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    VTA-70 Clone build - Power caps question.

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    JeffYoung

    Posts : 4
    Join date : 2014-10-21

    VTA-70 Clone build - Power caps question.

    Post by JeffYoung on Tue Sep 29, 2015 12:03 am

    Hi everyone...

    I am planning on building a ST-70 clone and have a question regarding the B+ capacitors.  I have some experience building guitar tube amps, but this will my first foray into HiFi.  I am planning on using a VTA-70 (octal) driver board.  For rectification, I'll be using a solid state board design that I have used in the past for guitar tube amps and a VTA-TDR to let the tubes warm up before B+ is applied.

    I have a CE 80/40/30/20 cap can or a blank Triode ST-70 cap board that I can use.  I'm not real happy with the 525v limit of the CE can... especially since I am using a SS rectifier.

    My question(s) are as follows...

    Since I am not using a tube rectifier, then I'm not limited to 40-60uF for stage 1, am l?

    Are my concerns about the 525 voltage limit of the 80/40/30/20 justified?  

    If the 80/40/30/20 is OK (voltage wise), would I be better to use the 80uF for the 1st stage, 40uF for 2nd, ..... Etc? Or should I connect it the standard 40/80/30/20 way? (With guitar amps, we 'usually' have the higher uF caps for the first stage, the lower ones for the following stages, when using SS rectifier)

    If the CE cap can is NOT the best solution, What would the 'best' capacitance for stages 1 through 4 be?   I can populate the Triode board with whatever radial caps (within reason) to come up the best uF for stages 1 through 4, and increase the voltage limit.

    Or.... Do you have a better solution?

    Sorry for the long post.... I just want to make the best HiFi amp using the VTA-70 octal driver, that I can.

    Thank you for your feedback.

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: VTA-70 Clone build - Power caps question.

    Post by peterh on Tue Sep 29, 2015 1:21 am

    JeffYoung,
    Define "best". Is it lowest cost, lowest hum or least space need ? Or is it reliability ?

    In my world, best is a mix of hum and reliability, thus i would use serial connected 400V caps
    with proper bleeding resistors for a B+ up to 800V without exceeding margins.
    This won't fit in a st70 case, and it will cost more. Then i would not need any time-delay ( + for reliability) . I would also skip the octal board and go for noval ( + reliability and ease of spares)

    But if you have space and cost concerns you will most likely design differently.

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: VTA-70 Clone build - Power caps question.

    Post by Bob Latino on Tue Sep 29, 2015 7:44 am

    The CE 80, 40, 30, 20 525 volt rated quad cap will work fine for ST-70 type B+ voltages. (430 - 450 VDC). Another option for a quad cap is the Dynakitparts 80, 40, 30, 20 550 volt rated quad cap that you could also use. This 550 volt cap is made in Germany and is more expensive than the CE cap but will also work fine.

    With either cap you should use the 40 uF section for the first section and the 80 section and the 20 section (wired together) to power the two output transformers IF you are using a VTA driver board.

    I also agree with Peter about the noval board vs. the octal board. If you are using a STOCK ORIGINAL Dynaco PA-060 power transformer, you should consider the noval driver board instead. The original Dynaco PA-060 power transformer was woefully undersized and ran pretty warm to begin with. The octal driver circuit uses double the current of the noval board and the amp will run even warmer. This is just my opinion now BUT - I would not use the octal driver board with an original Dynaco power transformer. What you might consider is to replace your original power transformer with the Dynakitparts version of the PA-060. This is an upgraded power transformer with a higher stack that runs much cooler. You could then sell your original power transformer on Ebay and recover maybe $50 - $75.

    Bob

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: VTA-70 Clone build - Power caps question.

    Post by PeterCapo on Tue Sep 29, 2015 9:01 am

    Either of the two versions of the quad cap should be fine.

    The 525VDC voltage rating of the CE quad cap is plenty if you are using a tube rectifier.  The question is exactly how much higher the B+ goes with purely solid state rectification.  If need be, you could bring it down by using one or more NTC thermistors in series with the power transformer primary, and/or resistors in with the solid state diodes to emulate the forward voltage drop of a 5AR4, and/or a new Dynakitparts power transformer that is wound with a 122VAC primary which will result in lower B+ than the original Dynaco power transformer.

    While I do not particularly appreciate Triode USA's recent idea of customer service, their "Upgrade Capacitor Board" for the Stereo 70 is quite good though a more complex affair.  It allows for greater energy storage with potentially better quality electrolytics (with datasheets available) and convenient placement of polypropylene bypass caps.  With the stock parts, its working voltage rating, while actually lower than the quad cap, is still fine with a tube rectifier.  Last I knew, the working voltage rating of sections two through four are 500VDC but with a surge rating of 600VDC.  Again, depending on how high the B+ goes with purely solid state rectification, and with some of the techniques mentioned above to reduce the B+, the voltage rating may not be an issue at all.  But, if you were nevertheless so inclined, you might be able to find different electrolytics that fit and have a higher working voltage rating, though you’d likely end up with a bit less energy storage.  This cap board is potentially a fine choice.

    Or, here's an idea from electrical engineering guru Dave Gillespie describing a way to use both solid state and tube rectification to good effect.  He suggests appropriately rated solid state diodes in each leg of the HV secondary before the 5AR4 and then tying rectifier pins 4 and 6 together to double the forward current capacity of the 5AR4.  Dave suggested actually using two solid state diodes per leg of the PT HV secondary before the 5AR4, though I don’t see why one per leg would not also work well.  He also said to tie a .05uF 600V or 1000V cap to ground from the joined #4 & 6 pins:

    http://www.audiokarma.org/forums/showthread.php?p=8760949#post8760949

    http://www.audiokarma.org/forums/showthread.php?p=9011018#post9011018

    JeffYoung

    Posts : 4
    Join date : 2014-10-21

    Re: VTA-70 Clone build - Power caps question.

    Post by JeffYoung on Tue Sep 29, 2015 3:37 pm

    peterh wrote:JeffYoung,
    Define "best".  Is it lowest cost, lowest hum or least space need ? Or is it reliability ?

    In my world, best is a mix of hum and reliability, thus i would use serial connected 400V caps
    with proper bleeding resistors for a B+ up to 800V without exceeding margins.
    This won't fit in a st70 case, and it will cost more. Then i would not need any time-delay ( + for reliability) . I would also skip the octal board and go for noval ( + reliability and ease of spares)

    But if you have space and cost concerns you will most likely design differently.

    Hi PeterH... Sorry, should have defined my desires better. What I am looking for is the best sound I can achieve using the parts that I already have. Best sound would include lowest hum with the best bass response. I know from building guitar tube amps that the uF of the different power stages can affect the bass response greatly. Often I would increase the capacitance of the first and second stage to 'firm' up the bass. This is why I was looking for what others have used when not limited to 40uF to 60uF that tube rectifiers require for the first stage.

    Unfortunately I already have the octal VTA-70 board in my possession and would like to use it.

    Thanks for your comments!

    JeffYoung

    Posts : 4
    Join date : 2014-10-21

    Re: VTA-70 Clone build - Power caps question.

    Post by JeffYoung on Tue Sep 29, 2015 4:12 pm

    Bob Latino wrote:The CE 80, 40, 30, 20 525 volt rated quad cap will work fine for ST-70 type B+ voltages. (430 - 450 VDC). Another option for a quad cap is the Dynakitparts 80, 40, 30, 20 550 volt rated quad cap that you could also use. This 550 volt cap is made in Germany and is more expensive than the CE cap but will also work fine.

    With either cap you should use the 40 uF section for the first section and the 80 section and the 20 section (wired together) to power the two output transformers IF you are using a VTA driver board.

    I also agree with Peter about the noval board vs. the octal board. If you are using a STOCK ORIGINAL Dynaco PA-060 power transformer, you should consider the noval driver board instead. The original Dynaco PA-060 power transformer was woefully undersized and ran pretty warm to begin with. The octal driver circuit uses double the current of the noval board and the amp will run even warmer. This is just my opinion now BUT - I would not use the octal driver board with an original Dynaco power transformer. What you might consider is to replace your original power transformer with the Dynakitparts version of the PA-060. This is an upgraded power transformer with a higher stack that runs much cooler. You could then sell your original power transformer on Ebay and recover maybe $50 - $75.

    Bob

    Hi Bob... Thank you for your comments...

    There is nothing about this Frankenstein amp that has anything original, except for the choke I purchased! This will be built from 'stuff' that I have acquired. Here are the parts that I have currently available to build this beast!

    The PT I already have is an Edcor XPWR233... it's a 360-55-0-360 (300mA) PT. It has a single 6.3V (5A) and a 5V (4A) secondaries. I know I'll need an additional 6.3V (I have a 4A sitting here) to power the rest of the filaments. Similar specs to the Dynakit PS060.

    The OTs are Edcor CXPP60-MS-4.2K. They are 60W, 4.2K primary with a 40% screen tap...4/8/16 secondaries.

    I do have a C-354 choke on order.

    The SS rectifier board I use has the capability to add different resistor to drop the B+ to where I want it to be. I figured the VTA-TDR would be a good way to allow the power tubes to warm up before the B+ came online.

    I have a large box of various Nichicon KX (450V) caps that I can build various power stage Uf's by mounting them in series on the Triode ST-70 cap board.

    If you think that the 40/80/30/20 is the optimal uF for the power stages, then that is the way I can go. And I do understand that this whole thing might end up the noisiest, hum monster in town... Razz

    Thanks for your input!

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: VTA-70 Clone build - Power caps question.

    Post by peterh on Tue Sep 29, 2015 4:51 pm

    JeffYoung wrote:
    peterh wrote:JeffYoung,
    Define "best".  Is it lowest cost, lowest hum or least space need ? Or is it reliability ?

    In my world, best is a mix of hum and reliability, thus i would use serial connected 400V caps
    with proper bleeding resistors for a B+ up to 800V without exceeding margins.
    This won't fit in a st70 case, and it will cost more. Then i would not need any time-delay ( + for reliability) . I would also skip the octal board and go for noval ( + reliability and ease of spares)

    But if you have space and cost concerns you will most likely design differently.

    Hi PeterH... Sorry, should have defined my desires better.  What I am looking for is the best sound I can achieve using the parts that I already have.  Best sound would include lowest hum with the best bass response.  I know from building guitar tube amps that the uF of the different power stages can affect the bass response greatly.  Often I would increase the capacitance of the first and second stage to 'firm' up the bass.  This is why I was looking for what others have used when not limited to 40uF to 60uF that tube rectifiers require for the first stage.

    Unfortunately I already have the octal VTA-70 board in my possession and would like  to use it.

    Thanks for your comments!

    That clears it up.

    As regards to hum, it's usually not a problem in a poweramp like this, the push-pull pair
    is naturally immune, providing the el34 are matched. Hum might get into play
    at any anbalanced stage, and the first triode is most likley that point. But that one comes
    last in the B+.
    Another common source of hum is (bad)grounding and maybe less then optimium cable ways.

    There is a point of less improvement, and for a 70w amp 100-200uF could be close to optimum,
    to much might give trouble, see :
    http://www.tronola.com/html/maximize_tube_life.html


    JeffYoung

    Posts : 4
    Join date : 2014-10-21

    Re: VTA-70 Clone build - Power caps question.

    Post by JeffYoung on Tue Sep 29, 2015 11:19 pm

    peterh wrote:
    .
    .
    There is a point of less  improvement, and for a 70w amp 100-200uF could be close to optimum,
    to much might give trouble, see  :
    http://www.tronola.com/html/maximize_tube_life.html


    Thanks PeterH... Great article.... I've stored it in my Amp Build library!

    100-200uF is in the same range that I use in guitar tube amps, for the first stage. I've got some Nichicon KX (450V) caps that I can put in series to get 110, 75, 60, 50, 41, 34, and 28uF at 900V. I'll set up something like 110/75/50/34.

    Thanks for your help!

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