Recommendations for a 15 to 30 W kit tube amp

[Levent]

I want to build a higher power amp than BH offers.  Chris at DIYAudio recommended Dynaco ST35 (and maybe 70) clones.  What does the forum members here recommend?  My speakers are not that sensitive, Alpair 7 with 85dB.

If you think this is an inappropriate post in BH forum I apologize.

Regards
Levent

[HF9]

Well there's a few fairly straightforward builds I can think of off the top of my head. The Dynaco ST35 as was mentioned, the Transcendent T16 OTL, and the Millett DCPP. Do you like lush and warm or crisp and detailed? I think the Poddwatt Series 1 would also fit your needs, although I don't really know much about it; perhaps the gents at DIYAudio would.

[Paul Joppa]

Google led me to the driver specs pretty quickly. An interesting driver! They are rated 20 watts nominal, which would produce 98dB. I have seen (and heard) enough over-driven drivers to convince me, I don't want to exceed the manufacturer's spec on that parameter. So I'd opt for an EL84 amp rather than a more powerful EL34 amp. You will not get the 102dB deemed adequate by "most audiophiles most of the time" but you will get quality sound and a long lifetime from the driver.

If you want to go push-pull, I still think that zero feedback triodes sound better - but without feedback, even triodes will not have enough damping for this driver which has a relatively high QE. If you are going to use enough feedback to get good damping, it's my experience that the EL84 is going to be very good sounding - a pair of 300Bs won't have much of an edge unless you give up the feedback.

My two cents.

[braubeat]

You can produce 20 watts with one 6550 tube running in class A single ended pentode mode. This type of circuit always sounds better to me than a push-pull amp. It is also much easier and cheaper to build. They are now producing some very nice 6550 tubes (same as KT88) in China and Russia because they are very popular in guitar amps.
Well that's my 2 cents worth.

Michael 

[Levent]

Thanks for the responses, looks like ST-35 with EL84's is the way go and it is reasonably priced.  Transcendent T16 is going to be over my budget, I will do some research on Poddwatt, price is in ST-35 range.  I will check out options on 6550 as well (need to find who makes them).

This setup doesn't need to produce loud music, I have my Crack and HD600 for big volume.  Loving the Crack BTW.

A friend of mine and I am building two sets of speakers, he is using Fostex FE125en, which is 93dB.  So I will have a chance to listen both Mark Audio Alpair7 and Fostex in identical cabinets (tapped hypex horn which they called Frugel-Horn Mk3).  I also have a set of Rega RS3 speakers to compare. Challenge is to make good sounding speakers for less than $250 a pair.

[Jim R.]

I don't know your budget, but the Audio Note Kits Kit2 6550 amp will give you 12-15 watts of SE power, and is a very sturdy, well-made amp.

Also the ST-35 kit is a very decent sounding amp.  And yes, I've heard both of these, but not with the alpair drivers.

The new Genalex gold lion re-issue KT-88s are very nice sounding tubes, but certainly not the only choice.

HTH,

Jim

[Paul Joppa]

Just a note, we have not been able to get 20 watts out of a 6550 with any feedback (necessary to get the damping factor to an acceptable level). That 20 watts on the data sheet is at 13.5 percent distortion; with feedback you would be lucky to get 11 watts at that operating point.

[rock4016]

Tubelab.com has SE circuit boards (you'd have to do the chassis work) for cheap.  If you source the parts you could build a SE amp that is 15watts for under $500.  I'd opt for the 6LW6 output tube and the one-electron UBT-1.  AES has the UBT-1 and you can read about the 6LW6 SE amp here.
http://www.tubelab.com/6LW6.htm

15 Watts at .8% distortion are outstanding numbers for a SE Triode class A1 amp.  I've built a similar amp using this tube as a cathode follower output, and that also sounded great.

[Levent]

Thanks for the tubelab suggestion.  I did quite a bit reading on the tubelab SE over the weekend, it looks like a very good option.  This might be more fun to build than ST-35.  Price wise it is going to be in the range I am targeting ($500-600). 

I also have an option to replace the speaker to a Fostex FE126EN, it has higher sensitivity 93dB with lower QE compared to Alpair 7.  This is a direct drop in into the enclosure I am building and would reduce my power need quite a bit.  Paramounts would be lovely with them but above what I can afford at this time, maybe in the future.

Thanks for all your suggestions.

[corndog71]

I've been looking at building an ST35 myself.  Dynakitparts.com sells a nice clone as well as the parts themselves.  I built a pair of Mk IV's from their parts along with Joe Curcio's upgrade circuit boards which work well.

I've heard many over the years say great things about those EL84 tubes.  If that's the direction you plan on going in then you should be aware of a recent modification that makes the amp more stable and improves the audible and measurable performance.  The best part is it only cost a few bucks in parts.

http://www.diytube.com/phpBB2/viewtopic.php?t=4374&postdays=0&postorder=asc&start=0

[Paul Joppa]

Thanks for posting - I had not seen that article and it looks like a very clever and useful improvement to EL-84 P-P amps.