MapleTree Audio Stealth60 to come out as a kit

knowledge base for the classic Dynaco ST70

Postby WA4SWJ » Mon Mar 20, 2006 9:50 pm

Hi John,

I see that no one has replied to your question so I thought I'd throw in my thoughts.

First, I have no experience at all with Mapletree Designs so I am not speaking from experience. I am only offering an opinion that to be honest is not based on much. So here goes.

How that amp sounds will depend on a large number of variables. The power supply, the output transformers, the circuit topology (and I have not experimented with that particular circuit as used by Mapletree), the quality of parts, the quality of the tubes, and on and on. It looks OK on paper. I would question why those particular tubes were selected. You don't often see a 6SL7 as a phase inverter for example. I happen to like 12BH7's or 6SN7's. But, it may sound just fine. Only your ears can tell. It would be very hard for anyone to praise or assail that design without actually hearing it. I don't think anyone can tell you how it will sound by just looking at the schematic.

A lot of this is subjective. An amp that sounds great to me or you might sound terrible to another person. Or the type of music you listen to might make a difference. Some speakers for example sound terrible on rock but great when playing jazz and vice versa. All I know is that the amps I've built using the circuit topologies Shannon has offered here do sound extremely good to me. I love my ST-70 clone built using his board. It is powerful and wonderfully clear. My eiclones are also very powerful, responsive and sound very, very good. They work great in my home theater setup and provide a lot of transient impact. Commenting on how the Mapletree amp would do with the movies and music I like to listen to would only be a shot in the dark without actually trying it. It would be nice if there were some impartial reviews available (maybe there are) or perhaps a try before you buy program or money back if you're not happy or something like that.

So, unfortunately, the only way I know to tell you how that amp sounds would be to listen to it. My opinion is that I could build a lot of amplifier from scratch for $1000.

Hope this helps but I doubt it did much.

Good luck with your decision. I wish I could offer more help.

With regards,
Ed Long
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Postby EWBrown » Tue Mar 21, 2006 8:03 am

I've used 12AT7s (and other similar mu triodes) as LTP or Schmidt type phase inverters with good results, 6SL7 isn't too far different specs than the 'AT7s. One modded ST-70 that I built very early last year, uses Roy Mottram's VTA-70 driver circuit, which uses three 12AT7s and it sounds darn good! I have yet to install a DIY70 board in another ST-70 that I have, which currently has an "exact duplicate" new PC board of the original Dynaco 7199 circuit, which reolaced a 38 year old crispy 'n' crunchy phenolic board complete with the original "black cat" caps (not firecrackers) Yellow_Light_Colorz_PDT_07

The oft-mentioned 6EM7 PP amp, the VA half of the tubes has a mu about 64, and two of these make a decent phase splitter, with no preceding VA stage. And I haven't even tried any tube matching or AC balancing to tweek them out, yet. No NFB in the circuit, either.

It will be interesting to run a side-by side of comparison of 3 12AT7s vs 12AX7 and 2 12AU7s, with all else being equal. I would guess one would need golden ears or some good test equipment to suss out the differences...

In either case, either of these all-triode drivers will definitely beat out the 7199s, or EF86s and 12AU7, or just about any other pentode input circuit.

From looking over the schematic, it looks like this Mapletree Stealth 60 amp uses fairly "generic" Hammond Iron, including 1645 OPTs. Nothing special, no custom wound high-dollar stuff here. Just using some "Brand T" Dynaclone iron would probably be a moderate upgrade, at added cost.

/ed B in NH
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Postby Hollow Fate » Wed Mar 22, 2006 8:06 pm

Hello John in Copenhagen. Which Mapletree headphone amp do you have? I am considering building an Ear+ Purist HD. I bought an Ultra 4A SE pre-amp off of Lloyd about 1 ½ year’s ago and it is a fabulous piece of audio engineering. Dead quiet even when the signal travels through the phono stage, which from what I read on the Web is not the easiest thing to do with tubes. Some folks insist that transistors are the only way to go for the phono stage due to noise issues. Not so the Ultra. I had Lloyd mod the Ultra to take Russian 6N2P (mu=100) tubes instead of the 12SC7’s (also mu=100).

As an aside (to anyone): beware of some Russian 6N2P’s. Mine are 6N2P-EV (OTK/military) tubes made in February 1991 by the Voskhod factory (Logo is a horizontal missile). These are great tubes. I’ve also bought four “normal” 6N2P’s (Novosibirsk manufacture, 1960’s). These are trash. Noisiest tubes I’ve ever heard. Just a warning. Another warning: 6N2P-EV tubes take a very long time to break in. They were good in the beginning but great after about 20-30 hours. Weird but true.

I find it interesting that every power amp Lloyd offers, including his ST-70 driver board, uses an input pentode instead of a high mu triode. My next power amp is one of Lloyd’s designs called the “777” Amp. It’s not a commercial offering but its input is also a pentode (EF37A or 6J7). Tube line-up in my version of this amp is a red Mullard EF37A input (beautiful tube), a pair of RCA 6J5’s as the phase splitter, and National Union 1625 (i.e: 12 volt 807) power tubes running in triode mode (as per the design).

Aside #2: The aforementioned EF37A was the tube that was the basis of the world’s first electronic computer developed by the Brits during WW2 and based at Blechley Park. Some folks believe the world’s first computer was Eniac, but in fact Eniac was preceded by several years by the “Colossus” computer which ran on banks of EF37A tubes (grey military paint, the red metalized tubes were for the consumer market.) Colossus was built during WW2 to crack the Nazi Enigma machine and its codebooks. Hitler declared than no human could crack Enigma. He was right, but in the end a computer did it instead. The Royal Navy succeeded in capturing an intact Enigma machine (Lorenz manufactured; like the tubes) from a German U-Boat that was then sunk to appear that all was lost. It is authoritatively stated that the information gained through the use of the Colossus computer and the access thus gained of “secret” Nazi communications had the effect of shortening the war by two years. Calculate that out in thousands of lives. More info at:

http://www.r-type.org/static/valvecpu.htm

Donald
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Postby EWBrown » Thu Mar 23, 2006 7:12 am

I have also found that true with the OTK and "diamond" marked 6N1P-EVs, they are much better and lower noise, and microphonics. I cloned one of the Decware "Plate" linestages, and the standard run of the mill 6N1Ps really s*cked in the VA stage, but the Mil-Spec OTKs were excellent. The CF stage, didn't matter if it was a permium or cheapo 6N1P.

I had bought the 6N1Ps on sale from AES a few years ago, as "Svetlana" 6N1Ps, four were "OTK" and the rest were "standard".

OTK stands for "Otdiel Technicheskovo Kontrolya" or Technical Control Bureau. Basically Mil-Spec standards. The "Diamond" is another internationally-recognised "quality" marking. I also look for these markings on the Russian capacitors (AKA Kommie Kaps) available on E-bay.

On the -EB tubes (EV), The E stands for extended life >5000 hours, and the B/V stands for Ruggedised construction - I have noticed that 6P14P-EV tubes are noticeably heavier than "plain" 6P14Ps. Some say that the EV's have a "darker" sound but I haven't noticed this (YMMV).

/ed B in NH
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Postby Hollow Fate » Sat Apr 01, 2006 11:22 am

There are actually two similar/different “diamond” marks in Russian Tube-ology. The most important one is the one Ed refers to as a “quality mark” applied after testing of the individual tube in question. The other diamond mark that is sometimes encountered is the old Reflector Factory mark.

The following Ebay listing shows a tube with both diamond marks together. The lower position “quality mark” diamond on the tube looks like it was applied by hand, because it was. Funny that.

http://cgi.ebay.ca/ws/eBayISAPI.dll?Vie ... otohosting

The “current” Reflector mark is a stylized thermionic device. I say “current” since part of this tube factory is now called “Xpo-pul “ and is owned by New Sensor Corp. that markets its production in the US and Canada under the name and logo of Svetlana (stylized “S”). Not to be confused with the real Svetlana factory based (as always) in St. Petersburg that uses a Winged “C” (the Russian “S”) as its logo.

Ref: Russian Tube Logos:

http://www.ominous-valve.com/russtube.html

The above site site shows both “S” and “C” Svetlana marks as belonging to the same company. This was only true for a short while. New Sensor/Electro Harmonix/Sovtek is the owner of the Svetlana name for the retailing of vacuum tubes in North America.

The confusion is diminished somewhat by this article attributed to the Svetlana Factory in St. Petersburg.

http://www.wingedc.com/

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