Recommendations on power tubes for my new amp project

I just got myself a power transformer suitable for makeing a a hefty SE amp and I am looking at options for output tubes. I am going to be running around 700-800 volts and I am looking for tubes that will get me at least 20 watts. I was going to use a KT88, I have been wanting to try one of these out running at its upper limmits for awhile now, but I decided to ask around and see what else is out there. This is going to be a utility amp pretty much, mostly for recording bass and guitar, but synths will probably get some use of it also. I mainly want to get an SE amp that will end up with a good range for bass. 6V6 and 6L6 SE amps just do not give a whole lot of breathing room with bass.

Also, is there some reason unknown to me why no one seems to run KT88s SE and with high plate dissapations? Even KT88 datasheets seem to either completely ignore SE amps or just list characteristics of the tube running well below its ability. I suppose I should do some math and figure some loadlines to see if i can answer my own question first. Or maybe its just that they assume normal people will just run Push-Pull instead of pulling 25 watts from a single tube? Would save quite abit of cash on the output iron, but I really perfer the sound of the SE amps.

adam

I built a fender style 2x 6L6 amp primarily for bass guitar. It has an oversized output Xfmr I pulled from an organ with the same power tube setup. Being oversized It gives me a lot of headroom for the low fundamentals.
For you, I would go with a 50w amp. 20w won't give you much when it comes to bass. My 50w works great in the studio and I can also use it for small coffee shop gigs.

As far as other tubes you can get a lot out of KT88's and 7027's (30-40w each) I would steer clear of EL84's and 6V6's (5-15w each) due to their low output. I would go with 6L6's (20-30w each) because, with bass you will want plenty of headroom, not just volume. As far as Xfmrs, unless you are going after a certain tone, always oversize your output transformer to maximize your wattage and it's life.

Oh, I read your post again and realized SE = Single Ended. There are some tubes for Hifi stuff rated at 125w each, I think JJ sells them.

Personally, I have never been a fan of single ended amps for instruments with low end, simply because most are low wattage, and they can break up on the low notes. I would definitely try using a KT88 or check out some HiFi amps and how they are built.

Now that I think about it, this guy makes a really cool single ended amp http://www.aikenamps.com/Europa_MKII.html
He is really cool and likes helping out DIYers. He probably has some good info on his page. If not I?m sure you could email him and he would point you in the right direction.
http://www.aikenamps.com/

The Aiken Amps site was one of my main sources of information back when I was just starting to learn to work with tubes. Alot of good information in the tech pages there. Unfortunately the Europa amp the KT88s well below their ability, to keep power levels low enough that the other tubes that can be swapped in do not get fried. I have been wanting one of these amps that you can just swap power tubes in and out for awhile now, but that would not get me the power I need. The highest powered tube JJ seems to make is the KT88, at least that is the highest powered tube on their webpage. I think you might be thinking of the Svetlana SV572-160 which has a plate disapation of 125 watts, but it looks like it will only get about 25-30 watts of actual power output before things really start distortion. They also have some even high power tubes, but to see any advantage of useing them I would have to go up 2KV or more, and I have no need for that much power anyways, nor could I addord the parts or the power bill. They do have High power triodes there though that might be fun to give a try. Thank for the responses.

adam

> they assume normal people will just run Push-Pull instead of pulling 25 watts from a single tube?

Over about 5 or 10 watts, any sane designer will go push-pull. It takes an extra tube, but the balanced DC makes the output transformer so much cheaper that it nearly covers the cost. The P-P design will make more output power with lower THD numbers on less power supply and filtering, saving enough extra bucks to more than pay for the extra output and driver tube.

If you are an insane designer, you don't care what the factory book says: you are going to do it anyway.

And there were no insane designers who sold enough amplifiers to raise a blip on tube makers' sales charts in the 1930s-1980s. The guys moving a lot of boxes (and buying a lot of tubes) were making as many watts per buck as they could: hence 6L6GC and 7027 (hot 6L6) and the 6550 and KT88 (essentially oversized 6L6) beasts working push-pull.

> 700-800 volts and I am looking for tubes that will get me at least 20 watts.

The transformer frequency response would be better at a lower impedance than you are going to need with 800V for 20W. If you mean 20 gross watts with large distortion, the tube will have to idle at least 40 watts (more than many of these types are rated). 40W at 800V is 50mA. An idealized 800V peak swing at idealized 50mA peak current is a 16K load impedance. Leakage inductance and stray capacitance will make the high end lump-up. And there is no need to run the big radio/PA tubes at 50mA. I'd be aiming at 400V, 100mA, 4K load, a more common impedance, less voltage stress. (Remember that with a 800V B+, the plate pin and one end of the transformer winding will swing to 1,600 volts!)

I'm insane, but not had a reason to run dozen-watt SE amps. I can tell you that a 6L6GC or 6550 working triode at 400V, 50mA, 10K load (underloaded) is a very sweet two watt amp. And two 6L6 or 6550 doesn't raise the power much (because load impedance was the same) but sounds better. So you might consider a quad of big tubes working 400V-600V at 50mA each (200mA total) into 2K or 3K load. It would be a heck of a lot of heat, but pretty authoritative, and I think you can buy a transformer like that.

The original 6L6 has a SE Pentode rating of 10.8W (at 15% THD!). With 6L6GC you should be able to make 15 watts the same way, with plate and screen voltages and cathode current 23% higher, same load. Two 6L6 with the original voltages and half the load impedance gives you your 21.6 watts on paper. (Remember that any hi-fi SE transformer has 10%-20% power loss. Takes a lot of copper to give bass response when the iron wants to saturate. 20% less power is hardly less to the ear, just don't be shocked when you measure.) Two 6L6GC, 6550. KT88 etc with slightly higher voltages should get you to 25, 30, or more watts, so 20 watts after transformer losses is a cinch.

Most poeple do not seem to understand why bother with higher power SE amps, I just really prefer the sound of the power tube distorting in SE amps. For some reason I get caught up in the idea of High power single ouput tube SE amps everynow and then, but someone generaly gets me back on the better track of Parallel SE operation. I think the Biggest advantage of running around 400 Volts would be it would give me alot more ability to experiment with differnt output tubes in the amp.

I experimented with dual 6V6 and 6L6 amps awhile back, i did get some nice sounding resaults, but the power increase was not really enough to deal with finding a place to store another amp. That is pretty much what i expected at the time anyways, it was more of just playing around for the sake of playing around.

A pair of KT88s might be the best way to go. I should be able to get the power I want with most likely better specs then I would have with jsut a single KT88 running at its extremes. It will deffinetly be easier on the tubes. After I get some sleep I can do some math and get this all figured out proper.

adam

> prefer the sound of the power tube distorting in SE amps. ... someone generaly gets me back on the better track of Parallel SE operation.

Not necessarily better. But the available tubes favor either very high impedances or under 10 watts per tube, with parallel to get more.

Yes, 400V is a nice number for most of the available tubes: 6L6GC, 7027, 6550, KT88. Since these are generally 35 watt plates, for best power you need to bias them to about 80mA per tube. The approximate load impedance for best power in pentode operation is pretty nearly the plate voltage divided by the idle plate current, or about 5K. (Graphical or test-bench operation will give a more exact number, but with available transformers and real speaker impedances, V/I is always a good enough approximation.)

When I think of an SE amp, I mostly think of triodes, the classic 300B SE for example. How much money do you want to spend? There are a lot of options.
A NOS KT88 in the box can run you 350 bucks! Same with the 300B.
Other triodes are the 211 and 825. As well as the Sevetlana stuff. There are many makers now for the 300B, including Chinese stuff.
Want to go low dollar? There are sweep tubes for 50 cents at Antique Electronics that have been used in SE amps. Some parallell circuits use these sweep tubes in SE.
You have to decide on the tube before you decide on the iron. The iron is half the sound in an SE amp.
I have some circuits for weird SE tube amps if yoyu are interested. The power supply is important in an SE amp, you will need a good choke, which can be expensive also.
cj

> When I think of an SE amp, I mostly think of triodes

Yes, but millions more SE pentodes were built. Think of all the tube radios 1935-1965. Rare to find anything but SE pentode.

There is a fad for SE triodes again in golden hi-fi. But I scent that adam is looking at a musical instrument amp. The "flaws" in an SE pentode can be virtues in some electric guitar work.

> decide on the tube before you decide on the iron. The iron is half the sound in an SE amp.

The iron is more important than the tube. However you don't really have to pick an exact tube before you begin, just approximate plate voltage and dissipations that you know you can find a tube type (or several
tube types) for. 300V-450V and 50mA-100mA per socket allows use of any of the large octal power pentodes with slight re-biasing and gain change. Once you know the B+ and the available supply current, you have pretty much set the load impedance needed to get best power from pentodes. Triodes are a little different: their losses are large so maximum power may not be B+/Ik impedance, and their distortion goes down as impedance increases so you have a judgement-call to make. Pentodes have low losses and their distortion never gets as low as an underloaded triode. Small shifts in impedance do affect a pentode's balance of 2nd to 3rd harmonic, but speaker loads make precise calculations futile.

I have thought about useing triodes, and one of these days I will most likely give a triode a try, but right now I just trying to get that SE sound with abit more volume. A pair of KT88s is looking like the best way to go, and as PRR said, I will also have the ability to swap other kinds of power tubes quite easily, new tubes, bias, check the load and off I go. I am not going to bother with triodes now, with the pentodes I have a good idea of where I am going and what I end up with. I do not have much of an idea of what to expect with triodes, someday I will build a single triode amp with a 2A3 or something and see how it works.

The only transformer I have picked out is the power transformer, and thats just any one of the many I have in my collection. I need to save money somewhere after all. The output transformer will most likely be either from the Magnaquest Air Gapped series of the Hammond 1600 series. So thats going to be around $200 or more. Two KT88s are going to be close to $100, I am not going to bother with NOS KT88s, I have to accept it eventualy and stop spending so much on NOS tubes. Then I still have the choke, filter caps and everything else to pay for. This is going to expensive for a SE instrument amp no matter what. I do not think I spent over $150 on any of my other SE amps, but those also used alot of parts that I had one hand. I do not have much for 600 volt caps sitting about.

I would be very intrested in seeing some of those odd SE amp schematics you have. I am always looking for those odd circuits to try out. Sweep tubes are one of those things I have thought about playing around with for awhile now, but have not gotten around to giveing a go. Tubes like the 300b I do not have much intrest in, I can not justify that amount of money into a single tube. Although what I can justify on a single tube seems to increase with price of black plate 6V6s and 6L6s.

adam

Adam;
Do yourself a favor and contact Kevin Carter at kandkaudio.com
He is known here as KevinC.
We have a small group Piedmont Audio Society. You pro
guys that do not listen and hate HiFi guys need to cover your ears now.

In the group we build about 6 large transformer coupled usually push pull
directly heated triode amps a year. That is 6 stereo channels.
Kevin has a method to his madness and the amps sound better than
any thing you can buy. Build and learn.
Kevin will glady talk give him a call (during business hours preferred).