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PostPosted: 03 Dec 2010, 22:04 
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Joined: 16 Nov 2010, 22:19
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Hello. I have just finished building my 2nd tube project. First i built a Valvecaster pedal and now the 12AU7 Tube / MOSFET Headphone Amplifier. So far I have been 100% successful on both. And for my engineering class at school, I got a good grade on both, so I am pretty ecstatic :mrgreen:
However, I need to build another project in the class, because the more projects I build, the better grades I get, and my teacher loves to see my projects a lot(especially the fact that its from a 15 year old...).

I wanted to build a tube amplifier, a high end one. My teacher has these big auditorium speakers in the classroom(Our auditorium is being reconstructed so he got to keep the speakers because they were going to trash them :eek: ), a professional rackmount Graphic EQ, and he said if I want, I can build a tube amplifier that can drive these speakers pretty well, and use the EQ. Well I just want to build one period, and He gave me the choice of the project. But I want it to be able to sound nice and bassy(Bot not overly bassy) and a good treble level for metal music. I was thinking about building the 6L6 / 5881 Single-Ended tube amplifier. However I dont have a lot of experience with tubes, just the 2 projects i built, and for the headphone amplifier, i had to sketch my own filter circuit from ExpressPCB to cut out the AC ripple, and to my surprise, it worked...

But anyway... would this be a good project to build for my kind of music? And if so, is it hard to build and understand how to?

Edit: Oh and before anybody notes this, i do credit everybody who makes these designs and I always make sure that the link, company name, project name, and original schematics are credited and shown.


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PostPosted: 03 Dec 2010, 23:08 
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Hi,

Auditorium speakers will be typically high efficiency, so a lot of power isn't needed.

They also are incapable of reproducing lows, unless they are bass drivers. Forget about trying to make them, you may make the voice coils jump the pole piece (then you're really in trouble).

Would be nice to know the brand/model.

Cheers!

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PostPosted: 03 Dec 2010, 23:14 
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idk what the brand was, but they were powered by a seriously high powered solid state amplifier(which weighs a ton)... idk but it is pretty big. I dont have to use those speakers, I also have 2 speakers laying in my garage that need some kind of use... They are 2 big speakers, 25W 4 Ohm. They were part of my old stereo system that broke due to cheap buttons, so i opened it up and siphoned the pieces out of it which helped in making a lot of my projects. I was planning on using those. Could this tube amp drive those?


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PostPosted: 04 Dec 2010, 22:14 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
Hi!

For that kind of music I strongly advise KT77 or KT88 tubes! :thumbsup: They´ll put a smile on your face and drop your chin! :)

Cheers,
Miguel


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PostPosted: 04 Dec 2010, 23:01 
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I have to agree with Miguel on that score. The KT88 is the same tube as the 6550 power tube and they make for amazing amps. You could even build a stereo triode strapped KT88 PP class AB1 amp and still get 30w per channel and have an all triode signal chain. The Edcor XPWR016-120 has the voltage and current required and even has a 70 bias winding.

Now for the important question. Exactly how much money :$: do you have to sink into the project? I'm sure people will suggest all manner of possibilities however, in the end, your limiting factor may well be your budget. Does this need to be a $250 type of amp or a $1000 type of amp? Or maybe somewhere in between? It also depends on how sensitive are the speakers. If they need 20W or 25W to really get rolling then a single ended amp is probably out of the question. If they sound good and produce enough volume at 8W or 10W then a SE amp would probably be a good choice.

You did say that you'd like to build an SE amp. In single ended operation the KT88 can deliver up to 20W but the distortion is a little high (13% - Ds=0.68%/W). If you drop the output power you can get 12.5W with a more acceptable 7% distortion at full output (mostly second harmonic - Ds=0.56%/W). The 6L6 will give you about 11W in single ended operation but the distortion is 15% (Ds=1.36%/W).

So the questions you need to ponder are: :confused:
1 - How much power is really required to drive these speakers?
2 - How much money are you willing to spend on the project?
3 - How complicated a project do you feel comfortable doing?
4 - Are you committed to an SE design or are you willing to consider something like an oddwatt where there are lots of people with all kinds of experience to answer questions?

Answer these questions and you'll be much farther along in making a decision about what project to build. ;)

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PostPosted: 05 Dec 2010, 14:31 
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Joined: 16 Nov 2010, 22:19
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Suncalc wrote:
I have to agree with Miguel on that score. The KT88 is the same tube as the 6550 power tube and they make for amazing amps. You could even build a stereo triode strapped KT88 PP class AB1 amp and still get 30w per channel and have an all triode signal chain. The Edcor XPWR016-120 has the voltage and current required and even has a 70 bias winding.

Now for the important question. Exactly how much money :$: do you have to sink into the project? I'm sure people will suggest all manner of possibilities however, in the end, your limiting factor may well be your budget. Does this need to be a $250 type of amp or a $1000 type of amp? Or maybe somewhere in between? It also depends on how sensitive are the speakers. If they need 20W or 25W to really get rolling then a single ended amp is probably out of the question. If they sound good and produce enough volume at 8W or 10W then a SE amp would probably be a good choice.

You did say that you'd like to build an SE amp. In single ended operation the KT88 can deliver up to 20W but the distortion is a little high (13% - Ds=0.68%/W). If you drop the output power you can get 12.5W with a more acceptable 7% distortion at full output (mostly second harmonic - Ds=0.56%/W). The 6L6 will give you about 11W in single ended operation but the distortion is 15% (Ds=1.36%/W).

So the questions you need to ponder are: :confused:
1 - How much power is really required to drive these speakers?
2 - How much money are you willing to spend on the project?
3 - How complicated a project do you feel comfortable doing?
4 - Are you committed to an SE design or are you willing to consider something like an oddwatt where there are lots of people with all kinds of experience to answer questions?

Answer these questions and you'll be much farther along in making a decision about what project to build. ;)


Ok here are my answers, hehe:

1. 25W is the highest/maximum rating to drive the speakers at full volume. They used to hook up to a stereo system i once had, but I cannot remember its output unfortunately. I have 2 of them, and I would like to use them with a stereo tube amplifier if possible.
2. My budget is not very high, so I was thinking around the 200-300 dollar range. If that is even possible :worried:
3. I want one that will allow me to learn easily. I guess im not a novice, but im not an expert... I guess somewhere around intermediate. I want to do one that is complicated, but not overly complicated where only a licensed engineer can handle(Plus I dont know much about inductors, which ive been trying to understand. Random i know, but i guess it kind of shows how much i know...).
4. I dont know much about what a SE tube amp is or an oddwatt. I know that oddwatt is a company that sells kits(It cant be a kit, i have to build it straight from a schematic to get a grade.). And is a single ended tube amp just mono?

I guess maybe i still am quite a notice at this kind of stuff...


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PostPosted: 05 Dec 2010, 17:32 
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Joined: 05 Nov 2010, 21:07
Posts: 702
Location: South East US - Tennessee
Since Matt and I piled up, I am pulling my input. His sounds better anyway - :)

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board


Last edited by Les on 05 Dec 2010, 17:53, edited 1 time in total.

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PostPosted: 05 Dec 2010, 17:41 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
Ok. This is a really good start.

Lets start with the terminology. A single ended amp is one that uses only one tube (or set of paralleled tubes) to drive the output transformer. Single Ended (SE) amps can be any number of channels (mono, stereo, quad, or n-channel studio amps). The other popular output stage topology is Push Pull (PP). In a push pull topology two matching tubes (or two sets of paralleled tubes) are used to drive the output transformer for each channel. In an SE amp the distortion is primarily 2nd harmonic and is roughly proportional to the volume setting (i.e. as you turn up the volume the distortion increases). In a PP amp the distortion is primarily 3rd and 5th harmonic (even order harmonics are cancelled in the output transformer) and the distortion is roughly constant throughout the amplifier's power range. The distortion is also generally much lower then the SE distortion at full power. PP amps are more complicated then SE amps but they also deliver a lot more power. SS and PP amps also have much different "sounds". Which is "better" is a topic of endless debate (and it's a debate that will probably never be settled).

1. The 25W limitation is a good thing. It gives you the choice of either SE or PP topologies. For comparison, given that 25W through your speakers will generate a certain SPL (volume), then a 12W amp will produce 3dB less and a 6W amp will generate 6dB less. So for these speakers you might want to start with a simpler SE amp that produces between 5W and 10W per channel, and maybe later build a PP amp with more power and get a set of more efficient speakers for the SE amp. The bottom line is that you have lots of options.

2. Your budget of $200-$300 is very reasonable. The thing to remember is that the iron (power and output transformers and maybe a filter choke) is probably going to consume the largest portion of your budget. So this is where the critical decisions will really effect the bottom line cost.

3. It sounds like you are a little leery of starting a project that's too complicated. SE amps are usually the simplest. Usually just one tube per channel for an output and single dual triode tube for a driver. (1.5 tubes per channel) PP amps are more complicated. The traditional PP topology uses a phase splitter to drive the output stage (this adds another triode or two per channel). So you end up with two power tubes, a phase splitter tube, and maybe an additional driver for two channels. (3.5 tubes per channel) The reason I mentioned the oddWatt is because it is a PP topology that does not require a phase splitter because it uses a constant current source in the cathode path of the two output tubes. It's a neat little trick that simplifies PP topology and there are a lot of people on the forums who have built amps in this fashion. This results in two power tubes, one driver element, and one CCS (usually a solid state voltage regulator wired for constant current) per channel. Regardless of which topology you choose, I guarantee that you'll learn a lot!

You'll need to choose a rough power level you want and then chose an output topology and tube. For lower power SE amps I really like the 6V6 family of tubes (4 to 5 watts) or the 6BQ5/EL84 series (5 to 6 watts). The 6L6 is also nice (5 to 10 watts) but I think the sound is not quite as smooth as the other two. For PP you could use any of these or the popular KT66 (although the KT66 and KT88/6550 seem to like higher voltages which will drive up the cost of your power supply significantly).

I hope this helps. Just make sure that what ever you build it's something that you'll enjoy building. :)

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It's all about the Glass!
http://www.CascadeTubes.com
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PostPosted: 05 Dec 2010, 18:20 
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Location: USA
OK! But Oddwatt produce 2-3 time less output power, than regular PP. With tubes, KT....You will have about 20 wtts
per channel

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PostPosted: 05 Dec 2010, 20:40 
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Joined: 05 Jan 2010, 21:16
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Location: Las Vegas Nevada USA
I've pondered the question of how to build an inexpensive class A PP amp in the 15 to 20 watt class for those on a budget. How about the following as a start for an Oddwatt type amp but with part substitutions?

First power. I'd use two 120/120 volt isolation trannys in series for a 240 VAC, 200VA, $20 power tranny. Three could be used for a voltage that exceeds 500 after rectification:

http://cgi.ebay.com/ISOLATION-TRANSFORMER-POWERTEC-USA-P-N-02-0010-PRI-120V-/220700656500?pt=LH_DefaultDomain_0&hash=item3362c8c374

"6L6 class" Russian 6P3S tubes in push-pull, a quad for $30:

http://cgi.ebay.com/4-x-6P3S-6L6-6L6GT-Tubes-1960-FOTON-pl-Military-OTK-/280588930387?pt=Vintage_Electronics_R2&hash=item4154674d53

and Edcor 15 watt output trannys for $37 each:

http://www.edcorusa.com/products/409-gxpp15-6-8k.aspx

You're now at about $130 for all the power tubes, and all the transformers except the heater supplies. These costs include shipping. You have a long way to go, but if you don't spend money where it's not needed you might come in under $250. If you're willing to go with Russian tubes and buy from an ebay store you can probably also find some inexpensive PIO caps and driver tubes and not have to pay additional shipping.

IMO your budget dictates a SS power rectifier and capacitive filtering, so your next big ticket item will be the high voltage filter caps for the power supply.

All I'll add is that IMHO I would build as powerful of an amp as my budget would allow. You never know what speakers you might wish to drive in the future, and building a 5 watt PP amp is just about as difficult as buildng a 25 watt PP amp, it just costs more for parts for the more powerful amp.

Even more importantly, over-building for power will allow you to run the amp at reduced power with a lower output impedance and a higher damping factor for tigher bass as long as you buy either a multi-tapped secondary or design for that mode of operation by purchasing an OT that has a secondary tap designed for speaker's with half the impedance. You'll also probably get less distortion overall since the plates of the power tubes will see a higher impedance load and the amp will sound better.

So if you don't use big power tubes and OTs for power, use them for low output impedance (within your budget).

BUT remember, a 20 watt amp is only 3dB louder than a 10 watt amp, so if you have to restrict yourself to lower power it's not that much in terms of output volume.

Just my 2 cents. :soapbox:

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