What can I expect from a Dynaco ST-70?

[mediumjim]

Happy New Year, survived another one! 

The guy who serviced it indicated that he feels that 5U4GB's are more stable that older 5AR4's. I will use a GZ34/5AR4.  It should also be noted that when real clean and original units comes his way he will service them from ones he is restoring, e.g., use resistors, caps, etc., that are well within spec from them.  This apparently is the case with mine.  This is more than fine with me. 

It will ship Monday and I should have it by the following Monday. 

I might need to sell part of my vintage tubes to afford a quad of Mullard XF2's or similar and a good duet of 7199's...only if the ST-70 has the goods in the tone department.   

Jim

 

[Paully]

I can't speak from real experience, but man I don't think a matched quad of Mullard xf2 are even remotely worth it!  Output tubes today are, by and large, considered every bit as good as most of the historic counterparts.  There are so many really good output tubes for the Dynaco; EH 6CA7, Genelex KT77 reissue, JJ E34L, etc...  Or even just some old stock Tesla EL34.  Try those first, they really are very good!  But of course its your money and you have to go at this hobby as you see fit. 

That is the first I have ever heard a 5U4 thought to be significantly more stable than a 5AR4.  Could be, I won't argue but I would want some real evidence to support that claim.  I would be interested to know actually.

That can cap is a ticking time bomb.  I can understand why you would accept a man's opinion who services these with regularity.  But please do go onto Curcio's forum and read up on "can cap / can capacitor" if that is the original can cap and see what other people whom I have come to respect a great deal myself have to say on the subject.  Anyway, don't want to worry you too much.  Listen and enjoy!  You made a good decision getting a Dynaco.  You will enjoy it. 

 

[mediumjim]

Not at all worried about it.  Sometimes on forums, a very few control the discussions and a bad experience can outweigh all the good ones.  I've been around vintage tube guitar amps for the last 20+ years and I truly believe in fixing only what is in need of fixing.  I've seem 50+ year old electrolytic caps that are still right and new amps less than 5 years old with bad ones.  Most of the old Guitar Tube Amps have selenium rectifier circuits that have presented no problems.  My point is simple, each amp has to be taken on a case by case basis. 

One of my friends is Aspen Pittman who is/was Groove Tubes and he (as do I) would disagree with you regarding vintage Vacuum Tubes versus new ones.  The metallurgy is far superior in vintage tubes, as is the overall construction and tolerances.  Find me a EL34/6CA7 with true welding that will last several thousands of hours in normal usage. 

I do agree that it would be a little foolish to spend $500.00 for a Dynaco ST-60 and then spend around 1K for tubes for it.  There are some nice EL34's from the late 70's to mid 80's that I will try to find, but these actually are more scarce.

Thanks,
Jim

I can't speak from real experience, but man I don't think a matched quad of Mullard xf2 are even remotely worth it!  Output tubes today are, by and large, considered every bit as good as most of the historic counterparts.  There are so many really good output tubes for the Dynaco; EH 6CA7, Genelex KT77 reissue, JJ E34L, etc...  Or even just some old stock Tesla EL34.  Try those first, they really are very good!  But of course its your money and you have to go at this hobby as you see fit. 

That is the first I have ever heard a 5U4 thought to be significantly more stable than a 5AR4.  Could be, I won't argue but I would want some real evidence to support that claim.  I would be interested to know actually.

That can cap is a ticking time bomb.  I can understand why you would accept a man's opinion who services these with regularity.  But please do go onto Curcio's forum and read up on "can cap / can capacitor" if that is the original can cap and see what other people whom I have come to respect a great deal myself have to say on the subject.  Anyway, don't want to worry you too much.  Listen and enjoy!  You made a good decision getting a Dynaco.  You will enjoy it. 

 

[Paully]

I won't come back with condescension to match, but I would hate for someone reading this to think that preventive maintenance on a Dynaco ST-70 is a non-issue and that 50 year old electrical parts should just be left until they present a problem.  I can't think of anyone on the Dynaco board who has ever suggested otherwise.  But perhaps that isn't really the point, we simply disagree on what "merits fixing" up-front and what can be left alone.  I do tend to err on the side of caution but then I get that from Grainger!  Regardless, enjoy!

 

[JC]

I would tend to agree with that assessment.  BTW, please tell your friend that I keep his book (The Tube Amp Book) close at hand!

I ran my Stereo 70 for many, many hours before I did anything to it beyond replacing the tubes, because one of the 7199's was defective and the EL-34s were a mismatched bunch of questionable lineage.

I eventually replaced the can cap, not because it wasn't working, but because a drop-in replacement of new manufacture became available and I really had no way of testing the actual capacitance of the sections on the original one at operating Voltages.  IIRC, the original was an AeroVox, and as far as I know, it is still a functional part.

The Selenium rectifier was somewhat of a moral dilemma once I read that one that gives up often spews toxins into the air.  Since I would be using the amp around others, I wasn't comfortable exposing them to an unknown risk.  So, as a compromise, I left the Selenium in place, but simply took it out of service.  Again, I had played the amp  "as is" for hours at this point.

My decision to retain as much of the original amp as possible centered on its heritage.  When I first acquired it, I naturally wanted to inspect it "under the hood".  When I turned it over and removed the bottom plate, I discovered a name and date scratched into the underside of the chassis with an engraver.  The date put it at Christmas of 1959, but the name was of an old acquaintance since passed who had owned the first music store in my town to sell drums, guitars, and amps!

At that point, I knew that fancy new driver and PS boards were out, and hunted up the NOS and new tubes I needed, set the bias, and put it into service!

Yours looks to be in very good shape, physically better than mine was.

The link should get you to a .pdf of the original manual, and the site has lots of good stuff:

www.curcioaudio.com/st7_mnl.pdf


Happy New Year!  

 

[mediumjim]

I won't come back with condescension to match, but I would hate for someone reading this to think that preventive maintenance on a Dynaco ST-70 is a non-issue and that 50 year old electrical parts should just be left until they present a problem.  I can't think of anyone on the Dynaco board who has ever suggested otherwise.  But perhaps that isn't really the point, we simply disagree on what "merits fixing" up-front and what can be left alone.  I do tend to err on the side of caution but then I get that from Grainger!  Regardless, enjoy!

Paully, I wasn't being condescending, merely an observation.  Sure, eventually everything will go bad, but since its all well within proper specs why mess with it?  I personally agree with the way Carl tastefully serviced the ST-70 he's shipping to me.  I could have easy opted for one that he fully restored, but that wasn't what I was seeking.  Our differences are of philosophy and personal choice and I tend to err on the side of originality on vintage gear when possible. Besides, I have one of the best amp guys if something goes wayward on it to put it back right. 

Who knows, I might get it and not like the way it sounds and then have to do some tweaking and upgrades.

On Audiogon right now is a couple of original ST-70's that I was considering...would you get one and then fix what ain't broke?

 

[mediumjim]

JC:

Pretty cool having one from 1959! I will have to check the EIA coding on the Tranny's on mine to see when the kit was assembled.  I may opt to do the same thing to bypass the Selenium and go with a new "Can" down the road.  Too bad Aspen sold GT to Fender as I used to be able to search the back room for tubes he had.  I'm sure that is where I got the EL34's that I hope will match nicely for ma output...  It is a conundrum to spend twice and much for tubes than the unit itself, but at the end of the day, tone is king. 

For the record, my McIntosh MX-110 is from 1961 and is very complete and original and was recently serviced and aligned and only needed a couple of caps and tubes...it would be real cool if the ST-70 I'm getting is from 1961! 

Jim

 

[Paully]

"On Audiogon right now is a couple of original ST-70's that I was considering...would you get one and then fix what ain't broke?"

Yes, I have a severly overstated tendency to do just that...  I do love upgrading parts and tinkering.  I have some Altecs I am getting ready to "fix" that play quite well.  Can't help it.  But I understand the tendency to keep it stock to a degree as I kept the stock 7199 circuit even if I did upgrade the board and its parts.  I wanted the orginal sound, just more of it!  Sorry if I misinterpreted condescension however, just how I read it.  But there are multiple issues.  The cap doesn't even have to fail to be taxing to the power tranny and rectifier as it ages.  Like you said, it is just which side we want to err on.  For me we buy the Dynaco for the output transformers mainly.  Leaving it stock is good, but not when the part in question most likely detracts from the sound as it was used as an economy and when its failure can cause siginificant damage I start upgrading.  My nature.  Maybe I get something better, maybe I have nothing like the designer intended.  But anyway, I have had two Dynacos, the can cap failed on one, the other I never gave it a chance.  I may be overeacting as others may be doing as well.  It doesn't matter, we have said our piece and its your amp and as you have made clear you have your own experience with components over a long period of time with guitar amps.  So again I say congratulations, I would love to know how you like the sound after you have listened for a while.  This is all a side issue and it really does come down to personal experience and opinion, I don't know of any hard data to back either of us up one way or the other.  So again, enjoy and please do post back on your assessment of the sound.  I think I will be plugging mine back in for a while when I get back home.

 

[JC]

I wouldn't be surprised if you learn that it is in that same neighborhood regarding its age.

The one I got had been serviced at some point by someone who learned about servicing those new-fangled printed circuit boards in military training: When in doubt, cut the trace!  I discovered that the ones Dyna used were remarkably tough, as I neatened up his work!

Yours, I'm happy to say, looks like it avoided such "service"!

 

[Wanderer]

IThat can cap is a ticking time bomb.  I can understand why you would accept a man's opinion who services these with regularity.  But please do go onto Curcio's forum and read up on "can cap / can capacitor" if that is the original can cap and see what other people whom I have come to respect a great deal myself have to say on the subject. 

Not to flog a dead horse but back in the mid 1970s to early 1980s I fussed about a number of Dynaco MKIII, ST-70s, PAS preamps and even a couple ST35s.

I agree whole heartedly with Paully here. Based on my experiance with a number of Dynaco (and Scott..and Fisher tube units) the quad caps and original selenium rectifier have a high probablity of failure.

One of the issues - at least for the quad cap - is that common domestic AC voltage has risen in most places in the USA from the 110 to 115 volts in use when the ST-70 was designed to 125 volts or even 130 volts now. The safety margin for over-voltage at start up has been reduced because of this. Keep in mind we are talking about a power supply that features a step-up high voltage transformer that only magnifies the change.

I have also seen bad connections in orignal old tube sockets cause a thermal runway and distruction of output tubes. Given the relative cost of tubes it seems odd economy to not replace some less expensive old parts.             

 

[mediumjim]

I love good passionate discussions...

Don't you think that if the Selenium/Quad Cap made it this far that it did so for a good reason?  As I said in one of my posts, I probably will do a selenium bypass and drop a new can in it, but only after listening to it.   I have full faith (right now) in the guy who did the work on it as he has good rep...if it was out of spec, he would have dealt with it.  

Jim

Note:  The Quad Cap Can was replaced for one that was tested and was in spec, the Black Cats tested in spec,  bias caps were replaced.  The only thing that has not been addressed (per the concerns) is the Selenium Rectifier and I will consider replacing it with a silcon diode say a 1N4004 or of higher value. 

 

[mediumjim]

Just to put it to rest, I'm having Carl replace the Selenium Rectifier with a Silicon Diode before he ships it to me.  As I posted already, the Quad Cap Can is in proper spec as are the Black Cat's.  

I already have a space made for it and will be using the Meadowlark Swifts as they have the sweetest mids and highs.  They have Vifa 5.5" drivers and 1" Soft Dome Tweeters that can reach down to 35hz  (with a 4db rolloff) and being 8ohm and 89db should be able to get to pretty impressive sound levels with the ST-70.  

Jim

 

[2wo]

Glad to hear that you are going with the diode. 0 down side and it may save you an expensive quad of outputs someday...John

 

[mediumjim]

2wo:

I agree, it is a smart move.  It may or may not change the tone, but as you say, if it goes it will do damage, especially to the output tubes.  As you may have already surmised, I fancy nice tubes. 

It will be interesting to compare it to my Marantz 9's.  It should be no contest, but that's what I thought when I A/B'd my Upgraded FP2 against my serviced and aligned McIntosh MX-110 and the FP was the clear winner. 

It's nice that I still get thrills and excitement over this stuff and hope I will always!  I admire you guys that actually understand the insides and can work and design upgrades.  Some day I might attempt building a kit or two....but I need to practice on my soldering skills first! 

Jim

 

[mediumjim]

Okay, one of my good friends has a great collection of NOS Vacuum Tubes and sold me a tightly matched quad of EL34/6CA7's  Two are branded Haltron and have welded plates and two small halo getters.  The other pair are RCA (Mullards) from 1970 that also have welded plates with a single large halo getter.  They tested 7400 7600 7700 7700, well above New.  They are also very tight ma wise 25.5 to 27ma...I believe they were tested on a Hickok 539A. 

This is the same guy who I got an octet for my Marantz 9's.  My justification is that if I ever need a tube for the Marantz's, I will have some to pull from the Dynaco and put something back into the Dynaco.  I have mixed double/single getter Mullards in my Marantz's and they sound great.  The plan for the Dynaco is to have one of each on each side. 

I should have it some time next week and really am getting excited about it...

Jim

 

[mediumjim]

I've had the Dynaco ST-70 for several days now and all the adjustments and set-up tweaks have been done. 

I rolled in a NOS matched quad of Mullard made xf2 EL34's, a NOS set of Sylvania 7199's and used a tests very stong 1957 metal base Amperex GZ34.  The bias point is 1.50v (sounds the best there).  I had to adjust the new powered subwoofer several times to get it to a place where it was perfectly integrated with the Meadowlark Swifts.  It takes several hours for a new speaker to break in and do its thing...

Last night was the pinnacle (so far), so it all came together and wonderful music was coming out of my new system.  The beauty is that as the tubes burn in, it will only get better!  Right now the Soundstage is every bit as nice as I get from my Marantz 9's.  There are subtle differences in instrument placement and depth. 

I love my Marantz 9's and they sound great and deserve their place in history as being one of the best amps ever.  At the same time, I also have to say that the Dynaco ST-70 is truly a great amp in its own rights. 

The 50's were a great time for Tube Amp Design and many simply cannot be improved upon IMHO.

Jim

 

[Doc B]

The 50's were a great time for Tube Amp Design and many simply cannot be improved upon IMHO.

I think you might be pleasantly surprised to find that some current single ended designs can show improved resolution over the classic push pull amps.

 

[corndog71]

I would also recommend the Dynaco Doctor as an excellent reference.  I've rebuilt an ST70 with Joe Curcio's boards and a new PT from Dynakitparts and it turned into a very seductive amp.

Last year I finally finished building a brand spanking new pair of Mark IV's.  Dynakitparts makes a really nice looking chassis.

 

[mediumjim]

The 50's were a great time for Tube Amp Design and many simply cannot be improved upon IMHO.

I think you might be pleasantly surprised to find that some current single ended designs can show improved resolution over the classic push pull amps.

Doc B.:

I have no doubt that SET amps have improved dramatically.  However, I do run my Marantz 9's in triode mode and they are pretty awesome.

My next system will be SET.

 

[mediumjim]

I would like to address the comments regarding the Quad Can as being a ticking time bomb.  There will tell tale things such as hum or white powder  residue leaking from the bottom of can.  If your Dynaco ST-70 is quiet and has a nice soundfloor, it is fine.  Moreover, there's a 3A Slo Blo fuse to protect the Iron and Tubes.

Jim