unless finding octal sockets troubles you...:)
cheers,
Douglas |
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| kstagger |
The EL156 does require a higher filament current. 1.9A (EL156) vs 1.6 (6550). It also biases a little lower - eg, when I tested them on a a pair of Dynaco Mark III refurbed amps, I replaced the 10K bias pot with a 25K. I may have increased the resistor before the bias pot, but I can't remember off hand.
Also, in my SE experiment, I never went over 500V, so I'm not sure how the Shuguang version hold up after that. |
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