SY said:
SY, I wish I had seen this a few days ago, I already ordered Curcio's premium driver board.
I'm a noob with a couple of kits under my belt and studying Rosenblit's tube design book. Can you provide me any more clues on improving the power supply? You mentioned the 317 which I assume is the LM317 regulator?
After the cap and premium driver board purchases, I'm feeling the desire to curtail further purchases for a while with other mouths (projects) to feed.
Thanks.
FYI
Free advice column for ST-70s
Assuming not a huge mod to be done, like changing the driver board:
- replace all resistors with MF/MO types
- replace all caps with film caps - the ceramics with Silver Mica
- put ~100ufd AFTER the choke in the PS.
- remove the cathode resistors - or reduce them by 10x in value
(they do serve as fuses) this will dramatically improve the bass
- remove the mono switch/replace RCA jacks
- add cap to the bias supply - replace the old caps there
- strap the tubes to triode (tie the screens to the plate with a current limiting resistor)
This will make the unit sound pretty decent.
The power tranny came in two sizes: small and larger
The larger one is more better.
There were several after market drop in replacements, higher current bigger core, which were suitable for dumping on the PS filtering.
What the above mods will do is to improve the amp to the point where it is listenable, and actually good, and not cost you very much money or time...
No doubt the worst part of the amp is the driver board & icky compensation circuit that resides there.
The second worst part is a slightly anemic power supply.
I have some definite ideas on a nifty driver board, but have yet to get through the 101 other projects that have priority over that. But if it ever got done, it would be a definitive approach.
Sy - neat cross coupled stage! Did you see the one that was around back 20 years ago? Andy something out of Jersey did that one...
_-_-bear
Free advice column for ST-70s
Assuming not a huge mod to be done, like changing the driver board:
- replace all resistors with MF/MO types
- replace all caps with film caps - the ceramics with Silver Mica
- put ~100ufd AFTER the choke in the PS.
- remove the cathode resistors - or reduce them by 10x in value
(they do serve as fuses) this will dramatically improve the bass
- remove the mono switch/replace RCA jacks
- add cap to the bias supply - replace the old caps there
- strap the tubes to triode (tie the screens to the plate with a current limiting resistor)
This will make the unit sound pretty decent.
The power tranny came in two sizes: small and larger
The larger one is more better.
There were several after market drop in replacements, higher current bigger core, which were suitable for dumping on the PS filtering.
What the above mods will do is to improve the amp to the point where it is listenable, and actually good, and not cost you very much money or time...
No doubt the worst part of the amp is the driver board & icky compensation circuit that resides there.
The second worst part is a slightly anemic power supply.
I have some definite ideas on a nifty driver board, but have yet to get through the 101 other projects that have priority over that. But if it ever got done, it would be a definitive approach.
Sy - neat cross coupled stage! Did you see the one that was around back 20 years ago? Andy something out of Jersey did that one...
_-_-bear
Never trust used parts
Well, I'm eating crow right now as I must admit an error.
I replaced the coupling caps with what I remembered to be good used pieces. I rechecked the amp again and found these to be leaking. I replaced this time with new ones. The result is amazing as it actually sounds good now. I added to the filter cap after the choke and there is some bass now.
Joe
Well, I'm eating crow right now as I must admit an error.
I replaced the coupling caps with what I remembered to be good used pieces. I rechecked the amp again and found these to be leaking. I replaced this time with new ones. The result is amazing as it actually sounds good now. I added to the filter cap after the choke and there is some bass now.
Joe
KT88's or KT90's?
Sy
Does the power supply have enough guts to run some higher heater current tubes?
I would like to try some 6550's,KT88's, and KT90's in it since I have an ample supply of them. This does boost the heater current need from 1.5A per tube to 1.6. Just wondering if there is enough left to run them.
Thanks
Joe
Sy
Does the power supply have enough guts to run some higher heater current tubes?
I would like to try some 6550's,KT88's, and KT90's in it since I have an ample supply of them. This does boost the heater current need from 1.5A per tube to 1.6. Just wondering if there is enough left to run them.
Thanks
Joe
If you want to do that, use an auxillary filament transformer. The stock power transformer will cover the stock tubes with little margin. And for bigger tubes, you might consider boosting the B+ with another auxillary supply- 50V@1A would be ideal. Get out the shoehorn, though.
What you'll find is that you can really pump a lot of midrange power out of those output transfomers; if you don't ask them to do bass, you can easily get 60-70 watts out of a pair of big pentodes or beam power tubes.
What you'll find is that you can really pump a lot of midrange power out of those output transfomers; if you don't ask them to do bass, you can easily get 60-70 watts out of a pair of big pentodes or beam power tubes.
Andy Fuchs it was...
As is usual, and why I do badly in interviews and tests, I remembered his name while driving in the car about 3 hours *after* I made the post! heh.
Too bad SY, you lost out on a perfectly good ST 70...
Burnedfingers, take heed of the the cathode resistor mod (the outputs, of course) and the extra 100ufd after the choke - too little is ng, and too much more is also not so good (with that choke and with the stock PS tranny). Should make a significant difference overall... also try strapping for triode.
As is usual, and why I do badly in interviews and tests, I remembered his name while driving in the car about 3 hours *after* I made the post! heh.
Too bad SY, you lost out on a perfectly good ST 70...
Burnedfingers, take heed of the the cathode resistor mod (the outputs, of course) and the extra 100ufd after the choke - too little is ng, and too much more is also not so good (with that choke and with the stock PS tranny). Should make a significant difference overall... also try strapping for triode.
bear
Do you have a suggestion as to how much capacitence to add after the choke on the Dynaco?
I installed the VTA ST70 driver board today and was pleasently surprised with the outcome. I must admit the 12AT7 isn't my favorite tube because it usually sounds metalic to me, this is different.
Joe
Do you have a suggestion as to how much capacitence to add after the choke on the Dynaco?
I installed the VTA ST70 driver board today and was pleasently surprised with the outcome. I must admit the 12AT7 isn't my favorite tube because it usually sounds metalic to me, this is different.
Joe
I usually add 100ufd in the form of a couple of SMPS caps in series... under the chassis held in place by a pair of "P" cable clamps... you can make it a little higher, but too much more and the rectifier toobe and the xfmr will not like it...
That seems to make the PS reservoir just about right.
You can switch to any of the toobes that fit in the same hole
as the 12AX7 for that unit, I suspect.
Did you reduce the cathode resistors yet?
Keep 'em lowish wattage, they are fuses and bias set devices - but with DVMs you don't need 1.56 volts to read it!
Did you remove the wires from the "mono" switch?? :0
_-_-bear
That seems to make the PS reservoir just about right.
You can switch to any of the toobes that fit in the same hole
as the 12AX7 for that unit, I suspect.
Did you reduce the cathode resistors yet?
Keep 'em lowish wattage, they are fuses and bias set devices - but with DVMs you don't need 1.56 volts to read it!
Did you remove the wires from the "mono" switch?? :0
_-_-bear
Hi bear
Yes, the wires and the mono switch are gone. Removed are the 15.6 ohm resistors and 10 ohm are in their place for each tube. The tubes are biased at 40mA each.
I tried tubes in the 12AX7 family namely the 12AT7, and 12AY7.
I paralled together the left over cap sections after the choke. I was thinking of adding 2)additional 47mfd caps as I believe the 5AR4 should handle them since they would be after the choke.
The transition was quite an eye opener and never again will I laugh at the little Dynaco as it went from being so so to actually sounding good.
Joe
Yes, the wires and the mono switch are gone. Removed are the 15.6 ohm resistors and 10 ohm are in their place for each tube. The tubes are biased at 40mA each.
I tried tubes in the 12AX7 family namely the 12AT7, and 12AY7.
I paralled together the left over cap sections after the choke. I was thinking of adding 2)additional 47mfd caps as I believe the 5AR4 should handle them since they would be after the choke.
The transition was quite an eye opener and never again will I laugh at the little Dynaco as it went from being so so to actually sounding good.
Joe
burnedfingers said:
My thinking is that going from 15.6 to 10 ohms is relatively meaningless. I'd go with a 1.56 ohm or a 1.0 ohm or a 0.1 ohm
resistor (assuming ur DVM works ok down to a low enough voltage).
Not sure what ur saying here?
Paralleled "left over" sections?? Didn't think there were any
left over... and are they high enough voltage rated?? None of
my ST70s had any left over sections...
Ur probably still not fully "there" but as SY mentioned, the output iron is respectable, and that is the weak link in any tube amp, other than the power supply - assuming you can fix the electronics if you need to.
_-_-bear
Thanks for the reply...
Quote:
My thinking is that going from 15.6 to 10 ohms is relatively meaningless. I'd go with a 1.56 ohm or a 1.0 ohm or a 0.1 ohm
resistor (assuming ur DVM works ok down to a low enough voltage).
Being that I had 10 ohm resistors I used them. Your correct it is meanless to go from 15.6 ohm resistors to 10 ohm. The voltage across the 1 ohm or .1 ohm resistors would be a lot smaller requiring the usage of a "good" meter to check the bias. Good 1%resistors in the 1 ohm to .1 ohm value range would need to be ordered. I used what I had. I also found it easy to dial in .400 accross each 10 ohm resistor. This is easy to do even with a Simpson 260 meter.
Quote:
I tried tubes in the 12AX7 family namely the 12AT7, and 12AY7.
I paralled together the left over cap sections after the choke. I was thinking of adding 2)additional 47mfd caps as I believe the 5AR4 should handle them since they would be after the choke.
If my memory is correct...in the original Dynaco circuit there are several voltages available to feed the front end (board) taps A&B and I believe (C) feeds the B+ to the output transformer. Therefore there are (3) different voltages available. In the board I used the 450Volt feed to the driver board. This leaves several voltage dropping resistors and several cap sections doing nothing. I paralled the cap sections after the choke and removed the resistors. The caps are rated 525V so I assume it will handle it ok.
Quote:
Ur probably still not fully "there" but as SY mentioned, the output iron is respectable, and that is the weak link in any tube amp, other than the power supply - assuming you can fix the electronics if you need to.
There is always a point where one has to make a decision as to spend any additional money on transformers or to keep the stock ones. There is a question in my mind if there would be enough difference to warrant throwing the money out for some. The power supply does suffer with its "little" transformer that runs hot and the output transformers could be improved upon. It is my belief that one could throw away another 5-6 hundred on different transformers and still not have an amp that would be half as good as my "Quicksilvers
With my original investment I now have $359.00 including shipping in the Dynaco. This includes the new driver board and parts to stuff the board. I can't forget the new driver tubes now either. Is it worth spending any more on? I think not!
Quote:
assuming you can fix the electronics if you need to.
Sounds like a slam...ouch!
Quote:
My thinking is that going from 15.6 to 10 ohms is relatively meaningless. I'd go with a 1.56 ohm or a 1.0 ohm or a 0.1 ohm
resistor (assuming ur DVM works ok down to a low enough voltage).
Being that I had 10 ohm resistors I used them. Your correct it is meanless to go from 15.6 ohm resistors to 10 ohm. The voltage across the 1 ohm or .1 ohm resistors would be a lot smaller requiring the usage of a "good" meter to check the bias. Good 1%resistors in the 1 ohm to .1 ohm value range would need to be ordered. I used what I had. I also found it easy to dial in .400 accross each 10 ohm resistor. This is easy to do even with a Simpson 260 meter.
Quote:
I tried tubes in the 12AX7 family namely the 12AT7, and 12AY7.
I paralled together the left over cap sections after the choke. I was thinking of adding 2)additional 47mfd caps as I believe the 5AR4 should handle them since they would be after the choke.
If my memory is correct...in the original Dynaco circuit there are several voltages available to feed the front end (board) taps A&B and I believe (C) feeds the B+ to the output transformer. Therefore there are (3) different voltages available. In the board I used the 450Volt feed to the driver board. This leaves several voltage dropping resistors and several cap sections doing nothing. I paralled the cap sections after the choke and removed the resistors. The caps are rated 525V so I assume it will handle it ok.
Quote:
Ur probably still not fully "there" but as SY mentioned, the output iron is respectable, and that is the weak link in any tube amp, other than the power supply - assuming you can fix the electronics if you need to.
There is always a point where one has to make a decision as to spend any additional money on transformers or to keep the stock ones. There is a question in my mind if there would be enough difference to warrant throwing the money out for some. The power supply does suffer with its "little" transformer that runs hot and the output transformers could be improved upon. It is my belief that one could throw away another 5-6 hundred on different transformers and still not have an amp that would be half as good as my "Quicksilvers
With my original investment I now have $359.00 including shipping in the Dynaco. This includes the new driver board and parts to stuff the board. I can't forget the new driver tubes now either. Is it worth spending any more on? I think not!
Quote:
assuming you can fix the electronics if you need to.
Sounds like a slam...ouch!
15.6 ohm vs. 0.1 ohm
This is almost meaningless. Either value will give the same accuracy and won't change the sound much, so don't waste too much ink on that topic...
I've played with a Welborne front-end (5751 dual-triode in parallel loaded by a 6U8 pentode CCS, using the 6U8 triode as a splitter). Most of the improvement is from the lower feedback of this design, but still the basic sound characteristics are the same as the original.
If you design your own front-end, I'd go with as little feedback as possible (zero would be nice) and definitely do the triode mod on the output section.
In comparison to its big brothers - a pair of MkIII's are much nicer sounding in the stock form and would be a much better platform for a high-quality tube amp.
YMMV...
This is almost meaningless. Either value will give the same accuracy and won't change the sound much, so don't waste too much ink on that topic...
I've played with a Welborne front-end (5751 dual-triode in parallel loaded by a 6U8 pentode CCS, using the 6U8 triode as a splitter). Most of the improvement is from the lower feedback of this design, but still the basic sound characteristics are the same as the original.
If you design your own front-end, I'd go with as little feedback as possible (zero would be nice) and definitely do the triode mod on the output section.
In comparison to its big brothers - a pair of MkIII's are much nicer sounding in the stock form and would be a much better platform for a high-quality tube amp.
YMMV...
Quote:
This is almost meaningless. Either value will give the same accuracy and won't change the sound much, so don't waste too much ink on that topic...
No need to reply to this as we both know there isn't any sonic difference between the 15.6 ohm resistors and the 10 ohm or .1 ohm for that matter.
Quote:
I've played with a Welborne front-end (5751 dual-triode in parallel loaded by a 6U8 pentode CCS, using the 6U8 triode as a splitter). Most of the improvement is from the lower feedback of this design, but still the basic sound characteristics are the same as the original.
Naturally the difference in overall feedback is going to change the sound. Fact is the front end design will also change the sound of the amp as well. Why do you think there are different boards available for the Stereo 70? Does each one not sound different? I think they do indeed. Sorry, not impressed with the overall quality of the "Welborne front end.
Quote:
In comparison to its big brothers - a pair of MkIII's are much nicer sounding in the stock form and would be a much better platform for a high-quality tube amp.
I would be really surprised if they didn't sound better. Humm, maybe because of better transformers, power and output.
If I'm going to pick a platform for a better designed amp I believe I will stick with my converted 8417 "Quicksilvers" a much better sounding amp.
This is almost meaningless. Either value will give the same accuracy and won't change the sound much, so don't waste too much ink on that topic...
No need to reply to this as we both know there isn't any sonic difference between the 15.6 ohm resistors and the 10 ohm or .1 ohm for that matter.
Quote:
I've played with a Welborne front-end (5751 dual-triode in parallel loaded by a 6U8 pentode CCS, using the 6U8 triode as a splitter). Most of the improvement is from the lower feedback of this design, but still the basic sound characteristics are the same as the original.
Naturally the difference in overall feedback is going to change the sound. Fact is the front end design will also change the sound of the amp as well. Why do you think there are different boards available for the Stereo 70? Does each one not sound different? I think they do indeed. Sorry, not impressed with the overall quality of the "Welborne front end.
Quote:
In comparison to its big brothers - a pair of MkIII's are much nicer sounding in the stock form and would be a much better platform for a high-quality tube amp.
I would be really surprised if they didn't sound better. Humm, maybe because of better transformers, power and output.
If I'm going to pick a platform for a better designed amp I believe I will stick with my converted 8417 "Quicksilvers" a much better sounding amp.
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