Hi, new to me ST-70 .. a question

Hi Everyone,

My dad back in the 70's built an ST-70, a PAS-3 and a tuner from the old Dynakit kits. And for the past 50 years its been powering my mom's organ that she used....

Well, the organ moved on ( fortunately not the parents )... and I found myself with a a new set of audio components to play with....



Cool,


I happen to know this hasn't been touched since the mid 1980's... and has sat in the back of a dusty organ since. Tubes and caps are all still original for the most part.... pots are scratchy and need cleaning, but overall it works and sounds pretty good. I'd like to restore and update this amp and put it to use in my house...


So.....I've been digging through online and going through the schematics with the intent of updating the components that at this point would be nearing failure... ( I have a degree in electrical engineering and experience with tubesamps... )


but.. There are two caps in this amp that I can't figure out, I don't see them on the schematics, and I don't see any "modification threads" that would point to what these are....




The two big caps,
one connects to pin 8 of the rectifier tube, and the other connects between ground and what I think is R31...




Any ideas why these are here?

And.. thanks in advance!

Hi,

I can't clearly see how the main cap is wired or where one of the additional caps is soldered, but it appears to me that at one time, someone added capacitance to the original 30/20/20/20 power supply cap. One of the resistors in the main power supply has been changed too. It also looks like one of the resistors in the bias supply was changed as well as those caps appear to be newer than any OEM caps I have seen.

I would consider looking into replacing all the caps in the high voltage supply as they are old, perhaps using an ST-70 power supply board such as is sold by Triode Electronics or others to replace both the high voltage and bias circuits in its entirety.

http://triodeelectronics.com/capboards.ht

I would also recommend replacing the selenium diode feeding the bias supply (remember it supplies negative voltage)and checking the values of all the carbon comp resistors in the amp as they don't age too well. Also add a thermistor on the power cord (see Dynakit Parts, I believe the value is CL-80 for a 3 amp thermistor).

I've rebuilt a number of these amps over the years, the basic amp can still sound very good, with some extreme modifications it can sound damn close to state of the art.

take out about half the stuff under the hood, put our VTA board in, which includes a diode to replace the selenium rectifier, also includes FOUR bias trimmers to replace the original two,
more than doubles the original power supply capacity, and it sounds 10X better. Also replace the quad cap, guaranteed bad after about ten years.
See this diagram www.tubes4hifi.com/VTA70wiringdiagram3.jpg
order at www.tubes4hifi.com/ST70.htm

I disconnected my big can cap (actually four caps in one can) and soldered in modern equivilants underneath. The rest is all original and still works & sounds great. Note that replacing the original oil caps on top board with modern type will change the sound quite a bit. I prefer the warm sound of the originals every time

I personally agree with just putting in the VTA board.  When I built my ST-70, I ordered the VTA board and installed it with the original assembly process and haven't looked back.  I also replaced the Selenium rectifier with a diode, replaced the quad cap with a replacement quad cap (new not used) and put in 2 switches where the current bias pots are so I can run in ultralinear or triode mode (you would need to use the VTA board to free up the holes where the bias pots are currently) ( https://dynacotubeaudio.forumotion.com/t41p25-run-your-dynaco-st-70-mark-ii-mark-iii-or-mark-iv-in-triode-mode-photo ).  

In addition I added 2 diodes on the rectifier (5AR4/GZ34) tube and haven't had a probelm with blowing the rectifier anymore ( https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-yellow-sheet-diode-mod ).  

I love my ST-70 and play it every day.  My best advise is to just set it up the way you like it and enjoy it!

P.S. I also discovered that I get a lot better sound (in my opinion) from the VTA board by using a 12AU7 (I use an RCA clear top) in the center position and 12BH7A (I have GE brand) for the 2 other tubes.  This really brought the mids and vocals to life.  This is just a suggestion.  Have fun and do some tube rolling of your own!

Hi mattnew,

A couple of additional observations based on the replies I've seen since I replied to you initially.

If you don't need the power, you can convert the amp to triode mode by removing ( and taping off ) the transformer lead to Pin 4 and then adding a 100 ohm resistor ( or higher ) from Pin 3 to Pin 4.

If you want to keep the amp Ultralinear, then insert a 100 ohm or higher resistor between the screen of each output tube ( Pin 4 ) and the transformer ( green or green / white ). This will help protect the tube and will "stiffen" the sound slightly. This is a feature seen in most modern tube type amps and has NO negative impact other then reducing the power by a small, fractional amount.

I also highly recommend my original thought about adding a CL-80 NTC to slow down the power inrush, preferably to the hot side of the power cord, Robert Tomer, a well known 1950's "Tube Expert" claims that an NTC will increase tube heater life by about 300% with no negative impact on the amp except a very slight delay ( if any ) when you power it on.

Finally, I would recommend a power supply makeup of 30 mfd initially after the rectifier ( no higher, assuming you use a tube rectifier, higher if solid state diodes are used ), at least 100 mfd 2nd section for the transformers / power tubes, then about 30 mfd for each of the final two sections, depending on the board and tube selection. This will firm up the bass and open up the sound in general. The original can cap was a plain electrolytic, not an "Oil Cap" as indicated by one of the previous posters.

Please replace the Selenium rectifier with a diode as well to supply the negative voltage.

As I indicated in my original post, if you are keeping the original circuit board, check the values on all the resistors, carbon comp resistors drift over time, considerably.

And if you replace the board, the VTA board is one option, I personally use the Triode Electronics board but have modified the values somewhat.

Enjoy the amp, I keep one as a backup to my very modified, triode Dyna Mark IIIs.

-Ed