What can I expect from a Dynaco ST-70?

mediumjim · 32231

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Offline Paully

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Reply #15 on: December 31, 2010, 01:13:01 PM
Correct me if I am wrong, but everything I remember reading was that the 5U4GB was in the manual as nothing more than a temporary sub.  It pulls 3 amps versus the 5AR4 1.9 and is therefore taxing the old power transformer which is rated for a max of 3.  Also it is direct heated and starts conducting very quickly also taxing the power supply.  It is not a long term option for a Dynaco without an upgraded power trannie.  I have the upgrade so I wouldn't be concerned.  When I had the old power transformer I looked into it and decided it wasn't a good idea.  It is cheap so people are tempted to use it but you have to do some upgrades to be safe doing so.  At least that is what I recall.



Offline JC

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Reply #16 on: December 31, 2010, 01:23:37 PM
Well, I believe you are correct about the respective heater currents, but I would have to dig up my manual to ascertain what Dynaco said about the matter.  Hey, I don't even remember the transformer specs off the top of my head!

Hopefully, I will run across that info before too much time has passed.

Jim C.


Offline mediumjim

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Reply #17 on: December 31, 2010, 01:49:40 PM
Grainger:

Thanks, I will have to give it a listen to to determine if anything needs to be corrected/replaced.  This guy has rebuilt/restored hundreds of Dynaco's and does only what needs to be done to put them back in spec or better than spec. 

Jim

Marantz Model 9's Bottlehead FP2 AH!Tjoeb 99
Dynaco ST-70 McIntosh MX-110 Rotel RCD1072
KEF 104/2 B&W ASW300 Subs
Dynaco A-150 University Medallion XII


Offline mediumjim

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Reply #18 on: December 31, 2010, 04:32:40 PM
Was going through my tube stash and a found a sleeve of NOS Richardson 5AR4's that are really Red Bank Bendix 6106's.  Made in 1987.  I also found a Amperex Holland Metal Base GZ34/5AR4.  The guide pin is broken, but it is nice and strong. 

On the other front, the GT-EL34B's tested strong for mutual conductance  from 116 to 138, with 80 being borderline weak.  Not a tight match for mutual conductance.   Next test will be to get them in the ST-70 and do a Mutual conductance test under real load. 

Okay, time for this west coast kid to prepare to celebrate the new year!

See you all next year!

Jim

Marantz Model 9's Bottlehead FP2 AH!Tjoeb 99
Dynaco ST-70 McIntosh MX-110 Rotel RCD1072
KEF 104/2 B&W ASW300 Subs
Dynaco A-150 University Medallion XII


Offline JC

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Reply #19 on: December 31, 2010, 06:31:01 PM
I dug up the Stereo 70 manual, and the 5U4 is indeed listed under the "emergency" replacements section.  I believe both it and the 5AR4/GZ-34 are directly heated rectifiers, so I don't think that part applies.

I have not yet located my specs on the PT 5V winding.  I can only say that I used a 5U4 in mine daily for some time while locating/waiting for a replacement 5AR4/GZ-34 without any evident issues. 

Happy New Year, all!
« Last Edit: December 31, 2010, 06:38:08 PM by JC »

Jim C.


Offline mediumjim

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Reply #20 on: January 01, 2011, 05:40:48 AM
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

Marantz Model 9's Bottlehead FP2 AH!Tjoeb 99
Dynaco ST-70 McIntosh MX-110 Rotel RCD1072
KEF 104/2 B&W ASW300 Subs
Dynaco A-150 University Medallion XII


Offline Paully

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Reply #21 on: January 01, 2011, 06:24:26 AM
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. 



Offline mediumjim

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Reply #22 on: January 01, 2011, 08:40:54 AM
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. 



Marantz Model 9's Bottlehead FP2 AH!Tjoeb 99
Dynaco ST-70 McIntosh MX-110 Rotel RCD1072
KEF 104/2 B&W ASW300 Subs
Dynaco A-150 University Medallion XII


Offline Paully

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Reply #23 on: January 01, 2011, 09:23:49 AM
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!



Offline JC

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Reply #24 on: January 01, 2011, 09:25:45 AM
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!  

Jim C.


Offline mediumjim

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Reply #25 on: January 01, 2011, 10:26:49 AM
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?



Marantz Model 9's Bottlehead FP2 AH!Tjoeb 99
Dynaco ST-70 McIntosh MX-110 Rotel RCD1072
KEF 104/2 B&W ASW300 Subs
Dynaco A-150 University Medallion XII


Offline mediumjim

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Reply #26 on: January 01, 2011, 10:37:35 AM
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

Marantz Model 9's Bottlehead FP2 AH!Tjoeb 99
Dynaco ST-70 McIntosh MX-110 Rotel RCD1072
KEF 104/2 B&W ASW300 Subs
Dynaco A-150 University Medallion XII


Offline Paully

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Reply #27 on: January 01, 2011, 11:25:49 AM
"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.



Offline JC

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Reply #28 on: January 01, 2011, 11:50:52 AM
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"!


Jim C.


Offline Wanderer

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Reply #29 on: January 01, 2011, 03:39:37 PM
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.             

Kevin R-M