|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
1st, thanks for those who answered my post below. that help a bit!I now need to decide what is the better use of money for restoring/redoing the Dynaco ST-70.
option 1: new tubes and 1 or more mods from CurioAudio.com Which ones would you choose for the money?
option 2: replace all the components except the large cap, PCB's, transformers and Chasis? I would get new Sovtek tubes, Audiocap Theta caps, metal film resistors. I figure I could do all the parts for ~$200
What would you do? Option 1 can get much much more expensive...probably to the point where I would just consider getting the Consonance Ella...but others here may have another opinion.
Follow Ups:
The original Dynaco ST-70, as designed by David Hafler, was a landmark of balance. Truly an example of the whole being greater than the sum of the parts. Each part was chosen with regard to price and performance. Too expensive, no dice. Didn't sound good no dice.To mod or replace the circuit is to alter the inherant balance and sound.
Right now, most ST-70 amps are at least 30yrs old, with most ~40yrs old. So the current sound, however good, is the result of 30-40yr old parts. Basicly a looonnngg burn in to translate into audiophile terms. I would be very surprised if most ST-70 amps can meet their original specifications for power, bandwith and extension. No problem with the design mind you, just a function of age. So even if everything is "working", you are not hearing the amp at its best.
To go ahead and modify the amp at this point will surely improve the sound. But this comparison is apples to oranges because you are comparing aged parts (and solder connections) to spiffy new ones.
Before you embark on changeing the basic design of the amp, which ALL of the mod kits do to a certain extent, how about listening for awhile to the amp as it was designed. You will learn a lot along the way, about the amp, electronics and the sound of your system.
So, what I advocate is a rebuild to stock specifications, with new close tolerance parts. I would not go for premium caps or resistors at this stage.
Take the time to match the resistors to at least 5% tolerance, better to try for 1-2%. Time consuming, but easy to do. Whether you opt for metal film or carbon comp is up to you. Metals tend to remain stable over time, but with a more SS sound. Carbons on the other hand, retain the vintage warm sound, but at the expense of instability over time. For circuit board caps, I would opt for Sprague 716P, but others like Illinois or Xicon. Definatly replace the selenium rectifier with at least a diode (1n4007 is fine), but others use Hexfeds. At this time, replace the bias supply circuit. Generic Rat Shack electrolyitics are fine here, same for the associated resistors. I would also replace the can cap at this time.
Pull all of the preamp socket wireing (except for bias pin !!) as well.For tubes, Svetlana EL-34 quad definately. But I would go used for the 7199pr and 5ar4. With patience, you can find a pair of good used 7199 for $10-15 and good used 5ar4 for $25. Both are better sounding choices than current production tubes.
Button it up, test it, bias it and be prepared for a revolation !
To my ears, the difference in sound between old parts and new parts is an order of magnitude. A stock rebuild also gets you right to the point of diminshing returns in terms of value. I would stop here and listen for a few months. If you want to continue, fine, but at least you have a fair base to work from. The Triode or Curcio circuit kits, combined with the SDS Labs power supply seem to be the most popular choices. But be forwarned- the difference from a stock rebuld to one of the kits is small, and definately on the order of diminshing returns.
Best,
rather than using a 1n4007, I would recommend 2 GI856's in series. I replaced the 1n4007's in my preamp with those and the difference is remarkable. I got mine from Jim McShane after a few emails back and forth (seems like a very knowledgeable fellow).
This is for the neg. bias supply in the ST70. Are you saying that your preamp has a neg. bias supply for the output tubes, and that changing diodes made an improvement?
Diodes in series is definately recommended (my OTL project taught me that lesson.) When they go they short rather than open so a second diode in series will save your butt and output tubes if one diode just happens to fail an early death. -jd
I just love it when people speak of ST70 upgrades that are really total circuit redesigns. Philosophically speaking, these are nothing more (or less) than some foreign amp on an ST70 chassis. Shades of xenomorphology. One may like the sound of what results but an ST70 it ain't.
The above posts have expressed beautifully the quality of a simple unmodified ST-70. I often wish I had my old 7199 and circuit board. My poor old board just couldn't take anymore and had to be replaced, but otherwise STOCK.
Charles
I love this kit and think the price is silly-cheap: It keeps the things you liked about the ST70 and fixes the not-so-nice parts. Sonically, it's lively and engaging and has no particular vices. This is complete rebuild, not a modification, and it does away with the exposed circuit board, so you can safely operate the amp without the cage. The hard-to-get 7199 tubes are also eliminated. I was happy enough with the sonics of a borrowed Welborne ST70 that I thought "I could stop right here". But the amp was only a loaner and I didn't have an ST70 of my own. So I bought a pair of Ron W.'s Laurel IIX SET amps instead and couldn't be happier: Though mine are the base models, I've felt zero need to change out any of the stock parts (Ron's idea of stock parts includes Elna Cerafines and Hovland Musicaps); I just like firing them up and listening to music. Sonically, the Laurels make happy sounds like the rebuilt ST70, only more so.
Hi,
For me the choice was to strip the chassis completley and install both triode boards. this allowed me to polish the chassis, install ceramic sockets, eliminate pam sockets, new power switch and speaker terminals-cardas. I am very happy with the results and learned valuable info along the way.
I just finished a Dynaco ST70 restoration that took about 6 months. I agree with Jim C's post regarding doing it slow and spreading out the cost over a period of time.My changes are pretty much as follows:
Replaced selenium rectifier $1
Replaced Bias Filter capacitors $2
Valve Art EL34 Quad $56
Svetlana EL34 Quad $56 (I had a valve art go bad so I bought these)
Replaced Bias Resistors $2
Replaced 7 tube sockets (4 output, 1 rectifier, 2 driver) don't change the sockets on the front to keep the look $25
Sovtek 5AR4 $15Replaced the quad capacitor with a replacement from Antique Electric Supply $35
Replaced the PC-3 Driver board with a new restoration board from www.dynaco-doctor.com. $45 (Joe's website should be the first step in your restoration. He has some great info for downloading available on the ST70)
Stuffed the restoration board with all new Riken resistors from Angela Instruments $2 each ($75 or $80 or so for all the resistors in the dynaco)
Replaced all driver board capacitors with Auricaps $35
New Input jack replacement part $7
New Output Gold Binding Posts from Radio Shack $16
As you can see the restoration didn't cost much and you certainly don't need to spend near as much money on resistors and capacitors as I did. If you were to replace with Allen Bradley resistors and Orange Drop capacitors you'd spend a fraction of what I did.
Change things slowly so you'll know what changes work for you. Just putting all new stuff in is no guarantee that it'll sound better than the old stuff. If the old stuff sounds better....keep it.
Good Luck on your restoration. Almost everybody has owned one of these at one time or another and the www.deja.com newsgroups are full of posts with every imaginable problem, solution, modifications, and theories regarding the ST70. No matter how bad it gets the answer is out there on the web.
John H.
Nice post! Very good advice for a Dynaco newbie, and great step by step directions.FYI, I have found that reducing the feedback amount oferes more magic and dimensionality to this amp.
What did you do specifically to drop the feedback?
Just a bit of an overview, a larger resistor between the output transformer and driver cathode will reduce the feedback, smaller will increase.The small pf cap many amps use across that resistor may also need tweaking for optimum square wave response and/or stability from HF oscillation if you change radically.
You can remove the resistor entirely, but with less feedback you get a narrower frequency range and more output impedance, generally considered a bad thing.
You can remove it and really like it better with an UL or triode amp like the ST70 because the damping factor should still be somewhat reasonable. This is generally not the case with pentode connected output stages, the output impedance just becomes too high, but YMMV.
If you want to experiment, try doubling the size of the resistor. If you hate it I would leave it where it was stock. If you like it try removing it all and see what you think. You can't hurt anything by removing it, although you head towards instability increasing the feedback. Some amps probably run more of a ragged edge in this regard than others with the parameters their designers settled on.
I didnt know it could be droped back. Maybe stopped completely.
Most people agree the Sovtek 5AR4 and 7199 tubes are at least OK. For the output tubes though, you can do better than Sovtek. Svetlana EL34 or JJ-Tesla E34L are much better regarded on AA. I agree with the other comments, go slow with your upgrades so you can savor each improvement (or know what to undo if it makes the sound worse!)
NB,If the filter cap cans and filter caps in the bias power supply are original or ove 20 years old---you'll probably need to replace them. Also, any electrolytic bypass caps on the front end tubes.
Before, you invest in any components---eyeball the circuit board, if you spot damaged tracing on the circuit board or burnt components---you might want to reconsider redoing the amp.
As a rule, I usually replace all filter caps (multi-section cans and single caps), plus voicing or non-polar caps, at the same time. As, it'll be easier to remember when the caps were replaced. And sometimes, when you replace the power supply caps the increase in voltage output stresses the other non-polar caps and may push a borderline component over the edge into failure.
Resistors get changed out, if they reside in heated areas---like the cathode tied resistors (those 15.6 ohm jobs off the EL34's). And ones in the power supply portion of the circuit.
My ST70 renovation started with new tubes out jof necessity, as it arrived with an incorrect rectifier, a bad 7199, and a mis-matched set of outputs.Before installing the new tubes (Svetlana EL-34s, JAN Philips 7199s, and a good used Mullard rectifier), I cleaned the sockets with Caig D-5.
I used the amp for couple of years with just the above service until recently, when I read a post here that explained that the Selenium rectifier used for the negative bias supply tends to spew vaporized Selenium when it fails, so that got replaced with a generic SS diode. The resulting voltage put the first bias filter cap right at its rated working voltage, so after a couple of days use, the caps and resistors in the supply were replaced. I used Mr. Curcio's idea for changing the range of bias control, and calculated new resistor values to replace the 10k carbons, although the old resistors measured very well.
I next replaced the input connectors with som nice gold ones from Radio Shack, which I selected for their mounting hardware as much as anything else. I was able to remove the old connectors from the phenolic boards, and modify the boards to accept the new connectors. In the process, I also relplaced the two 470k resistors with some metal films, although once again, the original resistors measured well within tolerance. In addition, I installed some very pure copper wire in the signal path from the connectors to the PCB.
I next replaced the 4 .1uF driver coupling caps on the PCB with some AXONs. The old caps, although they measured and worked well, were a tiny bit leaky.
Today, I will replace the can cap with a new manufacture one from Antique Electronics Supply. They sell these for under $35.00 (there is still a 10% off sale in effect for the next few days!), and even though the old cap seems fine, I thought it time to let it retire after more than 40 years of faithful service.
Next will be the octal sockets on the top of the amp with ceramic ones. As you can see, I am working my way toward the PCB, which I will probably replace entirely with a new reproduction from Curcio, stuffed with parts of my choice.
So far, not counting the tubes, I have well under $100.00 in parts; the Curcio replacement PCB will add another $40.00 plus parts. As is, the amp sounds wonderful, and I suspect an improvement from replacing the can cap.
This 'little bit at a time' technique has been beneficial to me, as it has allowed me to evaluate each change along the way, and it has kept the cash outlay spread out over a period of time.
IMO, this is a wonderful sounding amp, and well worth the effort.
Cheers!
I replaced the PCB with Curcio's design and I was floored. I liked the 7199s and thought there was little room for improvement, but WOW was I wrong. Go for the board and get the upgraded cap package from Joe-WELL worth it. I'm not a fan of radical renovations but if you're restoring the ST-70 everything on the PCB has to be replaced anyway. Why not benifit from some improved board design.I too have not changed the original can cap but I guess it's time. Even though there is no hum your right 40+ years-time for a rest.
Charles
I was in the same situation, a few weeks ago. People suggested to restore the Dyna first and then upgrade. Thats the way i did it and feel was the best. You learn as you restore and more so, you learn what the original Dyna sounds like (great, to describe it in short). Then you can choose if an upgrade is worth it. For me the Neds Triode board really does make the amp sound more like a modern upper end amp.Ask questions along the way, there are many many helpful people out there.
This post is made possible by the generous support of people like you and our sponsors: