VTA-70 Clone build - Power caps question.

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.

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.

.

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!

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...

Thanks for your input!

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

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!