What can I expect from a Dynaco ST-70?

[mediumjim]

I had my Parasound HCA-1200II in storage and pulled it out last week with the plan to build a 2nd system.  This particular Parasound was a good one.  Well I get around to hooking it up and the left channel is distorted and was cutting out.  I do a simple DC-Offset test on it and everything was in proper spec.  I cleaned the contacts but it still was a no go. 

This led me to procuring a clean and recently serviced original Dynaco ST-70. The power supply has been recapped, otherwise is all original.  From what I've read, these are great sounding amps.  Does anyone here have first hand experience with them?  If so, what can I look forward to?  It will be to me in about a week or so as it is shipping cross country to me. 

It will be mated with a McIntosh MX-110 Preamp/Tuner.  The speakers I plan on using are Meadowlark Swifts that are 8 ohm and a true 89db.  The other possibility is a restored pair of JBL L100A Century's that are 91db.  Any thoughts on what speakers will perform best?

Many thanks and Happy New Year! 

Jim

 

[JC]

Well, you should have plenty of power for either of your speaker choices. 

Beyond that, almost everyone has their favorite modification or new parts.  This was a very popular amp in its day, so there are still plenty around, and they end up in a lot of hands.

Mine is pretty much stock, too, and I love it for what it is: A very good push-pull tube amp.

Not the ultimate this or that, just a very good amp.

This means that the bass will likely not impress like a good Class A Mosfet amp, but it will be very enjoyable to listen to if the load is not too difficult, and you're not asking it to fill a stadium with 32' organ pipes!

Make sure you have nice matched output tubes, because you will be setting bias per pair, not per tube.

And, if you have pets or kids, keep the cage on; it isn't as pretty, but with the cage off, high Voltages are accessible on the top of the driver board.

If you do a search here, there was a discussion of this amp just a few days ago, with lots more information and opinions.

Enjoy!

 

[howardnair]

jc is correct it is a good amp-you will like it-i had 3 why i left the last one go i don't know--i got all crazy and redid all of them soup to nuts-ahhhh now i remember --HOW I GOT  HERE- using the dynaco and their incredible following and documentation as a learning tool and then looking for something else  led to bottlehead kits-a bottlehead foreplay ----you may get more bass response by changing the coupling caps to .47 or 1 uf-you can get more info at the dynacodoctor forum or dynacotubeaudio forum --there is a great deal of info on them and  a large assortment of mods and tweaks

 

[Grainger49]

Both Paully and I have ST-70s.  Mine is the stock circuit.  His uses Magnequest output transformer and a different power transformer.  I don't remember about his power supply caps.  Mine is an Antique Electronic supply drop in replacement.  Both have all new resistors.  The chassis, iron and circuit board are all that are original in mine but the circuit is only slightly modified.

Maybe Paully will post on his modifications.

 

[mediumjim]

Thanks for all the great input.  Mine was done by a guy in New Jersey named Carl and from I can gather he has been restoring them for a long time and has a good loyal group of followers.  The one I got is pretty much stock.  Many years ago when I was a just a teenager, I friend of a friend had built one and drove a pair of A7 VOTT's to impressive levels. 

I do know about tubes and I will be replacing the JJ's that it will be with if they don't do it for me.  The funny thing is that replacing the EL34's will cost more than the amp if I go with Mullards or Fat Bottle RCA's.  They are supposed to bias a 1.56 and I hear that running them a bit cooler, say 1.45 will increase the tube life and provide a better soundstage...will have to experiment. 

Happy New Year!

Jim

 

[Grainger49]

Jim,

Sorry, I just realized that I didn't  tell you what to expect.  Expect good power.  I had speakers that were mid 80s dBW in sensitivity and they rocked with the ST-70.  This was in a room of enormous volume.  These speakers, Fuselier 3.8Ds, are the other end of the scale from Altec A7s.  So power is there.

All the tubed goodness you expect was there.  There wasn't the SET purity but soundstage, imaging, detail, smooth high frequencies and pretty good bass was there.  The ST-70 and a Krell KSA-250S were swapping in and out of my system.  The ST-70 didn't compare under 250 Hz.  But the Krell ended up being a 145 pound ST-70 amp stand.

Bias can be where you want it.  There are two bias resistors.  As long as you measure and hand match a pair you just divide by 10 and you have the magic point.  As a matter of fact you should measure the resistors that are in the amp and go with 1/10 of the resistance in volts for an accurate measurement.  Many of us who don't need the maximum power have tweaked the bias lower than Dynaco's spec.

 

[mediumjim]

Hi Grainger:

Thanks for the advice on how to properly Bias them.  I often run my Marantz 9's Triode and with my KEF 104/2's that even though KEF says 92db, they're closer to 87db...if I really push them I get sibilence and some distortion.  The good thing about original Dynaco's is that they did use great iron and as you said, plenty of current.  

If the bass is lacking, I may run a pair of powered subs to address that issue.  Another choice for speakers is a pair of Energy 22 Reference Connoisseurs that have tight bass down to 40hz and can reach 32 or so if set up correctly and are 8ohm 89db so they will be perfect for the job.

At some point, I will get another ST-70 and set them up as close as possible and run them as Mono Blocks...

Jim

P.S.  I was going through my tube stash as came upon a quad of Groove Tubes GT-EL34B's that are power rated 7 (late breakup).  These are made in Czechoslovakia at the old Tesla facility and are from Telefuken tooling.  Also found was a NOS Tung Sol 5U4GB.  I will need to test the EL34's (means I need to dig out my Eico 667) and see how close they match up for Mutual Conductance, Leaks, Shorts.  If they pass at this point, I wiill heat them up and check the ma on each.

 

[Wanderer]

I used an ST-70 for years.

Assuming a stock circuit expect it to sound rather romantic, with "lush" rather then deep lows and soft rather then extened highs. It's faults tend toward the pleasant and musical, more so if still running the tube rectifier.

The selenium stack in the bias supply should have been replaced with a modern diode by the fellow doing the rebuild. The selenium diode WILL fail if not replaced.

Do make sure the bias resistors measure at 15.6 ohm if you are setting bias via the original Dynaco instructions. If the rebuilder used a different value make sure he tells you how to set bias. 

The 7199 driver tubes in the orginal circuit are getting hard to find. Sovtek did do a run a couple of years back but the orignal Dyna marked USA sourced tubes were better sounding.  There are tube sockets available to use 6U8 tubes in place of the 7199. Make sure the modder has not done this already. 

http://www.dynakitparts.com/store/product.aspx?id=277

I think a pair of ST-70 will be better used for biamping then as mono-blocks.  I don't know how loud you listen but I would guess the Meadow Larks or JBLs will go as loud as you need with one ST-70. If you really need more grunt I would look at getting a pair of Mark IIIs instead of running ST-70s in mono.

With updated driver and power supply boards, which can make the unit less "romantic", a ST-70 is competative in sound with very good modern amps. 


     

 

[mediumjim]

Hi Wanderer:

Here's a couple of pics showing the power supply from both sides.

 

Thanks,
Jim

 

[JC]

That 5U4 will most definitely work as a sub, but I think you will enjoy the GZ-34 more.

Dynaco originally used precision 15.6 Ohm resistors to feed the cathodes of each pair of EL-34s to facilitate a quick calibration of the Volt meters of the era before setting the bias.  1.56 Volts was the Voltage of a fresh carbon-zinc flashlight battery, so you set your Volt meter with that, then set the bias on your amp.

This set the bias, as you may have surmised, to 100 mA per pair, or 50 mA per EL-34, if they were well matched.

So, yes, you may indeed find that this setting is a little hot for your tastes.

Then again, maybe not.  Marshall guitar amps specified that one used a scope to set bias, and watched the scope until the last vestige of cross-over distortion disappeared on an amp being run just under clipping.  This method generally worked out setting the bias remarkably close to that 50 mA point.

You're in for some fun!

 

[JC]

OK, that looks like the original Selenium rectifier for the bias supply.  If it is still in use,  you will want to replace that with silicon.  I am told that you don't want to breath the results of the Selenium going 'poof'.

I just mounted a 3-lug terminal strip to the screw that holds the Selenium stack in place, and mounted a generic 1A silicone diode to that.  That way, I just removed the wires from the Selenium and moved them "up" to the terminal strip.

It looks like it has new electrolytics in the bias supply, a good thing.

 

[Paully]

I breezed through the posts, not sure if what I would suggest was mentioned.    If that is not a new can cap, that should be replaced sometime in the very near future.  The old ones go bad.  The old tube sockets can get loose and lose contact, so whenever I have rebuilt one I have replaced those.  Just something to watch if there is an issue.  Do some reading on using a 5u4, is that one that can tax the power transformer?  I can't remember to be honest.  But the rectifier does matter so use a good one (5AR4 probably being the best and what the amp was designed around) and make sure it isn't rated too high for the old transformer which is only 3A if you use something else.  The new ones are usually 5A. 

The 7199 are expensive.  I didn't want a new circuit for the driver board so I bought a new board using the old circuit for new parts and then also bought adapters from ebay to be able to use 5GH8A and I like those as much as the 7199, as far as I could tell.  And thems 5GH8A tubes is cheap!!!  So that is something you could try without changing anything other than buying an adapter and a few tubes.

I love my Dynaco.  Even though I use Paramounts now, I have no intention of letting it go.  Nice well rounded sound.  It is a great amplifier.  The output transformers are what everyone fights over, so you can mod other things and improve the sound.  I would recommend upgrading the power supply via Triode Electronics power supply or some such and then you don't have to worry about the can cap, which is something to worry about otherwise.

Don't forget the Dynaco Doctor forum and the Vintage Tubes asylum.  Lots of help and reading there.  Curcio, DynakitParts, and the Vintage asylum have all the parts, upgrades, and help you could want and then some.

That's all I can think of at the moment.

 

[mediumjim]

Thanks JC/Paully:

It is being shipped to me with a 5U4GB...I do have some nice Mullard 5AR4's that I can try.  The rebuilder is actually from NY, sorry and is well known for his excellent work.  With that said, if he felt the Selenium Circuit was risky or bad, it would have been replaced as he typically does.  At some point I will update the Selenium to Silicon as I will Speaker Binding Posts.  What's interesting, I'm seeing that he typically uses 5U4GB's. 

7199's are still out there and I plan on getting a backup pair and keeping it original.

Jim

 

[Grainger49]

Jim,

Your idea of "recently serviced" and mine are different.  No component there looks like it has been replaced with the possible exception of the bias caps, not the B+ cap.

The 10 ohm resistors that have caused problems with other ST-70s and Foreplays are on the circuit board dead center up near the input jacks.  You can try a jumper around them to see if it decreases hum.

 

[JC]

I think the Dynaco Doctor has replacements for the screw terminal strips for the speaker  connections, if you want to go that way.  Otherwise, I've seen people mount standard 5-way binding posts onto a small strip of plexiglass, and replaced the terminal strips with those.  They should also have a download available of a .pdf of the original owner's/construction manual at Dyanaco Doctor.  You'll definitely want to hunt that up.

The screw terminals, of course, allow for having all the different impedances from the output transformer available at the rear of the amp.

The 5AR4/GZ-34 was indeed the original in the amp, though, IRRC, the 5U4 was even in the manual as a usable replacement.  I know I used one once, when my last Mullard GZ-34 decided to give up one night.

Of course, that's the neat thing about tubes: If the manufacturer is OK with it, it's an easy matter to find out which you prefer!

OH, BTW, the Voltage to the plates of the output tubes will change, depending on which type of rectifier tube you choose.  So, be prepared to reset the bias each time you swap.

 

[Paully]

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.

 

[JC]

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.

 

[mediumjim]

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

 

[mediumjim]

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

 

[JC]

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!