Found the above artical while doing research on the amp and trying to determine if the amp needs a lot of changes or is it best to leave it stock. Looks like its either upgrade the power supply and run it or upgrade everything. For now I'm going to replace the cathode to ground resistors and the choke. In the future I might try the dynamutt board. I have ordered a new stock layout board with mf resistors. After that I'll do some listening tests and determine if I want to change the driver design or maybe build a 300 based amp. Thanks for all the information.
the previous post if the link dies was ST-70 baseline testing paper on audioregenisis.com by David Gillespie.
He points out that the ST70 is subject to LF oscillation if DC gets into the input stage, and recommends installiing an input grid cap to stop it.
He also points out the problem of the high wall socket voltage these days and recommends a solid state 5AR4 replacement not as good as Eli Duttman's CL90 plus diodes, plus a resistor to drop some voltage.
I'd like to point out that I replaced the 5AR4 for low power 3 times in 12 years of 2000 hours per year, mostly at 1-2 watt music level, but symphonic with the 50 db dynamic range. My criteria for examining things was the unit sounded "polite", confirmed by output was well below 17.5 VAC (35 w) on the 8 ohm speaker or later an 8 ohm log resistor. I replaced the B+ cap 3 times: just after I bought it with Atomlytics tied in a bundle with a plastic tie, then with a CDE cardboard tall "can" cap.
I replaced the EL34/6CA7's twice, once with Mullard EL34's and once with GE 6CA7's, also for low wattage out . I never replaced the 7199's.
Mr. Gillespie suggests the selenium rectifier doesn't need replacing. I disagree. When I bought my 1961 ST70 in 1970, the voltage at eyelet 6 and 21 (negative bias) was low and the selenium rectifier was measuring low ohms backwards, hundreds of ohms or low thousand ohms, whereas the new GI selenium rectifier I bought was high ohms. 4 years later the same symptom occured, with the selenium rectifier again leaking backwards, and I replaced it with a 1n4003.
My unit was operated with limited airconditioning in Houston, TX and some years with no air conditioning, so room temperatures as high as 80 were common sometimes. The power transformer is fine, don't worry about it unless you are using the unit in PA service at high watts. I've done an evening or two of PA service too, that melted solder in a ST120. The unit took a lightning strike once bad enough to arc across the power switch in the PAS2 and blow up the turn off pop cap across that, so insulation in the power transformer is stellar quality.
He points out that the ST70 is subject to LF oscillation if DC gets into the input stage, and recommends installiing an input grid cap to stop it.
He also points out the problem of the high wall socket voltage these days and recommends a solid state 5AR4 replacement not as good as Eli Duttman's CL90 plus diodes, plus a resistor to drop some voltage.
I'd like to point out that I replaced the 5AR4 for low power 3 times in 12 years of 2000 hours per year, mostly at 1-2 watt music level, but symphonic with the 50 db dynamic range. My criteria for examining things was the unit sounded "polite", confirmed by output was well below 17.5 VAC (35 w) on the 8 ohm speaker or later an 8 ohm log resistor. I replaced the B+ cap 3 times: just after I bought it with Atomlytics tied in a bundle with a plastic tie, then with a CDE cardboard tall "can" cap.
I replaced the EL34/6CA7's twice, once with Mullard EL34's and once with GE 6CA7's, also for low wattage out . I never replaced the 7199's.
Mr. Gillespie suggests the selenium rectifier doesn't need replacing. I disagree. When I bought my 1961 ST70 in 1970, the voltage at eyelet 6 and 21 (negative bias) was low and the selenium rectifier was measuring low ohms backwards, hundreds of ohms or low thousand ohms, whereas the new GI selenium rectifier I bought was high ohms. 4 years later the same symptom occured, with the selenium rectifier again leaking backwards, and I replaced it with a 1n4003.
My unit was operated with limited airconditioning in Houston, TX and some years with no air conditioning, so room temperatures as high as 80 were common sometimes. The power transformer is fine, don't worry about it unless you are using the unit in PA service at high watts. I've done an evening or two of PA service too, that melted solder in a ST120. The unit took a lightning strike once bad enough to arc across the power switch in the PAS2 and blow up the turn off pop cap across that, so insulation in the power transformer is stellar quality.
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I would also replace the selenium as well, it's decades old and if it fails, your seriously stress or damage the output tubes on the way to blowing the line fuse. They were never overly reliable, and adding many many years of age to them doesn't help.
Sheldon
In the 20 or more ST-70 I have worked on over the years I have generally found the selenium rectifiers to bad along with a large % of the bias filter caps. Replacement is a very good idea even if you have one that currently appears to be working.
Yes, however the bias voltage will now be more negative, and some tubes may not bias properly.
The resistors in the bias network may now need to be changed to get the right bias voltage range.
Usually adding a little resistance in series with the new silicon rectifier does the trick. I no longer remember the exact value but something between 220 - 330 ohms I think..
you Really Wanna fool with the St70?
Had one from new,, eventually Modded the hell out of it ... full Curcio mods Spare no expense Foolishness.
Happy as a Clam with it for years.
Then Brother brought over his brand New NAD.. to demo it for me.
Made MY St70.. modded, sound farty and frequency constrained.
Bit of an Epiphany. Sold the St70 and Never looked back.
YMMV, but it ain't all ALL roses.
Had one from new,, eventually Modded the hell out of it ... full Curcio mods Spare no expense Foolishness.
Happy as a Clam with it for years.
Then Brother brought over his brand New NAD.. to demo it for me.
Made MY St70.. modded, sound farty and frequency constrained.
Bit of an Epiphany. Sold the St70 and Never looked back.
YMMV, but it ain't all ALL roses.
I listened to a friends NAD power envelope something and wasnt impressed. I also have a Yamaha A-1020 amp which is ok and a few of the older crown amps but I keep going back to the tube amps. On my ST-70 amp I replaced the selenium rectifier with a diode and did not have to change anything else to set the bias at 1.56 volts. I did change the driver board to a new version of the original board. Power transformer is just warm to the touch. Currently running my Infinity RSb speakers but soon I'll have the cabs done for a couple EV 12TRXB drivers I bought a while back.
Amps cannot really be compared without consideration of the speaker part of the system.
That said, my amplifier, which started out as 2 broken ST70s, and now with little resemblance to a stock ST70 is quite good. I found the stock ST70, even with basic fixes, quite underwhelming.
dave
That said, my amplifier, which started out as 2 broken ST70s, and now with little resemblance to a stock ST70 is quite good. I found the stock ST70, even with basic fixes, quite underwhelming.
dave
So true, they are a pair, inextricably linked.
I know what you mean. Mine still looks like a ST-70 because I use the chassis (which also sucks, frankly).
Sheldon’s World Blog Archive Does the world need another Dyna ST-70 Input Board?
Sheldon
Sheldon, is there perhaps an error in the schematic of your new ST 70 input board? On the input tube, it appears the gridstopper resistor is on the wrong side of the input load resistor.
Hello seadoo, It looks like you are well one your way to great Dynaco! Would it be ok if I thrown in a few things to look for in terms of problems that just might surface?
I found the insulation of the red hookup wire in mine to have a low melting point. The gotcha here occurs if the wire insulation was touching a component lead when that component was soldered. That is about all it would take for a melt through and then a shorted circuit is waiting to happen, even 50 years later. It happened to me!
As to the 15.6 ohm resistors, I have not seen any available lately. These were drifting badly in my Dynaco when I first got it. A 15 ohm resistor works fine.
Lastly, in mine, one of the 10K bias pots developed (don't ask me how!) a short to ground with nasty results to the corresponding pair of EL34s. I recommend military grade pots as replacements. They work well and come with a lock nut that you can use if you wish.
Good choice on the K40 caps. They look great and sound good too.
Hope this helps you!
I found the insulation of the red hookup wire in mine to have a low melting point. The gotcha here occurs if the wire insulation was touching a component lead when that component was soldered. That is about all it would take for a melt through and then a shorted circuit is waiting to happen, even 50 years later. It happened to me!
As to the 15.6 ohm resistors, I have not seen any available lately. These were drifting badly in my Dynaco when I first got it. A 15 ohm resistor works fine.
Lastly, in mine, one of the 10K bias pots developed (don't ask me how!) a short to ground with nasty results to the corresponding pair of EL34s. I recommend military grade pots as replacements. They work well and come with a lock nut that you can use if you wish.
Good choice on the K40 caps. They look great and sound good too.
Hope this helps you!
There are a lot of magic cures for the aged Dynaco stereo 70. The best thing is to remove all the wiring and input board and go promptly to the garbage can with them. All caps should be removed and replaced. The "can" power supply cap should be replaced with a Triode power supply board or something similar with larger caps with higher voltage capacity.
So starting with a bare chassis with only the original output transformers on it carefully remove the power transformer and put it on ebay for a quick sale. Purchase a replacement transformer which has higher capacity and the ability to not heat up and possibly end up in a puff of smoke.
Some would have you believe the magic cathode resistor reduction. Let me laugh with that. Some sell driver boards that aren't well engineered (the ones with 3 12AT7's on them). Be careful if you purchase something like this and be sure to keep an eye on the plate resistors as they have a tendency to burn up or at least turn a nasty brown in color.
So, what are you left with if you get rid of all the troublesome items on the stock chassis? You have a bare chassis and a pair of output transformers and possibly a power supply choke if it hasn't been replaced already due to melt down. Rebuild now.
1.) Driver Board, 2.) power supply board, 3.) power transformer. 4.) A decent bias circuit and the list seems to continue.
So, what is all the hype about concerning the Dynaco Stereo 70? By the time you finish repairing it and building something you can actually listen to and enjoy you have put enough into it to buy a decent new amp. Been there about 20 times myself and tried ALL the different driver board combinations. What seems to surprise me the most is the companies that make a living off the interest in the Dynaco. How do these people sleep at night knowing they have ripped people off?
So starting with a bare chassis with only the original output transformers on it carefully remove the power transformer and put it on ebay for a quick sale. Purchase a replacement transformer which has higher capacity and the ability to not heat up and possibly end up in a puff of smoke.
Some would have you believe the magic cathode resistor reduction. Let me laugh with that. Some sell driver boards that aren't well engineered (the ones with 3 12AT7's on them). Be careful if you purchase something like this and be sure to keep an eye on the plate resistors as they have a tendency to burn up or at least turn a nasty brown in color.
So, what are you left with if you get rid of all the troublesome items on the stock chassis? You have a bare chassis and a pair of output transformers and possibly a power supply choke if it hasn't been replaced already due to melt down. Rebuild now.
1.) Driver Board, 2.) power supply board, 3.) power transformer. 4.) A decent bias circuit and the list seems to continue.
So, what is all the hype about concerning the Dynaco Stereo 70? By the time you finish repairing it and building something you can actually listen to and enjoy you have put enough into it to buy a decent new amp. Been there about 20 times myself and tried ALL the different driver board combinations. What seems to surprise me the most is the companies that make a living off the interest in the Dynaco. How do these people sleep at night knowing they have ripped people off?
So I have a "no longer an ST-70" that I've been using for many years now as my primary amp. I've replaced just about everything, only the transformers and chassis are original at this point. I put a Curcio board in for the driver, the one with 4 6DJ8s, the power supply caps are on a board, but most of all the power tubes are now Sovtek 6B4G DHTs running with no feedback. Did I spend too much on it? Maybe, maybe not. It didn't really cost that much, but I also got some pleasure out of doing all the mods over time, each one making a bigger difference than swapping cables or power cords or any of the snake oil changes ever did for me.
I would now do this on any piece of gear, looking for the next DIY upgrade. Isn't that why we're here on this board?
I would now do this on any piece of gear, looking for the next DIY upgrade. Isn't that why we're here on this board?
Ripping people off? 🙂 I don't think folks are naive enough to not understand the situation with these amplifiers and the vendors supplying mods.
IMO, people tend to fall into two groups......those who appreciate the nostalgic appeal the ST-70 has and choose to keep it stock (or nearly so), and those who just view it as a "platform" to construct a new amplifier. (I realize there is some gray area in between.)
However, in all cases I think objective and subjective performance is just one aspect of their decision. Tube amps have an attractive "quality" to them....even when turned off. 🙂
Dave.
Probably the best appeal of the 70 is when its off. There are companies that push their agenda what they feel is best for their customers and lines their pockets. Unfortunately a few of them are wrong and steer the unknowing into purchasing a product that is inferior. It is unfortunate that a few of these companies have a cult following and a web site for discussion of their product somewhat like diyaudio.
Two of the products that raise the hair on my back are the "original can capacitor" and the use of the delayed turn on rectifier in the copper can. Instead of telling their customers how to modify their new purchase from them they steer them toward the original type can capacitor which is totally inadequate for the amplifier or when the customer is the dark about the SS approach the push toward the copper clad $5 worth of parts rectifier with a 1-2 second delay.
Can I respect this? No
A add on power supply board will out perform the can capacitor and the copper clad piece of junk can be copied for $5 worth of parts.
Two of the products that raise the hair on my back are the "original can capacitor" and the use of the delayed turn on rectifier in the copper can. Instead of telling their customers how to modify their new purchase from them they steer them toward the original type can capacitor which is totally inadequate for the amplifier or when the customer is the dark about the SS approach the push toward the copper clad $5 worth of parts rectifier with a 1-2 second delay.
Can I respect this? No
A add on power supply board will out perform the can capacitor and the copper clad piece of junk can be copied for $5 worth of parts.
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