DIY Tube Amp Help...

J

Jeroth

New Member
Hey everyone, im new to the forum and also new to DIY Audio. I am currently building myself a pair of speakers, and am looking into building a 2 channel stereo Tube Amplifier to power them. I have experience in soldering and putting together electronics, so im not a total newbie to that. I am looking at building a Tube Amp that has more SQ than SPL, but also has a fair amount of power, know what i mean? My speakers are about 70W RMS each, so the amp needs to put out anything under 70W per channel, but preferably over 25W per channel. Can ANYONE help me out??

Thanks,
Jeroth
 
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Jeroth what you need to know is your speakers efficiency rating, not their power handling capability. Building a good DIY tube amp in the 30 watt range isn't all that complicated. In fact there are more than a few DIY kits on the market.

Mike
 
To get the power you want, you will have to build a push-pull amp. The 2A3 single ended triode I built only puts out about 3.5-4.0 watts per channel. So, what you see on the output chassis is one 2A3 for each channel.

A simple stereo push pull amp using EL84's I've seen done with a PC board layout, so you don't have to do all the point to point wiring. You can always find these on Ebay:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3044571624&category=39783

Here is one that was at the last LoneStar Bottleheads meeting:
http://www.wardsweb.org/LSB/08_09_03/Houston28.html
 
Thanks for the advice, but according to the E-Bay add, that design only puts out 17 watts per channel which is slightly more than only half of what my preferable minimum wattage is. Thanks for the info though, i now know that i need a Push-Pull design, thats one of my questions answered, anyone else??
 
Jeroth,

If you want a lot of power I guess I'd suggest buying a pair of Dynaco MK-III's on ebay and modifying/upgrading them to stock with new capacitors and a SS rectifier instead of the unreliable selenium one for the bias supply. These mono block power amps use a pair of 6550 output tubes and push about 60 watts RMS each I believe. The circuit is simple and the amps keep their resale value well.

If you want to build from scratch you probably could buy these Dynaco MK-III's for less than the parts separately. Strip them and re-install into a custom chassis/case of your own design to give it that do it yourself feel. :) I think you can buy these amps for less than $200 US each when they occasionally show on ebay.
$400 or less for a quality 60watt/ch stereo tube power amp is good value IMO.

If you can settle for less power go for the Dynaco MK-II mono power amp. Similar prices on ebay, 35-40 watts I think and 6CA7/EL34 output tubes in Ultralinear operation. Very good sound and I can help you tweak this circuit as I have replicated it here for my own use on a single chassis stereo amp.

I've never owned either Dynaco from the factory but think the MK-II would be capable of the higher fidelity. Others may be able to comment from experience.
 
thanks for the reply, i will look into that. One thing ive noticed is that most of you tube guys are dealing with wattages of about 8 watts per channel, why so low. i currently own a midi hifi system, 25W per channel, and this is a good volume level, but you guys are listening at levels alomst 5 times less, why!?! Do tube amps sound louder or what, im so confused!

Also, how loud is a 8W Tube Amp, could you describe it in terms of how loud someone must say something before you can here them properly(like wispering, shouting, etc), im really confused, but these wattages that most tube people talk about just sound, SO LOW, help me, lol.
 
Jaroth,

That circuit was published in two consecutive RCA receiving tube manuals that I have in my reference library. You are right it looks complicated, and would be a construction challenge indeed for a novice builder. The trend these days in tube amplifiers is 'simpler the better'. The less components between input and output, especially coupling capacitors, the truer the reproduced sound it is believed. Same goes for negative feedback. That is considered the big evil in any more than small doses.

The Dynaco circuit I spoke of is much simpler to build, and the ultralinear configuration of the output tubes eliminates the need for a regulated screen grid supply circuit as found in the RCA circuit.

Here is a site with Dynaco circuits and information, http://www.geocities.com/vintageaudio/
 
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Well, i managed to find the schematics for the Dynaco MK III, so could i use them to build it from scratch, seeing as though they are rarely available in the US, and therefor probably NEVER available here in Australia. Heres a link btw:

http://hififorever.myrice.com/doc/famous_mk3e.htm

What do you think, cos thats got just the right amount of power i need, and the design looks simple.
 
re: "Tube watts"... somebody has to address this :)

One "tube watt" RMS IS the same as one "solid state watt" RMS... the difference is the way tubes overload as compared to transistors. A tube amplifier IS a compressor... as the input voltage increases, the output will compress as it distorts, such that the sound's character will change, but you may well not be consciously aware of (harmonic) distortion. When typical ss amps "hit the rail" (all the P/S voltage they have to give), the clipping is agressive and audible... as well as dangerous to sensitive loudspeaker drivers.

Another, not unrelated issue, is the harmonic character of tube vs. ss distortion... but I don't want to go down that road right now.

I routinely use a ca. 14 WRMS per channel Magnavox EL84PP amp with a pair of the nortiously inefficent AR-3 acoustic suspension speakers. Common wisdom will tell you NO WAY is 14 WRMS enough for these power-hungry speakers, but I listen near field (say within less than 10 feet) and I think the combination sounds just dandy. I wouldn't recommend trying a 15 WRMS ss amp (say a Pioneer SA-5500) with AR-3's!

The eBAY PCB looks very cute... Another cute kit amp is the 11MS8 PP stereo amp on a board made by S-5 Electronics (http://www.s5electronics.com/gpage1.html) and also sold (with 11BM8 tubes) by Antique Electronics Supply (www.tubesandmore.com). This is a good sounding amp (8W per channel) and very easy to build. The only drawback is that the tubes run quite hot, so I don't know how long they'll last, and one has to be careful to provide adequate ventilation.

Another WHOLE topic is whether one really needs inefficnent speakers and high-power amps at all... My main amps are 3.5 watt monoblocks, but my speakers are ca. 100 dB and the system does the job quite nicely.
 
Thanks for that, sorry for posting a question to something thats already been discussed by the way. Oh yeah, and heres a link to a Dynaco MK II Schematic, now this is SIMPLE, i think im gonna go for this one, still thinking about it though, anyone else got any suggestions?

http://home.snafu.de/appelt/dynaco/dyna-mk2-sc.gif

JUST NOTICED THAT MOST OF THESE SCHEMATICS ARE DESIGNED FOR 117v OPERATION(AMERICAN) WOULD THE WHOLE SCHEMATIC CHANGE FOR 240V(AUSTRALIAN AND UK VOLTAGE) OR JUST A FEW THINGS?
 
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All ya really gotta change...

... is the power supply. You'll need a power transformer with a 240 V primary and the HV and filament secondaries that you need for the design. You can use multiple transformers if need be and you can't get them all in one package.

Is your power 50 or 60 Hz? Actually, I don't think it'll be an issue for an amplifier.

BTW: That Dyna schematic doesn't look so simple to me :) but then I have an aversion to NFB... Take a look at The 5W PP amp at http://www.the-planet.org/Shishido.html

BTW2, another nice primer on DIY tube amp construction is to be found at:

http://boozhoundlabs.com/howto/
 
Thanks everyone for the input and advice, i appreciate it. Well ive been doing a lot of looking arround at which Tubes are usually used for higher power amp designs, and then did a google search for free schematics and designs using these specific tubes, this is by far the est way to search, as it always returns results in my wattage range. I have found one design that REALLY interests me, because it looks so simple and puts out about 50 Watts. Bassicaly looks like all the components are grounded, which means there arent many parts of the amp where there are complex combinations of component connections, well that proly made no sense, but take a look for yourself and let me know what you think.

http://www.plitron.com/pages/Products/Audio/vtvkt88.htm

Thanks,
Jeroth
 
Works for me!

You might want to try doubling or tripling the value of the 680 ohm shared cathode resistor on the 2nd 6SN7 driver tube.
 
Originally posted by Rob
Works for me!

You might want to try doubling or tripling the value of the 680 ohm shared cathode resistor on the 2nd 6SN7 driver tube.
Click to expand...

So, you have one of these then? Or are you just saying that the design would work? Thanks for the reply, looks like ive found a winner, finaly, hehe. Why would i need to bouble or triple the value of the resistor??
 
Oh yeh, and seeing as though im gonna need to build 2 of these, to have a stereo amp, will i also need to build 2 power supply's, or do i just use onepowering both?
 
One other thing thats confusing me a bit is in the power supply schematic. In the area ive circled in red, does that mean that i need to buy another pair of each tube, or does it just mean that i connect pins 11 and 12 of that big curly lined symbol thing on the left (lol, newbie talk) to the displayed pins on the tubes in the actual amp, so is it just added into the power supply schematic to save them drawing the two schematics as one big schematic and having long lins going from one side of the page to the other. Or are those tubes shown in the power supply schematic area that ive circled seperate tubes? HELP
 
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