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    Will be getting my ST120 soon!

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    Jerryfan

    Posts : 7
    Join date : 2014-10-03

    Will be getting my ST120 soon!

    Post by Jerryfan on Thu Oct 30, 2014 12:36 pm

    My name is Mike and I'm from Chicago. This will be my first tube amp. I sold a couple of my best vintage receivers, including my beautiful Mcintosh 4100 and a very nice marantz 2270 to buy it. I'm missing them very much right now and hope I'll forget all about them when I get my completed amp next week.
    I had bob send a kit to my tech in New York . I'll be using gold lion kt88 with rca black plate 12bh7 and a weber SS rectifier. I also got the binding post that allow me to hook up 4 or 8 ohm speakers. I will be using the stepped attenuator controlling the volume from my yulong d18 dac. My tech will be doing some mods as well as adding a preout for my svs subwoofer. My speakers are either vandersteen 2ce or recapped yamaha ns690 mk1. Oh and I also got bobs power supply cap upgrade.

    This is from my tech describing his mods to the st120 board:

    I'll explain the ST-120 board. No doubt this is a well built amplifier. The 12BH7 phase splitters operate from about 190 volts plate with LM334s on the cathodes limiting the plate current to 5ma, Looking at the plate curves, the tubes are not quite operating in the linear part of the plate curves.

    The input 12BH7 (tube in the middle of the board) also is operating at a low voltage with high cathode bias.

    Looking at the power supply, the last 4.7K resistor is slightly high in value. Change this resistor to 2.7K @ 5 watts.

    Next. The LM334 (looks like a TO-3 transistor) uses a 10 ohm resistor across two of the leads. Two exist on the circuit board. Adjust for 11ma per tube plate (10.5 volts at the 12BH7 cathode).

    The coupling capacitors were changed to K40Y-9.

    The tubes were RCA 12BH7 black plate for the phase invertors and a Sylvania 12BH7 short black plate for the input tube. (owner reports even better results with three Hytron 12BH7s).

    After mods, I heard a much larger soundstage and the sound itself was more exciting vs ordinary.

    I just wanted to introduce my self.


    Last edited by Bob Latino on Sat Nov 15, 2014 10:00 am; edited 2 times in total (Reason for editing : Spelling)

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Will be getting my ST120 soon!

    Post by deepee99 on Thu Oct 30, 2014 2:03 pm

    Welcome to the loony bin.
    Interesting comments from your tekkie.
    You'll find some great expertise on this board. And if we don't know the answers, we'll make something up that's really cool.


    Jerryfan

    Posts : 7
    Join date : 2014-10-03

    Re: Will be getting my ST120 soon!

    Post by Jerryfan on Thu Oct 30, 2014 2:49 pm

    Thank you. I am worried about the problems that pop up with tube amps. I'm glad there is this forum. Anyone know a good tech in Chicago ?

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Will be getting my ST120 soon!

    Post by tubes4hifi on Thu Oct 30, 2014 3:29 pm

    Hi Jerry,
    glad to hear you are enjoying your new amp and this forum.
    The VTA70/120 circuit wasn't designed for use with a 12BH7, although a lot of people do use it as a sub, but yes, it does like a higher operating point.
    Unfortunately, the LM334 CCS also isn't designed for use over 10ma, that's one of the first specs you'll find if you look at the data sheet, it has a maximum
    spec of 10ma. As the circuit was designed for 12AU7, it was set for 8ma and works fine, in fact have never had a report of one going bad, but if your amp has problems
    in the future you'll know where to look. There are several other diodes and CCS parts that are designed for higher current operation, and your tech should have
    checked into proper operating conditions before making all those changes. That is a similar effect of running the output tubes at 70ma per tube instead of the recommended 40ma per tube. It may sound better for a week or a month, but the tubes will burn out 4X as fast.

    Jerryfan

    Posts : 7
    Join date : 2014-10-03

    Re: Will be getting my ST120 soon!

    Post by Jerryfan on Thu Oct 30, 2014 3:49 pm

    I won't have the amp until next week. I know of one other person(from another forum) who had this tech do these mods to his st120 in June of 2014. He haven't had problems yet. I guess I'll find out. Thanks for the info.

    j beede

    Posts : 317
    Join date : 2011-02-07
    Location : California

    Re: Will be getting my ST120 soon!

    Post by j beede on Thu Oct 30, 2014 4:10 pm

    Welcome to the land of ordinary sounding, narrow sound stage amplifiers Jerryfan.

    Alan-14

    Posts : 46
    Join date : 2012-03-31
    Age : 56
    Location : Melbourne - Australia

    Re: Will be getting my ST120 soon!

    Post by Alan-14 on Thu Oct 30, 2014 4:17 pm

    Welcome to the forum Jerryfan.

    Bob and Roy have been building and refining these amps for years ... they are reliable and perform and sound better than many more expensive amps.

    Having a tech without the experience of Bob and Roy on these amps modify the circuit design is going to be a risky excersise.

    Might have been better to ask about your planned modifications here first.

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: Will be getting my ST120 soon!

    Post by tubes4hifi on Thu Oct 30, 2014 7:06 pm

    after looking at J Beede's reply, I had to look thru some of his posts to see what amp he is using.
    Looks like he is not an ST70 fan, but loves his MK3 amps!   That's OK !!

    BTW, 25 years now selling the VTA70 mod, it's gone thru some changes over the years but still the same basic circuit,
    the major improvement being adding the CCS about 3 years ago . . .
    Lots of pretender competition, but I think our customers know what works the best compared to the other stuff out there.

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: Will be getting my ST120 soon!

    Post by mantha3 on Thu Oct 30, 2014 8:34 pm

    Jerryfan

    This is a new amp. Issues with tube amps are most often vintage where 30 years of age may have caught up... I have had this same amp you just purchased for 3 + years and it is rock solid. Get a spare GZ34 and maybe a quad of spare power tubes... A good set of preamp tubes last Gawd knows how many years.

    Enjoy! Solid amp and these measure up with the McIntosh gear. I have a pair of MC75 amps and this amp you have in ways sounds better to me

    Alan-14

    Posts : 46
    Join date : 2012-03-31
    Age : 56
    Location : Melbourne - Australia

    Re: Will be getting my ST120 soon!

    Post by Alan-14 on Fri Oct 31, 2014 5:49 pm

    Where's the like button on these forums ... Razz

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Will be getting my ST120 soon!

    Post by deepee99 on Fri Oct 31, 2014 6:19 pm

    Alan-14 wrote:Where's the like button on these forums ... Razz
    Alan
    There's no "Like" button but we have drones for "Unlike." Beware.
    Why not just start with Bob/Roy's components and designs?
    They've got a week or two's expertise here.
    You will have many tube-rolling options that will vary your experience: that's the beauty of their designs. I wouldn't suggest messing about under the hood until you've at least a year of listening, or until something goes pear-shaped.
    Your driver tube choices and how you wire your speakers, interconnects, and how you bias your output tubes will influence your experience than anything else, IMHO.
    Go with what's good, which is stock, have a smoke and a jug of wine, and enjoy what you've got.


    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Will be getting my ST120 soon!

    Post by zx on Sat Nov 01, 2014 11:47 am

    Jerryfan...............THanks for any an all info an pic.....on Dynaco are any other tube mod are stup you are having done are you do......so far look like a great teck you have....an any tube amp Made.... can be made to give better sound i have found.....an for the Cash.... it looks from other input cant beat Bobs setup as a base!
    I have a few 50year old Stock Dynaco MK3s ....an there a lot of fun ....an can sound great with a few MOds...
    So Welcome an keep it comeing.......an most of all have fun with tubes...............................



    Thanks for the Site Bob............

    Alan-14

    Posts : 46
    Join date : 2012-03-31
    Age : 56
    Location : Melbourne - Australia

    Re: Will be getting my ST120 soon!

    Post by Alan-14 on Sat Nov 01, 2014 7:37 pm

    deepee99 wrote:
    Our drones have a special warhead for blurkers. We know where you live and what animals and plants you eat.  We're just excessively polite, but even crazy people have their limits.

    Just as well I like polite, crazy people Razz

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Will be getting my ST120 soon!

    Post by deepee99 on Sat Nov 01, 2014 7:45 pm

    Alan-14, Jerryfan,
    Just messing with ya.
    The reason there's no "like" button is because it would just encourage Skiz -- like he needs any encouragement. Love his graphics but beware, he controls the drones.
    If you've been small tube-rolling, you've no doubt learnt the "house sound" of old-stock Philips, Mullard RCA, Tung-Sol, GE, et. al and it's at that point you've just gotta decide what you like best. We've all been through this. It's a Ford v Chevy thing. I run a 9-pin SP-13 with old RCA 6CG7 clear tops, tried many others, can't imagine any better than the 2nd-gen (grey plate) clear tops so I quit screwing around with 'em and just bought some more, in case I live into my 90s.
    Power tubes, a whole different story. While I'm partial to the original TS 6550s, the new KT-120s from New Sensor, especially in triode mode, are very, very sweet and at a fraction of the price. Jim McShane puts his through a torture test before matching, worth the bucks. The old NOS TS 6550s go for $150 or more apiece and they ain't getting cheaper and while not as robust bias-wise as the 120s, they'll live for decades. The old Tung-Sols, and later, transistors, are what put TV repairmen out of business.






    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Will be getting my ST120 soon!

    Post by deepee99 on Sat Nov 01, 2014 8:17 pm

    Bob, et. al,
    My Collins S-line, which was the pinnacle of tube ham gear, used 12AT7s, 12AX7s and 12AU7s in their circuits. Even into the late 1980s of my hamming days, there was no grief with these tubes.
    http://www.wa3key.com/tubes.html
    Why would Collins be using these tubes until the late 1970s? I'd like to see some documentation.
    Somebody's blowing smoke, and it ain't magic.
    -d-

    Jerryfan

    Posts : 7
    Join date : 2014-10-03

    Re: Will be getting my ST120 soon!

    Post by Jerryfan on Fri Nov 14, 2014 1:12 pm

    t



    My preouts


    http://i1368.photobucket.com/albums/ag170/smego8982/Mobile%20Uploads/65437CC5-41E5-435A-BAC2-675559183FE3_zpsmzqelun8.jpg

    http://i1368.photobucket.com/albums/ag170/smego8982/Mobile%20Uploads/FC2CB44C-849C-42CF-8E4B-8FDD93BEB06B_zpsjvklsgbl.jpg



    http://i1368.photobucket.com/albums/ag170/smego8982/Mobile%20Uploads/782A21CC-1761-4B29-96F9-11BD380F6936_zpsmsmftbdm.jpg

    http://i1368.photobucket.com/albums/ag170/smego8982/Mobile%20Uploads/D31571B3-395B-442B-8ECF-AF6D9DC5A971_zpsgupkiemv.jpg

    Well it's done and he is burning it in now. I'll have it by the middle of next week. My tech is smart guy an engineer and designer. He has lots of satisfied customers. Is it that outrageous to think this design can be improved upon? I'm not trying to start trouble here I just want to share what I'm doing with people that love music and fine audio. I will be sure to post my impressions after a day or so with the amp.  

    Here is an email from my tech:

    This is the build details.

    The stock power supply with factory upgrades produces 3 volts AC ripple. (I use software simulators). My redesign produces .3 volts AC ripple.  Downstream the ripple is better than .05 microvolts. No AC ripple to mix with the audio causing minute, but detectable intermodulation products (IMD).

    The stock power supply input filter is at the limit of ripple current. My input filter can carry 5x the ripple current. Cpacitors should last 40 years.

    The stock power supply with the upgrade capacitors on the preamp board at 385 volts using 400 volt capacitors. I modd'ed the input board voltage from the main power supply with 430 volts and used 450 volt capacitors. The extra voltage is needed to bring the 12BH7s into the more linear part of the plate curves that equals better sound.

    The phase splitter 12BH7s are re-biased from 4.5ma to 7ma each further bringing the 12BH7s into a more linear parts of the plate curves. Simply, standard operating voltages & current per the tube manufacture (attention to detail).

    The bias supply produced .3 volts AC ripple. I added a series resistor to reduce the bias AC ripple to .03 volts ripple.

    The stock coupling caps were K42Y-9 and a 'run of the mill' unidentified film cap. My mod uses K40Y-9 capacitors throughout.

    The wiring routing was performed in such a matter as to reduce coupling from other wires that could cause some degrading of the sound.

    About my rectifier:

    Permanent- SS only.  A GZ34 rectifier cannot handle the current of a ST-120. That rectifier tube is pressed as is with a ST-70 amplifier. Not only is the rectifier socket not wired for the 5 volts as (why stress the 5 volt winding with 535 volts DC riding on it), a tube rectifier will not sound better nor produce the peak watt output of the ST-120 capacity. The only GZ34 that has a chance is old Mullard GZ34 at huge cost. The Sovtek GZ34 would fail quickly. Perhaps immediately.

    GZ-34 specs-
    250ma maximum current with power transformers having a secondary winding of 125 ohms (peak current limiting by design)
    60uF maximum capacitor size (first filter)
    Dynaco ST-120 secondary winding of approximately 50 ohms violates the GZ34 250ma current rating reducing it to 160ma maximum in a ST-120

    ST-120 specs
    250ma current (average)
    60uF first filter (limited by a rectifier tube)
    155uF second filter (with upgrade option)

    Your ST-120
    227uf first filter. I installed a super filter. Same as Jim McShane performs in the H&K Citation II.
    110uF second filter

    Result- no tube allowed.

    The new SS rectifiers are quiet (no switching noise) and the same as tube rectifiers that are quiet. That is, a quiet power supply does not add IMD products in the sound. SS rectifiers have no series resistance (unlike tube rectifiers) and provide much higher peak to average current, This how a 60 watt H&K Citation II makes over 100 watts peak output.

    Jerryfan

    Posts : 7
    Join date : 2014-10-03

    Re: Will be getting my ST120 soon!

    Post by Jerryfan on Fri Nov 14, 2014 1:13 pm

    http://i1368.photobucket.com/albums/ag170/smego8982/Mobile%20Uploads/D31571B3-395B-442B-8ECF-AF6D9DC5A971_zpsgupkiemv.jpg

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