Dynaco Owners Opinions Please

[Shakey]

I have both a stock ST-70 and PAS2 & the SCA-35.

The PAS 2 needs some TLC. Probably should re-tube it before I pass judgement.

The ST-70 is stock, except for the power tubes, EI's, with original 7189's on driver board otherwise stock.

The SCA - 35 is stock with the exception of the filter cap I think, still hums a little. Mostly Russian tubes.

I also have a nice sounding pair of A-25's.

Which should I tackle first. I am thinking the PAS - 2 should be fixed up or should I get rid of the PAS-2, ST-70 combo and focus on the SCA-35 for a second system which most likely be CD based.

Where have any of you found good re-hab stuff. I know about Curio and I am somewhat reluctant to scrap the original circuits. I'd rather just keep as stock as possible. I will probably use the A-25's until someone suggests something better.

Anyway I am somewhat handy with the soldering iron, biggest project was a HeathKit Color TV.

My thinking is get these things running before they die on the shelf. Actually they all run, the PAS 2 limping the most.

[wallenjs]

I have the Triode Electronics upgrade board and capacitor upgrade.
Very nice additions to my ST-70

http://www.triodeel.com/dynaco.html

[Shakey]

Yes, I have seen it. They seem to be connected to Elite Audio near my home. Ted's and Triode and Elite seem to be associated. I bought the ST-70 from them but I am hesitant to go non-stock.
Looks like you have replaced the Filter Cap on this unit, amongst other things.
Looks nice!
Thanks!

[Sckott]

I'd keep the PAS & the Dynaco especially if you can put these things to use. I would tackle the Pre 1st. The ST70 should be a sinch for most with good technical background and a soldering iron.

But I wouldn't get rid of an ST70 :) There are many improvements you can do to it for very little money, but only mod the amp when parts fail like the 20/20/xx can cap. There's many snap in replacements that really trim up the performance of the amp, but many mainly don't change a LOT in the sound. They do improve reliability though! Big time.

Of course you're listening to the biggest fan of that amp. :rolleyes: ;)

[whaaat]

Originally posted by Sckott
There are many improvements you can do to it for very little money, but only mod the amp when parts fail like the 20/20/xx can cap. There's many snap in replacements that really trim up the performance of the amp, but many mainly don't change a LOT in the sound. They do improve reliability though! Big time.
Could you elaborate, please? I picked up an ST-70 about a month ago, and am picking up a PAS-3 this weekend. So far, I'm very happy with my intro to tubes. :love:
In fact, I have my computer hooked up to it, and a producer friend was asking me "why doesn't my copy of this CD sound this good?" while I was streaming mp3s through it. :laugh:
I know it'll be a while until I'm able to afford something in the range of a Mac or Marantz, but in the meantime, I'm more than satisfied.
Now I know that swapping out the EL34s will make a difference in the sound. Can I expect a change by changing out the 7199s or the rectifier tube?

[Clay]

I am in a similar situation, so I hope someone posts some of the upgrades to get rid of hum on the PAS2. I switched out the 6x4 (?) since the original one was weak and it improved it. It still hums, yet has the original telefunken tubes. I think I could use a upgrade kit with good instructions.
I have 3 st70's and a PAS2.
I used to listen to a Joe Curico modified unit (older version) and now listen to a rebuilt on the stock boards with all the best components unit with Mullards. One is dead in that it will blow one tube if you turn it on.

My son wants the Dynaco set up - It is cool that he likes the sound of tubes too.

[Clay]

I have my computer hooked up to it, and a producer friend was asking me "why doesn't my copy of this CD sound this good?" while I was streaming mp3s through it.
Totally Cool. I should try that.

[rickl]

Clay,
It is hard to tell but if you get hum out of both channels the quad cap might be bad. I'm using Joe's quad cap replacement board in my Mark IIIs with success.

Joe's site as some pretty good descriptions on troubleshooting that should narrow down the problem. There is also a dynaco lovers forum (asylum :-) that is pretty active.

These things are very simple and with a few ideas and time, you should be able to get your son listening to the Dyna gear.

http://www.curcioaudio.com

rick

[cwon]

Grover does work on stock ST-70's. I don't know what his backlog is like. I guess you hit the CONTACT US button at the bottom to ask him about it. If I had a stock ST-70, I would.

[Sckott]

Originally posted by whaaat

Could you elaborate, please? I picked up an ST-70 about a month ago, and am picking up a PAS-3 this weekend. So far, I'm very happy with my intro to tubes. :love: =

You can really tighten up the Dynaco ST70 electronically ONLY IF you are electronic savvy and know your way around a soldering iron (with the exception of #1).

1. Tubes - An easy, harmless upgrade
2. Solder joint examination - Check those dirty connections, if they exist. Touch up the suspicious ones.
3. Replace bad wiring - Rare ever that an ST70 is wired well AND has decent miles on it. I've owned 3, two of them I still have and ALL of them needed rewiring TLC. This is big. Make sure you replace the heater wires THRU AND THRU if you replace some of it, make sure you twist em.
4. Tighten tube connections - A very small pair of needle nose pliers are fine....

The rest like the SDS Cap Board, the funky 7199 board replacements don't seem to do an AMAZING job at a sonic performance difference, but the replacement 7199 boards make driver tubes cheaper and the rest DO make big improvements on the amp's reliability. The "Can Cap" does have a limited life, and a lot of them die after long storage or are found dead. The SDS replacement drop-in board makes the amp more "solid" electronically, and you might even find your tubes last a bit longer.....

I listen to my ST70's almost every day. Lately, my "Mut" one has been very strong with a few mods.

[Clay]

electronic savvy
Joe's site has instructions for how to turn on a st70 that has been sitting. It is IMO a intricate set of details. It sounds fun and I change light switches and outlets, yet there are a lot more volts on the amp than house current so I do worry a bit about a deadly shock. I get better with each solder joint so I am willing to solder.

I am very lucky that I can drive to Joe's in less than 30 minutes.

I listen to a st70 almost every day also. I temporally have a Fisher 400 system in the same room. It is fun to compare the sound.

These are all excellent ideas Sckott, cwon, rickl, wallenjs

[NOSValves]

I put in a strong vote for the Stereo 70 and the Pas 2 !! Although if it was me I would opt for a more modern preamp but the Pas 2 can be made to do very well with a minor cost involved. I definitely wouldn't sink a bunch of money into replacement boards for the Pas 2. The stereo 70 can do great things and the upgrade front end board from Ned at Triode Electronics is a great step in the right direction. I would also redesign the PS filter network if it was me! With the JJ Telsa cans available now its possible to do this with full size cans to really give the amp what it really needs. The SDS Cap boards are nice but only do the job half way in my opinion.

Craig
NOSValves.com

[The Viking]

HI Guys: Well I am in position of having both of these units. The ST 70 is just upgraded to replacement of socket ,wiring and caps, etc. The Pas 2 I went whole hog and did the Curico full mod. It is very quiet, but don't like the fact of no tone controls. I wouldn't do it again. All in all though it is a great sound.Soon as my daughter gets her digital camera for Christmas I will post a picture.
Love the St 70.
Dale
God is Great SABOO

[Shakey]

Viking - Do all upgrades to the PAS at Curcio involve removing the tone controls?

Thanks for all the input everyone, I always thought of keeping the ST and the PAS and I figured I'd have to do some upgrading. The ST Works fine as is and is completely stock with the exception of the power tubes.

The SCA-35 was what I really was interested in as far as getting opinions on what I might do. If any of you are familiar with it, let me know. I know this, it shares the iron of the ST-35 which seems much less common. I understand some think it to have a sound of its own, different that is, from its big brother and is well liked by some. It is very clean and is basically stock except for a replacement of the power caps with a newer configuration, not a can.

The PAS is what is in need of the most help. After I picked it up and listened to it, it seemed to have a couple of problems. One the overall gain is low and not well balanced. Changing tubes seemed to switch the imbalance with the tubes, but the low gain remained, so I suppose those ought to be replaced first.
What I'd like to do is not invest in tubes for the stock boards if I will be changing them soon. The boards themselves seem to have aged in a manner which caused them to depart from flat, I assume that they were flat at one time, and are now curved.
A couple members appear to think investing too much $$$ into it may be unwarranted on account of its age and what could be had for the equivalent $$$.

So I guess I'll focus on the ST-70 & PAS for now. I might add I do like the provision of a phono section on the PAS but at present do not have a second TT so I am on the fence about its necessity.

[The Viking]

Yes!!
Dale

[NOSValves]

I just feel by the time you spend all the money to upgrade the boards Curcio's stuff in a Pas 2 or 3 you have driven your cost up to a point where you could buy some really nice preamps. Take a look at Mapletree Audio he has some really great preamp for cheap but if you decide to go this route get his SE version of whatever preamp you buy. Well worth the extra few bucks. I have his Ultra 4A and thinks its a great package. When you factor in that eBay will net you near $200 for the Pas 3 it seems to me this is a noi brainer.


Craig

[electrode10101]

One problem routinely overlooked in the PAS series of preamps is replacing the selenium rectifier that provides DC to the tube's heaters.

These rectifiers lose voltage with age, and when they die, they emit toxic fumes. Additionally, they provide about 11-11.5 volts to the heaters of the 12AX7's, which helps control noise, but also runs the tubes out of their optimum range. As the rectifier ages, this voltage can go as low as 10, 9 volts or lower (actually, the voltage is double; the tube heaters are wired in series), which will definitely affect the operation of the tubes.

If you are not experienced in repair, pass this chore, and the heater filter capacitor replacement on to a tech. While this section of the circuit deals in low voltage, there are parts of the circuit that cointain lethal voltages.

That being said, a basic rebuild will bring a PAS back to life: replace the caps, check the resistors and the tubes. Enjoy the PAS for what it is; a bargain priced, very good sounding tube preamp.

The SCA-35 is similar to the highly coveted ST-35 in that it shares the same output iron. The driver circuits are dissimilar in that the ST-35 uses a triode input and driver, and the SCA-35 uses a pentode/triode driver. They sound different from one another.

Again, the SCA-35 will benefit from tube changes, cap replacement, and resistor checks, but magic-mojo mods should be avoided, as they tend to be costly, and don't always improve the sound of the amp. Enjoy this amp for it's nice mellow tube sound, nice looks, compact size.

AFA the ST-70 is concerned, I've installed the Triode Electronics board in one of mine, and it sounds very nice. Keep the rectifier tube in circuit; SS diodes let the magic out. I owned my first ST-70, built as a kit, in 1972. They are great little amps. There are scillions of mods for them. Get it running right first, then decide on an upgrade path, if you wish, and follow it.

Happy Listening,

John Diamantis

[Sckott]

Keep the rectifier tube in circuit; SS diodes let the magic out.

:agree: Yep. A very small diode worth 13 cents will replace the 5AR4 but I would NOT do that.... :)

[rickl]

I just feel by the time you spend all the money to upgrade the boards Curcio's stuff in a Pas 2 or 3 you have driven your cost up to a point where you could buy some really nice preamps.

Craig's point shouldn't be ignored. The Curcio stuff uses the enclosure and iron. If you pay $$$ for a PAS and plunk more $$$ into replacement boards, it will add up.

However, if you can read a schematic, fling some solder and can get your neighbor's PAS for free, upgrade boards might make sense.

One problem routinely overlooked in the PAS series of preamps is replacing the selenium rectifier that provides DC to the tube's heaters.

This rule applies to all vintage tube gear with selenium rectifiers. Even a minty Layfatte KT600 ;-).

When I got my Mark IIIs going after decades of storage, I added new 6550Cs, new quad cap and replaced the selenium rectifier. While they are not as nice as my friend's AtmaSpere M60s, they are still enjoyable.

I'm currently building from scratch the driver stages.

rick

[The Viking]

I totally agree. By the time mine was done( I had a Tech do the work)I ended up having $1600.00 Canadian in mine. Sounds good, but could I have done with the money for a new Pre?Retoricle question.
Dale

[indy mike]

Craig, could you fill us in with some info on those Tesla cans? I have a MK IV that I suspect has a weak/dying filter cap and wonder if there's a replacement available for that particular amp (and ST-70's are cool - my older brotehr snatched my dad's when he went to college - grrrrrrrr)....

[NOSValves]

Indy if I was you and you just want to replace the existing can with a slight upgrade in value I would go to AES www.tubesandmore.com and buy part number C-EC80-40-30-20 this can is 525VDC rated and has values of 80uF 40uF 30uf 20uf use the 20uF right on pin 8 of the rectifier tube and then 80uF on the far side of the choke use the 40uF this is the supply spot for the center tap of the output transformer very important spot then use 30uF and 40uF sections to fill in the final sections that filter the front end board. I take it you have 2 Mark IV's so 2 cans will be require.

JJ Telsa also makes a 40/20/20/20@500V can which would do the job but not really be a upgrade. Its cheaper though. In The Mark III's I use the above mentioned can then sneak 2 JJ Telsa 100/100uF Cans under the chassis to really give the amp some real slam and clean reserve power.

The Mark IV is actually a better amp then the ST70 the biggest weakness of the Stereo 70 compared to the Mark IV is the fact that it uses 1 5AR4 and very little filtering to supply both channels power. The Mark IV has a dedicated power supply for each channel which is a much better setup.


Craig

[NOSValves]

Oh and you can add the extra cans under the chassis if you like also. What direction you take there depends on if you value the life of your rectifier tube. The 5AR4 is only rated for 60uF of direct filtering but I run 140uF on mine without a problem and this is even with $12 sovtek tubes ! You can safely use without stressing the 5AR4 by using a 100/100uF and a 50/50uF JJ Telsa install 100 with the 80uF another 100uf on each front end section(couple the 50/50 can together for 100uF) You will have one stiff PS for great bass definition and a very clean top end.


While your at it buy the Mark IV upgrade board from Ned at Triode electronics.

The next time your brother comes by you can then tell him your sure glad he took the Stereo 70 !!!

I bet you can complete this upgrade yourself for about $350 bucks when done this amp will compete with amps in the 2 to $3K range no problem.


Craig

[indy mike]

I'm NOS on the 7199's (GE) and GZ-34 (Mullard/GE); is stiffening things up that much ok? I've read that doubling the value of the filter caps is safe, but that's a pretty good sized leap you've mentioned. Does this tighten the bass up? I'm running Dynaudio monitors and the poor woofers are looking for tighter control on the bass notes (my slightly modded Dynaco ST-150 puts a comparative stranglehold on woofer cone motion, but the magic mids/sparkly highs ain't there)....

As for the ST-70, sentiment reigns here - my dad built a MK II (which I have and has LOTS of hum - another project for another day), then went the ST-70/Pas-2 route when he went stereo...

[NOSValves]

Well allot of people get scared away by engineer or design specs. All I can tell you is my personal amps run the value I suggest and work flawlessly 16 hours a day 7 days a week and have for a long time. The only danger is to a $12 tube so what is the big deal. If you do not want to put your NOS GZ34 in harms way buy a Sovtek.


This will absolutely help put a strangle hold on the woofers !!


I gave you many options on what to do ! The decision is entirely up to you.

Most people that caution upping values by no more then double are talking about coupling caps and this would be absolutely true !! To large of a value on coupling caps can cause instability in the circuit. This is not true with the power supply.