Volume on an ST-70

knowledge base for the classic Dynaco ST70

Volume on an ST-70

Postby Clik2media » Wed Jan 06, 2010 7:08 pm

I find that I only start to "hear" music when my volume control is at about half way, this is true for my PAS, I swithed pre-amp to a McIntosh MX113 and the volume was the same - started at about half way. That being said I suspected my 7199 tubes, so I switched for another pair, then another pair. ......same results volume starts to build at about half way up the dial. Any thoughts

cheers
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Postby TomMcNally » Wed Jan 06, 2010 8:11 pm

Is this a newly built ST-70 ?

Did you set the Bias correctly ? If the tubes are
running too cold, you may experience that kind of
action ... nothing happens until you drive them hard.

Check the bias and make sure you understand what
you should be getting.

I don't know what variety of ST-70 or driver board
you are using, so I can't add much more.

Also, if it's a new build, double check your cathode
resistors and make sure they are 10 ohms, not 100 or 1000 !

... tom
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Postby Clik2media » Wed Jan 06, 2010 8:18 pm

It is a stock Dynaco, nothing new except a "new board" with new parts and the Auricaps I spoke of and got your advice on polarity orientation.
I will put back the original Dynaco 7199 tubes and check my bias [15.6?] volts right? Not sure where to check the cathode resistors but I will consult the manual to see what I can dig up.

thanks very much

TomMcNally wrote:Is this a newly built ST-70 ?

Did you set the Bias correctly ? If the tubes are
running too cold, you may experience that kind of
action ... nothing happens until you drive them hard.

Check the bias and make sure you understand what
you should be getting.

I don't know what variety of ST-70 or driver board
you are using, so I can't add much more.

Also, if it's a new build, double check your cathode
resistors and make sure they are 10 ohms, not 100 or 1000 !

... tom
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Postby TomMcNally » Wed Jan 06, 2010 9:03 pm

It wasn't me who helped with the cap polarity ... so are the
boards just "new" versions of the originals, using 7199's ?

I would suggest if the amp worked fine with the new boards,
and doesn't now - you may have some wrong resistor values
someplace ... double check each one against the manual with
an ohm meter ...
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Postby Clik2media » Wed Jan 06, 2010 9:17 pm

Sorry about that but of course I still very much appreciate the advice. All emails come from "Separks"

It is a 7199 new board, I will revisit my resistor install one by one... grrrr LOL... Afterall this is DIYTube...smiles

thanks


TomMcNally wrote:It wasn't me who helped with the cap polarity ... so are the
boards just "new" versions of the originals, using 7199's ?

I would suggest if the amp worked fine with the new boards,
and doesn't now - you may have some wrong resistor values
someplace ... double check each one against the manual with
an ohm meter ...
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Postby mesherm » Thu Jan 07, 2010 12:27 am

http://www.dynakitparts.com/store/ProductZoom.aspx?pSrc=http://www.dynakitparts.com/store/images/productimages/zoom/15-A.GIF

Compare your board and parts layout with the one above. Its easy to get resistor values switched when loading one of these boards.
Also check to see all ground connections are correct.
Film capacitor do not really have a "polarity" as such. Any orientation refers to which side of the cap is connected to the foil layer closest to the outside. On the ST70 each pentode section of the 7199 has a 470K grid leak resistor that is located off the board soldered to the input RCA jacks.
This has to be there. It connects the pentode control grid (pin 7) to ground. On the original it is simply wired across the RCA inputs to their grounds.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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