Push-Pull KT88 poweramp

[JacksonRR]

Hey guys. New here. I'm looking for a "simple" schematic for a power amplifier circuit that can run a pair of KT88s in push-pull. Looking to achieve 90 watts. Presence control. I know pedals are like 4 piece jigsaw puzzles to you guys and biasing is like shaking it after, but that's my experience. Well that and changing some cathode resistor bypass caps in my preamp as well as swapping blown parts. Thanks and I promise your patience will not be tested if you decide to help.

[überfuzz]

Sounds like a fun project. Are you aiming for a pcb or point to point? I've seen loads of schemes on KT88 amps, but without any ton-control. Seems the KT88 is every hifi nerds wet dream. Have you tried searching the net? You might get more hits if you search for 6550. Depending on how you wire your beast the 6550 and the KT88 are interchangeable.

[defaced]

A pair to get 90 watts is going to be fun. I'm not sure I've seen a design that has achieved that. You're probably looking at a dual rail PSU and high plate voltage. Someone who knows how to draw/read load lines will be better to offer specific advice though. Are you dead against a 4 x KT-88 design? Things get significantly easier when you do that.

[J M Fahey]

As a general idea: search for a Marshall or Hiwatt schematic with 4xKT88, and scale it down for 2.
Whichever you build, you'll have to figure the output transformer.

[jrfrond]

The most you are going to squeeze out of a pair of KT88's is 80W in Ultralinear mode, run balls-to-the-wall at 600V. If you are looking for the 100W region, then 4 x 6550 or KT88 is really what you will need to do.

[Wilder Amplification]

Actually...I'm thinkin' 100 watts out of a pair running under these conditions should be easy -

Dual rail PSU

700V Plate/350V screen

Output Transformer Primary Z = 6.8K plate-plate

However, I highly advise using ceramic octal sockets with plate voltages in that range.

[jrfrond]

Actually...I'm thinkin' 100 watts out of a pair running under these conditions should be easy -

Dual rail PSU

700V Plate/350V screen

Output Transformer Primary Z = 6.8K plate-plate

However, I highly advise using ceramic octal sockets with plate voltages in that range.

I'd worry more about running any NEW KT88's at that voltage, which is why I suggested a 4 tube approach. But yes, ceramic sockets are definitely called for, and preferably not the cheapos.

[imaradiostar]

The Reeves custom 225 and the Hiwatt 200 come to mind as 4XKT88 amps that easily do over 200 watts with modern tubes. I'd say it's about proper screen and plate voltages. I agree that 700/350 volts is a good starting point with a nice high plate voltage. You could achieve that with an Antek Inc toriod. You could also use an Edcor 100 watt output transformer- seems like they weigh 12 to 14 pounds, plenty of iron for big power. You could easily add two extra tubes if you decided to do so later, assuming you had the filament current to supply them.

jamie

[Wilder Amplification]

I honestly don't think there's any sufficient data that backs up the whole "current production tubes can't handle the plate voltage" other than what has been found in high voltage single rail amps. People run CP EL34s in a vintage Marshall that has a PT that is wound for a 110VAC primary, which will put out 500+V easy at that wall voltage and will fry the screens of ANY EL34 running on a single rail supply, then claim that "Oh...EL34s claim to handle 800V on the plates yet I've seen them fail at 500V" when in reality it's because the screens are running on the same rail just a few volts below that, which will fry the screen on ANY EL34.

With this design, sure there's 700V on the plate, but you're running the screens at 1/2 that, which is well below the 600V screen voltage rating of a KT88. On top of that, the Primary Z has been sized to allow for no more than 72 watts instantaneous dissipation @ 1/2 swing, which will amount to 36W average plate dissipation since the tube is below dissipation/in cutoff for 1/2 the swing so the plates won't melt down. Assuming a 600V plate drop, average cathode current should be no higher than 176mA, which is well below the max DC cathode current that a KT88 is rated for.

[imaradiostar]

Hey guys. New here. I'm looking for a "simple" schematic for a power amplifier circuit that can run a pair of KT88s in push-pull. Looking to achieve 90 watts. Presence control. I know pedals are like 4 piece jigsaw puzzles to you guys and biasing is like shaking it after, but that's my experience. Well that and changing some cathode resistor bypass caps in my preamp as well as swapping blown parts. Thanks and I promise your patience will not be tested if you decide to help.

For the record I think building amps is easier than pedals- bigger parts, no pcb's to make! I can knock out a turret or eyelet board in no time flat.

I think a simple KT88 based design is a good start. Just make sure you ALWAYS probe at the amp with one hand behind your back, especially with higer voltages.

jamie

[JacksonRR]

Oh my. I had selected this thread to notify me by email.. I thought noone was interested!! Come back 2 months later...

I never thought about looking up 6550 designs instead, that was dumb of me. It's the little connections sometimes... I'm going to do that when I wake up or else it'll be 5 am before I know it. Again. The only amp I know of that runs 2 KT88s at 90 watts is the VHT 2:90:2. It's a stellar sounding poweramp. Well I've ordered Merlin Blencowe's 3 book package from Amazon after reading a sample chapter. I was so surprised to be able to follow the math formulae. I'm in college now, but was a mediocre student. Have you guys heard of these books? Have I wasted $150? I'm really excited to get busy once I understand a little more than EQ filters. I'm still interested in building a KT88 amplifier. 4xKT88 would be just fine, but expensive that's all. Well thanks for the words of advice, opinions and nudges.

I've heard that "one hand behind your back" a thousand times. My father is has an electrical engineering degree, but he's a dick. He'd rather play golf than help me out with this. He said I'll get a hold of the knowledge if I want to understand it bad enough... Ex-marines, lol. He was a little correct though. Had I seen the replies in August, I would have made small use of them. I've been at it reading theory. I have a problem amp, but that's another thread.

Hey Imaradiostar: The Antec powers supply you mentioned: It's a toroidal. I've read that design introduces less noise and something about the magnetic flux being less intrusive. True? False?

[imaradiostar]

Hey Imaradiostar: The Antec powers supply you mentioned: It's a toroidal. I've read that design introduces less noise and something about the magnetic flux being less intrusive. True? False?

Not really a magnetics guy- I can't answer your question except to say that's what I've heard as well. I built an amp for a guy in an old Valvestate Marshall chassis and was able to package the transformer inside the chassis. I thought it worked well and was easy to mount.

I'm also a student though I'm getting a late start. I was a "music business" major in the late 90's. I built guitar amps in my freshman dorm- had a drill press in there and everything. Now I'm working toward an engineering degree and kicking myself for not doing it when Calculus was still fresh in my mind.

As for your Dad- maybe he's trying to teach you something. My parents got a lot smarter during the 12 years I was away in Nashville. Something tells me they were always wiser than me but I couldn't see it. Of course, it's also possible he's just being a jerk. I've seen that in plenty of dads but thankfully not mine!

What is your power goal? An antek AN-2T230 with the secondary windings in series should produce around 640 volts for the plates and half that for the screens. This kinda of voltage with a 6550 should produce 90 watts without any trouble, given the correct output impedance. It's such an inexpensive transformer that it's worth purchasing one just to try it. Based on the numbers on the datasheet it'll probably sag to just about 600 volts at full power running a pair of big bottles- pretty decent regulation. FWIW, I used a substantially smaller AN-1T200 this way in a cathode biased 6L6 design and it worked well.

I know you may not want to hear this but you might be better off just building an 80-100 watt Fender or Marshall output section. Big bottles (6550/kt88) generally require higher voltages to make big power. That's why I recommend the Antek PT's. Heck, with the 275 volt antek you could get 90 watts out of a pair of EL34s! Plenty of things to try...which one makes you feel warm and fuzzy?

If you're in the mood for simplicity and you're willing to experiment it would be worth it to build an amp with 100 watt iron and a pair of EH KT90's or tung-sol KT120's. Both have substantial plate structures and are likely to do OK pinch hitting for a pair of smaller bottles. If nothing else it would be a good starting point. Make sure you get an output transformer with a few taps so you can mismatch up or down if needed.

JT

[überfuzz]

My father is has an electrical engineering degree, but he's a dick. He'd rather play golf than help me out with this. He said I'll get a hold of the knowledge if I want to understand it bad enough... Ex-marines, lol.

Lol - My fathers interest was somewhat cool when I told him I was building an amp. When it was done, working, he came running like a child for candy. Turns out hes dad, granny, tried to build an organ back in the days, that never saw the brake of dawn.

Back to the amp. Do you have any hardware etc as of now?

[defaced]

Hey Imaradiostar: The Antec powers supply you mentioned: It's a toroidal. I've read that design introduces less noise and something about the magnetic flux being less intrusive. True? False?

Not that Wiki is the best resource, but this will get you going. The answer is yes, they do leak less flux thus do not create as much magnetic noise, thus are "quieter". But if you follow proper chassis layout with an EI core transformer, you shouldn't have a problem. Look under "construction". Transformer - Wikipedia, the free encyclopedia That said, I like the Antek transformer I'm using. 50 bucks for a 400VA transformer with enough HT and heater current is a steal.

[JacksonRR]

Lol - My fathers interest was somewhat cool when I told him I was building an amp. When it was done, working, he came running like a child for candy. Turns out hes dad, granny, tried to build an organ back in the days, that never saw the brake of dawn.

Back to the amp. Do you have any hardware etc as of now?

Yeah, I have a blank head-type chassis, a 4 unit rack-mount enclosure, plenty of here and there components, PCBS for etching, enough sockets for the project. I don't know whether rack-mounting it is best for my beginner situation. I just grabbed it anyways. I kinda just bought with the intention of figuring it out as I went.
I'm having some difficulty in finding a 2x6550 based schematic.
I found:
http://www.drtube.com/schematics/ampeg/svt6550-jp.gif

Since we already mentioned that avenue of research, aren't 6550 and KT88, KT77 replacements for EL-34? Can I just use a modified EL-34 design?

Ok, if I was to maybe add a preamp to the project... hehe. Do you guys see any flaws in this?:
http://www.mif.pg.gda.pl/homepages/t...l_Scr50pre.gif

It's one that a member did and built, not one of those wacky schems ENGL had on their site.