• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Dynaco ST-70 / Mark III / Mark IV - Which board to accept 12ax7?

gvocale

gvocale

Active Member
I came in possession of a Dynaco ST-70, a Mark IV and two Mark III.

I am considering modding these amps, currently using their stock boards, with input boards which would allow me to use 12ax7 and 12au7. I have in fact a good collection of old ones of those and I would like to use them.

I stumbled across the Poseidon board, which uses a 12ax7 and 12au7, but seems available only for Mark III and Mark IV.

I also found the KTA-HiFi ST-70 Driver PCB, which uses one 12ax7 and two 12au7, for the ST-70.

Finally, I have at hand an old GSI Dynaco Input board, which takes 6922, of which I have some too, which I may consider installing into the ST-70.

Do you have any experience with these boards, or do you know of any other that uses 12ax7 and 12au7, and possibly the ability to bias each power tube independently?

Thank you
 
Register to hide this ad
I'd stick with the boards that use 12au7s, very good 12au7s are inexpensive and widely available. Try to get organ pulls, since the organ manufacturers demanded very tightly matched sections so the organ mfg labeled tubes tend to be very high quality. I've purchased several here in bartertown and have been very happy with them.

fwiw, I use the VTA board in my ST70 and am very happy with it. The ability to individually bias power tubes is a great feature.
 
Thing is I have plenty of good old 12ax7 (Mullard, Telefunken, Philips etc..)

I also have many RCAs 7199.

Would RCA 7199 perform same or better then vintage 12ax7, after same upgraded filter and signal capacitors and separate bias capability for all 4 power tubes?

I could stick with 3x 12au7 too, as I have a vintage mullards, tele, philips of those as well, but my 12ax7 collection is definitively larger, so I would prefer to have a board that use 12ax7 + 12au7, so I can use rotate and use them all.

I have some documentation for the GSI board, but it doesn't seem very clear. I also have some RCAs made by Siemens 6922 which sounded incredible on any amp I tried them on. I would be curious to install the GSI board on the ST-70 and see if they do perform better there too...
 
I upgraded two Dynaco ST70 amps with new driver boards not logn ago and I can say there is a night and day difference from original to new boards. The first board I used was Triode Inc, which uses 12AU7 and EF86. The second board, from Tube4HiFi uses three 12AU7. The difference between the sound are not huge, but if I had to do it again, I would pick Tubes4HiFi, if only for the ease of install, a very clear manual and because it's widely used. Only needing one type of tube (12au7 are easy to find and cheap) is a bonus.
 
I don't own and have never owned an ST70, so take this for what it may be worth.

One thing I'm not over fond of with these alternate boards is they never seem to provide specs on how the amp actually performs. I've only really seen one well documented set of test data for the ST-70, and that was on a dead stock unit. It measures quite well in stock config. I'd really like to see how the other boards do by comparison.

Yeah yeah, obligatory comments about how you can't measure everything and the numbers are meaningless, etc. I'm a data guy, its just how I process life.
 
gadget73 said:
I don't own and have never owned an ST70, so take this for what it may be worth.

One thing I'm not over fond of with these alternate boards is they never seem to provide specs on how the amp actually performs. I've only really seen one well documented set of test data for the ST-70, and that was on a dead stock unit. It measures quite well in stock config. I'd really like to see how the other boards do by comparison.
Click to expand...

I would imagine at least the output wattage would be the same as stock. I can tell you non-scientifically, the hum and other noises were greatly reduced after both upgrades. This would possibly be due to all new resistors, caps and pots.
 
ikorman said:
This would possibly be due to all new resistors, caps and pots.
Click to expand...

Very possible indeed. This is also sort of why I wonder if these other boards do truly perform better, or if its a case of taking a dysfunctional amp and making it functional again that accounts for the improvement. I know I've rebuilt amps that kept to the stock design and gotten very significant improvements just by replacing faulty parts.

All this isn't really to dissuade people from changing the amp to how they want. Frankly I've got no skin in the game here.
 
Imagine the different versions along with a freshened stock 70 being benched at a McIntosh clinic.
Many aftermarket boards follow the Mullard LTP driver design, with a constant current source, a long established standard ckt. Marantz used it, f.i.
 
peterh said:
I am amazed.
What is most important : using your current heap of tubes in a munged st-70
OR
using your ST-70 as designed ?
The problem with ST-70 is the 7199 tube, which is expensive and soon exhausted.
But 6u8/ 6gh8 may be used with a proper adapter,
http://www.ebay.com/itm/2Stuck-verg...656727?hash=item2ed5372597:g:txQAAOSwd0BVvCFc

But rebuilding a good amp just to be able to use a heap of tubes is strange...
Click to expand...
Some of us rebuilt our Stereo 70s because we disliked the sound of this design (even after a recap, and retubing). If you want an inoffensive sound amp with mushy bass, rolled-off sounding highs, no ability to image at all, and a very unremarkable midrange to compensate you for the rest of its weaknesses, then the stock stereo's 70's design is exactly what you want. If you want something which sounds subjectively better, and which at least has some of the plusses associated with tube sound to offset some of its other weaknesses, then buying one of these completely re-designed boards may well be the ticket for you.
 
WOW gold ear...no luv for the stock venerable st70...sorry t hear that.My experience with them is upping capitance and recapping will give an admiral performer.I liked my Dyna MKiv' mono's better and my MKiii better yet but on the right speaker the st70's will sound more detailed then many competitors and ive never bought or changed a board in any of them.
Just my opinion with dyna gear
Is it up to par with my Fisher mono's..no..but there a lot cheaper and easier to source...

hunter
 
Last edited:
I have two ST-70's the first one is an original from the 50's and it has been upgraded almost completely, the only original parts are the chassis and the output transformers. This amp has the DIYTUBE board that uses a 12AX7 and a pair of 12AU7 tubes in it. My second amp is a new build completely with Dynakit Parts power transformer and Transcendar output transformers. I was looking for a second DIYTUBE board but they were not available so I mounted a plate in the gaping hole and used the circuit from DIYTUBE and went point to point. Very happy with both amps.
 
Dave G has a thread on the stock Dyna driver that is quite interesting. Fresh tubes and in tolerance passive components are a must for optimum results.
 
I moved on from the el-34 tube as much as I enjoyed my dynaco amps.,Moved back and settled on the 6L6 gc family and settled there.I do enjoy the 6550's in my HHscott lk-150 but I dig mono blocks to much for the dynamics and separation of channel content. in triode there performers...Different strokes for different folks...as always

hunter
 
hunter00 said:
WOW gold ear...no luv for the stock venerable st70...sorry t hear that.My experience with them is upping capitance and recapping will give an admiral performer.I liked my Dyna MKiv' mono's better and my MKiii better yet but on the right speaker the st70's will sound more detailed then many competitors and ive never bought or changed a board in any of them.
Just my opinion with dyna gear
Is it up to par with my Fisher mono's..no..but there a lot cheaper and easier to source...

hunter
Click to expand...
I know that lots of people love these amps. But I've honestly never understood why. I've yet to hear one which gives me any of the aspects which I love out of tube sound when using the stock circuit. But I've heard some marvelous sounding modified units. But all of these threw-out the stock circuit entirely. This includes MKIIs, MIIIs and MkIVs, all of which feature this same mushy boring dynaco house-sound which I dislike.

The stereo 70 is an amp which features a brilliantly elegant design from an engineering point of view. And I totally respect its measured performance, as well as its impressive stability, as well as the really nice iron Haffler gave you at such a low price point.

But if the stereo 70 represented what all tube amps sounded like, then I would never have become a tube junkie. I am of the opinion that using a split-load phase splitter as the driver was, and always has been, a bad idea. I'll take a good mullard circuit variation any day of the week.

YMMV.
 
Last edited:
petercapo said:
I expect that the sound of the Stereo 70 may not be everyone’s personal preference, but your description of the sound of a Stereo 70 even after a recapping and retubing is the polar opposite of what I can report from my experience.

If what you say is inevitable for the original Stereo 70 circuit, then my Cary SLI-80 Signature, my circa 2006 production McIntosh MC275 Mk IV, and Parasound Halo A21 must also have “mushy bass, rolled-off sounding highs, no ability to image at all, and a very unremarkable midrange” since my Stereo 70 does a remarkable job of keeping up with their sonic qualities.
Click to expand...
Perhaps in different systems these amps sound wonderful. But I've clearly never heard these systems.

I've tried one od the MC275 reissues in my system at one point, and I definitely liked it. It was not a very tubey sounding design to my ears though, and I distinctly preferred my CJ Premier 1B over the 275 for its glorious midrange, sweet top-end and gigantic and deep soundstage. But I've never heard any Cary design inside of my system, and so I have no reference for comparison there.

But I own an MC225 which I am extremely fond of. I really like the sound of that little amp. It is not as tubey as my Eico HF89, but it ranks as one of my favorite amps for its great all around sound.

YMMV.
 
Last edited:
We are discussing a subjective matter. It seems to me that we find ourselfs in one of two corners :
either we know how acoustical music sounds and strive to reproduce ( in vain) this natural sound as close as possible
or
we like "impressing sound" of some music we never heard ( or never existed ) in the open, a studio sound.
Depending on which corner we choose we like one or the other types of reproduction apparatus.

Both these corners are valid and respectable.
 
You must log in or register to reply here.
Back
Top Bottom