DIY 300b SET monoblock

Derek Walton's DIY 300B SET amp mk1 is one of the most popular and respected freely available designs on the web. He provides all the info anyone would ever need - schematic, parts list, chasis layout drawings, and step-by-stp instructions with photos.

It provides excellent performance using non-boutique, affordable parts. And there is plenty of room for future upgrades as need be.

thanks herman and darkmoebius, great links.

Jamem6,

My only major suggestion forthe Walton amp is that you start out with much better output transformers and chokes, they will have the biggest effect on sound and will provide a solid foundation for future upgrades to be revealed.

Magnequest makes fantastic X-formers. Their TFA-204 output tramsformers (3K:16,8,4 ohms, 8 watts, 60 mA) - $240 pair and BCM-19 filter chokes (10H, 200 mA, 109 ohms DCR) - $150 pair are perfectly designed for the job. You can buy them from Bottlehead (2/3 down page)

Of course, this adds significantly to the cost of the project at which point it might be worth it to consider a kit which comes with everything you need for a 1st rate set of amps. At the top of my list of kits would be the Diy HiFi Supply Lady Day amps.

About a 30-40% jump on price to the $1,200-1,500 range, but solid design with excellent parts and literally hundreds of already built kits. Diy HiFi has a builder's forum on Audio Asylum that is highly active for advice and ideas. Check out the forum and post a few questions.

More importantly, the Lady Day is designed to be an experimenter's paradise - the output transformer comes with multiple taps from 2k to 8k allowing the amp to easily be modified to run up to 8 different output tubes (10Y/205D/45/50/PX4/PX25/2A3/300B) and a myriad of input/driver tubes. You can try several classic circuit designs like WE91, MU, and SRPP.

The advantage of a kit is that:
1) you don't have to fiddle with finding, drilling, and modifying a chasis - BIG hassle,
2) no ordering or guessing on parts from 10 different sources, all w/ minimum order requirements
3) complete building instructions with photos and a support forum with lots of other beginner and expert builders
4) a tested and proven upgrade path for endless improvement. no flying blind.
5) Built-in resale value if you ever decide to move on to another amp.
6) Ultimate versitility

Hi Darkmoebius, I wish I knew what your moniker meant so I would know how to abbreviate it. Darkmoebius is way too hard to write, and I'm sure I could have some fun with it if I understood it.

Thank you, this is great stuff. I have had a pair of Cary 300B SET monoblocks in my system. There are some down sides to the low wattage, but I have had many times where my mood for music would fit the 300B perfectly. $1500 would be a reasonable amount for me to have a second amp option in my system. I'll read over all the great info you provided.

jd

Aaah Jadem6, I'm an insufferable scifi-techno dweeb. So, my moniker is cross between "Dr. Morbius" from the 1956 movie "Forbidden Planet" and the famous mathematical model Mobius Strip made famous in MC Escher illustrations. Add a heavy dash of admiration for Jean Giraud's fantasy/futuristic/scifi Moebius comics from 60's-70's.

I think you might find the Lady Days vastly superior, in many respects, to the Cary 300B's. I had a Cary 300SEI in-house for a bit and while I found the initial lush/richness intoxicating, it later revealed itself to be bloated, colored, and slightly slow in the low frequencies. A little too much of a good thing, in my opinion. (of course, your mono's may not have had this problem) On the other hand, the LD should be much more neutral and transparent with more "jump".

But, your Dunlavys 5 ohm load probably isn't the best match for low-power SETs, even at 91dB. Those 4 10" woofers have got to draw some current.

The best of all worlds would be bi-amping with SET for mids/highs and your Plinius on the woofers. There's a nice used pair of Welborne Labs 300b DRD's listed for ~$1,100. I own them and they are the most balanced/neutral/revealing 300B SETs I've heard in the sub-$2,500 range. Far better than the Cary's I've heard. Also, saves you the building hassles and hold their resale value extremely well.