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Been a long tome coming but I finally made the 91A clone from SP magazine a real amp instead of just a bread board amp. Glad I spent so much time auditioning parts though because I really like the sound of this one. Joe Roberts is right, this design is something special.I am pleased with the obbligato oil film caps; they sound great and they are not too terrible on price. Better than having to trust old military oil caps like I have been doing for years. I will save the old timers for bread boarding.
Edits: 04/20/15 04/20/15Follow Ups:
Nice job!
When I first wrote that article I didn't think anyone would ever read it. Xeroxed 200 copies of SP #1 to pass around as a semi-joke among friends.
I intended that essay as a thought-provoking piece about technological paradigms more than a DIY project, but I'm happy that hundreds of amps have been built in the aftermath.
The cherry red EF37A are a nice touch. Only time I ever saw those was in a Leak TL-12 I had. 6C6s are really good subs too.
I always felt that a pentode + triode was a great combo: 91, Radiotron AA3, etc. Sure there is room for refinement and further lessons, but I experienced massive enjoyment with such simple amps. Always a good idea to build one!
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Free your mind and your ass will follow -- Parliament/Funkadelic
Hey Joe,
Since you jumped into this thread, I have a couple questions about those SP 91's you built lo so many years ago. First, you were somewhat coy about which transformers you chose for your build. IIRC, you auditioned several, but never said which ones settled on your amps. Second, do you still have the amps? I'm sure I'm not the only one who'd love to see a photo. You know, for posterity.
There were very few SE transformers at the time. I used MQ FS030s in the ones I played long term. I don't even know if Mikey wound FS030s yet at the time I wrote that article. I think he only had those Altec 91 replacements TL217 or something..
Those amps no longer exist as far as I know, although I do not recall what exactly happened to them. I think I gave them away to a certain friend. Built a nicer pair with FS-030s eventually, then don't recall what happened to those. I used to build a lot, one project after another or two at a time. Now I am lazy as hell.
Now there are 1000 transformers available, so what I liked back then is quite out of date info. Lately I have been playing with Silbatone transformers using Finemet cores, silver wire, and silver foil secondaries. The DIY world has come a long way as has the transformer market in general.
I must say that I was uncomfortable about the article being read as a parts review. As mentioned, I meant that piece as a study on shifting paradigms in design, in the form of a DIY article. Fortunately, the main message somehow took hold.
Reviewing isn't my thing. I suffered for years when I read Hovland ads quoting me with some sound bitey quip about Hovland caps like "If you want see- thru transparency with juicy tone, try Hovland" blah blah blah. Some line I gave two seconds thought to...haunted me for years! ;op
So, looking back, I realize that publishing an article or book is like throwing a note in a bottle into the waves. You just can't know where it will end up and what people will do with it, but the history can be extremely interesting to watch unfold.
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Free your mind and your ass will follow -- Parliament/Funkadelic
As much as anything can be, it was merely idle curiosity that motivated me asking about it. It seemed like an obvious omission in the original article, when you were so specific about the other components you used. Thanks for the straightforward reply.
I had already been made vaguely aware of the Western Electric 91 (and 86) amplifiers by the time I saw the first issue of SP, and was thrilled to learn more about something that was, at the time, somewhere between myth and chimera. No one in 'mainstream' audio had ever heard of such a thing, and information was scarce, to say the least. I think it was probably several years afterwards before I even saw a photo of a 91.
If it makes you feel any better, I interpreted the article "as a study on shifting paradigms in design, in the form of a DIY article". That sums it up rather nicely, I think. Before learning of SP magazine, I had already rocked my hi-fi paradigm when I built my first SE45 amplifier on the advice of a more experienced and eccentric hi-fi mentor. I built it for the fun of it, expecting little. How could I have known that an amplifier with less power and more distortion could sound SO much better than the latest in 1980's silver-box-blinky-light amps?
Over the years, I've built 91-like amps several times, using a variety of transformers. I have a pair of amps underway currently using the little MQ transformers you mention. For the record, they're TFA-204's.
Oh...and I still have a pair of NOS Hovland caps I bought on your recommendation! ;-)
Yeah, Hovland owes me. Too bad they are out of business, so I will never collect.
TFA=204...that's it. They were not bad. That was the first modern US made SE trans and I got one of the first pairs. I liked the later DS-050 Magnequest too.
There were a couple of us Audiomart guys messing with SE in the late 80s. Larry Rubins up in NJ, Andy Bowman, Art Loesch, and a few others, but getting Hifi transformers was a problem. There were none.
I had a pr of real 91s and my first DIY sounded better! That was encouraging. It was at that point I switched from vintage to DIY. Still say DIY is the way to go! Keep it up!
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Free your mind and your ass will follow -- Parliament/Funkadelic
I built this amp with the c3m in pentode, k&k CCS and OD3 regulation on the screen. cap coupled with a copper .1uf Duelund to a EML300bxls with AC heat and luckily no hum pots needed. Used Audio Note Kaisei caps in the power supply and throughout along with some BGs I had on hand. Audio note opt trannies into audio note an e speakers. Big involving sound with great pacing dynamics and texture. Pretty much in love with the sound with no interest in up grading.
I forgot to mention that I built it as an integrated amp with a 50k Goldpoint attenuator. The icing on the cake is kondo silver interconnects and audio note Lexus speaker cables. This wire combo really brought the sound together.
Thank you Joe Roberts for you influential schematic and article! Made a difference for me!
Hello, I am planning on building this amp also. Where do I find 15mf 1000v caps from. I cannot find them anywhere.
Benniehanas
Thanks for posting. Nice work.
One of my systems is running 310A/300B monos that I built from a diyhifisupply kit(Ladyday 91). Top quality iron and parts. Probably the best amps I have owned and there have been many. I plan on building the 4 Pi speakers and matching subs which should be a great match for these amps.
My amps are similar: pentode (6bq5) driving 300b with magenquest iron. Love it! And I'm driving 4-pi clones. Amps do a great job with that speaker!
nt
Sounds like a fun project. Make sure you post some pictures.I had 310A's in my amp back when i was bread boarding it, but I liked the sound of the ef37a's so much I tried them that I decided to sell the 310's to help finance the project. The ef37a's aren't cheap, but they much less than the 310A's. Post a pic of your amp if you get a chance.
Edits: 04/21/15 04/24/15
nt
nada aqui
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Buy Chinese. Bury freedom.
Well done Vinnie - looks really nice and neat. I like how you have labelled things as well.
Agreed, there is something nice about the low Gm pentodes driving a DHT. Well, of course I think so.
Congrats.
91.
Someday, I will build one. Nice pics!
The article from Sp and the schematic are on line at the address below
http://timebanditaudio.com/300b/WE91A.pdf
Yes, thank you! I have the SP DVD's since 2008 (if I am not mistaken). I bought my copy from JR himself.Abe
Edits: 04/20/15
Glad to hear it. Joe worked very hard on that magazine, and never got much back from it financially. A great many of us started in diy audio because of him and SP.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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