DIY 300b SET monoblock


My father is an experienced amatuer radio operator who has a large, self restored radio collection of both solid state and tube. In addition he is an electrical engineer of 40+years.

He has offered to build me a tube amp and preamp if I can do most of the research for parts and schematics.

I am wondering if anyone has an experience in this and can suggest a very good set of plans that sound great. I believe that the plans can either be a simple design that a beginer could handle or something for someone who has ALOT of experience, depending on which final product is better. Preferably, 300b would be good, but other tube types are acceptable.

The same goes for a preamp.

put your post on in the diy section if you haven't already

Check out this page. Lots of info and links. Some links are dead ends but lots of good ones.
Nice link Herman
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.

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.

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.
There is a website called, Check it out. There is a tube section. There you will find some great people who really know there electronics and tubes. If it can be built they just may have what you are looking for including suppliers.

Mr. dark and morbid, you told me, but I still don't understand. "I'm not a smart man." Oh, Morbius...

... never mind.

That would be an interesting experiment, the Plinius and SET. I'm not right now in a position to add a set of interconnects and speaker cables, but I love the idea. I did not however feel I was lacking with the eight watt Cary's, at least where I would use them. I would use them on acoustic, female and intimate recordings. Rock and classical would be the Plinius. I'm going to do some research, I may contact you off thread if I decide I could afford your amp.

Thanks for all the great discussion.

jd with SET and the Plinius would allow you to enjoy the benefits of single-ended midrange/highs for all music instead of just small scale acoustic and vocals. Rock and classical would still be just as dynamic and powerful with the added benfit of greater space, air, and tonality.

I run a very similar setup with my Cain & Cain I-Bens being sriven by the DRD 300B's or the Art Audio PX-25 that I own and my dual passives subwoofers being powered by an IcePower 500ASP module or 250wpc Hafler DH-500 SS amp. Due the lone 6.5" single-driver (super tweeter >12kHz), the six foot I-Bens provide extremely tuneful, but not powerful bass. Their stated frequency response is 45Hz-36kHz w/ super tweeter, but in my room the lower rolloff point is closer to the >70Hz range. My dual subwoofers do the dirty work from ~80Hz don.

That seems to be somewhat identical to your Dunlavys in a bi-amping situation. In Robert Deutsch's 1999 Stereophile review(1/2 down page) of your speakers, they set the biamping crossover point at 70Hz:

"The biwiring terminals allowed me to investigate the crossover between the twin woofers and midrange units. Set at 70Hz, with the expected first-order slopes, it allows the woofers basically to act as subwoofers."

I thik that setup will blow your mind on all types of music. It could end up being a near perfect situation. And while I know your Kubala Sun cables are some of the best currently available, in this situation you could easily get by with less expensive cables on the woofers and Plinius. At elast for tsting purposes and the short term.

"I may contact you off thread if I decide I could afford your amp."

Ooops, I hope I didn't give ou the impression that my amps were for sale - they're not.

But, there is a set for sale here on Audiogon right now for $1,100 with some important upgrades that mine do not have. The amps don't come with 300B output tubes, so would to buy some TJ Meshplate 300Bs for ~$275.

Feel free to send my an email via Audiogon if you have anymore questions.

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