Just bought a Modded Dynaco ST-70: I hope I did the right thing

[chadbang]

I'd been watching an auction for a Dynaco ST-70 on ebay and no one was bidding. It was for a Dynaco Stereo 70 amplifier that had "its input driver card modded to a triode design". Frankly, I didn't know what the hell that means. But I found the Dynaco Doctor page and I guess that mean substituting tubes in the input stage for capacitors? Looks that way, anyway. The mod was done by "Richard Gray".So I wrote the seller and he said that the amp was built and modified by Richard Gray of the Power Company (they make power conditioners). Richard Gray called it his "baby gray". He also installed an upgraded power cord on it. The seller said the amp was Biased by Richard Gray last year (I wish I knew what 'biased' meant. Is this something I have to do frequently). He's a friend of Grays. Anyway, the seller seems genuine enough and he's going to build an custom crate to ship it. I bought it for $250. Does this seem like a good deal? Anyone familar with this mod?

 

[rickcr42]

the ST-70 is as designed an "ultralinear" circuit design and from the description it has been modded for "triode" design

the jury is out on which is better , i have heard opinions from both camps and I can not say from personal experience which is better

the "biasing" is something you need to do every time the output tubes are replaced-you can find some good info in the DIY Forum-see the "DIY Links Database"

you can not suibstitute tubes (active device) for capacitors (passiver device) unless you are either gaining or losing an entire gain stage .

More likely it is an entire front end rebuild to replace the ST-70 weak point which is ...yup...the front end !

plus the original gain/phase splitter tube is very hard to come by so a replacement is a good and not bad idea

If the cosmetics are good and the amp works you can consider the price not unreasonable at all-a bone stock ST-70 can and does run as much or more

hope this helps

rickamundo

 

[rickcr42]

some background

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

http://www.curcioaudio.com/st7_mnl.pdf

http://www.geocities.com/vintageaudio/st70a.JPG

http://www.geocities.com/vintageaudio/st70b.JPG

http://www.audioasylum.com/forums/dynaco/bbs.html

http://quadesl.com/schematics.shtml

http://tinpan.fortunecity.com/saints...mer/index.html

http://home.indy.net/~gregdunn/dynaco/

your education begins young man !!!!!!

 

[chadbang]

Thanks, Rick!

Damn, one of those links just bummed me out. Curico Audio used to be five minutes away from where I'm moving to in Pennsylvania. Now they're in San Jose, Calif. Bummer.

 

[mkmelt]

The Dynaco ST-70 is the Chevy/Ford small block V-8 engine of the tube audio world. There are more modifications available for this design than any other tube amplifier ever sold.

If you read up on the ST-70 you will learn a good deal about tube amplifiers. When you buy any tube amplifier, but especially an ST-70, what you are really buying is the power supply transformer and the output transformers. As long as these parts are working and undamaged, you can rebuild and customize the ST-70 and keep it running practically forever.

When set up for normal pentode mode, the ST-70 is rated at 35 watts RMS per channel. When rewired for triode operation, the same EL34 output tubes develop much less power, however, some prefer the sound of triode operation of the amplifier. If you need very little power such as for headphones, or if you have fairly efficient speakers, the available power may be adequate for your needs. With a bit of work, you can always rewire it for standard pentode mode.

 

[chadbang]

Hm, this has me confused because now the seller says it is putting out better than 35 watts

"ST 70 originally built and modified by Richard Gray of New Orleans. The input side has been converted from solid state to tube. The amplifier now does 44 watts per channel rms."

Is that possible?

 

[rickcr42]

the seller has no clue about the ST-70 topology

Never was this amp offered with a solid state front end but has always been 7199 tube from the first unit produced to the last.

A bone stock Dyna ST-70 is 35 W per side hance the St "70" designation

and no way it puts out 44W per CH unless the entire circuit has been replaced with something else

The good news is you can do an entire rebuild for not a whole lot of loot and make changes that are very easily reversed-exept for the front end if a new board/circuit is used

I have a Dyna MK-ll single channel amp that is the first Dynakit (yeah ,that is how it is tagged-not dynaco) offered.

I use it for the center channel in my home theater setup and that puppy puts out 50W and uses EL-34 tubes PP instead of the KT-88/6550 of the MK-lll monoblock

i guess all you can do is take delivery of the amp amp check it against the schematics for changes then if there are balatant circuit changes try to figure out the what and why at that time

Still,the iron (transformer) and the chassis alone are worth the $250 if in good shape

 

[rickcr42]

yo man !

you need to read this , really......

http://www.avahifi.com/root/equipmen...uild_plans.htm




rick

 

[chadbang]

Rick,
Well, I tried to give it a read. Oh brother, am I brain-dead. Call it advancing age, acute laziness, or just plain old dense -- I don't get it. I think I better find a good repairman.

You guys are geniuses to me.

I do know I want this done, since this quality is what I value greatly in my Spicas

Quote:

Inasmuch as the perceived "image" and "depth" of an audio system is dependent upon both channels having exactly the same gain and phase response, and because the resistors in the St-70 (and other tube amps) may have drifted out of specification over the years, it is helpful to replace all of the resistors with new tight tolerance parts.

Okay, got that part.

Now this seems to be important concerning mods to the driver board.

Quote:

I assume you have noticed we have not spent much time on the inner details of the circuit topography of the St-70. There may, or may not be "better" input, phase inverter, and output circuits available. The point is that almost all tube amps are mistakenly DC coupled and whatever the internal circuits are, they are driven into gross nonlinearities. The important concept is that any tube amp in which the input is limited to within the internal capabilities of the circuit will outperform any tube amp that can be driven into internal overload, no matter how expensive or complex the circuits may be. And the final limitations of a tube amplifier are the output transformers. Lots of money spent trying to achieve a "better" drive circuit probably is of little value, because the output transformers still are the limits of performance.

I take it to say, improving the driver board isn't really that important of a mod. Correct? Well, that shoots that whole deal...

Anyway, thanks, Rick. I'll go through that material again. I'd really like to learn this stuff. I would give me a useful new hobby.

 

[rickcr42]

what that last meant was sometimes limiting the bandwidth of an amp is a good thing .

Not all designs are meant to reproduce ultrasonic or subsonic frequencies and when they try to get good on paper specs or serve the ad copy for "DC coupled" it can be detrimental to the actual operating points of the amp

Since the power supply was one area where the ST-70 is not up to modern standards and since bass uses the most power it makes sense to try and prevent the amp from reproducing something it is not meant to do and in the process robbing other areas of the power needed to reproduce music .

So the simple high pass filter solves that problem without adversely effecting actual music bass which is way different than on paper bass

The front end is where if someone WERE to improve on the overall design it would be there

and considering how difficult and possibly expensive it is to obtain the original tube it makes sense to attack the design there

What would i do ?

First check the amp out with music but on a pair of backup[ speakers and if it works and if you are happy with it leave it alone !

Listen to music and don't sweat the details.

Later on dowm the road if you think you want to attempt to upgrade the amp do the simple first-connector upgrades ,filter cap replacement (AFTER DRAINING THE CHARGE !) ,cosmetics,line cord,switches,maybe do some tube rolling (and you will man ,trust me)

other than replacing bad filter caps and the rolling the updates will have little effect on the sound but the reliability will be enhanced


If after that you are comfy with your skills and you are a CAUTIOUS person you may want to do a total rebuiuld

do not attempt this if you are careless or absent minded

these voltages can be lethal man !!!!!!

Those big caps hold the full voltage even when off for a while and can put a person down , permanantly !

But say you go with it and you take the care required


First is the chassis/cosmetics

no point in doing a rebuild and having it look like crap so polish and paint as required until you are satisfied with the look

maybe even find a harley shop and have them chrome the chassis

again it is choice

Then I would print out a copy of the assembly manual and proceed to tear down the amp to individual parts and lay them aside as in the picture

I would then decide what stays and what goes

Tube sockets ?

Gone ! There are far better replacement sockets out there for cheap guy

Obvioulsy all mechanical parts would be in for upgrade/replacement and all caps would be improved

Don't get crazy here-orange drops are fine exept for the main coupling cap and that one is a matter of taste.

The resistors don't really matter either as long as they are matched in sets with an ohmmeter for left/rightchannel balance

the biggest decision is

Keep the original front end or replace it with something else ?

Personal choice and one you have to do the research on to decide what you want.Lotsa choices out there

Triode Deal ,SDS ,Curcio ,AVA ,Wellborne Labs.....

but if you keep the original topology i suggest a brand new board for the rebuild,they are available at various places

this can be fun or a nightmare depending on how you take it


i think it will be fun and you will learn along the way

or you could pay someone to do it for you

 

[chadbang]

Thanks Rick, that sounds like a smart plan of attack. The seller contacted me today and is building a crate for the amp this weekend (nice of him, for no charge) and it'll soon be in my grasp. Hopefully I'll be happy with it "stock". I figure Richard Gray must have some idea about what he's doing - or at least knows how to solder. I haven't picked up a soldering iron since 9th grade shop class (actually it was this huge lump of steel you heated in an oven, not exactly made for precision work). Now tube rolling, I can handle.....

 

[rickcr42]

quick update man ,meant to add the links earlier

Full Manual-
http://www.curcioaudio.com/st7_mnl.pdf

TroubleshootingRepair Guide-
http://www.curcioaudio.com/Stereo%20...ir%20Guide.pdf

Restoration Manual-
http://www.curcioaudio.com/Stereo%20...0Manual-R0.pdf

Output Stage Triode Conversion-
http://www.curcioaudio.com/ST70 OP ...://www.curcioaudio.com/ST7 Bia...sion Mod.pdf

you can never have enough information and even if you do not yet need the info you will at some point so I would download the files and print them out for reference later

rick