Big Cap PS Supplies
I've been researching larger caps and improvements in PS. Bottom line is more caps helps speed up the amp improving highs and lows. Quote "Music is mainly transients, nothing to do with the usual steady 1KHZ sine wave used in power tests... The only storage component that are sufficiently fast are the caps. Electrolytics have a big capacity but a lower slew-rate(Slow cap), higher ESR and inductance. Polypropylenes have a low capacity but a high slew rate(Very Fast Cap), low ESR and inductance." They go on to say the best compromise for cost and space reasons is to bypass electrolytics with polypropylene caps.
Result is depending how much capacitance is used mids and highs are improved in an impressive way with much more clarity instrumental focus and an impression the amp goes higher. The bass changed a lot, incredibly more tight giving more air to the mids and highs. Dynamics have increased a lot, as there is much more difference between soft and loud sounds. Hum was lowered even further.
They also found many small caps were faster in the PS as they could charge and discharge quicker. For example use TEN 50 uF 500V electrolytics instead of one 500 uF 500V. You can wire caps in series with low voltage to give lots of punch from small caps if that's what you have or can only buy. Example TWO 100 uF 350V will give you 50 uF 700V. You can use more in series if needed. You can make up a board with snap in caps for low cost.
Coupling caps make also a big difference and you can voice it with paper in oils like Jensen PIO (Russian K40 great bang for buck) or poly caps like Mundrof Supremes (Auricaps less $ and almost as good) depending on sound preferences.
I see Gumby is going for the VTA board and a new chassis. If you haven't ordered, you may consider the low gain 12AU7 board as it offers lots of tube rolling for sound improvements over the basic 12AU7 one. Info in their Forms. Also Diodes added in series with todays Rectifiers is probably a good idea.
Regards,
Randy
I've been researching larger caps and improvements in PS. Bottom line is more caps helps speed up the amp improving highs and lows. Quote "Music is mainly transients, nothing to do with the usual steady 1KHZ sine wave used in power tests... The only storage component that are sufficiently fast are the caps. Electrolytics have a big capacity but a lower slew-rate(Slow cap), higher ESR and inductance. Polypropylenes have a low capacity but a high slew rate(Very Fast Cap), low ESR and inductance." They go on to say the best compromise for cost and space reasons is to bypass electrolytics with polypropylene caps.
Result is depending how much capacitance is used mids and highs are improved in an impressive way with much more clarity instrumental focus and an impression the amp goes higher. The bass changed a lot, incredibly more tight giving more air to the mids and highs. Dynamics have increased a lot, as there is much more difference between soft and loud sounds. Hum was lowered even further.
They also found many small caps were faster in the PS as they could charge and discharge quicker. For example use TEN 50 uF 500V electrolytics instead of one 500 uF 500V. You can wire caps in series with low voltage to give lots of punch from small caps if that's what you have or can only buy. Example TWO 100 uF 350V will give you 50 uF 700V. You can use more in series if needed. You can make up a board with snap in caps for low cost.
Coupling caps make also a big difference and you can voice it with paper in oils like Jensen PIO (Russian K40 great bang for buck) or poly caps like Mundrof Supremes (Auricaps less $ and almost as good) depending on sound preferences.
I see Gumby is going for the VTA board and a new chassis. If you haven't ordered, you may consider the low gain 12AU7 board as it offers lots of tube rolling for sound improvements over the basic 12AU7 one. Info in their Forms. Also Diodes added in series with todays Rectifiers is probably a good idea.
Regards,
Randy
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I hope this doesn't lead into a discussion of which type or brand of caps to use. I'd rather poke red hot forks in my eyes than read that.
But when it comes to building an ST-70 (with whichever driver board one chooses) I think using a solid-state power supply is worth considering, and for those of us wanting to go that way, there's no sense doing it wrong or halfway, so I value all the advice I can get here.
..Todd
But when it comes to building an ST-70 (with whichever driver board one chooses) I think using a solid-state power supply is worth considering, and for those of us wanting to go that way, there's no sense doing it wrong or halfway, so I value all the advice I can get here.
..Todd
Todd I think you mean eliminating 5AR4 for diodes and not a SS switching PS.
It might be wise to keep 5AR4 for slow start up and use SS diodes to take all of of the load away from it. I think Greg may be showing a way on his site in the preliminary schematic for his boards. SS diodes also help bass response. He too recommends a bigger choke such as a 4H, 225ma Hammond 159S .
Triode electronics has PS boards with bigger capacitance if you want to go that way.
Randy
It might be wise to keep 5AR4 for slow start up and use SS diodes to take all of of the load away from it. I think Greg may be showing a way on his site in the preliminary schematic for his boards. SS diodes also help bass response. He too recommends a bigger choke such as a 4H, 225ma Hammond 159S .
Triode electronics has PS boards with bigger capacitance if you want to go that way.
Randy
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I think a properly designed SS (not switching) power supply involves a lot more thought than just slapping in diodes, increasing the size of the choke and arbitrarily adding capacitance.
Eli's suggestion to add an NTC thermistor is a great example. Boywonder has done wonders with PSUDII for other projects here, and I'd love to get some direction from him for work on that for my ST-70. My immediate questions: Is a hash choke worthwhile? How does pre-choke capacitance affect the voltage and is there any equations to apply to determine an optimal value for a given requirement; Same question for post-choke capacitance, which brings us back to the question I posed earlier.
..Todd
Eli's suggestion to add an NTC thermistor is a great example. Boywonder has done wonders with PSUDII for other projects here, and I'd love to get some direction from him for work on that for my ST-70. My immediate questions: Is a hash choke worthwhile? How does pre-choke capacitance affect the voltage and is there any equations to apply to determine an optimal value for a given requirement; Same question for post-choke capacitance, which brings us back to the question I posed earlier.
..Todd
Taj: One way to get the B+ voltage down using SS rectification is to use a CLC and reduce the value of the first cap until you get the desired B+ voltage. C1 usually ends up somewhere between 1uf (or less) and 10uf. PSUDII is your friend here, model the stock 5AR4 supply in PSUD, adjust transformer parameters and load current to get the stock B+, then change the rectification in PSUD to FWCT SS. The B+ should be higher by around 40V or so. Then reduce the value of C1 to get back to the original B+ voltage with the same load.
You could also use a higher inductance choke, which will have higher DCR for a given current rating; that will also drop some volts, and provide less ripple V.
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One disadvantage of doing this is the extra power being wasted by the tube. Both the heater current, and the voltage dropped across the tube. ST-70 driver boards are typically designed to replace the existing driver board and use the rest of the amp as-is.
I guess in a perfect world (one that meets my requirements at least,) an SS (diode) power supply, driver board, operating points, and output bias scheme would be all designed together, to work well together.
I'll keep learning until I can do this.
..Todd
Thanks BW. I don't have my workbench back yet (it's still a home-reno sawdust generating/storage room), so I can't measure the DCR of the power transformer or choke. Do you happen to know those values?
..Todd
Why use a SS diode? I see no good reason. Get a Mullard GZ34, put a Pearl tube cooler on it, relax, and enjoy the music. Or just replace them as they go.
Yes, ST-70s eat new production 5AR4/GZ34 for lunch. Especially the Chinese ones. The amp runs the rectifier right at the limit, and only the old production tubes can handle it. (The current Reflector-built (Sovtek) 5AR4s seem decent, however...) The tube cooler really helps with the life of the tube. I have been running a Mullard GZ34 in mine for 8 years now. It has outlived 2 sets of output tubes!!!
I would also whole-heartedly suggest and recommend the Welborne Labs upgrade. Welborne Labs
Mine has been wonderful for the last 12 years, and never fails to impress. let my say this - it is the one single piece of hi-fi gear that I will not sell! It addresses all the problems of the original - driver, bias, psu... really all that remains of the original is the chassis and transformers... but that is a good thing!! The ST-70, as good as it is, was built to a very tight budget and has some interesting engineering choices, that served a purpose in the day, but are now obsolete. (I'm thinking of the bias)
The good news is that you can't really go wrong with an ST-70. It is great amp, and a great place to dive in and have fun with DIY. Enjoy your journey!
Yes, ST-70s eat new production 5AR4/GZ34 for lunch. Especially the Chinese ones. The amp runs the rectifier right at the limit, and only the old production tubes can handle it. (The current Reflector-built (Sovtek) 5AR4s seem decent, however...) The tube cooler really helps with the life of the tube. I have been running a Mullard GZ34 in mine for 8 years now. It has outlived 2 sets of output tubes!!!
I would also whole-heartedly suggest and recommend the Welborne Labs upgrade. Welborne Labs
Mine has been wonderful for the last 12 years, and never fails to impress. let my say this - it is the one single piece of hi-fi gear that I will not sell! It addresses all the problems of the original - driver, bias, psu... really all that remains of the original is the chassis and transformers... but that is a good thing!! The ST-70, as good as it is, was built to a very tight budget and has some interesting engineering choices, that served a purpose in the day, but are now obsolete. (I'm thinking of the bias)
The good news is that you can't really go wrong with an ST-70. It is great amp, and a great place to dive in and have fun with DIY. Enjoy your journey!
You seem out of touch with important facts. Ron Welborne has been involved in bad behavior, taking people's money and not shipping what was paid for.
Mullard made GZ34s are SCARCE and VERY expensive. Even the 5AR4s Matsushita made with Mullard tooling have become difficult to source and costly when found. Across the board, quality NOS is fast disappearing and current production tubes are the only realistic option for popular types. HIGH PIV Silicon carbide (SiC) Schottky diodes are every bit as quiet as vacuum rectifiers and, unlike current production 5AR4s, they are reliable.
I have dealt with the man off and on for 12 or so years and have had no problem. That is what I know first hand. Have you, personally, had a problem with him? I have not.
Regardless, the Allen Kimmel designed upgrade kit that he sells has performed admirably for me for all that time.
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So far in this thread I have learned that it is essential to use a Mullard $125
5AR4 rectifier tube, that if I go to SS rectification then I need to slow down the voltage inrush to avoid what must be cathode stripping and that I need to do a bias modification that results in replacing the stock 15 ohm resistor with a 100 ohm one in order to lower the output tube strain.
Well, god knows that I am no expert unlike some here but I have formed my opinion on the above based on my slim to no experience I've only built up a half dozen stereo 70's some here have built a lot more. I have two Mullards sitting on my shelf and I can tell you that they go poof also. I use two diodes in series with the AC feeding the 5AR4 and no more poof and I would rather use two 7 cent diodes than gamble a $125 tube. I've also used 5U4's and I've used 5U4's in conjunction with a damper diode tube, SS rectification straight and about anything else I could fit on the chassis. The result has been NO cathode stripping. I have used the higher B+ available as a result of SS rectification and I lowered the bias and the output tubes work within recognized guide lines. I also made the needed resistor changes to make my non-stock driver board voltages correct. I don't now or will I buy the idea that a large cathode resistor will do anything of value except to raise distortion to a higher figure. I believe that Broskie (of the Aikido fame) may have miss spelled his name has an article that explains that this practice does result in higher distortion. At least I believe I read something there. I'm sure that someone here will look it up and post it.
So I guess it all boils down to use what you want and use what you feel works the best for you. At least this is my opinion. I don't feel that there is a best rectifier tube, a best driver board, certainly anything works better than the stock board again my opinion, we could argue about power supply boards and or capacitence increases for example or if adding an additional choke in the power supply so that each channel was the best way to go.
I have learned one thing since purchasing my first stereo 70 and that is they sound very bad in stock form but there is hope for them and you can end up with a good sounding amplifier by modding the hell out of it. Keep the chassis or better yet purchase a new stainless steel chassis, keep the output transformers and junk everything else and then start building.
5AR4 rectifier tube, that if I go to SS rectification then I need to slow down the voltage inrush to avoid what must be cathode stripping and that I need to do a bias modification that results in replacing the stock 15 ohm resistor with a 100 ohm one in order to lower the output tube strain.
Well, god knows that I am no expert unlike some here but I have formed my opinion on the above based on my slim to no experience I've only built up a half dozen stereo 70's some here have built a lot more. I have two Mullards sitting on my shelf and I can tell you that they go poof also. I use two diodes in series with the AC feeding the 5AR4 and no more poof and I would rather use two 7 cent diodes than gamble a $125 tube. I've also used 5U4's and I've used 5U4's in conjunction with a damper diode tube, SS rectification straight and about anything else I could fit on the chassis. The result has been NO cathode stripping. I have used the higher B+ available as a result of SS rectification and I lowered the bias and the output tubes work within recognized guide lines. I also made the needed resistor changes to make my non-stock driver board voltages correct. I don't now or will I buy the idea that a large cathode resistor will do anything of value except to raise distortion to a higher figure. I believe that Broskie (of the Aikido fame) may have miss spelled his name has an article that explains that this practice does result in higher distortion. At least I believe I read something there. I'm sure that someone here will look it up and post it.
So I guess it all boils down to use what you want and use what you feel works the best for you. At least this is my opinion. I don't feel that there is a best rectifier tube, a best driver board, certainly anything works better than the stock board again my opinion, we could argue about power supply boards and or capacitence increases for example or if adding an additional choke in the power supply so that each channel was the best way to go.
I have learned one thing since purchasing my first stereo 70 and that is they sound very bad in stock form but there is hope for them and you can end up with a good sounding amplifier by modding the hell out of it. Keep the chassis or better yet purchase a new stainless steel chassis, keep the output transformers and junk everything else and then start building.
So far in this thread I have learned that it is essential to use a Mullard $125
5AR4 rectifier tube, that if I go to SS rectification then I need to slow down the voltage inrush to avoid what must be cathode stripping and that I need to do a bias modification that results in replacing the stock 15 ohm resistor with a 100 ohm one in order to lower the output tube strain.
Well, god knows that I am no expert unlike some here but I have formed my opinion on the above based on my slim to no experience I've only built up a half dozen stereo 70's some here have built a lot more. I have two Mullards sitting on my shelf and I can tell you that they go poof also. I use two diodes in series with the AC feeding the 5AR4 and no more poof and I would rather use two 7 cent diodes than gamble a $125 tube. I've also used 5U4's and I've used 5U4's in conjunction with a damper diode tube, SS rectification straight and about anything else I could fit on the chassis. The result has been NO cathode stripping. I have used the higher B+ available as a result of SS rectification and I lowered the bias and the output tubes work within recognized guide lines. I also made the needed resistor changes to make my non-stock driver board voltages correct. I don't now or will I buy the idea that a large cathode resistor will do anything of value except to raise distortion to a higher figure. I believe that Broskie (of the Aikido fame) may have miss spelled his name has an article that explains that this practice does result in higher distortion. At least I believe I read something there. I'm sure that someone here will look it up and post it.
So I guess it all boils down to use what you want and use what you feel works the best for you. At least this is my opinion. I don't feel that there is a best rectifier tube, a best driver board, certainly anything works better than the stock board again my opinion, we could argue about power supply boards and or capacitence increases for example or if adding an additional choke in the power supply so that each channel was the best way to go.
I have learned one thing since purchasing my first stereo 70 and that is they sound very bad in stock form but there is hope for them and you can end up with a good sounding amplifier by modding the hell out of it. Keep the chassis or better yet purchase a new stainless steel chassis, keep the output transformers and junk everything else and then start building.
99% of the GZ34 failures can be traced to a few factors....
The original tubes data sheet specifies a minimum of 100R DCR equivalent for the power supply if 40uf of input capacitance is used without a choke to prevent arcing at full power start up. If a choke is used then less DCR is necessary. If the input capacitance is less than 40uf then even less DCR is necessary. This is why an original ST-70 with an old GZ34 is reliable with about 50R DCR power supply equivalent, ...AT 117v....
Now that line voltages are nearly 125v. when people mod the amps and add input capacitance they are asking for trouble from the GZ34.
If the new production GZ34's are inferior to the old stock tubes then the risk of arcing goes up again.
The solution is to get the input capacitance back as designed, and to counter the higher line voltage somehow by either adding a power resistor to the PT centertap to bring the powersupply DCR up to the GZ34 requirement or swap to a tube with a higher voltage drop and arc resistance and having 2A filament current requirements, like a 5R4 type.
My stock ST-70 sounds absolutely wonderful. I marvel at its state of the art fidelity and compare it to my Sansui G-6000 every time I listen to it with SACD, LP, DVD-A source material through some 93db Technics speakers.
The slippery slope of modding can start a snowball of requirements. Since the line voltages are now higher than in 1959 the B+ is higher. When a solid state mod is performed the B+ goes even higher and risks peaking above the input capacitor voltage rating. Those conditions then have to be countered with additional mods.... The snowball grows and soon you don't have an amp the even closely resembles an ST-70.
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The solution is to get the input capacitance back as designed, and to counter the higher line voltage somehow by either adding a power resistor to the PT centertap to bring the powersupply DCR up to the GZ34 requirement or swap to a tube with a higher voltage drop and arc resistance and having 2A filament current requirements, like a 5R4 type.
Well, I guess it would make a lot of sense to use a 5R4 and then drop the cap to something in the 5mfd range to avoid arch over or maybe use two 7 cent diodes and take the strain off the 5Ar4 while still using the slow start capabilities. Any run of the mill 5AR4 will work and live after modifying the power supply by adding two little diodes. The little increase in B+ won't hurt the output stage but should be dealt with if need with respect to the driver board.
Never heard a stock 70 that I felt like listening to period. Mod the hell out of them and you finally have something worth turning on. Then again its my opinion.
The solution is to get the input capacitance back as designed, and to counter the higher line voltage somehow by either adding a power resistor to the PT centertap to bring the powersupply DCR up to the GZ34 requirement or swap to a tube with a higher voltage drop and arc resistance and having 2A filament current requirements, like a 5R4 type.
Well, I guess it would make a lot of sense to use a 5R4 and then drop the cap to something in the 5mfd range to avoid arch over or maybe
use two 7 cent diodes and take the strain off the 5Ar4 while still using the slow start capabilities. Any run of the mill 5AR4 will work and live after modifying the power supply by adding two little diodes. The little increase in B+ won't hurt the output stage but should be dealt with if need with respect to the driver board.Never heard a stock 70 that I felt like listening to period. Mod the hell out of them and you finally have something worth turning on. Then again its my opinion.
I'm not getting how a diode on the secondary limits the inrush current when a diode is not a current limiting device. About all it it can do is drop the forward voltage the usual @.7v which is not the problem with inrush current. The reverse peaks are not the problem through the GZ34, which is what a diode would supplement.
A good old GZ34 doesn't need any help. However the new crop of reissues if they are truly subpar could benefit from a power resistor on the CT to bring the DCR up a bit, to make them happy.
Sorry, I misplaced my ST70 manual when the wife moved in. I'm getting ready to replace the B+ capacitors and I bought axial 47's and 20 mf's, that were in stock. Were does the 100 ohm resistor go, the 5v transformer winding or the 475 v winding?
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