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Old 19th February 2011, 04:50 AM   #1
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Question [TubeAmp] Beginner's questions

Hello,
I am new to this forum, so allow me to quickly introduce myself so you know where I come from... (Feel free to skip ahead to my questions, if you don't want to read all that)
I am a 45yr old German living in San Francisco for the past 10 years. I started out with soldering and electronics tinkering when I was 14yrs old and slowly gained experience by taking anything and everything apart that had a power cord going into it, just to see how it works. Eagerly offering up my growing knowledge to my family, neighbors and friends whenever something stopped working, my ratio of successes to failures slowly, but steadily increased. When I was 16 and guided by a teacher at my school, I entered the world of computers and built an Apple ][ (remember those?); financing the required parts by fixing TV's, Amps, Receivers, etc. my dad brought home from coworkers after telling them that I can 'fix anything' (no pressure, right!? ).
I feel very confident around devices that carry a meaningful combination of Volts, Amps and Joule, have never lost my respect for them (which may be why I am able to write this... ) and am very aware of the required safety precautions needed to work with such equipment. I also read through the complete sticky thread on here about safety and still learned a couple more things I hadn't thought about. So, that's enough to frame my goal/questions:

My simple goal:
I would like to build a beginners-adequate Tube amplifier that will be used mainly to listen to Jazz/Soul and some classic rock. If it all works out, it is intended as a gift to a good friend turning 50 soon, who is a musician with a keen sense for music details. (The plan may die right here... )

What I am thinking:
I wanted to start with a limited investment and acknowledge my lack of foundational knowledge to design a circuit myself (the power supply would be easy for me, but that by itself typically doesn't sound all that great ). Given what I have read here thanks to all of you willingly sharing your knowledge and experiences, I am considering this kit Untitled and some of the recommended modifications described on the net (snubbers, bleeders, diodes, different caps, etc.).

My questions are:
1) Does this amp sound/perform well enough to satisfy a reasonably sensitive music lover's ear or should I merely expect it to serve as a means to gain experience for higher-quality designs?
2) Could the sound quality of this specific kit be improved by using different tubes or would higher-quality tubes require a fundamentally different circuit? How about better transformers?
3) What equipment other than the obvious (isolation transformer, soldering station, an active brain) would I need to get the most out of this kit? I do own a dual-channel scope (good old trusted Hameg), but I don't have - for example - a function (sine/square wave) generator.
4) Does anybody use/have experience with a software sine wave generator that creates a test signal on the audi output of a computer or is that a bad idea for this application?

OK, I think I've eaten up enough of your time for now. I would appreciate your thoughts very much since this forum seems to attract a lot of very knowledgable and experienced people.

Thanks very much in advance,
Stefan
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Old 19th February 2011, 06:24 AM   #2
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I'd be looking at the version of that amp without the paralleled output stage. Half the power, but better sound. From those that have these they are reasonable for almost no money. A dual mono would be where i'd spend more money, if that is what you want.

If you are willing to do a bit more work, look at a variation of RH84 (you'll need efficient speakers with a benign load) or El Cheapo.

For a kit i'd consider TubeLab SimplePP or Simple SE.

dave

PS: As a German in SF do you fly a German or a US flag?
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Old 19th February 2011, 07:27 AM   #3
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Quote:
Originally Posted by planet10 View Post
I'd be looking at the version of that amp without the paralleled output stage. Half the power, but better sound. From those that have these they are reasonable for almost no money. A dual mono would be where i'd spend more money, if that is what you want.
Hmmm, never thought about building two mono amps. Interesting thought...

Quote:
Originally Posted by planet10 View Post
If you are willing to do a bit more work, look at a variation of RH84 (you'll need efficient speakers with a benign load) or El Cheapo.
I'll read up on that. Not sure what the target speakers are yet; I don't want to raise any suspicions by asking my friend, so I'll have to wait until I get to his house again and can stealthily 'check the plates'...

Quote:
Originally Posted by planet10 View Post
For a kit i'd consider TubeLab SimplePP or Simple SE.

dave

PS: As a German in SF do you fly a German or a US flag?
Looks promising, although not available as a kit. But George sells the PCB, so that's a good start. And there is some great advice on choosing the various components.

Regarding the flag: I've never been a big flag flier; probably remnants of times where flying the flag of the fatherland made too many people inhale sharply. I have two small versions of each flag (5"x3" or so) that sit happily and peacefully together in a big flower pot in my kitchen. That's gotta do it.
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Old 19th February 2011, 07:28 AM   #4
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Ooops.... where are my manners!?
Dave, THANK YOU for your response!
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Old 19th February 2011, 08:22 AM   #5
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Buying a PCB-based kit will get a lot of 'problems' out of the way, but the chassis will be left to do...

Do you have a good shop where you can fabricate a chassis/base?

(Especially important if the amp is to be a gift- it must be properly enclosed for safety's sake.)

Very nice project idea, BTW.
John
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Old 19th February 2011, 08:31 AM   #6
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Quote:
Originally Posted by dagerman View Post
But George sells the PCB, so that's a good start. And there is some great advice on choosing the various components.
I bought (actually traded for FE126eN) for a SimplePP board with a parts kit (and an early test build of a SimpleSE)

dave
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Old 19th February 2011, 09:10 AM   #7
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Hi, and welcome.

I would definitely recommend a project where you have a printed circuit board to work on. Makes things much easier especially for a first build. Not having the components included in a kit is less of a problem -- ordering them yourself adds to the fun

I think for the music genres you mentioned, anything from 15W would do quite okay with all but the most inefficient speakers. Not knowing the speakers' parameters in advance, I would avoid the low-power single ended designs and go for a push-pull design.

To answer your questions:
- How the amp sounds depends primarily on the topology (circuit diagram) and the amount of feedback used. Secondarily on the tube types used (somewhat related, because some types do not do well in some topologies); and finally on the quality of the components used. In that last category, output transformers are the most important, followed by interstage coupling capacitors.
- If you want to use a different tube type you will need to rebias the stage, by recalculating and changing the anode and/or cathode resistors. So for a first build, I wouldn't recommend it. It's something you can play with afterward, once the amp works as-is.
- Most kits or PCBs will not require adjustments for which you need a 'scope or signal generator. Once you start designing your own circuits, a generator, scope, and distortion meter will come in handy.
- Many people here use the PC as a signal generator and scope. You just need the appropriate software and, if you use it as a scope, an interface box to allow you to measure several 100V signals without destroying the PC's audio interface.

Kenneth
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Old 19th February 2011, 11:26 AM   #8
Ian444 is offline Ian444  Australia
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I've been through the mill so to speak, if I were in your shoes I would buy the Simple PP and partial parts kit and build from there. As VictoriaGuy said you still need to build a chassis, usually a simple wooden box, and an aluminum top plate seems to be standard. Or I could say it like this - I don't know of any other amp that is easier to build that sounds as good and puts out 15W per channel
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Old 19th February 2011, 02:49 PM   #9
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Dagerman,

Another poster remarked about speakers. That is spot on! Paul Joppa has provided us with a useful rule of thumb. Joppa's Rule states that in a "typical" listening space an amp/speaker combo should be capable of 102 dB. SPL peaks at a 1 M. distance. A big space requires more power and background/easy listening requires slightly less.

The claimed efficiency, nominal impedance, and (if available) the impedance curve of the speakers are data needed to make a reasonable amp design selection. Be careful, as more than 1 speaker maker claims nn dB. sensitivity for 2.83 V. into 4 Ω. The "true" sensitivity in those cases is nn - 3 dB., as 2.83 V into 4 Ω is 2 W., not 1 W.
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Old 19th February 2011, 10:11 PM   #10
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Wow, you guys are great! Thanks for all the responses! You covered all my questions and even those that I didn't know to ask yet...

Yes, the amp will be enclosed in a case I am going to build myself. My wood- and metalworking skills and available tools far exceed those needed for tube amp design. I got a lot of interesting ideas for cases on here as well. It's probably going to be a wooden frame with aluminum bottom, top and frontplate (brushed).

In the meantime, I found out what the speakers are that the amp would have to drive; they are Monitor Audio Silver S8, the exact tech specs of which are not easily located, but it looks like they are close to:
Impedance: 6 Ohm
Sensitivity (1W@1m): 91dB
FreqRange: 33-30k Hz
Recommended Power Amp output: 40-175 WRMS

I am going to see how that will work out. I already know that if the amp I come up with is not adequate for those speakers, it will end up on my desk to beef up my iPod with a set of single-driver homebrews.

Looks like I am going to decide between the smaller S5 kit and the TubeLab SimpleSE/TubelabSE.

Thanks again for all your thoughts and advice, much appreciated!!
Have a great weekend and - for those in the US - Happy President's day,
Stefan
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