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    ST70 power supply questions

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    ureche

    Posts : 9
    Join date : 2016-09-11

    ST70 power supply questions

    Post by ureche on Mon Sep 12, 2016 4:13 am

    Hello everybody,

    I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

    I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

    So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

    Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

    Thank you.

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST70 power supply questions

    Post by peterh on Mon Sep 12, 2016 4:36 am

    ureche wrote:Hello everybody,

    I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

    I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

    So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

    Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

    Thank you.

    Yes and yes.

    The rectifier ( if you use tube ) is the limiting component in the st-70 power supply, doubling
    the number ( and reducing the loading on each) will lift this.
    Don't forget that you need 5 independent filament windings in that case.
    Maybe the simplest and topologically easiest is to use 2 separate transformers ...

    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Mon Sep 12, 2016 5:20 am

    But are there any sonic advantages for using two 720v secondaries instead of one with higher current and maybe two rectifiers in parallel ?

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST70 power supply questions

    Post by Bob Latino on Mon Sep 12, 2016 10:17 am

    peterh wrote:
    The rectifier ( if you use tube ) is the limiting component in the st-70 power supply

    If you reduce the bias to 40 milliamps per each output tube then this is not true and is one of the most often repeated falsehood about the original Dynaco ST-70. What is true is that Dynaco (for whatever reason) chose to operate the amp right at the limit of a 5AR4's ability to supply DC current. A 5AR4/GZ34 can supply steady state about 225 milliamps of DC current. Dynaco operated the 4 EL34 output tubes at 50 milliamps each which is 200 milliamps. I am guessing each of the two 7199 driver tubes used maybe 8 to 10 milliamps each SO > we are up to near 220 milliamps. Yes - this is at the limit of a 5AR4's ability to supply DC current. To get around this all you have to do is reduce the bias level on the original Dynaco ST-70 to 40 milliamps per each output tube. To this, set the bias level at about 1.25 VDC for each channel instead of 1.56 VDC per channel.

    The key thing that limits overall power in an original Dynaco ST-70 is the lack of DC storage. Dynaco use that 30, 20, 20, 20 microfarad quad cap to run the ENTIRE amp except for the bias system. Of that 40 microfarads (20 + 20) was used to run the driver board and the remaining 50 microfarads (30 + 20) was used to power the primaries on the two output transformers. Using just 50 uF for two output transformers gave just 25 uF per each output transformer. Set up this way the original Dynaco ST-70 could only make 20 to 25 WPC before it "ran out of gas". The original ST-70 would make a cracking noise on loud passages. Years ago I would play Strauss's "Also Sprach Zarathustra" on my original ST-70 and above a certain volume level the drum strikes would make that cracking noise on each drum beat. I proceeded to add extra capacitance (about another 50 uF) to the second section of the quad cap much like the SCM is now in the VTA ST-120 and VTA M-125. The amp would now play much louder before the cracking noise started.

    On the VTA ST-70 we use an 80, 40, 30, 20 uF quad cap and a driver board which has its own power supply. The driver board on the VTA ST-70 does not have to draw DC from the quad cap. This leaves the quad cap free to power both output transformers or about 85 uF per each of the two output transformers which is about 3+ times the 25 uF per each output transformer on the original Dynaco ST-70. Set up this way, the VTA ST-70 will make its power level of 35 WPC from 20 Hz to 20 KHz. The VTA ST-70 can put out about 45 WPC @ 1000 Hz in pentode ultralinear mode before clipping. See test link below.

    Independent testing of the VTA ST-70 amp

    Bob

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: ST70 power supply questions

    Post by deepee99 on Mon Sep 12, 2016 10:55 am

    ureche wrote:Hello everybody,

    I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

    I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

    So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

    Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

    Thank you.
    Why re-invent the wheel? Bob and Roy have been there, done that. It would be my advice to buy their kit.

    Dave_in_Va

    Posts : 128
    Join date : 2013-04-02

    Re: ST70 power supply questions

    Post by Dave_in_Va on Mon Sep 12, 2016 12:21 pm

    Just my own experience but when I tried a set of Gold Lion KT 88's in my VTA ST70 I didn't think they sounded as nice as the Valve Art EL34's that came with the amp. I sold the KT 88's. Maybe they are a better fit for the VTA ST120.
    I eventually ended up with NOS Mullard EL34's and that was an improvement over the Valve Arts.

    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Mon Sep 12, 2016 12:35 pm

    deepee99 wrote:
    ureche wrote:Hello everybody,

    I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

    I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

    So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

    Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

    Thank you.
    Why re-invent the wheel? Bob and Roy have been there, done that. It would be my advice to buy their kit.

    I would have bought the kit in a heartbeat, but as i am located in Europe, the shipping cost and customs tax are setting me back a lot.

    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Mon Sep 12, 2016 1:04 pm

    Bob Latino wrote:
    If you reduce the bias to 40 milliamps per each output tube then this is not true and is one of the most often repeated falsehood about the original Dynaco ST-70. What is true is that Dynaco (for whatever reason) chose to operate the amp right at the limit of a 5AR4's ability to supply DC current. A 5AR4/GZ34 can supply steady state about 225 milliamps of DC current. Dynaco operated the 4 EL34 output tubes at 50 milliamps each which is 200 milliamps. I am guessing each of the two 7199 driver tubes used maybe 8 to 10 milliamps each SO > we are up to near 220 milliamps. Yes - this is at the limit of a 5AR4's ability to supply DC current. To get around this all you have to do is reduce the bias level on the original Dynaco ST-70 to 40 milliamps per each output tube. To this, set the bias level at about 1.25 VDC for each channel instead of 1.56 VDC per channel.
    [/size]

    I saw in a post somewhere, the recommended bias for KT88 in ST70 would be 50ma. So i was wondering if setting the bias to 40ma would have an impact on the sonic output?

    Bob Latino wrote:
    On the VTA ST-70 we use an 80, 40, 30, 20 uF quad cap and a driver board which has its own power supply. The driver board on the VTA ST-70 does not have to draw DC from the quad cap. This leaves the quad cap free to power both output transformers or about 85 uF per each of the two output transformers which is about 3+ times the 25 uF per each output transformer on the original Dynaco ST-70. Set up this way, the VTA ST-70 will make its power level of 35 WPC from 20 Hz to 20 KHz. The VTA ST-70 can put out about 45 WPC @ 1000 Hz in pentode ultralinear mode before clipping. See test link below.
    [/size]

    As i'm trying to build the amp from scratch i will go with the cap configuration you have outlined.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST70 power supply questions

    Post by Bob Latino on Mon Sep 12, 2016 1:13 pm

    ureche wrote:

    I saw in a post somewhere, the recommended bias for KT88 in ST70 would be 50ma. So i was wondering if setting the bias to 40ma would have an impact on the sonic output?


    You can use KT88's in the VTA ST-70. Understand though that if you do use a KT88 in the VTA ST-70, the KT88 is not running in a range that is fully optimal for this tube. A KT88 does work better in the VTA ST-120 with its higher B+ voltages ..

    Bob

    MarcVBelgium

    Posts : 81
    Join date : 2013-12-06
    Age : 62
    Location : Antwerp, BELGIUM

    st70-power-supply-questions

    Post by MarcVBelgium on Mon Sep 12, 2016 2:15 pm

    ureche wrote:
    deepee99 wrote:
    ureche wrote:Hello everybody,

    I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

    I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

    So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

    Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

    Thank you.
    Why re-invent the wheel? Bob and Roy have been there, done that. It would be my advice to buy their kit.

    I would have bought the kit in a heartbeat, but as i am located in Europe, the shipping cost and customs tax are setting me back a lot.


    Hey Ureche,
    I live in Europe (Belgium) too. I have the ST-120 from Bob.....With shipment, import duties and VAT it comes to approx. 1.750 €
    with tubes (matched quad of KT-120)
    All I can say is that it is a magnificent amplifier and.... doing things right from the start, usually saves money !
    There is no tube amplifier that I know of that is this good at this price level.

    I can recommend the Bob Latino kit.....

    All the best, MarcVBelgium


    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: ST70 power supply questions

    Post by deepee99 on Mon Sep 12, 2016 2:32 pm

    MarcVBelgium wrote:
    ureche wrote:
    deepee99 wrote:
    ureche wrote:Hello everybody,

    I'm planning to build an ST70 from scratch, most likely i will use the KT88 tubes.

    I'm going to order the power transformer locally, and i want to clarify a few things before, as i can custom order the pt to my specs.

    So first question, would be better to order the 720VCT (360-0-360) winding with 400ma rating instead of 300ma ?

    Are there any advantages to build the pt with two 720VCT secondary winding and 200ma current rating each, and use one for each channel? And of course double the power supply circuit (two caps, two rectifiers).

    Thank you.
    Why re-invent the wheel? Bob and Roy have been there, done that. It would be my advice to buy their kit.

    I would have bought the kit in a heartbeat, but as i am located in Europe, the shipping cost and customs tax are setting me back a lot.


    Hey Ureche,
    I live in Europe (Belgium) too. I have the ST-120 from Bob.....With shipment, import duties and VAT it comes to approx. 1.750 €
    with tubes (matched quad of KT-120)
    All I can say is that it is a magnificent amplifier and.... doing things right from the start, usually saves money !
    There is no tube amplifier that I know of that is this good at this price level.

    I can recommend the Bob Latino kit.....

    All the best, MarcVBelgium

    I would second MarcVBelgium's comments. Consider the time you're going to spend finding parts, chassis, and transformers alone and bill yourself an hourly rate for all that effort. It will easily better the VAT and customs charges even if you're paying yourself minimum wage. I built a pair of Bob's M-125s and they are easy to assemble. Yes, I got stumped a couple of times but Bob is an email away and never seems to sleep, so you get an answer pretty quickly and a wonderful-sounding amp at the end with bullet-proof chassis. Plus you get Roy's boards which are top-drawer. I would think in Europe you could find tubes for them very easily, locally. There's a lot of good former military tubes out there. Bob's chassis and transformers, and Roy's boards, are hard to duplicate.

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST70 power supply questions

    Post by peterh on Mon Sep 12, 2016 4:21 pm

    I'll aggree with both Marc and depee, by the kit and you will have every little component.
    Tubes can be bought in europe ( they are made there :-)

    Save your build desires for you next amp !

    Tube Nube

    Posts : 603
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: ST70 power supply questions

    Post by Tube Nube on Mon Sep 12, 2016 5:58 pm

    If you watched that mk III restoration video on you tube that was posted on this forum 2 weeks ago, you can steal the only good idea in the video, which was the way the fellow used a spreadsheet to detail a few different cost projections for his rebuild. Like him, you could incorporate links to all the suppliers.

    Its a fun way to use spreadsheets, if that kind of thing turns your crank. You'll have all your research organized and stored in one place, so you can fret and deliberate, and come back to it later without having to re-assemble your understanding of where you were when you left it.


    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Tue Sep 13, 2016 12:03 pm

    Tube Nube wrote:If you watched that mk III restoration video on you tube that was posted on this forum 2 weeks ago, you can steal the only good idea in the video, which was the way the fellow used a spreadsheet to detail a few different cost projections for  his rebuild. Like him, you could incorporate links to all the suppliers.

    Its a fun way to use spreadsheets, if that kind of thing turns your crank. You'll have all your research organized and stored in one place, so you can fret and deliberate, and come back to it later without having to re-assemble your understanding of where you were when you left it.


    Didn't know about the video, i will be sure to watch it. I will be doing a spreadsheet with all the parts, source, shipping costs etc. But first i have to sort all the facts. Smile

    Tube Nube

    Posts : 603
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: ST70 power supply questions

    Post by Tube Nube on Tue Sep 13, 2016 12:09 pm

    Here's a link to the thread with the video link, as well as a few posts with constructively critical thoughts on the video from a couple of forum members:


    http://dynacotubeaudio.forumotion.com/t3127-mkiii-restoration-video

    I do like the guy's spreadsheet!

    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Tue Sep 13, 2016 12:19 pm

    peterh wrote:I'll aggree with both Marc and depee, by the kit and you will have every little component.
    Tubes can be bought in europe ( they are made there :-)

    Save your build desires for you next amp !

    I agree with you and Marc and depee Smile, the problem here are the money. But, i won't start the venture until November or December, in the meantime i'll try to raise the money for the kit. It would be a lot easier than sourcing the parts from 10 suppliers, also building the case will be challenging.

    deepee99 wrote:
    I would second MarcVBelgium's comments. Consider the time you're going to spend finding parts, chassis, and transformers alone and bill yourself an hourly rate for all that effort. It will easily better the VAT and customs charges even if you're paying yourself minimum wage. I built a pair of Bob's M-125s and they are easy to assemble. Yes, I got stumped a couple of times but Bob is an email away and never seems to sleep, so you get an answer pretty quickly and a wonderful-sounding amp at the end with bullet-proof chassis. Plus you get Roy's boards which are top-drawer. I would think in Europe you could find tubes for them very easily, locally. There's a lot of good former military tubes out there. Bob's chassis and transformers, and Roy's boards, are hard to duplicate.

    I wont be trying to replicate Roy's boards or anything like that. Even if i chose to build the amp by sourcing some of the parts myself, i will be buying Roy's board with the parts. For the opts i would be going with Edcor cxpp60-ms-4.2k. For the PT would be going locally, as i have a company who can build the PT according to my specs and it would cost ~ $70.

    Anyway, i have to make a list with all the parts, costs and see if it is worth the trouble.

    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Tue Sep 13, 2016 12:26 pm

    Bob Latino wrote:
    ureche wrote:

    I saw in a post somewhere, the recommended bias for KT88 in ST70 would be 50ma. So i was wondering if setting the bias to 40ma would have an impact on the sonic output?


    You can use KT88's in the VTA ST-70. Understand though that if you do use a KT88 in the VTA ST-70, the KT88 is not running in a range that is fully optimal for this tube. A KT88 does work better in the VTA ST-120 with its higher B+ voltages ..

    Bob

    As i didn't order the PT and i can order one according to my specs. Theoretically speaking, what if i order one with 400V-0-400V main voltage? This will raise the B+. Then use a zener or increase R15 to lower the V going to the board, back to ~430V.

    Are there any other aspects that i have to take into consideration ?

    Tube Nube

    Posts : 603
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

    Re: ST70 power supply questions

    Post by Tube Nube on Tue Sep 13, 2016 12:27 pm

    As you construct your build plan, or plans, I hope you'll considering posting them here for input. Members might know of alternate parts sources that can save you money, or offer other helpful suggestions. Good luck with planning this out. It can be an exciting phase of a project so long as you have more patience than me.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST70 power supply questions

    Post by Bob Latino on Tue Sep 13, 2016 1:20 pm

    ureche wrote:

    As i didn't order the PT and i can order one according to my specs. Theoretically speaking, what if i order one with 400V-0-400V main voltage? This will raise the B+.

    Are there any other aspects that i have to take into consideration ?

    You can use a custom transformer to raise the secondary voltage to 400-0-400 BUT > This attempt to increase B+ and power will be defeated by the fact that the original Dynaco ST-70 output transformers OR any replacement Dynaco ST-70 output transformers will saturate at low frequencies at 40 to 45 watts. You won't really be able to increase the power of the amp unless you use larger output transformers that will not saturate at low frequencies and higher volume levels. If you do get a custom power transformer with 400-0-400 secondaries, you should use a pair of Edcor CXPP60-4.2K output transformers. (about $85 each + shipping). The Edcor CXPP60-4.2K along with a 400-0-400 PT will allow the amp to reach 60 WPC.

    BTW, if you intend to do a scratch build like you mentioned in your first post and are seriously considering larger power and output transformers, it will probably cost you less money to just purchase one of our VTA ST-120 amp kits. I am pretty sure this will cost noticeably less than a scratch build where your source all the parts on your own ...

    Bob

    Tube Nube

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    Location : Calgary, AB

    Re: ST70 power supply questions

    Post by Tube Nube on Tue Sep 13, 2016 3:02 pm

    This is the kind of expert input you can count on here!

    I second the wisdom of aiming for the VTA ST 120 kit. It'll be an interesting exercise (for us all) if you source the parts or some of the parts, vs. a complete kit (or complete kit, sans tubes).

    The way it seems to be shaping up, youre comparing a kit against the cost of

    Roy's driver board (including parts) and the additional costs of sourcing:
    -chassis
    -trannies
    -components not on the board (choke, a few caps & resistors, fuse & hardware, tube sockets).

    A completely 'nother way of going about this might be to check the for sale thread, or appealing on the forum for anyone eager to sell what they've got in service of pursuing their own upgrade.

    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Fri Sep 23, 2016 6:13 am

    Hi, i was wondering, if there is any sonic difference in using a solid state vs tube rectifiers?

    I was thinking of going with ss rectifiers, this will also have an impact on PT cost, as i get rid of the 5v heater.

    So, if i go solid state, i will add a switch for B+, or a 60 seconds delay circuit.

    Should i increase the capacitance ? What would be the recommended values for 80/40/20/30 ?

    Thanks,

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST70 power supply questions

    Post by peterh on Fri Sep 23, 2016 7:17 am

    ureche wrote:Hi, i was wondering, if there is any sonic difference in using a solid state vs tube rectifiers?

    I was thinking of going with ss rectifiers, this will also have an impact on PT cost, as i get rid of the 5v heater.

    So, if i go solid state, i will add a switch for B+, or a 60 seconds delay circuit.

    Should i increase the capacitance ? What would be the recommended values for 80/40/20/30 ?

    Thanks,
    A 60s delay _might_ create more problems then it solves . Think for a moment what
    happens when 60s has occured, all cathodes are hot, all electron clouds are
    saturated. Then a sudden surge of B+ will cause all coupling caps to charge, but as they
    are uncharged they will for a moment drive the el34 grids to a positive voltage, a
    surge will flow in the el34. Not good. Especially if someone has "improved" the circuit
    with larger coupling caps. The above will cause surges and eventually flash-over.
    The same goes for a coupling switch for B+.

    The beauty with rectifier tubes is that they will gradually increase B+, and so
    slightly ahead of the cathodes heating, coupling caps will charge slow.

    The next best is to apply B+ at once and have the tubes gradually start conducting.

    Look around. Lots of amps ( or devices) uses diodes and no delays, with no harm.

    ureche

    Posts : 9
    Join date : 2016-09-11

    Re: ST70 power supply questions

    Post by ureche on Fri Sep 23, 2016 8:49 am

    I read somewhere, that ss rectifier on cold tubes could lead to something like cathode stripping. Well i didn't find a post, yet, where this has happened, but i thought i should do the delay to be safe.

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST70 power supply questions

    Post by peterh on Fri Sep 23, 2016 11:39 am

    ureche wrote:I read somewhere, that ss rectifier on cold tubes could lead to something like cathode stripping. Well i didn't find a post, yet, where this has happened, but i thought i should do the delay to be safe.
    It's true - when tubes use B+ of above 1000V . But not with our "audio tubes".

    The one thing that could be of concern is that the B+ caps might get higher then usual
    voltage during the time between power on and the moment the power tubes will draw
    full current. The sensible solution to this would be a bleedresistor that is
    controlled via a powertransistor, ( not a relay but a device that gradually turns off )
    But most caps have enough voltage reserve to keep things running.

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