Curious if anyone has tried any circuit mods or have links to some
on the original 70's curcuits that made any decent improvements?
I know the original isn't anything great and plenty of good circuits
out there for these amps (have done a few myself) but I have this
newer green pc board that's in nice shape and so I thought about
messing around with the original circuit to see if anything can be
squeazed from it any further. 🙂
Yah I've seen the Van Alstine little tweak for frequency limiting but
that's about all I found and frankly not what I'm after here, I'd just
leave it completly stock instead of that thing.
on the original 70's curcuits that made any decent improvements?
I know the original isn't anything great and plenty of good circuits
out there for these amps (have done a few myself) but I have this
newer green pc board that's in nice shape and so I thought about
messing around with the original circuit to see if anything can be
squeazed from it any further. 🙂
Yah I've seen the Van Alstine little tweak for frequency limiting but
that's about all I found and frankly not what I'm after here, I'd just
leave it completly stock instead of that thing.
1976 Audio Amateur
"Dyna Stereo 70 Amp Mods"
http://www.audioxpress.com/magsdirx/backissues/BISAAM.htm
H.L. Eisenson descibes replacing the tube rectifier with diodes, replacing some of the caps and diodes, upgrading some resistors, and dialing in the square wave response.
Sorry, I don't have a link to the article, but it is included in the "Audio Amateur Power Amp Projects" book. (I have the book, but have never had a Dynaco ;-)
in 1977 (and the book) an article: "Audio Research Modifies the Dyna Stereo 70" - well, they all but completely redesign it really.
the amp project book:
http://www.audioxpress.com/bksprods/products/bkaa40.htm
-Chris
"Dyna Stereo 70 Amp Mods"
http://www.audioxpress.com/magsdirx/backissues/BISAAM.htm
H.L. Eisenson descibes replacing the tube rectifier with diodes, replacing some of the caps and diodes, upgrading some resistors, and dialing in the square wave response.
Sorry, I don't have a link to the article, but it is included in the "Audio Amateur Power Amp Projects" book. (I have the book, but have never had a Dynaco ;-)
in 1977 (and the book) an article: "Audio Research Modifies the Dyna Stereo 70" - well, they all but completely redesign it really.
the amp project book:
http://www.audioxpress.com/bksprods/products/bkaa40.htm
-Chris
Those might be cool, all those you have to pay for correct?
I've got a power supply rebuild I normally do, I'm just wondering about
the audio circuit and the original PCB as to any improvements to be had.
I've got a power supply rebuild I normally do, I'm just wondering about
the audio circuit and the original PCB as to any improvements to be had.
Go with an aftermarket driver board. I used one that had (3) 12AT7's. It also had bias adjustment for each output tube instead of each pair. I also added more capacitence to the power supply.
There is also a triode modification you can make. I wired in a switch to flip from normal to triode mode. Some people swear by the triode mode on these things. It may have sounded slightly better but I'm not really sure.
Replace the diode in the bias supply for sure.
Just Google Dynaco Stereo 70 and you should hit some free information.
There is also a triode modification you can make. I wired in a switch to flip from normal to triode mode. Some people swear by the triode mode on these things. It may have sounded slightly better but I'm not really sure.
Replace the diode in the bias supply for sure.
Just Google Dynaco Stereo 70 and you should hit some free information.
As I said I've used aftermarket boards and have designed my own which
work very well but I'm curious as to some tinkering on the original circuit.
I'd just like to see what a tweaked out/modded version of the original curcuit could do.
I'll probably have to devise my own mods to it but was curious any info allready done.
I'm looking for mods to the original PCB in the amp, plenty of replacement boards out
there but I'd like to see what can be done on the original.
work very well but I'm curious as to some tinkering on the original circuit.
I'd just like to see what a tweaked out/modded version of the original curcuit could do.
I'll probably have to devise my own mods to it but was curious any info allready done.
I'm looking for mods to the original PCB in the amp, plenty of replacement boards out
there but I'd like to see what can be done on the original.
I've redone at least a hundred ST-70s. The Ike Eisenson mods retain the basic circuitry but upgrade some parts and the power supply. Most bang for the buck. While you're in there, you should also replace sockets, switches, jacks, and put in a three wire power cord with the safety ground lead firmly attached to the chassis. Ike did not redo the goofy grounding scheme of the ST-70, but you should.
A step up is replacing the driver circuit board with something different. There are about a million of those out there. I've had limited experience with them since I generally do my own.
Another step up is putting in regulation for the power supplies. The best I've used is the Curcio design. I think regulation is the single most useful mod.
If it were me, I'd strip the chassis bare, replace all the low bid sockets and switches, redo the power supply, add a regulator, replace all the coupling caps and critical resistors on the stock driver board (be warned: you will destroy some traces so be prepared to do some repairs), then live with that for a while. It will be pretty darn good.
A step up is replacing the driver circuit board with something different. There are about a million of those out there. I've had limited experience with them since I generally do my own.
Another step up is putting in regulation for the power supplies. The best I've used is the Curcio design. I think regulation is the single most useful mod.
If it were me, I'd strip the chassis bare, replace all the low bid sockets and switches, redo the power supply, add a regulator, replace all the coupling caps and critical resistors on the stock driver board (be warned: you will destroy some traces so be prepared to do some repairs), then live with that for a while. It will be pretty darn good.
Thanks SY, before this gets out of hand, I to have done a bazillion mods
and or upgrades to the 70 but have never given the original circuit the time
of day and just once at least I'd like to see what can be done on the original
PCB to see if anything decent can come from it. (I've used it stock)
I'm not expecting earth shattering performance by any strech of the imagination
but I would like to give it a go and see how things work out, but I find little to no
work in this area on the 70 so I figured I'd post about it before I went and did it
myself anyway. Just figured if I could find a decent looking framework might make
my work a little less is all, but it looks as if I'll have to do this one on my own here. 🙂
and or upgrades to the 70 but have never given the original circuit the time
of day and just once at least I'd like to see what can be done on the original
PCB to see if anything decent can come from it. (I've used it stock)
I'm not expecting earth shattering performance by any strech of the imagination
but I would like to give it a go and see how things work out, but I find little to no
work in this area on the 70 so I figured I'd post about it before I went and did it
myself anyway. Just figured if I could find a decent looking framework might make
my work a little less is all, but it looks as if I'll have to do this one on my own here. 🙂
there is a mod to replace the 7199 with a 6GH8 as the driver tube. It requires cutting and jumping some traces. Other than what SY mentioned that probably about covers it.
You just can't make a silk purse out of a pigs ear.
You just can't make a silk purse out of a pigs ear.
You'll be surprised how well the unit works with the original 7199-based driver once the parts are refreshed and the supply tightened up.
Yah not trying to make these world class amps here, just want to experiment
with the original PCB/Circuit and see what can be achieved with it, that's all. 🙂
As I said, I personally have never tried to get the most from these original boards
and at least I'd like to give it a go once to say I've done it and see what happens.
----------------------------------
I hear yu SY, but I'm thinking there's probably still some tweaking that can be done on
the original PCB/Circuit to improve things a bit and like to try and find out what's possable.
with the original PCB/Circuit and see what can be achieved with it, that's all. 🙂
As I said, I personally have never tried to get the most from these original boards
and at least I'd like to give it a go once to say I've done it and see what happens.
----------------------------------
I hear yu SY, but I'm thinking there's probably still some tweaking that can be done on
the original PCB/Circuit to improve things a bit and like to try and find out what's possable.
Could be, but really this is a very well-thought-out and balanced design. That's why it's a classic. If there were an easy tweak, be assured that Hafler would have thought of it and done it. The only real circuit change that will actually improve performance (besides regulation and parts changes) is to diddle the feedback and compensation components a bit- they're chosen for worst-case, and you can often squeeze a bit more bandwidth out without compromising stability. Will that be audible? Not to me, but others may differ.
Hi Kegger,
No, it will not be audible. Once folks start tweeking, it is surprising what will be perceived to be audible, even when judged in totally the wrong way to be significant. In this division of amplifiers it pays to be sceptical. What SY said is generally true - unless one builds out a circuit to include more than originally intended (thus really "upgrading" to something different), it often is not worthwhile. Listening tests are simply too subjective to serve as sole standard of whether something has been improved.
I have not rejuvinated as many ST-70s as SY, but I can for the love of anybody not think what really perceivable improvement can be possible here (parts-wise yes - I will come to that). How replacing the 7199 by a 6GH8 would improve anything is also a mystery to me. (Just for the record, Kegger, I grew up in the valve era - have been an EE for over 50 years. Designed a lot, do know a little.)
I have fiddled with the 390pF compensating cap (from transformer to cathode of 7199). As SY said, one could optimise slightly - and be careful, there is HT there! Also, with present-day miniaturisation and materials advances, one could comfortably replace the 20uF power supply caps with say 100uF/500V types.
One thing I cannot recall that puzzles me now, is the heaters apparently going to HT via an 18K resistor, without a further resistor to common (what am I missing where?) Heaters at +120V would eliminate some heater-cathode noise induction, but not higher than +150V.
And, of course, one can get more accurate more stable resistors and capacitors these days. That I do replace, but that is not a circuit upgrade. And, oh yes, that selenium rectifier in the bias circuit - it was all there was then, but really not suitable in this day of Si-diodes! So might the bias adjustment pots be - again all there was then, etc.
Bidding you success.
No, it will not be audible. Once folks start tweeking, it is surprising what will be perceived to be audible, even when judged in totally the wrong way to be significant. In this division of amplifiers it pays to be sceptical. What SY said is generally true - unless one builds out a circuit to include more than originally intended (thus really "upgrading" to something different), it often is not worthwhile. Listening tests are simply too subjective to serve as sole standard of whether something has been improved.
I have not rejuvinated as many ST-70s as SY, but I can for the love of anybody not think what really perceivable improvement can be possible here (parts-wise yes - I will come to that). How replacing the 7199 by a 6GH8 would improve anything is also a mystery to me. (Just for the record, Kegger, I grew up in the valve era - have been an EE for over 50 years. Designed a lot, do know a little.)
I have fiddled with the 390pF compensating cap (from transformer to cathode of 7199). As SY said, one could optimise slightly - and be careful, there is HT there! Also, with present-day miniaturisation and materials advances, one could comfortably replace the 20uF power supply caps with say 100uF/500V types.
One thing I cannot recall that puzzles me now, is the heaters apparently going to HT via an 18K resistor, without a further resistor to common (what am I missing where?) Heaters at +120V would eliminate some heater-cathode noise induction, but not higher than +150V.
And, of course, one can get more accurate more stable resistors and capacitors these days. That I do replace, but that is not a circuit upgrade. And, oh yes, that selenium rectifier in the bias circuit - it was all there was then, but really not suitable in this day of Si-diodes! So might the bias adjustment pots be - again all there was then, etc.
Bidding you success.
It was worse than that. The heaters were not referenced to DC; the heater winding CT was returned to ground via a ceramic cap. Setting them up on a bypassed 90V rail is a much better idea.
Well, in the diagram I have there is a 18K resistor from designation B (terminal 20) to des. square (terminal 3) in the power supply diagram. Then the only other squares I found were the heaters on the amplifier diagram - but it is not altogether clear. I can recall that I did something there - hopefully to put it right as you suggested. I hope friend Kegger can get that right.
Gents,
Check out "Audio Basics July 1990" and you can judge the Frank Van Alstine 6GH8 mod. It was a suggestion and just a suggestion for you to take into consideration. It has been rumered by some that the 6GH8 is a far better tube and also less costly. I personally don't know. I do however have a working Dynaco stereo 70 board that you could try it on if you would like to conduct a test to see if the mod holds any merits.
SY, If you would like to drop me an email I would be more than happy to ship you the driver board and some 6GH8 tubes so it could receive a fair review and testing.
Check out "Audio Basics July 1990" and you can judge the Frank Van Alstine 6GH8 mod. It was a suggestion and just a suggestion for you to take into consideration. It has been rumered by some that the 6GH8 is a far better tube and also less costly. I personally don't know. I do however have a working Dynaco stereo 70 board that you could try it on if you would like to conduct a test to see if the mod holds any merits.
SY, If you would like to drop me an email I would be more than happy to ship you the driver board and some 6GH8 tubes so it could receive a fair review and testing.
Burnedfingers,
Apology if I detect that I may have offended. 🙂 Totally not the intention; I also simply expressed an opinion. 6GH8 would certainly also work. You did state that it is "rumoured" that ....
I am unable to access the Van Alstine article. Is it possible for you to give a synopsis of why the gentleman considers it an improvement? My comment was based on finding the pentode characteristics of the 7199 and 6GH8 very close, very possibly within the manufacturing spread of either tube. The triode of the 6GH8 differs in that it has twice the mu of the 7199 triode, but in a concertina phase splitter that would be of secondary importance only.
Expense yes! Locally it was double the cost of an ECF80 (6BL8) at the time, and I "modded" in the latter, but only for that reason. The ECF80 is also not very different from the 7199, though the latter specifies better hum/microphony characteristics, important for pre-amp use. In distortion measurements of the whole amplifier I could not detect a difference between using the ECF80 and 7199. (Changing the anode load slightly made more of a difference, though not drastic. I believe boot-strapping would make still more difference - one can go on and on.)
Regards.
Apology if I detect that I may have offended. 🙂 Totally not the intention; I also simply expressed an opinion. 6GH8 would certainly also work. You did state that it is "rumoured" that ....
I am unable to access the Van Alstine article. Is it possible for you to give a synopsis of why the gentleman considers it an improvement? My comment was based on finding the pentode characteristics of the 7199 and 6GH8 very close, very possibly within the manufacturing spread of either tube. The triode of the 6GH8 differs in that it has twice the mu of the 7199 triode, but in a concertina phase splitter that would be of secondary importance only.
Expense yes! Locally it was double the cost of an ECF80 (6BL8) at the time, and I "modded" in the latter, but only for that reason. The ECF80 is also not very different from the 7199, though the latter specifies better hum/microphony characteristics, important for pre-amp use. In distortion measurements of the whole amplifier I could not detect a difference between using the ECF80 and 7199. (Changing the anode load slightly made more of a difference, though not drastic. I believe boot-strapping would make still more difference - one can go on and on.)
Regards.
I'm an experienced 70 modder here Johan so no worries on that front. 🙂
(Probably done over a dozen 70's to date with many variations)
I do believe (in my experience with other amps as well) the 7199 tube
is a better sounding tube then the 6GH8 tube is so I plan on retaining.
Yes it's definatly more expensive but I have a few of them around here.
I'm just going to give the old circuit a thourough examination and checkup.
Thanks for the suggestions.
(Probably done over a dozen 70's to date with many variations)
I do believe (in my experience with other amps as well) the 7199 tube
is a better sounding tube then the 6GH8 tube is so I plan on retaining.
Yes it's definatly more expensive but I have a few of them around here.
I'm just going to give the old circuit a thourough examination and checkup.
Thanks for the suggestions.
Johan Potgieter said:
Johan I believe what your refering to is the takeoff for the front sockets to feed other things.
SY said:
My thinking is essentially parallel to SY's. I like the Triode Electronics replacement driver board equipped with 2X EF86s and an ECC99, as the OEM topology is retained. However, the active devices are better than those in the 7199. The EF86 is (perhaps) the best small signal audio pentode ever and the HIGH gm of the ECC99 is protection against slew limiting in the "concertina" phase splitter.
In amps that have a NFB loop around the O/P trafo (including the ST70), I think that rolling off infrasonic trash at the I/P is important. Rolling the junk off protects the O/P trafo cores from saturation due to a massive NFB error correction signal. Set the "corner" freq. just below 20 Hz.
Increase the value of the coupling caps. between splitter and EL34s to make the high pass pole fall at or slightly below 5 Hz.
Johan,
No apology needed at all. No offense taken. I will try to send the article to you for your evaluation. As a diyer I am always interested when someone comments that that have indeed discovered the best circuit and or the best way of doing something. In keeping with keggers goal of trying to get the most out of the Dynaco Stereo 70 amplifier I thought that maybe the PC board mod with the 6GH8 tube would still be within the goal.
I don't mean to sound negative when it comes to the Dynaco Stereo 70 amplifier so please take me with a grain of salt. The Dynaco has a lot to offer as others have stated. It has very good transformers and a good logical simple circuit that is capable of receiving and enjoying mods. Personally I guess I expected more of it than I received. I felt the low end left some to be desired.
- Status
- Not open for further replies.