Dynaco ST-70 kits

[Kegger]

Agreed feedback doesn't have be a bad thing, when sparingly used and around
less stages is what yields the best sonic results in the amps I've built/rebuilt.

Small amounts and around single stages is where I try to stay, if I can that is.

 

[Wolf012558]

Shelly - I don't really agree with you ..

Robert:

Try something simple before replacing the driver board. Install a capacitor between the input jacks and the imput of the first tube. Then listen and decide. The cap should be between .05 microFarads and .1 microFarads.. It's cheap and it really does great things for the sound of the amp. I know it doesn't do anything about the feedback, power supply or anything else, but if you feel you are ready for the challange of installing a new board (with additional parts) then the capacitor shouldn't prove too much or a challange, its cheap ($1-$2 per channel depending upon where you source the caps from) and maybe you'll like what you hear. IF you don't you can always go on to the new board and get those improvements.

The reason I sugest this is I'm one of those people that think feedback can be a good thing when properly implimeted. I realize that is a sore point these days, but consider this, the Stereo 70 has a reputation for not delivering "tight" bass. One of the benifits that feedback does do is lower the output impedence of a circuit. In this case that would improve the damping factor for the amp. For those who are not familiar with damping factor, it is one way the amp controls unwanted excursions of the speaker. Generally, the higher the damping factor the better the control. (Every mod has some kind of trade off, the trick is to find one you like). Lowering the ouput impedance will increase (improve) the damping factor.

The biggest reason to try it is it's cheap:thmbsp:, easy to do, easy to undo if you don't like it, and (in my opinion) has results far beyond the cost and effort involved) why not try it.

Whatever you try - good luck to you and happy tinkering.

Shelly_D

Shelly,

I don't really agree with you ..

Putting caps on the input does restrict the frequency range that the amp has to work in. But by doing that you are not helping the REAL problem which is a mediocre design for the original driver board with the 7199 tubes. The sound from an original Dynaco ST-70 is smooth, mellow but has a restricted frequency range (top and bottom end rolled off), flabby bass etc. It is a non offensive sound you can listen to all day long - BUT it isn't an ACCURATE sound. Dynaco got away with this because the main signal source of the day was vinyl records which have a low (30dB ?) dynamic range. Those records with their limited dynamic range never really taxed the amp' s weak power supply and many of those records (especially the non audiophile records) had weak output on the bottom and top end of the frequency range anyhow... If you compare an ORIGINAL Dynaco ST-70 with a modern tube amp like a Rogue, Audio Research, Conrad Johnson etc. these weaknesses in the sound from the original ST-70 are very noticeable.

When people hear an original ST-70 for the first time they go "Ahhhh - that's a nice smooth sounding amp" - Yes it is nice sounding but again - It's not accurate. But their ears are so attuned to their hard sounding Pioneer, Kenwood etc. solid state receiver that they "think" they have reached audio nirvana with a stock 70.

As I said above if you really want to hear the 70 at it's best - beef up the power supply with the SDS under chassis cap board (this has already been done on your amp) or pull the old quad cap and install the 80, 40, 30, 20 cap AND get a driver board upgrade - either the Curcio, Diytube, Triode Electronics or VTA driver boards will be a huge sonic upgrade for any original Dynaco ST-70.

If you are doing a complete rebuild then I would also pull the power transformer and replace that with upgraded PA-060 from Triode or Dynakitparts. The transformer on my Latino Dynaco ST-70 kit amp runs so much cooler than the transformer on the old original amp - you can put your hand on it and hold it on there even after hours of continuous play. It's warm but not "hot" like the original Dynaco ST-70 amp I have.

Putting caps on the input as you suggest is like putting a band-aid on the finger of a person who has a much more serious internal problem. No real help IMHO.

Charlie

 

[shelly_d]

Shelly,

I don't really agree with you ..

Putting caps on the input does restrict the frequency range that the amp has to work in. But by doing that you are not helping the REAL problem which is a mediocre design for the original driver board with the 7199 tubes.

The caps I suggest put the low frequency roll off down about 5 to 10 Hz. I don't know any instruments with a fundamental that low do you? The real problem is NOT the design of the original driver board. A pentode input first stage with a Williamson style phase splitter is a fine, workman like design with no inherent flaws. The flaw is the presence of the global feedback and the absence of a DC blocking capacitor. The combination drives the input tube into saturation. Inserting the capacitor removes the DC component and therefore brings the input tube down out of saturation, allowing much better response to the mid and high frequency transients that are so poor in the unmodified design. If you try this you will see how dramatically the sound changes. The fuzzy and somewhat indistinct midrange and treble sharpen into intense focus and become crystal clear - without negative effects on the bass.

... If you compare an ORIGINAL Dynaco ST-70 with a modern tube amp like a Rogue, Audio Research, Conrad Johnson etc. these weaknesses in the sound from the original ST-70 are very noticeable.

I agree that the ORIGINAL Stock ST70 pales against the likes of the amps you mention. I would contend that with the input caps it would compare much more favorably to them. (Perhaps not their equal, but this is a 40 year old design and the mod I'm suggesting is $2 to $5 of improvement).

When people hear an original ST-70 for the first time they go "Ahhhh - that's a nice smooth sounding amp" - Yes it is nice sounding but again - It's not accurate. But their ears are so attuned to their hard sounding Pioneer, Kenwood etc. solid state receiver that they "think" they have reached audio nirvana with a stock 70.

It sounded muffled to me.

As I said above if you really want to hear the 70 at it's best - beef up the power supply with the SDS under chassis cap board (this has already been done on your amp) or pull the old quad cap and install the 80, 40, 30, 20 cap AND get a driver board upgrade - either the Curcio, Diytube, Triode Electronics or VTA driver boards will be a huge sonic upgrade for any original Dynaco ST-70.

If you are doing a complete rebuild then I would also pull the power transformer and replace that with upgraded PA-060 from Triode or Dynakitparts. The transformer on my Latino Dynaco ST-70 kit amp runs so much cooler than the transformer on the old original amp - you can put your hand on it and hold it on there even after hours of continuous play. It's warm but not "hot" like the original Dynaco ST-70 amp I have.

Putting caps on the input as you suggest is like putting a band-aid on the finger of a person who has a much more serious internal problem. No real help IMHO.

Charlie

Ultimately it is your amp. The mod I'm suggesting WILL dramatically improve the sound of your stocker. It will do it for a very small effort in time and money. I like those kind of improvements. If you don't then by all means do what you feel best. But remember, at a roll off of 5 to 10 Hz, the output transformers can't respond that low anyway, so in truth, how much of the audio frequency spectrum are you going to be lacking.

This is not a band aide. It is adding the one part that Dynaco left out that would have made a good mid priced amp far better then it was.

 

[Wolf012558]

I did the input cap mod on my stocker

Shelly,

I added a .05 Mfd cap between the input jacks and the driver board on my old stocker. I listened to my stock amp for a couple of hours with music I am familiar with. I have to say there is *maybe* a VERY subtle difference in sound quality for the good - BUT IMHO this is a mod that is akin to putting a higher quality air cleaner on a stock car and getting maybe TWO more horsepower. You can't turn a STOCK Dynaco ST-70 into a world beater with this input cap mod. The mod does not "*dramatically* improve the sound of your stocker" as you said. The difference is subtle. The stock Dynaco ST-70 amp needs a modern all triode driver board and a better power supply to really address the demands of todays digital music sources that have a high dynamic range. Of course if you listen at low volume levels and not at realistic sound levels you *may* be satisfied with the input mod as you suggest.

After listening to the input capped stock ST-70 for a couple of hours I cycled in my Bob Latino upgraded ST-70 amp with the VTA driver board and upgraded power transformer that I built from one of his kits. I then listened to the SAME music with SAME components. The bass is tighter and goes deeper on the new amp. The top end has much more extension and the new amp has much greater resolution. This amp sounds more tonally accurate than the stock amp. There is no real comparison between the stock amp and the new amp. I have an audiophile friend who has listened to my new amp and has told me that my new ST-70 kit amp sounds every bit as good as the Rogue 88 that he bought new and which cost him about DOUBLE the cost of my amp kit. I think he paid like $1300-$1400 for his Rogue.

IMHO the input cap mod is a mod whose overall affect is subtle and a mod that cannot turn a Dynaco ST-70 stock amp into an amp capable of dealing with the dynamic ranges found todays digital music sources.

Charlie

 

[PakProtector]

hey-Hey!!!,
The parts from a St.70 can make a very nice amp. Get rid of stuff like global/loop NFB and high impedance driver stages and you're off. The amp can be done in two stages *WITH* an adequate amount of FB to tame the high damping factor an open-loop U-L amp exhibits.

The claims made in the Dynaco TX advertising said they were good enough to support a nice amp, and not hold it back. Now if you want better than nice, one needs to look elsewhere.

I still have the first tube amps I ever got, a pair of Mk.III's. They're hobbled by wimpy power Iron, high B+ and OK outputs. They do sound a whole lot better than they did with blueprinted 6AN8 front ends. Hopefully they'll make good dorm amps in about a decade and a half...
cheers,
Douglas

 

[shelly_d]

Shelly,

I added a .05 Mfd cap between the input jacks and the driver board on my old stocker. I listened to my stock amp for a couple of hours with music I am familiar with. I have to say there is *maybe* a VERY subtle difference in sound quality for the good - BUT IMHO this is a mod that is akin to putting a higher quality air cleaner on a stock car and getting maybe TWO more horsepower. You can't turn a STOCK Dynaco ST-70 into a world beater with this input cap mod. The mod does not "*dramatically* improve the sound of your stocker" as you said. The difference is subtle. The stock Dynaco ST-70 amp needs a modern all triode driver board and a better power supply to really address the demands of todays digital music sources that have a high dynamic range. Of course if you listen at low volume levels and not at realistic sound levels you *may* be satisfied with the input mod as you suggest.

After listening to the input capped stock ST-70 for a couple of hours I cycled in my Bob Latino upgraded ST-70 amp with the VTA driver board and upgraded power transformer that I built from one of his kits. I then listened to the SAME music with SAME components. The bass is tighter and goes deeper on the new amp. The top end has much more extension and the new amp has much greater resolution. This amp sounds more tonally accurate than the stock amp. There is no real comparison between the stock amp and the new amp. I have an audiophile friend who has listened to my new amp and has told me that my new ST-70 kit amp sounds every bit as good as the Rogue 88 that he bought new and which cost him about DOUBLE the cost of my amp kit. I think he paid like $1300-$1400 for his Rogue.

IMHO the input cap mod is a mod whose overall affect is subtle and a mod that cannot turn a Dynaco ST-70 stock amp into an amp capable of dealing with the dynamic ranges found todays digital music sources.

Charlie

Your experience and mine are VERY different. Right now I cannot account for the difference between the two. On mine, there was little to no difference in the bass but the midrange and treble opened up and became clear sharp and reacted fast. I have gone over some of the parts on my main board and know that some (haven't tested all of them) are within spec. For example, the matched resistors around the phase splitter triode on mine are less then 100 ohms apart. The 3 of the coupling caps are spot on and the 4th is close enough to not affect the output any.

Clearly your experience is not the same one I have had. I do listen at higher volume, loud enough for my 12 year old to have to yell at me to turn it down. The speakers are sealed 3 way boxes from the mid 70's that don't put out more then about mid 80's dB for 1 watt input. I push the amp hard (when it is up).

I guess we will have to agree to disagree.

Robert:

You have my opinion and Charlie's. They are very clearly in disagreement. Neither one of us can resolve that issue since we don't live near enough to hear each other's system. Ultimately it is your system. The input cap is a very easy mod. Maybe you will see my results maybe Charlie's. If mine, you've got a good, cheap upgrade. If Charlie's, you've lost maybe $5 and half an hour to an hour to install. You can still go on to the more extensive upgrades.

Whatever you do good luck to you and please let us know what you decided and what your results were.

By the way PakProtector: I am very much an outcast here as I like what global negative feedback can do for an amp.


Have fun all

Shelly_D

 

[Kegger]

"I guess we will have to agree to disagree."

Agreed, couldn't have said it better myself.

Everyone hears different and looks for different things in there system, not all
will ever agree what is right and what is wrong.

I don't think Global NFB is the "Monster" many feel it is but when I can avoid it
in a piece/system I generally find it very beneficial to do so, sometimes I've had
to leave it/tweak it for the system to perform upto MY satisfaction but I build with
the intent on not using/needing it to start with and proceed from there.

I for one don't find a stock 70 Gross but ultimatly when using one have the urge over
time to want something that performs better, so I either have to change amps or mod
a 70 to do what I'm looking for. Redone quite a few and still have 1 stocker left as well.

The cap on the input of the 70 will help to if your preamp has some residual dc left at it's
output that upsets the front end stage of the amp, just something to think about there to.

Take care guy's!

 

[shelly_d]

"I guess we will have to agree to disagree."

Agreed, couldn't have said it better myself.

Everyone hears different and looks for different things in there system, not all
will ever agree what is right and what is wrong.

...

Spoken like a true peacemaker. Maybe we should turn you loose on the middle east. Thank you.

Shelly_d

 

[lorne]

Hi Shelley_d: Earlier this year you wrote this in regards to some controversey about the ST-70

I have a stereo 70 which I had been running with bandwidth limiters set to 12 Hz and 22KHz and it sounded absolutely glorious. — SNIP— In the rebuild I intend to add an input cap to the circuit forming a high pass filter - still a form of bandwidth limiting. If you ever have the opportunity to listen to a stock stereo 70 vs. one with the filters you will hear the difference. In my case I built the filters outboard and plugged them into the inputs and then the preamp into the filters.

Firstly, do you have any drawings or pics of your plug-in filters that are convenient to upload? Secondly, have you yet incorporated a filter in the rebuild you intended to do. Can you list the parts and their values. Or is it simply a .1uF cap in series?

Yep, I know the controversy, but I have collected all the parts for a sort of improved stock rebuild — some mild value changes and a beefier supply filter blah blah. I have a new stock configured board and some NOS 1199's. IMHO, the stock/near stock can benefit from bandwidth limiting. And I was thinking of putting in the van Alstine filter circuit that has been mentioned already in this thread.

OK guys — I'm not trying to stir up argument again. I think all of us who have contributed so far know the pros and cons. I'm basically experimenting, and if the reformed ST-70 turns out to be less than a sterling performer, it will still be fun and an option to using SS power. I'd like to see what someone else has done using an input filter.

 

[131east23]

So, I was reading the assembly manual of the original ST 70, then I started in on the ST 70 MKII and all of its upgrades. This thread mentions Bob Latino and his ST 70 and its upgrades. Would Bob's kit be the best upgraded version of the ST 70 to go with or is there something else out there that I should consider?

 

[Ray7027]

About 5 months ago I built one of Bob Latinos' kits. I recommend going with the
capacitor upgrades. That included Russian military PIO capacitors. I am amazed everyday
when I play something I've played for years and hear things I never heard before.
It is a great kit with excellent instructions and very high quality parts. Bob is always
available for any questions. I highly recommend him.

 

[JHoman]

I'm still trying to make up my mind about this. I spoke with Bob and after he and I talked about my listening room (20' x 20') and the fact that I have a set of Klipsch Cornwall I's, he suggested that I go with his ST 120, has anyone had any experience with this amp, what is your take on this.

I also am not a electrical tech, but I can follow directions and solder. I just don't want to spend money on a kit that I might mess up.

Still trying to figure this all out.

Jeff

 

[Endspec]

Truly these st 70 and st 120 kits from almost any supplyer are the simplest kits to build, i say go for it and just take your time and if in doubt ask on this forum and you will get any help you need.

 

[Ray7027]

I found that BOB Latinos' instructions are excellent. I followed very slowly step by step
and checked off when I was sure it was correct. It took me longer than the time Bob
said it would, but I've had no problems and it sounds fantastic.

 

[Reid]

Bob Latino ST-120

I purchased an assembled ST-120 from Bob last Feburary. To date it is one of the best purchases I have made. I was looking at a MC275 but couldn't get past the price. This was to be my first tube amp. The ST-120 has more detail and low end grunt than my Rotel power amp. I enjoy this amp everyday. I just can't see how you could go wrong. This amp will just amaze you.

 

[boycephoto]

I just started reading this thread this evening, obviously it spans a couple of years.

I have a Van Alstine Ultimate 70. It sounds fantastic, a dramatic improvement over a stock Dynaco 70. This thing is really good, it won't replace my Audio Research VT200, 35 watts vs. 200/ch, but...it's almost that good. The Audio Research is a $10K amplifier. It has no compairison to the earlier Van Alstine Super 70i from the early 90's. I have not heard of anyone not liking it once they have tried it. Just my 2 sence.
www.avahifi.com

 

[hotroady]

"With room for a better power supply yet, we were able to widen the power bandwidth to 3 dB down at 50K Hz and 5 Hz and still maintain our goal of no feedback related overload under any condition of use. More engineering math provided us with better solutions for the bandwidths of the feedback and coupling circuits. Our tube choice has kept up with all the new audio circuit engineering even better than expected." Oh..now, finally: AVA gets with the program..http://www.avahifi.com/index.php?option=com_content&view=article&id=250&Itemid=235

 

[Kegger]

GEEE, interesting how NOW magically those crappy output transformers HAVE the Wider Bandwidth..
(his original bandwidth mod probably helps original circuits, but it ain't "because of transformer" bandwidth)

 

[shelly_d]

GEEE, interesting how NOW magically those crappy output transformers HAVE the Wider Bandwidth..
(his original bandwidth mod probably helps original circuits, but it ain't "because of transformer" bandwidth)

If you read it carefully, he never states what the bandwidth of the audio outputs is. He talks only of the bandwidth of the power supply. Pure marketing BS. If the bandwidth of the PS is a limiting factor for either high or low frequency output, then fixing that simply moves the limiting factor to something else. By his lights (and Dynaco's) using those output transformers at a 35 watt max output will put you in the range of 20 to 20K Hz.

See Here for (A470). But you already knew this.

http://www.the-planet.org/pdf/transformer.pdf

I do try to not get upset over the BS I see on the websites of people trying to sell audio gear. It is usually so self serving that if I took it seriously I would want to get out the hobby. About the only ones I take really seriously are Rod Elliot and John Broskie, in my book they talk straight and tell you just what they find in a circuit or situation. I have a great deal of respect for both of them.

Shelly_D

 

[shelly_d]

Looked at his slide show for the amp just now and he could, and should have done a better job on the chassis layout. He had what is basically new design. The chassis looks like it is deeper then the original. He could have done a new layout to 1- move the bias adjustments away from the hot tubes and 2) place the test points away from and close to the relocated bias adjustments. Fat fingered folks - like me - can burn their fingers checking the bias real easy on that sucker. I've also seen nicer internal wiring layouts as well.

All in all, for what he wants for that amp, he should have put more time and care into the physical layout and wiring arrangement. (IMHO)

Shelly_D