From: awelker@dretor.dciem.dnd.ca (Andrew Welker) Subject: Dynaco ST70 (LONG) Date: 3 Dec 92 19:24:41 GMT To all of you that responded to my request, thank you! Here is the list of responses that I recieved for those that are interested. I would still like to hear from more of you about modding tube equipment. Andrew Welker >From sbradley@scic.intel.com Fri Oct 30 10:59:17 1992 To: awelker@dretor.dciem.dnd.ca Subject: Re: Dynaco ST70 Newsgroups: rec.audio.high-end Organization: Intel Corporation Cc: I installed the Sutherland mods in my ST70. This consists of a classA FET input board, new B+ and bias power supply boards, and LED bias indicators. Quite a few people have been impressed by the sound, and I've gotten years of trouble-free service. Sutherland is out of business, but I have blank boards and complete info if you were interested. -- Seth J. Bradley, Senior System Administrator, Intel SCIC Internet: sbradley@scic.intel.com UUCP: uunet!scic.intel.com!sbradley ---------------------------------------- "A system admin's life is a sorry one. The only advantage he has over Emergency Room doctors is that malpractice suits are rare. On the other hand, ER doctors never have to deal with patients installing new versions of their own innards!" -Michael O'Brien >From sbradley@scic.intel.com Fri Oct 30 11:43:32 1992 To: awelker@dretor.dciem.dnd.ca (Andrew Welker) Subject: Re: Dynaco ST70 Andrew Welker writes: > Thanks for the response Seth! It's good to know that there are actually people > out there that will go to the trouble of responding. I am interested in the > boards. What do they cost and how much to stuff them? I have them basically to make copies of. I'd be happy to loan you the boards and you could copy them. Their layout is simple and would be easy to duplicate (there are kits on the market for making PC boards). You could probably stuff everything for about $50/set. > What do you think of changing to a different phase-splitter, al la 6AN8 ? I wouldn't change phase-splitter unless you go to a whole new input board (there are several out there at various prices). The 7199 is a lousy tube, and is becoming almost impossible to find. If these amps are "keepers", I'd either go with the Sutherland or another tube-based board. The 7199 will soon be impossible to get, as its out of production. Most of the tube-based boards replace the 7199 with 2 (sometimes 3) tubes. Seth >From tim@me.rochester.edu Fri Oct 30 13:18:56 1992 To: awelker@dretor.dciem.dnd.ca Subject: Re: Dynaco ST70 Newsgroups: rec.audio.high-end Organization: Univ of Rochester, College of Engineering and Applied Science Cc: I'm in the process of re-engineering my ST70 replacing the driver tubes (7199) with a 12ax7 and a pair of 12au7's. The amp is 1/2 complete.... I can keep you posted if you are interested. tim -- +----------------------------------------------------------------------------+ | Timothy Takahashi tim@fulcrum.me.rochester.edu | | Research Assistant | | Dept. of Mechanical Engineering | | University of Rochester (716)275-4072 (W) | | Rochester, NY 14627 (716)442-5832 (H) | +----------------------------------------------------------------------------+ >From raymond@casbah.acns.nwu.edu Fri Oct 30 14:05:54 1992 Subject: Re: Dynaco ST70 To: awelker@dretor.dciem.dnd.ca (Andrew Welker) X-Mailer: ELM [version 2.3 PL11] There was an article in Glass Audio (published by the same editor as Audio Amateur) that gave a comparison of three (or was it 4) ST-70 mods. The article appeared about 1 year ago, if my recollection serves me well. I have modified Mk III's using the GSI mod, but have nothing to comapre it to except for the original Mk III. The improvement was amazing, though I still feel there is room to do better,, in particular in the midrange. The GSI bords were not always reliable (parts mounted backwards), resistirs underpowered for the application getting so hot as to melt the solder, etc.My experience may be an exception, and if you know what you are doing, it gives you a great sounding amp for not too much $$$. When you have compiled a list of responses, could you send it to me? Thanks! RAY. >From rons@tv.tv.tek.com Fri Oct 30 14:55:58 1992 To: awelker@dretor.dciem.dnd.ca Subject: Re: Dynaco ST70 Newsgroups: rec.audio.high-end Organization: Tektronix TV Products I have a ST-70 also. It is basically stock with the only upgrades being metal film resistors and better caps. I also added biasing for each output tube individually. I use it to drive a pair of highly modified Altec A7's. The two compliment each other very well. The Altecs are HIGHLY efficient so the 35 watts is perfect. Of course I also use the PAS3-X preamp. What I wish to do is replace the 7199 with a dual triode input section. Future plans include bi-amping using the ST-70 to drive the horns possibly in triode mode and I have a pair of Bigg of California 50W mono tube amps to use for the lows. The Bigg amps are classic williamson with EL34 outputs. So yes! include me in the club so to speak. Love this tube stuff, especially DIY projects! ron >From jas@proteon.com Mon Nov 2 11:35:08 1992 To: awelker@dretor.dciem.dnd.ca (Andrew Welker) Subject: Dynaco ST70 I've been very happy with the Curcio modificaiton (see Glass Audio). However, he didn't stabilize the feedback, and it rings as designed. Put at least 75 pF across the feedback resistor, and it will be stable. (Without these, the highs are extremely over-etched). The lows are great. The regulated power supply makes all the difference. It may lose some of the depth, atmosphere, and palpability of the stock Stereo 70, but it is much more precise in so many ways that you won't mind this. The stock Stereo 70 just does not have enough open loop gain for the feedback to do much of anything at the frequency extremes, especially when you add the HF rooloffs needed for stability. The Curcio Stereo 70 has a lot more open loop gain (I'll be measuring it soon, to improve the stability). >From NICKERSO@SSCvax.CIS.McMaster.CA Mon Nov 2 18:49:08 1992 Subject: Dynaco Mod To: awelker@dretor.dciem.dnd.ca X-Envelope-To: awelker@dretor.dciem.dnd.ca X-Vms-To: IN%"awelker@dretor.dciem.dnd.ca" Mime-Version: 1.0 Content-Type: TEXT/PLAIN; CHARSET=US-ASCII Content-Transfer-Encoding: 7BIT I modded my Dynacos back in '86 and have since sold them. The only note I have to make (other than the standard cap replacement with high quality polystyrenes - I used Aselco brand) is the rectifier tube. A solid state high voltage supply would be good - that old can electrolytic is probably not in great shape. It would get rid of hum. The filament of the 6AN8 tube should probably be fed off of regulated power. For long filament life on this tube, feed the filament from a 12V regulated source through a 20 ohm resistor to effect a slow turn on (easy on the filament). This is for a 6V, 300mA filament (so the resistor drops 6 volts stedy state). The resistor should be rated at least 5 watts. If you don't replace the high volt supply completely, the 5AR4 rectifier can be replaced by a pair of solid state diodes with a common resistor equal to (or maybe half of) the 5AR4 operating plate resistance to simulate it. I found that raw diodes fed straight to the cap resulted in a voltage dangerously close to cap maximum and made the pwoer transformer buzz (from high current surges). I can get schematics. Kent >From alexmi@tv.tv.tek.com Mon Nov 9 17:06:21 1992 To: awelker@dretor.dciem.dnd.ca (Andrew Welker) Subject: Re: Passive Bi-amping There are two circuit boards you have to buy them from Old Colony. I vaguely remember that I paid $60 for both. You'll need two pairs of 6DJ8, $10 to $15 per pair, two power transistors, one regulator, couple of small signal transistors, a couple of pots, metal film resistors, good film caps, maybe $50 depending on your source. You can probably rebuild them for $150 to $200; I don't know how much Old Colony sells the kit, my guess is around $300, the advantage is that all parts will fit just right on the board. Sound quality? I did not have a chance to compare to a unmodified ST70 so I can't say "great improvement". It does measure though very vell, flat from 5HZ to well over 100kHz. I hope I'll have soon the chance to compare to a stock ST70 and I'll let you know. Cheers, Alex ------------------------------ From: Andy Moss Date: Sat, 5 Jun 1993 20:00:00 -0400 Subject: dynaco mods SHAHIN AL RASHID proclaimed.... ARp I have recently purchased a Dynaco ST70.It is a stock,factory wired ARp model without modifications.It came with matching quad Phillips EL34s ARp and stock unused (2 pairs) Dynaco 7199.It sounds great,and cost $400CDN. Drats! Another one gets snapped up! 8*) ARp 1)Which tubes are audibly better the EL34s or the 6CA7s (GE stock)? ARp (I also wouldn't mind suggestions on any other brand of tube) I don't know about your first question, but the ST-70 I picked up in April at a flea-market(yes, really)had Mullard tubes in it and sounded great! Mine had been modified a little when I got it. Specifically, the coupling cap's had been changed to some real antiques which I'm sure did NOT improve things. Also, the input had been modified for a higher impedance - why I don't know! Since then the amplifier has be returned to stock condition so that I can get some voltage readings to base my own modifications on. ARp 2)Amongst the many mods suggested to replace the 7199s there was ARp one that caught my attention.It was suggested by an old technician ARp which seemed extremely simple.He claims to have outfitted one ARp of the ST70s with 6U8s instead of 7199s.This was done directly with ARp just modifications to the pin connections. Hmmmm....this sounds very familiar. I *was* going to replace the 7199s with two seperate tubes but may try this route now. ARp 3)Is there any other brand of 7199s I should look into? Mine has RCAs in it, FTR. ARp 4)Anybody know where I can lay my hands on some 6BX7s (BTW were there ARp any JAN or industrial equivalents made?) 6BX7GT (cylindrical) new 6.50$ US. If you are interested, let me know. Not aware of any heavier duty models. ARp I feel its some kind of microphonic ARp effect,is it?If so is there any way of shielding these tubes? Could be due to proximity. Can only speculate. Try putting chimneys on the tubes, that may help. Thank's for the 6KD8 idea! Cheer's, Andy --- ~ Magical Mail ~ Klipschorn - Honk if you're horny. -- ------------------------------ Date: Tue, 8 Feb 1994 10:42:00 +0000 From: "henry (h.) pasternack" Subject: Converting an ST-70 to indirectly-heated triodes. It would be a nuisance to convert an ST-70 to indirectly- heated triode outputs. I'm sure it has been done, but I ask if it it's worth the effort. You'll need to put in another filament transformer for the output tubes, and you'll need to satisfy yourself that the driver stage is able to deliver about twice the stock output voltage. You'll need to think about the B+ voltage, which is on the high side for 2A3's. In the end, you'll put a lot of work into designing what is essentially a whole new amplifier around transformers of dubious quality. Better to save the indirectly-heated triodes for a whole new project, in my opinion. -Henry ------------------------------ From: AXLEROD@ubvm.cc.buffalo.edu (Harvey Axlerod) Subject: JAMES BRODSKY - readme Date: Wed, 9 Feb 1994 19:51:42 GMT Sorry to waste the bandwidth, but I can't my mailer to send this reply. Jim... $175 is an excellent price! Most of the used dealers want 2-3x that amount, and they are hard to come by. In fact, AUDIOMART often has "wanted" ads for W5M and W4M's. KT66 are somewhat hard to come by, they are equivalent to 6L6, which is an easy tube to get. Never heard about the OPT blowing. I would never run this stuff with the cages on. If you have young kids or cats, set the equipment on high, hard to access shelves. Even better, run them in the basement, where it will be cooler, and drill small leads through the floorboards. Be creative. I have 2 Dyna MkII monoblocks sitting around, as I prefer the more delicate sound of an older and less powerful H/K amp that puts out 12 amps. The DYNAs sound warm but muddy to me, and yes, they are up to specs. The reason there are so many mods to the ST70's is because they were not a great design. In fact, many mods bring the circuit back to a Williamson circuit. The W5M is a Williamson circuit (that's what the model number means -- Williamson, 5th version, Mono), and needs no mods except perhaps some basic parts replacements. Happy listening. ----------------------------------------------------------------------- Harvey Axlerod, Sr. Programmer-analyst, User Services, Univ. at Buffalo axLErod@ubvm.cc.buffalo.edu (716) 645-3570 FAX: (716) 645-3734 ------------------------------ From: peterca@ento.csiro.au Date: Thu, 10 Feb 94 10:17:42 EST Subject: Re: Converting an ST-70 to indirectly-heated triodes. : It would be a nuisance to convert an ST-70 to indirectly- :heated triode outpu.... :about the B+ voltage, which is on the high side for 2A3's...... : Better to save the indirectly-heated triodes for a whole :new project I have no problem with the above advice, but have we got the jargon right. I thought the 2A3, 300B etc were "directly heated" triodes because the cathode and heater were one and the same piece of wire -the "filament" whereas more modern valves were "indirectly heated" because they had a separate, electrically insulated heater inside the cathode. Peter Campbell (peterca@ento.csiro.au) C/O Div. Entomology, C.S.I.R.O. Box 1700 Canberra A.C.T. 2601 Ph.61-6-2464158 (w), 61-6-2516213 (h) Australia 61-6-2464173 (fax)