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Old 4th April 2013, 12:37 PM   #1
PFC is offline PFC
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Default Tube amp for a small room (newbie-like)

Hello everyone,

Does anyone have a suggestion for a tube amp for a small room? I have been looking for schematics on the internet but i dont know if they work or not. I have limited budget (around 700$) for an amp. So i want something best that i can do. Any suggestion?

I have limited knowladge about tube amps so be easy on me 3 years ago i have build Fender 5F2-A for my guitar and this time i want to listen music via CD player.

PS:I dont have speakers and CD player
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Old 4th April 2013, 01:00 PM   #2
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Let's start with Paul Joppa's 102 dB. rule. The 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. Adjust down a bit for your situation. However, reserves of power are a good thing.

$700 for an amp is a "piece of cake". There will be money left over for other purposes.

Build an "El Cheapo" with the excellent Dynaclone Z565 O/P transformers. You can incorporate triode/ultralinear mode switches. Triode is more refined, while UL is more powerful. For instance, listen in triode mode to a small jazz combo or string quartet and listen in UL mode to "Heavy Metal".
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Old 4th April 2013, 04:24 PM   #3
tomchr is offline tomchr  Canada
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102 dB, eh? Much beyond 85 dB(A) SPL at the listening position makes my ears ring. But let's just stick with the 102 dB for a second. That gives us some margin.

There are two speakers, so you get 3 dB gain there. If you have a relatively live room, you'll likely have a few dB (let's say 5 dB) of room gain as well. So when cranked to 11, each speaker will have to produce 102-3-5 = 94 dB.

My KRK R6 bookshelf-size studio monitors are 87 dB efficient (i.e. you apply 1 W, you get 87 dB SPL at 1 m). This is pretty typical for a bookshelf 2-way. So to get them to produce 94 dB SPL, you'll need to apply 10^((94-87)/10) = 5.01 W. Of course, if you're using, say, a 91 dB efficient full range speaker, you'll only need 10^((94-91)/10) = 2.00 W.

Building a tube amp that can deliver 5 W can be done in many ways. You can have a look at my 6LU8 Spud, Tubelab's SSE or Tubelab SE. The 4P1L SE is also a low-cost option for a few watt of power. Any of these amps can be built for $250-ish and up depending on your choice of output transformers, chassis, etc.

I'll be happy to point you to my latest pride and joy, the Damn Good 300B SET Amp. But given that the 300B tubes alone will set you back over $200/pair for low cost ones, I think a $700 budget will be rather tight. My build is probably $1200 per stereo amp. That's a guess. I'm afraid to do the math... My 300B does provide 10 W of sweet sounding SET power, though.

For a first build, I'd go with a $200 Spud type amp. If you master that, save up some cash for a bigger build.

Modulus-86: 40W/8Ω @ 0.000061% THD. HP-1: 3W/20Ω, 450mW/300Ω @ 0.000032% THD. DG300B and Other Tube Circuits.
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Old 4th April 2013, 04:47 PM   #4
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Originally Posted by tomchr View Post
102 dB, eh? Much beyond 85 dB(A) SPL at the listening position makes my ears ring.
Eli said 102dB *peaks*.

85dB average SPL is loud enough allright, but music has short, high amplitude transients which are way above the average SPL.

If the amplifier isn't powerful enough, it will clip and it may need a few R-C time constants to recover from overload, during which time it distorts.
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Old 4th April 2013, 05:44 PM   #5
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It's a 102 dB. SPL peak for each speaker. Mating 97 dB. sensitive horn speakers to 2A3 amps is done for good reason.
Eli D.
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Old 4th April 2013, 06:17 PM   #6
bondul is offline bondul  United States
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I built ST-35 from dynakit part: couple years ago and love it so much. I am ending up upgrade it fit fixed bias. Now working on Aikido preamp to pair it.

If I have to do it again, I would buy the integrated amp. model from ebay like this:
and upgrade it with better part. (I have no affiliation with either dynakit nor the ebay seller).

Good luck!

Last edited by bondul; 4th April 2013 at 06:19 PM.
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Old 4th April 2013, 06:29 PM   #7
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Darling, but only if you are prepared to build or buy some sensitive speakers, and pretty certainly not for real noisy stuff
"It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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Old 4th April 2013, 08:33 PM   #8
45 is offline 45  Italy
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In a small room bookshelf loudspeakers rule and 10W with 84-87 dB/1W speakers will sound pretty loud. Anything else won't sound just as good and will only eat space. Speaking for myself bookshelf loudspeaker (2 or 3 ways) always rule unless one has a concert room!!
Buy yourself a good EL84 push-pull or similar and you will be fine. Musical performance is more important than power. You just need enough power.
Recently I made a quasi-replica ( i.e. with a better power supply than the original and minor tweaks to the circuit) of the Mullard 5-10 for a friend of mine and I got 12W at the onset of clipping! Ultra-linear with zero feedback. Sounds really nice with both my Spendor SA1 (85dB/1W) and his Spendor SP100R^2 (89 dB/1W). The difference in efficiency of 4 dB's really doesn't make a difference in a small room. Actually the SA1's are quite better in such situation....
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Old 4th April 2013, 08:49 PM   #9
45 is offline 45  Italy
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This is it:
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Old 4th April 2013, 09:57 PM   #10
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Default 12W6 PSE

Here is another idea for you.

This is a 12W6 parallel single-ended using paralleled 6N1P for the driver. Sounds absolutely fantastic. The 12W6 is a very nice sounding tube. Of course the same holds true for the 6W6. Plenty of power for my 87dB bookshelf speakers in my small apartment.

The 12W6 tubes are $3 each. You can get four 6N1P-EV for $10 on epay.

You can easily build this under $200 if you have a few parts laying around.

The PT I used is an Antek AS-2T230, so you probably can't get that for about another month or so. The heater current for the *W6 tubes is kind of a downside unfortunately.

The OTs are Edcor XSE15-8-2.5K. The choke is a Triad C-14X.

I ended up using 220uF for the 12W6 cathode bypass caps.

Post #6 here: parallel single-ended 12W6

Last edited by scott17; 4th April 2013 at 10:01 PM. Reason: additional info
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