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Old 15th January 2004, 07:28 AM   #1
DVDHack is offline DVDHack  Australia
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Default Build with 300B or something else???

In another thread on SE OTFs Angel (surely a deceptive tag with these sort of questions) asked if I had considered SV572-3 over 300B. Given I know nothing about valves and chose 300B due to the reputation and level of support available on the web I thought I'd start another thread.

I want to build the best amp I possibly can, I don't want to build amp after amp. (may not end up true but I'd like to start there)

What is the best set of tubes to base my amplifier on keeping in mind that I really want to use an existing design and not have to become a valve engineer.

At this stage I'm not considering cost but woundn't mind feedback on value for money.

Thanks a lot


Ralf
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Old 15th January 2004, 08:12 AM   #2
angel is offline angel  Norway
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I'm glad you have decided to search for more information before choosing a path.

First off, if you are considering value for money, the 300B is certainly not the way to go. It is one of the most expensive valves around, largely due to its cult-like following.

Secondly, you need to decide whether you want a romantic sound, or a neutral/accurate sound. A romantic sound is typically acheived by using valves that have relatively high levels of distortion, and not properly using them to their maximum potential.

If what you want is neutral/accurate reproduction, I would suggest you forget the 300B. While it is more linear than many valves that are used for audio, it is not nearly the most linear valve available. And its power output is limited to about 8W when operated with a sensible load.

Taking cost into account, you have certain alternatives. I'll list them here, and bear in mind that power output for push-pull is typically four times that of single ended. I'll be listing approximate values for single ended.

(A) You could go with a triode of some sort. In doing so, I would carefully consider linearity, lifetime, availability and cost. Good examples of readily available triodes for audio use, that are not overly expensive, include the SV572-3 (25 watts), SV811-3 (15 watts), 6C33C or 6C33C-B (15 watts), 6AS7 (2-3 watts) or 6C45P (1 watt). There are other valves that would be even better, such as the broadcasting triodes by Svetlana, but those require special care, and are likely unsuited as a first project.

(B) You could go with a pentode. Mostly the same concerns, but they tend to be even more common. Pentodes have more odd ordered distortion than triodes, but universally labeling them as bad is just yet another audiophile myth. I could nominate e.g. 6550 (15 watts), EL34 (10 watts), EL84 or SV83 (3 watts). Note that the EL34 designs tend to be a bit low on detail, and slightly 'fat' sounding. I'm not convinced this needs to be so, and the Lundahl application note makes a good case for the EL34. Note also that the 6550 could easily be accomodated by changing that design slightly, which I'm sure people on this forum can help you with.

If you intend to use this amplifier for high-sensitivity speakers, I would recommend going with the 6AS7 (very available, low cost, rugged and durable, offers 2-3 watts, which is enough for speakers with a 90dB sensitivity or better, if you don't require very loud listening volumes), the SV83 (very available, low cost, rugged and durable, also offers 2-3 watts) or the 6C45P (not so available, high cost, extremely rugged, but only about 1 watt, which ups the requirement to about 95dB sensitivity)...

For medium sensitivity speakers, I would go with the 6550, 6C33C or 6C33C-B. Either way, you get about 15 watts of single ended power at low distortion. The 6550 was used to great effect by Cayin in one of their amplifiers, while the 6C33C-B is routinely employed by Balanced Audio Technologies. Both these valves are readily available at reasonable prices. The 6C33C-B has a low life expectancy, however, so beware.

For regular speakers, the SV572-3 is the only 'regular' valve with good availability at reasonable prices, capable of delivering enough power, about 25W in single ended configuration, to drive speakers whose sensitivity can be as low as 85dB (if you don't need a disco in your home), or maybe even 80dB.

If going for a push-pull solution is acceptable to you, the options are a bit freer, ranging from a few watts with two 6C45P, to a whooping 120W with two SV572-3. In push-pull, you also get the (realistic) option of building output transformerless amplifiers, although I would not recommend it out of concern for their reliability.

Considering that this is your first project, I would suggest you go for either the 6AS7 or 6C33C-B, as they are quite comfortable with voltages under 200Vdc. 6550 if you want to be a bit more ambitious about the voltages involved. The SV572-3 is absolutely lethal, with a recommended voltage in the range of about 800V for high output power.

Another question you're going to bump into, is that of driver valves. Along the same lines, for the 'low voltage' designs, I would recommend the 6N1P or 6N30P. These are readily available (although the 6N30P is a bit expensive), and have a medium gain, which is okay, since you can use two voltage gain stages, or an interstage transformer. (Both have low impedance, and are therefore suited to stepping up with a transformer)..

For semi-high voltage designs, the 6SN7, 6FQ7 or 6CG7 (the latter two are technically the same valve) should do, as far as triodes are concerned. For pentodes, I seem to recall there are a great number of low-capacitance, relatively linear, high voltage pentodes available.

I am unfamiliar with suitable driver valves for high voltage designs, such as the SV572-3, and would recommend an interstage transformer to solve that problem.

Either way, single end or push pull, high voltage or low voltage, triode or pentode, I would recommend a small amount of local feedback, at least around the driver stages. Valves have a high gain variation, and if your gain is off in the slightest, stereo imaging will suffer. For a first project, you don't want to get into having to rebalance your amp all the time.

As an example, the 6N1P has a nominal gain of 33. Using local feedback (a cap and a resistor will do this for you), you could cut it down to a gain of 20-25, which is less than 6dB feedback, but is more than sufficient to compensate for variations between the tubes.

When selecting output transformers, there are a number of good choices out there, and I doubt you would go wrong with any of the ones recommended here. In case no-one else responds, I will recommend Lundahl, who have single-ended and push-pull output transformers with simple wiring and excellent performance, in addition to their more flexible offerings. These simpler transformers come with impedance ratios of 5Kohm to 8ohm, 3Kohm to 8ohm, and 5500ohm to 5ohm.

The 5Kohm to 8ohm will be suitable for a 300B, and should be suitable for the SV572-3 and a number of others. The 3K version is useful for somewhat lower impedance valves. If your speakers are 4 ohm, the 3K:8 version is (IIRC) about ideal for the 6C33C-B.

Hope this helps.
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Old 15th January 2004, 08:13 AM   #3
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The only tube-amp I have ever listened to is my own SEKT88-amp. As I understand, the 300B is magical and probably not of this world. It was given to the the people of earth by kind extra-terrestrials who heard our cry for the highest possible form of fidelity. But that's just what I've heard

No, seriously... The 300B seem to be a great tube, but I suspect that are other tube that also sound good, but maybe not as good as 300B. I don't think I'll ever build another amp than my KT88-amp.
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Old 15th January 2004, 08:15 AM   #4
SHiFTY is offline SHiFTY  New Zealand
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The main problem with valves is that there are many ways to skin a cat. A person such as yourself may build a great 300B amp, and be happy with it forever....

Some of us, especially on a DIY site like this, have a lot of bits sitting around, might have some random transmitter tubes that we know a guy has, plus some interesting tube from the 1920s, and a constant current source, and amplifiers become Works In Progress for many years!!! Its a hobby thing.

300Bs certainly have a great reputation amongst single ended tubes. They also have a decent amount of power available, unlike many SE amps. You can't go wrong with them really, with the simplicity of circuits and the knowledge available. They are also not that expensive, maybe $US 100 for a pair, not too bad.

Its all in the output transformers if you ask me... Don't skimp too much on them, and you can reuse them if you want to.

BTW My 6B4G SET amp sounds fantastic, and does have all that "ultra smooth midrange" that they talk about... SETs can be very sexy to listen to.

Just make a decision and build the thing already!!

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Old 15th January 2004, 08:24 AM   #5
angel is offline angel  Norway
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Just one last point regarding actively supported projects, you might try searching for Andrea Cuifoli (spelling?). He has a ton of different projects.

As to the SV572-3 being out of production, that is not the case, as far as I can tell. Besides, either way it will be readily available for years to come.

The 300B does have a cult following.

However, its 'magic' has nothing to do with reality. It is a colouration. A darned cozy one, mind you, but a colouration nonetheless. And not a very subtle one.

I, for one, prefer to have my valves reproduce the soul of the music. Not infuse it with one that is not its own.

You would get a lot less colouration with the 6C33C-B, with more output power. Same goes for the 6550. You could also use the SV572-3, and get more than twice the output power.

Proper use of feedback, with a good output transformer (one with low static resistance, and a low, constant group delay), can linearize just about any valve, however.
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Old 15th January 2004, 08:34 AM   #6
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The 6C33C-B has a low life expectancy, however, so beware.
I have seen this mentioned many times but AFAIK it has no bearing in reality. The source of the supposed short life time of the 6C33 tube is from what is mentioned in the original Russian data sheet where it is mentioned life time of between 400 and 1000 hours depending on type which is indeed very low, BUT this is the guaranteed life time when fulfilling the complete specification including for instance the ability to withstand a 100 G shock and other spec points that is important when the tube is used as a series voltage regulator in a MIG29 fighter aircraft.

However when used sensibly as in an audio amplifier there is no reason to expect that a 6C33 will not have similar or probably better life time than other power tubes, this is also reported by many users including BAT which uses quite a lot of these tubes.

I have used the same set of 6C33C in my OTL now for more then 3 years without any drift in bias or any tendencies towards failure.

Regards Hans
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Old 15th January 2004, 10:00 AM   #7
DVDHack is offline DVDHack  Australia
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I've done a very quick search on the SV572-3 using google and while it turns up some references there certainly aren't the nice full on stories of how someone has put it together. With 300Bs I've turned up about 6 full projects with a lot of design discussions. That is pretty important to me.

I will go out and buy some good valve book to see if I can deviate from 300B - want to fully research rather than just jump in. Any recommendations for a particularly good book(s) to start with.

I'm not worried about voltage - was an electrician in a previous life before becoming a Technical Officer in a power station - worked on a lot of control circuits. I worked with very high voltage so that's not an issue. Also worked in microwave comms for a while.

I want to thank everyone, particularly Angel for the comprehensive coverage of alternatives. Will spend the week end researching.

It would be really nice if there was just a really weel accepted design with parts that are recognised as excellent to start out with - dare to dream.

Regards

Ralf
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Old 15th January 2004, 11:09 AM   #8
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Some people really love the 300B, but I'm not one of them. It's not at all "magical" although it can be very beguilling on a lot of material. However, it's way too expensive for what it is, and an 845 amp for the same money (cheaper tubes but more expensive PSU) would likely better it* and prove a cheaper long term solution.

* to my ears and tastes.
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Old 15th January 2004, 11:26 AM   #9
Mike C is offline Mike C  United Kingdom
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Default 300B SETs

Ralf,
Those are very wise words from Angel, and I will be bearing them in mind for my next project.
However, I'd still suggest a 300B for your first project for several reasons.

Mainly, there is a mass of information out there on 300B SETs. Circuits galore, but more importantly the acquired expertise on how to drive it well; and many discussions on finer points such as ac vs dc heating, fixed vs cathode bias, RC vs LC vs IT coupling, etc.

You will have no difficulty finding OPTs that are an ideal match for 300Bs.

Given all the above, I think you have a better chance of success if you go for the 300B.

A 300B amp can have plenty of 'drive' and good sound quality; results are pretty well known and virtually guaranteed.

From people's circuits and descriptions of the sound they get, you can try to pick factors to give you the 'flavour' of sound you prefer.
Romantic or neutral; dynamic/forceful or polite; etc etc.

I appreciate the concern that the 300B valve, in itself, may be nonlinear and 'romantic', but with respect I don't fully agree. I'd suggest that the distortions may be more to do with the driver stage.
For a neutral result, I suggest a driver stage has to be very linear; has to work over +/- 150V (yes, that's much more than in theory); and has to have a low output impedance so as to drive the grid of the 300B (with its high Miller capacitance) *and* a low value grid leak resistor.
A tall order, almost needing a power amp as a driver.
With such a driver stage, a good PSU, say TJ mesh 300Bs (neutral and extended), and a good OPT at 5kohms, a 300B amp can be highly neutral, fast, clean, dynamic and *powerful* rather than warm and cuddly.
And *musical* with it; really awesome.


OK, experts like Angel can do better by intelligent use of different valves, but I don't think it's a good first project.
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Old 15th January 2004, 11:56 AM   #10
SY is offline SY  United States
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I want to build the best amp I possibly can
The best amp or the best SE amp? As soon as you limit yourself to SE, you've made a bunch of compromises- you're going to have relatively low power, relatively high harmonic distortion, and relatively high source impedance. OTOH, if that's the sort of coloration you're seeking, a good push-pull design will assuredly NOT give that to you.
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