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Old 27th May 2004, 09:31 PM   #1
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Question Tube amp design characteristics?

So I am wondering if you kind folks will help me select a first, total DIY project. I want to build my first tube amp from scratch. Now, I am kind of poor compared to most so I canít afford to make mistakes or to buy something only to decide I should have gone a different way and have to buy something else. So Iíll probably be asking quite a lot of questions calling for opinions or characteristics so I can make an educated decision on what to build and component selection for it. I want to be able to build a good sounding amp for as little money as possible but I donít want to cheap out and end up with something completely substandard.

First thing I want to ask is characteristics of SE versus PP. Some generalizations I think I have heard is that a SE amp will reveal more of the micro details in music. And that a PP amp has more bass slam and dynamic power. In these themes Iíd say micro details are good but Iíd weigh more towards the more powerful sound of a PP amp. Iím wondering if these ďstereotypesĒ (hahaha, pun intended) are somewhat accurate or if there are others I need to take into consideration. Iím also wondering on what sort of scale these apply. Such as if I have a SE amp and a PP amp maybe the level of micro detail on the SE amp is a 9 out of 10 and the dynamics are a 6 compared to the level of detail on a PP design being 8.25 and the dynamics being a 9. So in comparison Iíd be loosing a very small amount of detail to gain a good bit of punch. So I am asking you guys if youíd take your vast experience and make some generalizations like these to help me decide which direction to take. Sort of a list of proís and conís but also with a level of trade off.

Right now since I am leaning towards a PP design but I still want some style or class so to speak I am thinking of making a PP 2A3 amp. I like the idea of the groovy shape of the 2A3 along with the triode mystique in combination with the additional power delivery and dynamics of a PP design. I donít find myself listening to music at top volumes all that often so I am not looking for megawatts of power but most of the speakers I have wouldnít be considered super high efficiency so I am not interested in a flea power amp either.

Thanks in advance for your input.

Bryan
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Old 28th May 2004, 06:55 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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Just remember that the transformers make the amp. I would buy the best transformers you can afford and build around them. Vintage transformers can be got quite cheaply.

Design or use a circuit with very little open loop distortion. What distortion there is should be mainly 2nd and 3rd harmonics. Above that gets objectionable to the ear.

The traits of PP vs SE is a whole debate in itself. A good amp is a good amp, you can get very detailed PP and very punchy SE.

I like the idea of a PP 2A3, your 2A3s generate mainly 2nd harmonic distortion, which is mostly cancelled in a PP design. So its inherenly low distortion!
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Old 28th May 2004, 12:35 PM   #3
SY is offline SY  United States
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Basic rules:

1. Ignore 99% of what "people say."

2. Decide if you want the amp to be neutral or to have a sound.

3. Base your design decisions on the answer to #2.

4. Get a good, reliable book on design and absorb it thoroughly before proceeding. (Morgan Jones's book has been highly recommended by many people)

5. Make sure you have the minimum complement of tools needed. That includes a good multimeter, an oscilloscope, and a signal generator, besides the usual construction equipment.
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Old 29th May 2004, 07:19 AM   #4
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Default What would be a good PP 2A3 OPT?

Something inexpensive but not the bottom of the barrel? It'd be tuff for me to justify and figure out how to budget $100 each so less would be good and more would be pretty much impossible. Am I dreamin?
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Old 29th May 2004, 12:05 PM   #5
SY is offline SY  United States
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Unfortunately, you're dreamin'. Tubes have many, many virtues, but low cost is not among them.
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Old 29th May 2004, 01:23 PM   #6
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For 100$ you can make a good headphone amp with tubes. But a little more and you can make an excellent little amp. As for sound quality SE usually outperforms PP, including bass respons and certainly dynamics. But given the same tubes, a PP amp will give more power than SE amp, so it will seem more bass capable and maybe even more dynamic, simply because it has more power. But a 10W SE amp will be more dynamic than a 10W PP amp.
As for cheap tubes the 6AS7 is a great triode. It is supposedly based on the 2A3 but is indirectly heated and has two smaller triode halfs in each bulb. The bulb is that nice looking coke bottle shape as well, except for the 6AS7GA version which is straight bottled. Many people think cheap tubes are bad for audio. Cheap is bad for sound right? Well it happens to be that prices of tubes is set by demand and what is on stock. The fabricators couldnt care less how a tube sounds, so they price them simply after demand and how many are in stock. The 6AS7 is plentyful, and not in such demand as 300B or 2A3s etc, so its cheap.
Admittedly a DHT is probably the best, but the 6AS7 is still a great sonicly tube, teh most under rated power triode if you ask me.
Do a search for 6AS7 based amps and ull find many SE and PP designs. A single tube will give you 10-12W in PP, and 6-8W in SE. It has low plate impedance and can be used in output transformer less amps if a couple of them are paralleled. Four of these tubes will set you back less than a decent 10W SE tranny will.
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Old 29th May 2004, 01:44 PM   #7
SY is offline SY  United States
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Quote:
As for sound quality SE usually outperforms PP, including bass respons and certainly dynamics. But given the same tubes, a PP amp will give more power than SE amp, so it will seem more bass capable and maybe even more dynamic, simply because it has more power. But a 10W SE amp will be more dynamic than a 10W PP amp.
Other than the power issue (P-P is much more efficient and will produce more power), all of this is quite arguable.

It is not arguable that the output transformer is the heart and soul of any tube amp, and that $100 will get you a decent output transformer- but not all the rest of the parts, the power tranny, the tubes, the electrolytics, the chassis, the case... Honestly, I'd be challenged to even get a tube-based headphone amp together for less that $100 without investing serious time, patience, and diligence in tracking down surplus.
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Old 29th May 2004, 02:12 PM   #8
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Default SEMPER BIASED...

Hi,

Quote:
As for sound quality SE usually outperforms PP, including bass respons and certainly dynamics
Bass response? Dynamics? Hmmm...One would think SE amps are at a disadvantage in those departments.
Needless to say, I think there are all sorts of both kinds, SE and PP.

Oh, let's not forget you'd better have some easy going speakers for that SE amp. preferably in the 90dB/m + effeciency too.

As for the 6AS7, I love those and yes they're based around the 2A3 but:

They have only a mu of 2...O.K., 2.5 when you drop to very low plate voltages, so they're going to need some driving.

Secondly, they are DUAL triodes not TWIN triodes so you'd better think of something to adjust bias (read the datasheet about this) or select from a ton of them, especially when thinking PP or shared PS SE.

Maybe the 6AS7G from Svetlana are better matched, dunno.

If you're willing to pay a little more there's the 7236 which is a twin triode according to the datasheet and it sports a little higher mu.
No SY, I won't mention that other one...

You can still make an excellent little amp with a 6AS7 and say a 6922, for little money. But 100 peanuts? Fuggedaboudit.

Cheers,
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Old 29th May 2004, 02:32 PM   #9
benny is offline benny  Australia
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SE output will give a more detailed acurate sound because there is no cancelation of the waveforms hapening.

we all know that PP gives less hum than SE as we get a cancelation of the inverted waves... but we also get a bit of cancelation of our audio waves too. so yes, SE is more detailed.

As for more bass response, i'd also say this is arguable, and i say it will come down to the quality of the OT used. how good a tranny is isn't just it's high frequency response, but also it's low frequency response. you need maximum bandwidth, not just maximum frequency.
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Old 29th May 2004, 03:42 PM   #10
SY is offline SY  United States
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Quote:
SE output will give a more detailed acurate sound because there is no cancelation of the waveforms hapening.

we all know that PP gives less hum than SE as we get a cancelation of the inverted waves... but we also get a bit of cancelation of our audio waves too. so yes, SE is more detailed.

The waves that are cancelled are the even order distortion and the common mode noise. Signal is reinforced. Your implication is correct- SE amps tend to have a lot of additive distortion. Many find that pleasing to the ear.
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