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    Barn find ST70

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    jrubin

    Posts : 50
    Join date : 2015-08-28

    Barn find ST70

    Post by jrubin on Sat Oct 03, 2015 11:39 pm

    Im working on a barn find - video coming shortly. Im trying to keep the unit stock with the exception of some 7199 converter sockets. I have a nice modded ST70 for regular use, this one landed in my lap and will be used for testing and understanding to compliment my new Hantek 5072P


    A few questions however.......

    The two 50MFD caps on the power supply. Rated at 75V. These need to be replaced. One has an ESR off the charts and measures in at 11MFD. What do you feel a good replacement should be?
    Should I stay at 50MFD and 75VDC rating or could I benefit from a higher capacitance?

    The output transformer inductance readings show the following. The right jack seems to have higher inductance. Does this reading look ok?

    LEFT
    16 OHM - 70mh
    8 OHM - 28.8mh
    4 OHM - 14mh

    RIGHT
    16 OHM - 85mh
    8 OHM - 33mh
    4 OHM - 17mh


    The RF choke measured 65 OHMS at the multi capacitor can and .87H.. I cant find the specification for this

    Whats a good way to measure the selenium rectifier. I generally replace them, but want to keep it in this unit.


    If you are still reading this... does anyone have the NO-LOAD voltage readings for the power transformer... That is to say. what it reads with all tubes removed.


    Thanks much, in advance


    Jordan


    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Sun Oct 04, 2015 12:12 am

    If you can get a hold of a variac, set it so you get 360VAC to ground from each of the two high voltage secondary leads off of the power transformer (two red wires at pins 4 and 6 of the 5AR4).  This will establish the baseline reference for checking all of the other internal voltages (with all of the tubes installed) against the chart in the original manual found on the same page as the schematic for the power supply.

    To determine the health of the selenium rectifier, you’d make reference to the voltages in the chart to check what you are getting from the bias supply.  If the selenium rectifier is well used, its voltage drop will probably be more than it should be (or so I understand).

    For the two electrolytics in the bias supply, you can increase the capacitance a bit, say to 100uF or so, each.  The voltage rating is important here, with two factors contributing to it needing to be higher than 75VDC: 1) depends on how your AC mains runs, 2) if the selenium rectifier needs replacing, you’ll very likely replace it with a diode that will have less of a voltage drop than even a new selenium rectifier (if you could even find one).  So, I suggest a 100+VDC rating apiece for the bias caps, and I base this on measurements I have taken.

    Have a look at Mouser Part #: 80-PEG124RL3150QL1

    peterh

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

    Re: Barn find ST70

    Post by peterh on Sun Oct 04, 2015 2:45 am

    I would advice NOT to increase the capacitance of the bias caps. Reason is that it is importent
    that bias is established before B+ on every possible sequence of events.
    The existing cap ( 50uF) is more then sufficient. 100V as petercapo suggests , is a good advice.

    I would also recommend to replace the selenium diode. It will fail, just wait.
    If you want to keep it for display, at least parallel it with a Si diode ( and adjust the resistor chain)

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Sun Oct 04, 2015 1:54 pm

    peterh wrote:I would advice NOT to increase the capacitance of the bias caps. Reason is that it is importent
    that bias is established before B+ on every possible sequence of events.

    Can you say more about this, please?

    peterh

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

    Re: Barn find ST70

    Post by peterh on Sun Oct 04, 2015 2:08 pm

    PeterCapo wrote:
    peterh wrote:I would advice NOT to increase the capacitance of the bias caps. Reason is that it is importent
    that bias is established before B+ on every possible sequence of events.

    Can you say more about this, please?

    If the cathodes are hot and B+ comes before bias then the tubes might be overloaded and
    flashover. This will destrouy the output tubes if unlucky.
    Thus and measure must be taken to make shure bias is at correct level before B+

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Oct 04, 2015 2:40 pm

    Heres a video of some of the work ive done thus far.......



    Guest
    Guest

    Re: Barn find ST70

    Post by Guest on Sun Oct 04, 2015 2:46 pm

    with any restoration, I feel, always start at the power supply, especially the caps and older resistors, then work your way to and through the rest of the circuit, again giving good attention to caps, solder joints, wires etc..
    Once up and running properly, then maybe fine tune it with better coupling caps, maybe some good quality film resistors and tube rolling as well.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Sun Oct 04, 2015 2:57 pm

    peterh wrote:
    PeterCapo wrote:
    peterh wrote:I would advice NOT to increase the capacitance of the bias caps. Reason is that it is importent
    that bias is established before B+ on every possible sequence of events.

    Can you say more about this, please?

    If the cathodes are hot and B+ comes before bias then the tubes might be overloaded and
    flashover. This will destrouy the output tubes if unlucky.
    Thus and measure must be taken to make shure bias is at correct level before B+

    I do remember having read some things along the way about not making the bias capacitors too large. This is an interesting possibility.

    But, I’m wondering how a somewhat larger value bias capacitor might make it more likely that there would be a runaway bias condition, or some sort of flash-over, for the power tubes.  

    Do you mean that the initial charging cycle for, say, a 150uF vs. 50uf bias capacitor would be sufficiently longer such that the power tubes could have a run-away condition?  Seems like the charging cycle of even a larger capacitor would be quite short compared with the time it takes for the filaments to heat up, wouldn’t you say?  Or, is there something else I am missing?

    Thank you.
    Peter

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Oct 04, 2015 3:20 pm

    Unfortunately the 7199's (at least) one of them cannot be tested in curcuit as it has a short.


    Im going to load in the EL34's and turn up the voltage to see what I get for heater and grid bias

    peterh

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

    Re: Barn find ST70

    Post by peterh on Sun Oct 04, 2015 3:40 pm

    PeterCapo wrote:
    peterh wrote:
    PeterCapo wrote:
    peterh wrote:I would advice NOT to increase the capacitance of the bias caps. Reason is that it is importent
    that bias is established before B+ on every possible sequence of events.

    Can you say more about this, please?

    If the cathodes are hot and B+ comes before bias then the tubes might be overloaded and
    flashover. This will destrouy the output tubes if unlucky.
    Thus and measure must be taken to make shure bias is at correct level before B+

    I do remember having read some things along the way about not making the bias capacitors too large.  This is an interesting possibility.

    But, I’m wondering how a somewhat larger value bias capacitor might make it more likely that there would be a runaway bias condition, or some sort of flash-over, for the power tubes.  

    Do you mean that the initial charging cycle for, say, a 150uF vs. 50uf bias capacitor would be sufficiently longer such that the power tubes could have a run-away condition?  Seems like the charging cycle of even a larger capacitor would be quite short compared with the time it takes for the filaments to heat up, wouldn’t you say?  Or, is there something else I am missing?

    Thank you.
    Peter
    In normal start the bias would come up even with very large caps. But, power glitches
    might unload both B+ and bias, but as cathodes are still hot the b+ might come up
    faster.

    And as 50uF is more then needed , why increase it ??

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Sun Oct 04, 2015 3:55 pm

    peterh wrote:
    PeterCapo wrote:
    peterh wrote:
    PeterCapo wrote:
    peterh wrote:I would advice NOT to increase the capacitance of the bias caps. Reason is that it is importent
    that bias is established before B+ on every possible sequence of events.

    Can you say more about this, please?

    If the cathodes are hot and B+ comes before bias then the tubes might be overloaded and
    flashover. This will destrouy the output tubes if unlucky.
    Thus and measure must be taken to make shure bias is at correct level before B+

    I do remember having read some things along the way about not making the bias capacitors too large.  This is an interesting possibility.

    But, I’m wondering how a somewhat larger value bias capacitor might make it more likely that there would be a runaway bias condition, or some sort of flash-over, for the power tubes.  

    Do you mean that the initial charging cycle for, say, a 150uF vs. 50uf bias capacitor would be sufficiently longer such that the power tubes could have a run-away condition?  Seems like the charging cycle of even a larger capacitor would be quite short compared with the time it takes for the filaments to heat up, wouldn’t you say?  Or, is there something else I am missing?

    Thank you.
    Peter
    In normal start the bias would come up even with very large caps. But, power glitches
    might unload both B+ and bias, but as cathodes are still hot the b+ might come up
    faster.

    And as 50uF is more then needed , why increase it ??

    Not sure how B+ could come back faster than the bias supply in the event of a power fluctuation - and, again, I don't see a relationship to the value of the bias cap in this situation.  But, as far as why to increase the value of the bias cap, a couple of thoughts come to mind.  First, it's a half wave rectified supply for the bias, so a larger value capacitor should smooth out the bias control voltage some more, perhaps decreasing noise (not that noise has been a problem in the first place with 50uF).  Also, just making the bias supply stiffer might be advantageous to keeping the bias stable.  Certain currently available parts substitutions, like the one I suggested in my prior post, have much lower ESR than others, but it happens to have 150uF.  If the 150uF won't hurt anything, perhaps it would be advantageous to have the low ESR quality.  In any case, I agree that the original 50uF is perfectly adequate, but if there could be an improvement by going to a larger capacitor without causing problems, why not?  It might help the amp to work/sound better - or not.


    Last edited by PeterCapo on Sun Oct 04, 2015 4:25 pm; edited 3 times in total (Reason for editing : Clarification.)

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Barn find ST70

    Post by sKiZo on Sun Oct 04, 2015 4:07 pm

    All the points about power fluctuations should be moot if you're using a circuit interrupter with a manual reset? Any brown out strong enough to cause problems should trip that.

    Theoretically, of course.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Sun Oct 04, 2015 4:17 pm

    sKiZo wrote:All the points about power fluctuations should be moot if you're using a circuit interrupter with a manual reset? Any brown out strong enough to cause problems should trip that.

    Theoretically, of course.

    Absolutely. And, I believe I use the same kind of GFCI that I've seen you post either here or AK. They work great. Not everyone uses them, though.

    peterh

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

    Re: Barn find ST70

    Post by peterh on Sun Oct 04, 2015 4:17 pm

    I would'nt increase the bias cap. But thats me. YMMV.
    Enough said.

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Oct 04, 2015 4:44 pm

    I ve reached an issue today. while setting up the grid bias I have no voltage on the 1.56 terminals nor pins 1 or 6 on the EL34's



    Changes to the trim pots cause a change in voltage to el34 pins 5 and 6 which show about 33 volts.

    Also I was able to test the selenium rectifier good which matched the manual at top 65vDC and bottom 50VAC



    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Oct 04, 2015 5:13 pm

    Wait a minute.....

    The manual says to set up the bias before putting in the rectifier tube.  Without the rectifier tube there is no high tension DC.  Therefore no current flow... so there would be no way to set up the bias.



    Just to be sure I pulled the rectifier from my other ST70 and sure enough, no readings on the 1.56 terminals without the rectifier.   Why would they write that in the manual?



    EDIT:   Ahhh ok , it only says to center the pots without the rectifier..... not that you would get a reading... only after installation would the reading be present when current flows


    I find it interesting that I cant find a 50uf 75v or 100v capacitor on any of the dyna parts websites

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Sun Oct 04, 2015 6:33 pm

    Try Mouser or Digikey

    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Barn find ST70

    Post by Bob Latino on Sun Oct 04, 2015 7:06 pm

    You can use the caps at the link below . It is Nichicon 100 uF @ 100 volt cap. Two of these are used as the bias caps in all the VTA amp kits.

    Nichicon 100 uF @ 100 volt caps

    Bob

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Oct 04, 2015 8:13 pm

    Hello Bob,



    So what you are saying is to use 100uf vice the original 50uf ones in the amp?

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Barn find ST70

    Post by Bob Latino on Sun Oct 04, 2015 8:33 pm

    jrubin wrote:Hello Bob,



    So what you are saying is to use 100uf vice the original 50uf ones in the amp?

    Yes - It won't really make any difference. Dynaco use 50 uF caps back then because electrolytic caps were more expensive in relation to the total cost of the amp. Two 50 uF caps are probably minimum capacitance for DC storage for the amp's bias system. Modern electrolytic caps are physically smaller for their capacitance and a lot less costly in relation to the total cost of the amp. Those Nichicon 100 uF @ 100 volt caps are smaller than Dynaco's original 50 uF caps. They are also better made, more reliable and will last longer than Dynaco's two original 50 uF caps. That statement is not meant to knock Dynaco. It is not fair to compare what is available today to what was available 50+ years ago. Dynaco did the best they could to sell a decent tube amp to the masses. A lot of people back then could not afford McIntosh and Harmon Kardon gear. In 1959 the Dynaco ST-70 kit sold for $99 with a complete tube set plus a tube cage. It is hard to believe that they could sell the ST-70 at that price but they did and sold 350,000+ ST-70 amps ..

    Bob

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Oct 04, 2015 9:06 pm

    Is this an electrolytic cap... i see that its polarized....

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Oct 04, 2015 10:07 pm

    Part II posted..... I should have realized that without the rectifier there would be no current flow.....



    mazeeff

    Posts : 122
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: Barn find ST70

    Post by mazeeff on Mon Oct 05, 2015 7:17 am

    Very nice video! If I read the date codes on the bias pots correctly, this amp was kitted in 1963. Both pots have similar date codes, which supports 1963. I like the patina on the amp. Any idea how long it sat idol?

    Mike

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Mon Oct 05, 2015 5:55 pm

    I am told 40 years.... Mostly as a display piece and possibly in storage too

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Mon Oct 05, 2015 8:26 pm

    So Ive purchased some go-no-further parts for the unit.

    2x 100uf 450v caps

    20/20/20/30 cap -   I figured, if im gonna stay original.....


    Since Im going no further till the parts arrived I though I might ask a few questions.  In looking at the schematic...and wondering

    1.  The input terminals TIP/RING are tied together with a 470KOHM resistor.   What is its purpose, and why 470KOHM




    2.  With the discussion about the 50uf caps:

        The AC connection RD-BL supplies 50vAC to the rectifier with 65VDC out.  The output after both Caps goes directly into
    the 10K POTS    to connections 6-21   onward to connections    1-2   and 22-23    to the EL34's pins 5-6.  So any voltage
    change with the larger caps can be offset by the bias pots.    Correct? Also I imagine the caps are flipped around because the negative voltage is lower than the chassis....




    3. What biases the 7199's?  There is no negative value for any test points on the tube. Pin 2 plate of the 7199 connects to (A) 305VDC which is connection 19
    after a further drop in voltage by a 270KOHM resistor but its also connects to the grid pin 9 on the triode side of the 7199. does that 82mmf cap stop the DC flow into pin 9?




    4.   Since there is no coupling cap between pins 9 and 2 of the 7199, am i to assume the DC Plate voltage from pin 2 is injected into the grid at pin 9
    or is there some internal capacitor to separate the DC from the audio signal within the tube.  It seems to me that the plate voltage of the pentode side[A] is
    different from the plate voltage on the triode side [B].

    5. Is a reliable way to test the choke C354 by measuring  pin 8 on the GZ34 [435VDC] and expecting a 20VDC from on the output of the RF choke where it connects to the 20uf cap?

    6.  What is the purpose of the two 270K OHM resistors in series that connect the output signals to the EL34 grid after the .1mf coupling capacitors? Does this match the peak to peak size of the wave forms going to the EL34 after they are phase inverted?



    7. what is the purpose of the 1K resistor before the input signal into the EL34. Is it to match the expected output signal to an appropriate input level for the EL34?




    8.  The pin 4 and pin 3 get a positive dc voltage from C power which is 415v  i see that pin 3 is lower probably due to the resistance of the audio transformer coil
    and should read 5 volts less as a result.  Where is the return path for the primary side of the audio transformer, is it fed back into the EL34




    sorry for all the questions, just curious

    Jordan


    Last edited by jrubin on Mon Oct 05, 2015 8:45 pm; edited 7 times in total (Reason for editing : added pics)

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