Have you seen this pic?

[DaveV]

I wouldn't snicker at a nice Dyna ST70 no matter what you hear from the higher end people.
I rebuilt one to "stock" except for the output tube bias supply that needed to be adjusted to compenstae for the increase in voltage due to the new rectifier having less loss than the selenium.
I cleaned the board, replaced out of spec resistors, replaced all the caps, signal and lytic except for the pf size ones on the board, replaced the 7199 tube sockets, reworked all the solder joints, used NOS 7199's and a fairly well matched EL34 quad.
On my Altec Model 19's it sounded much like my rebuilt Mac 225 tube amp except for the bass.
Had I kept it I might have added more UF to the supply to see if the bass would come up some. It did have sort of a nice mid range "bloom" that the 225 doesn't have.
I looked at the available "mods" and if I were to mod an ST70 I thought the VTA mod board from Bob Latino sounded good and it's gotten some very good reviews.

[spkrman57]

Dynaco made really nice ouput trannys which employed special windings that made them capable of very good frequency response.

I had a pair of output trannys from a Dynaco SCA35 that had a fried power tranny and the rest of the unit was pretty well beat up.

I had a friend of mine build me a new amp using these trannys. I asked him to design the amp to my requirements:

1. Cathode bias
2. Less than 300VDC across the 6V6 output tubes(Dynaco used 6BQ5/EL84 output tubes)
3. The only negative feedback to be used would be the ultra-linear taps for the screen grids and a cathode bypass cap across the cathode resistor on the 6V6 tubes.
4. Octal tube (6SN7) for 1st stage audio and phase splitter(split-load).

He did this and resulted in 275 volts plate-cathode and 25ma for 7 watts plate dissapation(6V6 is rated for 14 watts), I wanted a conservative amp that would allow my vintage tubes to last a very long time.

The only signal path caps are between the phase splitter and the 6V6 control grids, the rest is direct coupled. Currently I have orange drops installed. I would like to use Russian PIO, but they are 4" long and might not fit in the chassis, but in the future I will try and find a way to make them work.

The power transformer is special order Edcor 500V CT and 6.3V for the filaments. The PS choke is Hammond

I'm using RCA 6V6GT (SMOKED GLASS) tubes from an old Bell amp that needs too many caps right now. I found a pair of RCA 6SN7GTB NIB that were in my stash collection. All were tested and measure like new!

I have 40 hrs of burn-in on this amp and it sounds wonderful. The secret is the Dynaco ultra-linear output trannys.

If you find and old Dynaco amp with the ouput trannys still working, there is no limit to how you can rebuild them.

It's my opinion that the octal tubes sound more "big-tone" sound than the skinny pentode 6BQ5/EL84 and little 9 pin 12AX7/12AU7/12AT7/etc small signal tubes do.

I'm sorry for going on so far on this, but I really like this amp. The pic I attached is not a great one, but the camera batteries went dead on me before I could get a better shot.

The tuner/preamp is a MX110 tubed unit, CD player is Audio Note (tube buffered output), and the speakers are JBL 2225/2426/2370A 4 cu ft cab tuned to 40hz and utilizing Pi crossover at 1.6khz.

The max ouput is only 10 wpc, but it makes a great rock and roll amp!

Sorry to be so long winded, but I thought some tube audiophiles might be curious of the circuit design.

Regards, Ron

[TonyM]

I've noticed there is a big revival in 6SL7 and 6SN7 tube designs lately and not so much emphasis on 12AX7s and 12AT7s. From what I'm seeing tube designs are fairly simple now days with more emphasis on power supply quality than they used to have. This has got to leave more room to go back to those bigger octal triodes. There are still a lot of these running around as NOS plus new tubes out of Russia.

Gotta love the Mac...

[rustnrot]

Hi everyone, I just completed a 6sl7 6sn7 octal driver ST70 using a board from Classic Valve Design. More info here:

http://www.diytube.com/phpBB2/viewtopic.php?t=3643

[Ducatista47]

To be fair this is not the only ST70 kit out there. Triode Electronics has one and so does Dynakitparts (there may be others). As best as I can tell the biggest difference between the kits is in the driver board used.
Now I'm a firm believer that accurate reproduction is a pipe dream.

I know of one driver board kit that will be a lot more accurate. It was designed years ago by Alan Kimmel for Welborne labs. They have updated the parts and are again offering it. It has gotten to the point where I want all my tube amps to have an Alan Kimmel Mu Stage. And no, I am not a tube only guy. I will use MOSFET and JFET outputs, but bipolar transistor output will hopefully never again darken my door.

http://www.welbornelabs.com/st70.htm

I would not recommend their optional tube set. The Electro Harmonix EL34s suck; use the Winged C (real Svetlana from St Petersburg) EL34 tubes. Antique electronic Supply.

Clark

[spkrman57]

I have had great results with EH EL34's. I still prefer Old Stock when I can find them.

I used the EH EL34's in a Cayin TA-30 amp and preferred the sound to the original tubes Cayin used.

What was it you did not like about them, the bass response, overall presentation or did they sound too laid-back!

Just curious,

Regards, Ron

I would not recommend their optional tube set. The Electro Harmonix EL34s suck; use the Winged C (real Svetlana from St Petersburg) EL34 tubes. Antique electronic Supply.
Clark

[Ducatista47]

I have had great results with EH EL34's. I still prefer Old Stock when I can find them.

I used the EH EL34's in a Cayin TA-30 amp and preferred the sound to the original tubes Cayin used.

What was it you did not like about them, the bass response, overall presentation or did they sound too laid-back!

Just curious,

Regards, Ron

EH power tubes have not served me or any of my audio friends well. They do not sound nearly as high fidelity to me as the Winged C tubes. They have also proved to be unreliable relative to Winged C. I would say the EH tubes sound fine until you hear something better. The St Petersburg tubes are better in overall presentation and detail. I also think the bass is higher fidelity; the mids & highs certainly are.

The Cayin line is Chinese, isn't it? If it came with the original Chinese tubes, EH would indeed be a big step up the sound ladder. But Winged C is the best of the new tubes available in EL34s and do not cost that much. For my ears it is no contest.

While the Chinese tubes are the worst sounding, beware of JJ power tubes. They sound nice, are cheap, and they are known for taking out amps with them when they blow up - which they frequently do. FWIW, the last tube that blew up on me was an EH 6550.

While I am not a hard core tube roller - I think amp design & parts quality is more influential for the sound than tube brand - tube reliability is important to me. These fuses are expensive!

Clark

[PonMan3436]

I know of one driver board kit that will be a lot more accurate. It was designed years ago by Alan Kimmel for Welborne labs. They have updated the parts and are again offering it. It has gotten to the point where I want all my tube amps to have an Alan Kimmel Mu Stage. And no, I am not a tube only guy. I will use MOSFET and JFET outputs, but bipolar transistor output will hopefully never again darken my door.

http://www.welbornelabs.com/st70.htm

I would not recommend their optional tube set. The Electro Harmonix EL34s suck; use the Winged C (real Svetlana from St Petersburg) EL34 tubes. Antique electronic Supply.

Clark

Defending EH EL34's.

I'm new to this forum but when I saw your response to EH EL34's I had to respond. Are you sure you were not using EH 6CA7 Fatboys? They sound like the tube you described. I have been using the slim bodied Electro Harmonix EL 34 EH (in matched Quads) for a long time. It is the tube of choice for my modified Dynaco ST 70. I purchased mine from Jim McShane and I have only had 1 to fail in over 4 years and it was replaced by Jim. They usually last at least 2 years and still test good enough to listen to them longer when they are replaced.

All the following were matched quads: 1.we tried the Wing C's which sound warm and romantic and rolled off on the top end. My friend drove 2 hours to hear his amp vs mine with a matched quad of Wing "C"s under his wing ready for a shoot out vs the EH EL34's and my Stereo 70. His ST70 was armed with the Triode input board and mine Sound Valves. I assure you after we swapped his wing "C" for my EH's in his amp he had two hours on the way back to think about what he heard. Yes, he ordered a quad of EH EL34's.

We also listened to a pair of Tesla NOS Vintage early 90's excellent top end, excellent bass and a wonderful soundstage. Very pricey if you can find a geniune quad.

Next was a quad of Mullard reissues. Excellent bass, great midrange but not as articulate as the EH or the TEslas and but a bit rolled off on the top end. The funny thing all these tubes had a different flavor. They were all very listenable but in my system the EH EL34 EHs were almost as good as the Tesla's I mentioned above. The EH's are a little more extended on the top end and they don't have the bass of most of the EL 34's. But they are better than the Mullard reissues and the Winged "C" in midrange articulation, top end extension and the bass is a little light compared to the other tubes in our shoot out.

Just a couple of days ago another member heard my Dynaco.
He was impressed. But that's another story.

I also saw where you mentioned that Alan Kimmels input board may be one of the more accurate of the input boards. I have one thing to say on that subject and that is; if you like the sound of a solid state amp then yes you would be correct. I found them to be bright bordering etched (it was a new install and had not broken in). I have heard several versions of that board and they all sound like a solid state amp to me. I have heard them with Tube and Solid state rectification.

There are several other boards out there like GSI (they make guitar amps now), SoundValves (no longer in business but a board will show up every now and then from their site), Curcio (still formidable), Mapletree Audio (Octal tube set), Classic Valve (octal tube set), Kevin Kennedy ( I have heard some of his other mods and I like them) You have to hear his Dyna FM3 mods. Van Alstine (still one to be reckoned with, it's been a while but I remember good sounds with this one), and many many other mods for the ST70.

There is an injustice is when someone rebuilds a ST70, covers it with a fresh paint job but they still use 7199 tube circuit. Add to that prices as high as $800 on them. The magic in the ST70 is in the midrange. They are hard to beat in that area. I have been trying to replace mine for several years. I have owned it for over 11 years and I don't see it going anywhere. The ST70 biggest weakness is in bass control. They are not the best amps for some full range ported speakers. But they sound much better than many of the solid state amps up to nearly $2000. I haven't compared them to every brand but accurate this amp isn't. It is however
very musical which is what this business is all about. It's a matter of preference!

[TonyM]

What was the difference you heard between the EH EL34 and 6CA7?

So far I've been only running the 6CA7, which I have no complaint about, but I also have a new quad of EL34s that I haven't tried yet. Since the amp I have is supposed to take the KT88 tube I've been thinking about giving them a try also. It's not really supposed to increase the output power just change the sound of the amp. The output power is probably limited by the power supply voltage and current. I'm just playing around with it.

As far as the EH tubes are concerned I had a Heathkit AA100 that regularly turned the plates of the EH 7591As a nice toasty red and they always survived. Killed one of the RCA tubes that were in it before (that was a waste of a good tube).

[Steve Schell]

The picture looks like a hotrodded Dyna Stereo 70, probably the most prolifically built tube power amp ever. When I became interested in audio in the late 1970s these were still everywhere and a decent stock one cost $50 - $75. As many folks discovered, the transformers were better sounding than the rest of the circuit, and many an amp designer cut his/her teeth on modding Dynas. I used to run stacked Quads with mono-configured '70s, and I wish I still had that stack of them in my closet. They kept many tube fiends alive during the dark ages of early solid scrape when very little tube gear was made either by manufacturers or hobbyists.

[PonMan3436]

Hi everyone, I just completed a 6sl7 6sn7 octal driver ST70 using a board from Classic Valve Design. More info here:

http://www.diytube.com/phpBB2/viewtopic.php?t=3643

Gee, I don't know where to begin. Last night I had the pleasure to listen to this amp. I call it the Rustnrot Octal. I put it in my main system and was pleasantly surprised. There were minor differences between Rustnrot's amp and my Sound Valves input board tweaked by Tom Tutay. Actually, everything that has been done to my ST 70 was done by Tom Tutay.

First, I was impressed with the height of the soundstage this amp presented. Secondly, the depth was impressive for a new amp that hasn't had the time to burn in and I expect it to get better in time.
The midrange had excellent clarity playing through my Spica TC60's. The Rustnrot Octal impressed the both of us as we listened to several recordings. Mainly CD's. The speakers disappeared and the vocalist as well as the instruments occupied their own space in the sound stage. Both very satisfying to the ear.

The minor differences I heard in my system are as follows: My ST70 with "EH EL34's" had just a bit more extension on the top end but both were equal in the air around the instruments. The Rustnrot Octal unit appeared to have a taller sound stage. The bass of both amps presented a different flavor on each unit. Rustnrots' Octal reached down a little deeper with emphasis on deeper bass while my EH placed emphasis on mid to upper bass which seemed to be a littler puncher. But as the Rustnrot warmed up the bass got better and better.
I'm impressed with the KT77's. I may purchase a quad of KT77's just to see how they sound in my amp. Time did not permit us to swap tubes. I choose the EH's as I said in an earlier post because they were not fat on the bottom like the Svetlanas or Mullards, etc. I wanted emphasis placed on the mid bass, mid range and top end of my Spicas. The EH addressed that. The deep bass of the EH is simply a matter of taste but the mids and highs are better than most and equal to many of the NOS tubes out there in my amp.

I have a pair of Dali Suite 1.2 acoustic suspension subs that fill in the rest of the spectrum. I could easily live with either of these two amps. They are more similar in it's presentation than they are different.

My rig is housed in a 13' x 14' room with a 14' A frame ceiling. I have had problems placing fullrange speakers in this room in the past. I have owned Von Schweikert VR3, Vandersteen, Living Voices, Magnepan, Polk Audio, Klipsch, Tannoy, Eminent Technology, and many other brands the past few years. None have given me the joy as the Spica TC60's (and 50's) have.

I have a very modest system: VanAlstine Super PAS3si modifed by Transition Audio Design (Tom Tutay), Dynaco ST 70 with Sound Valves board by Transition Audio Design, Arcam DV78 DVD with a Scott Nixon Tube DAC, and a Systemdek IV (The Beast) turnatable with a Rega RB600 with Audio Technica OC9ML II and a Hagerman Piccilo Phono Step Up device. Speakers, Spica TC60's, Spica TC50's cap upgrade, Polk Audio SDS 2B's with cap upgrade and a pair of Eminent Technology LFT16 monitors.

With all that said, great job Rustnrot on the Octal. I may try the Octal tubes input board my ST70 in the very near future. It caught my attention.

[PonMan3436]

Hi everyone, I just completed a 6sl7 6sn7 octal driver ST70 using a board from Classic Valve Design. More info here:

http://www.diytube.com/phpBB2/viewtopic.php?t=3643

I made a mistake and said Genalex but I meant Gold Lion KT77's.

[Ducatista47]

Defending EH EL34's.

I'm new to this forum but when I saw your response to EH EL34's I had to respond. Are you sure you were not using EH 6CA7 Fatboys? They sound like the tube you described. I have been using the slim bodied Electro Harmonix EL 34 EH (in matched Quads) for a long time. It is the tube of choice for my modified Dynaco ST 70. I purchased mine from Jim McShane and I have only had 1 to fail in over 4 years and it was replaced by Jim. They usually last at least 2 years and still test good enough to listen to them longer when they are replaced.

Just a couple of days ago another member heard my Dynaco.
He was impressed. But that's another story.

Thank you for your responses! I was speaking about the EH EL34. I am sorry the Winged C product did not work out in your Dynaco. As I mentioned, I think the EH is a big step up from most OEM Chinese tubes. I am sure your ST70 does sound impressive. I do think a really well built power tube should last a lot longer than what is considered acceptable life. Two years is OK for a guitar amp, but I like to see hi fidelity amps get a lot more service than that from a tube. Not many currently produced tubes will do that, and Winged C is the least costly of any that I am aware of that consistently will.

EH has in the past been beset with problems not of their making that compromised their best efforts. Perhaps they are getting better. (Not meaning to be mysterious. Their plant in the former Reflector complex was forced to work in deplorable conditions - no gasses, no power - by powerful Russians who coveted their property and had them surrounded. A big amp manufacturer had to reject 50% of some power tube shipments for a while. EH knew it and had to ship twice as many tubes as ordered to make it right, until they could straighten out their problems. I hope he does not have the Russian labor problems some firms have there. A famous quote of soviet era workers - "We pretend to work and they pretend to pay us" - and one from Cindy Lauper after her first visit there - "First, you have to get used to everybody being drunk.") I have only my own and my friend's experiences to draw upon, and my experience is that of never having had any other brand of power tube fail catastrophically. That 6550 I mentioned almost destroyed a 2245H. It wouldn't die outright; arcing would generate the most godawful noises but take its time blowing the fuse. Not the first EH that did that to me, either, just the highest power one.

I also saw where you mentioned that Alan Kimmels input board may be one of the more accurate of the input boards. I have one thing to say on that subject and that is; if you like the sound of a solid state amp then yes you would be correct. I found them to be bright bordering etched (it was a new install and had not broken in). I have heard several versions of that board and they all sound like a solid state amp to me. I have heard them with Tube and Solid state rectification.

It is however very musical which is what this business is all about. It's a matter of preference!

I have found over the years that we all like our favorite distortion. It is what we are used to. I have also found that a nearly distortion free tube amp (some of Alan's preamps qualify) does not sound at all like a low distortion transistor amp. I like tubes because I think music does sound more natural and accurate through them. I have also found that a dose of audible second harmonic distortion is not necessary for the experience. The presentation of Alan's Mu Stage circuits does take some getting used to, but it does not sound like transistor amps. Close to JFETs though, I admit.

I have listened to a lot of systems that were considered startling by their owners, but none as uber as Alan's PHI-26 used as a preamp to the FirstWatt F2 JFET. That would be a tube preamp and a MOSFET/JFET current source power amp. Too bad the rig is only for Full Range drivers and dynamic headphones.

By the way, while I think that cables require no break in and speakers precious little, in other words I don't go to that church, many new film capacitors sound to me a lot less hot in the treble after a few hours to a day of use.

A tiny bit off topic, I think the ST35 sounds better than ST70 right out of the box. Its mids are more high fi and always were. I think there is little commerce in mods for them because so few of them were produced. They have plenty of power for a high efficiency speaker. Even a Model 19 sounds sweet driven by an ST35.

Hey, welcome to the forums!

Clark