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The BIG Dyna ST70 driver board thread

Brinkman

Brinkman

Analog loyalist
Hello everyone.

I'm too broke at present to embark on a monoblock project, so I'm succumbing once again to the siren song of Dynaco/Dynakit ST-70 modification...

As many of us know, there has for years been a venerable cottage industry of aftermarket drop-in replacement boards for the much-maligned driver board (or "front end", as it were) of the stock ST70. Most faithful to the topology of the original with it's pentode input stage and Concertina phase-splitter are the two offerings from Triode Electronics. A former employee of Triode has started his own company, Blackburn Audio, and up until recently, offered his own Blackburn Audio Dynaco Upgrade. Going back a ways is the controversial distortion-reduction-via-bandwidth-limiting Van Alstine Ultimate 70 as well as the modestly modified or total upgrade (transformation) offered by Curcio Audio. Currently used by many are the two all-triode variants offered by Vaccuum Tube Audio, using either three 12at7s for the standard version or three 12au7/12bh7s for the low-gain version. Also in the "long-tailed pair inverter" camp is the affordable DIYtube.com board, which also employs a Constant Current Source (or CCS) in the inverter tails. Lastly, and most recent, are two more: the point-to-point wired (unavailable as a kit), all-octal Mapletree Audio Special Red Driver and the brand-spankin'-new K&K Audio Upgrade Kit which uses interstage transformers for the phase splitting and turns the ST70 into a Class A 10WPC amp.

I probably forgot to mention a few, and didn't even bother mentioning power supply upgrades.

I think it would be useful for the ST70 enthusiast, new or old, if everyone shared their impressions of, before/after comparisons, shoot-out results, tube rolling experiences, component upgrades and/or tweaks performed on of any of these boards, as well as any I've neglected to mention.

I've been happy using the VTA 12bh7 board for a while now, but I am heeding the call of the soldering iron and will report back soon with a few comparisons.

Meanwhile, hopefully a few veterans will share...
 
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Well I have not herd a st 70 yet but have one sitting here waiting on a re build and have been looking at different boards for it I like the idea of 12au7's and the like. But am trying to do this and keep the price down, like trying to find some one that upgraded there unit and or has an extra board. Also looking at the bias supply mod to get rid of the rectifier. If some one has the parts list and schematic for that I probly have all the parts for that with out ordering any.
 
Endspec said:
Also looking at the bias supply mod to get rid of the rectifier. If some one has the parts list and schematic for that I probly have all the parts for that with out ordering any.
Click to expand...

I have been using the Weber Copper Cap WZ68 in place of the tube rectifier. All it is is four diodes in bridge formation and a wirewound resistor for voltage drop. Also, I am using the older version of the SDS power supply board from Triode Electronics (with the new version almost ready to be installed).

I have not installed a Triode/Ultralinear switch or performed the bias expansion modification (see Curcio Audio for details).
 
Hey! groovy idea for a thread... I have 3 stereo 70's in various states of decay that I want to rebuild and have wondered about all different options.....now if only i had a spare $500!!.......oh well, will watch with great interest, so chime in! I'm anxious to hear everyone's opinion on the best choice's.
cheers,
Kirk
 
I agree, great idea for a thread. I just finished installing the K&K Audio ST70 mod board. As you can tell from the pictures on K&K's website, the board is very well made. The design is really advanced in that it employs differential drive and output stages with constant current souces (CCS). The idea behind CCS is that it provides a more ideal current source to the amplifier. This design places the output tube's operation mode in Class A as oppose to Class AB1 of the original. I think I'll let you read the rest of this at K&K's site.

A while back, I went to this gentleman's house to audition a pair of Altec Model 19 that I eventually bought. We kept switching back between his custom made 300B SET amp (with very nice custom made OPT, btw) and a pair of monoblock Class A 6550 tube amps. Both amps have their strong points and were good in different ways. I really like how that class A PP amp sound, but due to my hectic work schedule, I haven't been able to put together a design of my own.

I have been hanging onto a mint condition ST70 with the intention of adding an upgrade board to it. I have also been keeping an eye out on new ST70 developments but it seems like that's all we are going to get (most of the list above). I wasn't aware of the Blackburn and Mapletree boards. I have heard the VTA board on my friend's ST70 and it sounds very good. When I ran across the ST70 board teaser on the K&K forum, I decided to wait to see what it's all about. I ended up buying the board with the amorphous signal transformers.

Unfortunately, I ran into a ground loop issue earlier today so I couldn't listen to it. I finally had it figured out late tonight so I haven't crank it up at all. I am burning in now with low volume music. I must say that the sound at low volume is very SET like with nice details. I hope to provide an update in a day or two.

And a side note, the power supply for the whole amp is included on the board so that you don't need to add anything underneath. Biasing is accomplished via the CCS so you don't need to mess with it much nor do you need to old bias circuit.
 
I am currently using with my restored Mac MX110 a Dynaco series II clone produced in the early 90's. I purchased it as a domo unit. it is similar in circuitry, and looks but uses all SS rectification and 6GH8A driver tubes. it has been rock solid dependable. The only upgrades I've done was to replace all coupling caps with MIT's which resulted in more detail and better bass definition. I'm using Russian Mullard EL34's and Sylvania labelled drivers. I've been very happy with this amp.
 
I wanted to maintain a somewhat stock configuration without using the 7199 tubes. As a result, after much research I finally went the full SDS route using both the cap board and the EF86 driver board. I've been very happy with them - they were easy to assemble and the ST-70 sounds much better now. I'm sure most of the improvement was due to the new parts, but overall I'm very happy with the upgrade.

The only issues I had during assembly and installation were with the SDS cap board which is large and very difficult to squeeze in. It took a lot more effort and planning than I originally expected to get that cap board installed.

Unfortunately, about a month after I purchased my SDS boards they announced version 2, which meant (among other improvements) plated through holes and a slightly smaller size for the cap board. Bummer. But I can't complain as my ST-70 works and sounds great now with the version 1 boards.

-Matt
 
Great thread OP!

:thmbsp:

I had the Van Alstine bandwidth mods on my first ST70. I felt this was a definite improvement over the stock circuitry. It is not the last word in what can be done to the ST70 to be sure but a great place to start especially if you are new to modding. These mods are easy to do along w/ the regular passive parts replacemen that must be done. I added the parts right at the RCA jacks terminals on the inside of the amp.

My baseline mods to the front-end were -

Strip board & reflow all traces with silver solder. (Some of the thin traces were beefed up w/ jumper wire laid over the original traces and soldered in place.)

Replace all passive components w/ new, hand matched to the original* schematic values.

Replace 9P sockets with ceramic PCB mount ones.

Add Van Alstine mods** to input of front-end.

Crack open beer & enjoy!

* There were component value change mods recommended to me for the front-end that I did not do. If anyone knows changes of this type it would be interesting to post them for others doing an original front-end rebuild.

** I have long lost the component values for the Van Alstine front-end mods. Does anyone have them to post to this thread?
 
Wow, there's a few on there I didn't know about. Thanks for the info.

I employ the V.T.A. version with three AT7s. I use it in a ST-70 and am very happy with it. This amp is fast, clean and detailed. I recently upgraded the board with higher value Caps; from four 22uFs electrolytics to two 47ufs and two 100ufs on C13-16, and replaced the 10% 0.10uF Orange Drops with 5% sonicaps on C1-C4 , and replaced the 10% 0.22uF Orange Drops with Russian Paper in Oils on C5-C8 . The result of all this was what seems to be a little more punch and attack when a fortissimo passage is encountered and more pronouncement in instruments that were hiding in the background.

Of all the tubes I have tried on this board; telefunken, mullard, among others, the best I have found are two GE 12AT7s on the sides with a triple mica GD 6201 in the center. The bass response is better with this combo and everything else; mids/highs are no noticeable difference. I would love to find three Siemens 12AT7 triple micas to place in this. Changing the EL34s or, in my case, the KT-88s made a much bigger difference. More change comes from tube changes in the pre-amp than in the amp's driver board. The best thing I did for this amp was to put some good KT-88s (genelex re-issues) in it.

After studying other options, but not having heard other variants, I chose and have now built another V.T.A. board using low esr electrolytics, PRP resistors, silver solder, along with all of the same above mentioned caps, and will put this in a 60 WPC version amp I plan to start building on Sunday. I went with the V.T.A. again becuase "if it ain't broke, don't fix it" philosophy I subscribe to. I was happy with the one V.T.A. I have and know it works, so I went with it again.

For the money you cannot beat this driver board (or amp), especially if you populate it with your own parts.
 
I've got a Van Alstine Super 70i setup in mine. I'm currently running a JJ 5AR4 rectifier, a quad of SED (winged-C) EL34's, and a pair of NOS RCA 6GH8A's. It sounds great to me, very clear and detailed. I'll never really be able to tell until I get my speakers finished, as I'm running Sansui SP2500's right now, which kinda suck on the bottom end. However, I'm very happy with the amp itself.

rooster.
 
rooster18 said:
I've got a Van Alstine Super 70i setup in mine. I'm currently running a JJ 5AR4 rectifier, a quad of SED (winged-C) EL34's, and a pair of NOS RCA 6GH8A's. It sounds great to me, very clear and detailed. I'll never really be able to tell until I get my speakers finished, as I'm running Sansui SP2500's right now, which kinda suck on the bottom end. However, I'm very happy with the amp itself.
Click to expand...

I like that you mention this, as I read (and contribute to) threads that remark on how a simple tube or cap exchange in some specific component significantly alters the sound in one way or another.

But I wonder: if a simple pair of cap exchanges can be so affecting, how controlled and accurate was our assessment of the parts selections prior to the change? And for that matter, after the change?

Thats what I like the best about the ST-70; exchanging driver boards while maintaining the same power and output transformers keeps a degree of control, but system diversity among users allows for enough component-dependence to keep things lively.
 
Brinkman said:
I like that you mention this, as I read (and contribute to) threads that remark on how a simple tube or cap exchange in some specific component significantly alters the sound in one way or another.

But I wonder: if a simple pair of cap exchanges can be so affecting, how controlled and accurate was our assessment of the parts selections prior to the change? And for that matter, after the change?

Thats what I like the best about the ST-70; exchanging driver boards while maintaining the same power and output transformers keeps a degree of control, but system diversity among users allows for enough component-dependence to keep things lively.
Click to expand...

In my case, I got the ST70 as a basket case, and built it up from the bare bones. Since I would've had to buy a driver board, sockets, jacks, etc., I figured, "why not just get Frank's board?" I knew that the original ST70 had some shortcomings, and I took a chance with his stuff. However, I'm very happy I did. Sounds nice.

rooster.
 
gener8r said:
I chose and have now built another V.T.A. board using low esr electrolytics, PRP resistors, silver solder, along with all of the same above mentioned caps, and will put this in a 60 WPC version amp I plan to start building on Sunday.
Click to expand...

I have an extra VTA board that I plan on building with quality components so I can compare it to the low-gain version I'm running right now. I also have the newer version Triode board as well as the DIYtube board but have not amassed all the parts to assemble them. The phono-preamp project I have going comes first though.

I'm really tempted to get another set of ST-70 iron and chassis so I can have two identical amps to compare driver boards with. It's educational to build a driver, live with it a while, roll tubes, then move on to a different design to learn what topology sounds best to your ears. A direct comparison would be most effective for this persuit.

Realistically, higher-wattage and/or better transformers exist than those offered in the Dynakits or their clones. In fact, I'm curious as to how much wattage one could get if they mated a set of ST-120 iron (Bob Latino kit) with the K&K Audio driver. It's not than 10 watts Class A isn't enough, but if more could be had I'd welcome it.
 
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Another VTA board user

I'm running the VTA board as well, on a ST-70 clone kit that I built a few months ago (the Bob Latino package). I am using the VTA board with paper-in-oil coupling caps, and the complete package sounds astoundingly good - so much so that I have dumped all ideas of "One day I will buy the Incredibly Expensive Amplifier that the Audiophool magazines tell me sounds the best".

The board as part of the overall package is VERY clean and VERY quiet. I have done side-by-side comparisons with a Chinese Class A SET amp (Dared MP-7 running a pair of KT66 output tubes), and the ST-70 is every bit as detailed as the SET amp - perhaps even more.

I wish I could do a side-by-side comparison with the original driver board.

-- JunkyJan
 
JunkyJan said:
I wish I could do a side-by-side comparison with the original driver board.
-- JunkyJan
Click to expand...

I originally built the stock driver board and I really woundn't encourage using it if the ST70 is your first and/or only tube amplifier. Even upgrading the power supply and tube-rolling could only take the stock board so far. Then again, there's a number of folks who love the sound of a stock '70, but I suspect they probably also have a higher-end tube amp around that gets the most out of their record collection.

Honestly, I don't know what about the stock driver schematic that makes it sound so veiled and sluggish; the 7199 pentode/triode is a rather good audio tube. Of all the replacment drivers, the Triode board is the most similar to the original (yet gets positive reviews), so if you really would like to hear a driver that performs like the original should have I would think this would be a closer, more worth-while comparison. I can't say for sure because I haven't built the Triode driver yet ;)

Congrats on the Latino ST70. I've reccommended it (and the ST120) to people who've expressed interest in building a tube amp after seeing and hearing my modded ST70. I'm pretty sure Bob gets most his components, save for the driver, from Kevin at dynakitparts.com.
Kevin is good people.

BTW, if you were to upgrade the power supply I'd be interested in what differences you'd hear. As you may have deduced, I installed mine before I upgraded the driver. As an added bonus, having a stand-alone power supply saves you the trouble of rebuilding the bias supply (and/or cap supply) every time you try out a new driver, as some of the drivers have you do. That's assuming you want to bother with "driver-rolling".

Best.
 
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Hey all.

So a few things I thought I'd post:

First, I finished installing the revised SDS cap board for my ST-70. The revision was to add bypass caps and increase the overall PS capacitance to 500uF. My personal upgrade was changing the big electrolytics to Panasonic TS-ED series (lowest ESR). So would I say my amp has better bass, "smoother" tone and increased transient detail?

...:scratch2:...

Of course I would.

There's a thread detailing how to install it in conjunction with the VTA driver boards here.

Secondly, for the sake inserting some objectivity into board comparisons, I purchased a Radio Shack Sound Level Meter. All measurements will be made with my phono preamp (Bellari VP-129) set at twelve o'clock, with the SLM 20 inches off the ground, and three feet from the center of one of my speakers (Klipsch Fortes), both of which measured within a decibel of each other during a fixed tone experiment I conducted prior.

VTA Driver Board: Low-Gain Model
89-92dB average, C-Weighted.

Reference: Sonic Youth, first side of "Daydream Nation"

Lastly, I have purchased the K&K Audio upgrade board. The purchase was kinda a cart-before-the-horse thing, but whatever. It will likely be rolled in sometime after I assemble an active crossover I am totally convinced I must have.

Gonna do some bi-amping with these drivers.
 
Brinkman said:
...

Honestly, I don't know what about the stock driver schematic that makes it sound so veiled and sluggish; the 7199 pentode/triode is a rather good audio tube. Of all the replacment drivers, the Triode board is the most similar to the original (yet gets positive reviews), so if you really would like to hear a driver that performs like the original should have I would think this would be a closer, more worth-while comparison. I can't say for sure because I haven't built the Triode driver yet ;)

...
Click to expand...

Read Frank Van Alstin's rebuild article on the Stereo 70. It explains why that circuit does have that "muddy" sound in the mid-range and treble. The short version is that the lack of a capacitor on the input to the first tube allows signals as low as DC into the amp. Those signals are missing from the output, the amp and output trannies can't respond that low. Therefore the extreme lows are missing from the feed back signal. Like any good feedback mechanism the input tube tries to compensate and drives the tube current up in the extreme base. This drives the tube into saturation where it could no longer respond to the smaller mid range and treble signals. That's why he advocated the band pass filters on the input.

You can read it here:

http://enjoythemusic.com/magazine/manufacture/0902/index.html

By the way - I tried his band pass filters set to 17 Hz and 21,000 Hz and the muddiness went away totally. Tells me he knows what he is talking about.

I'm not advocating doing this and I'm not going to start a discussion on the merits of the filters, I just wanted to answer your question on why the stock input board sounds the way it does.
 
I'm not going to get into a debate over this either.......

But the following clipped from above:

"the amp and output trannies can't respond that low"

Is just not true. A good working front end and adequite power supply, the ST-70 does
not "need" to be bandwidth limited. But in it's original form I could see it being a benefit. ;)
 
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