Dynaco st-70 series 3 where to buy

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1. 

   mikerm AK Subscriber Subscriber

   Did anyone grab one of those new ST 70 Series 3 amps while there were available? Dynaco was selling them direct for 50% off ($1,499 instead of $2,999) but sounds like they're sold out & the next batch will be regular price.

   I'm curious to hear how it compared to a Bob Latino ST 70 or standard/rebuilt ST 70. Is it a worthy successor? I'm considering taking a dive into the tube world and this seemed like a good place to start, but I haven't been able to find any real reviews or feedback so far!

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2. 

   heyraz AK Subscriber Subscriber

   I think you can pick up an original ST70 and repair it. New tubes, new driver board, new sockets, new everything for roughly a thousand.
   All you'll be keeping is the chassis, transformers and cage.
   That's how I started this hobby and it was fun.

3. don't waste your money on the Series 3......mutton dressed as lamb! Even at 50% off, you're better off with a new generation Latino ST70 or even ST120. The original price of $2,999.00 is/was an absolute ripoff aimed at audio-fools and covered with snake oil!

4. 

   mikerm AK Subscriber Subscriber

   Good to know. The Latino version looks sweet - is there a big difference between the sound quality of a Latino ST70 and one that's received a "regular" restoration (i.e. new caps & tubes)? I noticed there are a few different options to replace the entire circuit board, but the advantage of swapping out the circuit board isn't really clear to me.

5. 

   Lavane AK Subscriber Subscriber

   The Latino version uses a power transformer that produces more current. Thats nice if you want to try different types of drivers tubes that draw more heater current. You can order his amps with octal boards to run 6sn7 tubes too. Another feature I liked was bias adjustments for each power tube. You don't have to buy matched sets that way. Plus there is tons of support for these amps. The assembly instructions are very good also. Even if you bought it preassembled it would cost less than the series lll. I've been using VTA ST-70 as my daily driver for several years now.

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6. 

   Lavane AK Subscriber Subscriber

   That's just it. You can get an assembled VTA for around the same price or less than what the other is on sale. And there is great support from Latino and tons of useful info on the ol' interwebs. The slick new new look of the series lll isn't worth all the extra cash you have to shell out.

7. I have heard both the original Dynaco ST70 and the Latino ST-70...no comparison, period. The modern CCS (Constant Current Source) driver circuit is heads and shoulders above the original driver circuit and I don't care how much some prefer 'original', it is like night and day.
   And as has already been mentioned, individual bias adjustment for each output tube, larger PA-060 power transformer with higher current, 5A, 6.3V filament windings so you can try different tubes (there is of course a limit of the types of tubes you can safely try)
   Sure, if you have some kind of desperate need for 'original', go with a Dynaco ST70, otherwise, the Latino ST70 or even the 60W/CH Latino ST120, can't go wrong.

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8. my goodness...take a chill pill there!!

   

   I am purely giving my personal opinion and what I myself experienced between the Dynaco ST70 and the Latino ST70. And I absolutely stick by my opinion. You yourself quoted that you have not heard the Latino ST70.......
   No argument, the original Dynaco ST70, for its time, was a very good circuit, but there are a lot of great 'modern' tube circuit options available these days that just happen to outperform, in MY opinion, the original Dynaco ST70.
   By the way, I did not even hint at my gear, so lets stick with the discussion here, shall we?

9. indeed we have and this makes for good discussions with quite different opinions, likes, dislikes and so on!

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10. 

    peterh AK Subscriber Subscriber

    For best accuracy you still need tubes matched in Gm, this is a parameter that is independent of bias.

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11. While I have not heard the Latino amp, not the Series 3, I can offer some personal insight into my experience with the original circuit and small changes to it that can be done cheaply and easily. The bad rap that the Dynacos had came from a circuit that allowed the very first tube to get driven into current situation, causing poor transient response at higher frequencies, audible as muddy or laid back midrange and high frequencies. Frank Van Alstine wrote a comprehensive article on it here. In many ways he is highly opinionated but with his analysis of the amp, he was spot on. All this is by way of stating that by changing the circuit to avoid the saturation of the first gain tube (pentode section of the 7199), that muddy highs issue went away and you got an amp that offered much more clarity and brighter response, sounding much more like a modern amp. For me, all it took was to increase the coupling capacitors between the phase shifter (triode section of the 7199) and the output tubes. I used .47 uF. Van Alstine recommended 1 uF in that article but I was in a hurry and the local shop only had .47 in the right voltage (non-electrolytic).

    Initially I built and used a pair of external bandwidth limiters that could simply be plugged into the amps input. they provided the same improvement that the coupling caps I mentioned above did. At the time I built them, I installed one on only one channel and asked my wife and son to walk back and forth between the speakers and tell me which one sounded better, without telling them what was going on. They both heard the difference, and both prefered the modification.

    There were real issues with the original circuit, but they are easy and inexpensive to fix.

    Shelly_D

12. Do you have the low frequency roll-off points you get now compared to the stock .1uF cap?

13. I have not done that analysis. Since this is in the middle of the feedback loop, and the amp runs great without the bandwidth limiters I mentioned in my post, (no longer needed with the larger caps) I would expect it to be just about the same. TheVan Alstine article describes what was happening much better then I can, but I will take a shot at it.

    The coupling cap and the bias resistors following them, forms a high pass filter that rolled off the lows into the output tubes. The output of the output tubes then whet through the transformer and was fed back to the input tube. That signal, being deficient in the bass, forced a correction to increase bass output at the amp's output, pushing the tube into saturation. Improving the bass response of the forward path of the amp reduces the dependency on that feedback correction allowing the tube to come back down out of saturation. Since the amp was designed without DC blocking capacitors on the input, the output of the preamp in use can make a difference in this. The more low base (lower freq content) the more you need to fix the forward path of the amp. Also critical is eliminating any DC offset from the preamp output.

    I hope this helps.

    Shelly_D

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14. There has been a lot written about his limiting the highs as well. I used his formulas to redesign his filters to move the limits on the filters I built. His was something like low 20;s to high teen's, something like (I think) 21 Hz to 17K Hz. I set mine to about 17 Hz to 21 Kz. It did the trick just fine.

    I have not seen any sign of instability with .47 uF in place. That of course is not by any means definitive as speaker load can affect this as well, but there is no sign of oscillation with a fairly well behaved, 3 way speaker as load.

15. The high pass filter does roll-off the lows @ 40hz with the stock .1 cap. So those frequecies below that are at a lower level as a part of the NFB signal picked off. That means when the FB is applied, the frequencies above 40hz are attenuated more than those below 40hz. The lows are not increased, they are just not attenuated as much by the negative gain from the FB. There is nothing that pushes the output tube or transformer into saturation. The high pass filter does it all over again into the output tubes and the system finds a stable operating point. By changing the coupling cap to .5uf you move the filter down to @ 8hz and so any of that subsonic stuff that may find its way in will not be filtered.

16. My Series 2 (purchased very lightly used) has the "Van Alstine inspired" frequency limiting, all coupling caps are .68. It has been a very dependable amp and sounds very nice, though certainly not the last word in tube amps. My AES SE 1 and Superamp both have better sound.

17. 

    mikerm AK Subscriber Subscriber

    Thanks everyone for your responses. I think I’ll forget about the Series 3 & work towards one of those Latino amps. Now to figure out which one, which tubes & which preamp!

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18. Very much this. More to matching tubes than idle current. You can do 2 pairs vs 1 quad if you wish but best performance will be obtained when the tubes within a channel are reasonably matched.

19. I am not an expert in how negative feedback works, but I can understand the explanation of the process in the article I linked to. That explanation clearly stated that the front end tube was trying to increase the low bass response and it was that correction attempt that drove the first tube into saturation. As I understand it, negative feedback works by providing a signal to the circuit element (in this case the first tube, the pentode in the 7199) with what in essence is a corrective signal, focing the tube to correct it's own output to compensate (Note, this is the output of that tube I am talking about here, not the overall amps output.). That being the case, the feedback signal has an awful lot to correct in the original circuit. The feedback is deficient in the bass so the forward signal bass needs to come up to correct, the treble is unclear, (this is a form of distortion) that needs to be corrected. If the input to the amp does not contain the low bass signals that are rolled off by the stock coupling caps/bias resistors, the amp is crystal clear because the first tube is not driven into saturation and the feedback gets you great midrange and treble. With that low bass present, the tube is pushed into saturation in the attempt to correct the low bass deficiency that comes about from the content below the 40 Hz knee in the forward filter that you pointed out.

    Now perhaps .47 uF is too large to provide the best balance between the need for full frequency response in the forward direction and the need to remove unwanted subsonic frequencies, I did not work out the filter calculations when I did it, I followed the recommendation in that article after verifying for myself with the plug in band pass filters that he seemed to know what he was talking about.

    Please note that at no time did I say that either the output tube or the transformer was driven to saturation. If I was unclear in how I stated it, I do apologize for any lack of clarity, but I stand by my contention that the input stage in the stock circuit is driven into saturation by low bass content when using the stock value coupling caps.

    Shelly_D

20. To further clarify the point that I am repeating, I am posted a short excerpt from that Van Alstine article:

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