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I am interested to get into the world of tube amp., and I have a pair of B&W CDM9NT. I have several questions.


Based on my research, it seems the Dynaco ST-70 is a very popular choice for entry tube amp. It is only 40 wpc, would it be enough to drive my CDM9? And why the output power of a tube amp is so much lower than SS amp?


Besides the Dynaco ST-70, any other recommendation for similar price range? (

Is this hobby takes alot of time and effort? I read something saying that every time I change the tube, I have to re-bias. I haven't finish reading yet. Do I need extra tool to re-bias?
 

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Actually it closer to 35 wpc and even that is pushing it a tad, if you dont like it real LOUD you should be fine, and a completely refurbished AND upgraded st 70 should be a lot loss then $1000
 

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Yes, tubes are a hobby in themselves. There are lot of dedicated forums on older tubes. Consider that any amp over 7 to 10 years old needs re-capping, and tubes degrade the moment you first turn them on. Yes, any tube change requires adjustments.


The ST-70 was popular. Not sure if the odd input tube is still available. There are a lot of mods for them. Even in tube amps, a lot of progress has been made since the 60's. Basically, you buy a unit that old just to get the transformers, then build an entire new amp in the chassis.


I have been playing with a 10W ( actually about 5.6 max) cheep Chinese amp. Absolute garbage as delivered, but with extensive mods can be made pretty musical. But, when all said and done I will have about $500 invested. One can but a modern amp that is much , much cleaner and more musical. Even my old B&K or Haflers will easily sound smoother than any tube amp until you get into the mega-buck range. Five grand or so. I am keeping tubes as a hobby as they are fun, and solid state in my system for music.
 

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I can vouch for the sound of Hafler amps (XL-600, XL-280). I used them with older AR and B&W speakers. Very smooth and tubey sounding, lots of them around used and plenty of power for the CM9's. David Hafler had a big hand in the ST-70 design.
 

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Quote:
Originally Posted by gte357s /forum/post/18125231


I am interested to get into the world of tube amp., and I have a pair of B&W CDM9NT. I have several questions.


Based on my research, it seems the Dynaco ST-70 is a very popular choice for entry tube amp. It is only 40 wpc, would it be enough to drive my CDM9?

It's hard for someone else to say for you waht is enough, but for me, a 90dB/W speaker and 40W would not be enough based upon my own personal experience. But it might be for you.
Quote:
Originally Posted by gte357s /forum/post/18125231


And why the output power of a tube amp is so much lower than SS amp?

Transformer design gets harder as the units get bigger, and cost and weight factors go up. More output tubes are needed, so size, weight and cost go up etc. High power amps only came in the 70's (SS) and from then there was little development of tube designs, and most modern units for sale are simple derivations on classic topologies.

Quote:
Originally Posted by gte357s /forum/post/18125231


Is this hobby takes alot of time and effort?

What's a lot of time? Want to build an amp in the duration of a football game? Probably not going to happen.
Quote:
Originally Posted by gte357s /forum/post/18125231


I read something saying that every time I change the tube, I have to re-bias. I haven't finish reading yet. Do I need extra tool to re-bias?

No, as I have stated below, this is not the case except for some fixed bias output stages. Then you typically need a cheap DVM and a screwdriver. It's not hard to do on most amps.


If you want to get into tube amps on a DIY level, you must buy Morgan Jones' Valve Amplifiers. It is extremely well written in a clear style by a former BBC Engineer, that is easy for anyone to understand and it explains how all the component devices work and the factors and considerations involved in designing an entire amp, with a couple of fully worked examples.

Quote:
Originally Posted by tvrgeek /forum/post/18127634


Consider that any amp over 7 to 10 years old needs re-capping, and tubes degrade the moment you first turn them on.

Not true about the 7-10 yr recap timeframe; I have plenty of units and worked on plenty more that show this is incorrect. Good to do, but not needed in most cases.
Quote:
Originally Posted by tvrgeek /forum/post/18127634


Yes, any tube change requires adjustments.

This is only true for fixed bias stages, typically outputs. Almost all preamp and driver tubes will need no adjustment at all upon replacement, nor cathode biassed output stages.

Quote:
Originally Posted by tvrgeek /forum/post/18127634


The ST-70 was popular. Not sure if the odd input tube is still available. There are a lot of mods for them.

The tubes are available, as well as a ton of mods for them.
Quote:
Originally Posted by tvrgeek /forum/post/18127634


Even in tube amps, a lot of progress has been made since the 60's.

Sure, visit Tubecad as see what Broskie has come up with in the past decade.
Quote:
Originally Posted by tvrgeek /forum/post/18127634


Basically, you buy a unit that old just to get the transformers, then build an entire new amp in the chassis.

Again, not necessarily true. You can buy new replacements for the Dyna trannies from Triode Electronics and the power trans have been redesigned onto slightly bigger core so they don't run so hot. As well as that there are better units available from Lundahl, Heyboer, Electraprint and others.


Sometimes it's just cheaper and easier to start from scratch.
 
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