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    Barn find ST70

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    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Fri Oct 09, 2015 4:51 pm

    jrubin, here's some highly recommended reading material on this and related topics.  The first link is the definitive study of the original Stereo 70.  Second and third links talk about how to mitigate the 5AR4 bottleneck in conjunction with "the yellow sheet diode mod:"

    http://www.audioregenesis.com/documents/ST-70%20Base%20Line%20Testing.pdf

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

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

    Yellow sheet diode mod: http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod


    Last edited by PeterCapo on Fri Oct 09, 2015 5:20 pm; edited 1 time in total (Reason for editing : Add link.)

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Fri Oct 09, 2015 5:05 pm

    Bob Latino wrote:You are probably correct about the current draw being at the limit of a 5AR4 rectifier BUT > Dynaco used the excellent Mullard 5AR4's which would probably take the 250 milliamp or so current draw in stride. On the VTA ST-70 amps we recommend a 40 milliamp bias setting for each output tube. The 40 milliamp setting is probably easier on today's 5AR4's which are not as "sturdy" as Mullard's 5AR4.

    Bob

    Although, I recall that Jim McShane feels the new 5AR4s from New Sensor are up to the task, such as the reissue Tung Sol, Mullard and especially Gold Lion...

    Bob Latino
    Admin

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    Location : Massachusetts

    Re: Barn find ST70

    Post by Bob Latino on Fri Oct 09, 2015 5:26 pm

    PeterCapo wrote:

    Although, I recall that Jim McShane feels the new 5AR4s from New Sensor are up to the task, such as the reissue Tung Sol, Mullard and especially Gold Lion...  Yellow sheet diode mod should help, too.

    A good point .. A few customers have mentioned to me that the Tung-Sol and Gold Lion 5AR4's have worked very well in their amps. I have not heard anything about the Mullard reissue version as yet ..

    Bob

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Fri Oct 09, 2015 8:04 pm

    The results are in.

    Ive made some very specific measurements and comparisons of the whole bias circuit with the existing capacitors and then with their replacements.
    The biggest change I noted was one I wasn't testing for; my air conditioner.

    When the AC unit cycled on during testing of the old caps, I recorded a change of 20volts DC during its initial turn on and naturally leveled back to normal

    With the new caps later, the air conditioner cycled on and I recorded a voltage change of less than 4V DC.


    As for the 100uf caps having any notable effect of the bias circuit....... No. The DC waveform did smooth out as a result

    BEFORE Old 50uf's


    AFTER New 100uf's



    Of course there's a movie!!!! Its rendering now and will be posted tomorrow


    The GZ34 has been sitting in this thing for over 40 years and its a bit tired. I did expect lower than normal performance from it


    360VAC measured an output of 424VDC instead of 435VDC
    D - obviously was 424VDC
    C - after the choke was 412VDC

    B and A measured higher than expected. I believe its because I have no 7199's in to load down the A and B rails

    B - read 398VDC
    A - read 344VDC


    Im going to put 1 7199 in (i only have 1) and remeasure A and B to see if there is a notable change in them with the 7199 in. Also I should consider a new GZ34 as they are cheap







    PeterCapo

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    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Fri Oct 09, 2015 8:35 pm

    jrubin wrote: ...measured an output of 424VDC instead of 435VDC

    That's less than 3% low - it's fine.



    Last edited by PeterCapo on Fri Oct 09, 2015 8:41 pm; edited 3 times in total

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Fri Oct 09, 2015 8:38 pm

    PeterCapo wrote:
    jrubin wrote: ...measured an output of 424VDC instead of 435VDC

    That's less than 3% low - it's fine.




    I agree its sufficient but I cant accurately measure A and B till I get the 7199s in the curcuit

    PeterCapo

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    Join date : 2008-12-05

    Re: Barn find ST70

    Post by PeterCapo on Fri Oct 09, 2015 8:42 pm

    You might want to monitor the voltage across the large coupling caps on the PCB to make sure their voltage rating is not exceeded if running the amp with one or both 7199s removed.

    GP49

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    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Barn find ST70

    Post by GP49 on Fri Oct 09, 2015 11:51 pm

    The B+ voltage is OK. No need for concern.

    If that is an original Mullard GZ34, keep it. Treasure it. Treat it well.

    Even 40 years old, it has a good chance of outlasting a new current-production GZ34 that you buy and install now.

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sat Oct 10, 2015 8:55 am

    Movie Posted



    PCB testing has started.... All CAPS measure good
    I imagine the 330K resistors cannot be testing in circuit. It seems that a capacitor , perhaps .05uf one, is messing with the test set by loading up

    Interestingly both sets 270ks tied together on both sides measured 300K each



    Im concerned that the 10ohm resistors on the PCB were off. one was good at 10.4 and the other one was 14.1 (40% out of tolerance)



    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Mon Oct 26, 2015 8:55 pm

    So the 6GH8A tube converters are in and voltage testing was re-done. Everything seems more in-line with all of the tubes in place.

    On the B supply specifically, I am showing 360V which makes its way to pin 20 on the circuit board. At pin 1 on the 7199 Id expect to see the
    same voltage, or roughly the same voltage showing up on pin 1 of the 7199's. However Im seeing an 18 volt difference between the two.

    I see that B splits off to two resistors 18K to point 3 and 47K to pin 1 of the 7199.

    It also seems like pin 1 of the 7199 is directly connected to pin 8, which is not shown in the schematic.

    Something is surely dropping voltage in the area of one of the 7199's. Ive ruled out the resistors, but not the .1mfd coupling cap which could be leaky

    Adding to the confusion, if pin 8 is connected to pin 1, then pin 8 is seeing the D voltage after another 47K resistor.

    Any thoughts..... Movie to follow


    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 27, 2015 9:40 am

    Here is the latest video



    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Oct 27, 2015 10:55 pm

    jrubin wrote:So the 6GH8A tube converters are in and voltage testing was re-done.  Everything seems more in-line with all of the tubes in place.

    On the B supply specifically, I am showing 360V which makes its way to pin 20 on the circuit board.  At pin 1 on the 7199 Id expect to see the
    same voltage, or roughly the same voltage showing up on pin 1 of the 7199's.  However Im seeing an 18 volt difference between the two.

    I see that B splits off to two resistors 18K to point 3 and 47K to pin 1 of the 7199.  

    It also seems like pin 1 of the 7199 is directly connected to pin 8, which is not shown in the schematic.

    Something is surely dropping voltage in the area of one of the 7199's.  Ive ruled out the resistors, but not the .1mfd coupling cap which could be leaky

    Adding to the confusion, if pin 8 is connected to pin 1, then pin 8 is seeing the D voltage after another 47K resistor.

    Any thoughts..... Movie to follow


    Yup, I make my share of mistakes too.   Long day of work and snuck away from the repair of my wifes computer to do some quick testing.  This
    is what happens when you lose concentration...

    I don't want to spoil the answer to this question because I put it at the end of the movie.....  however ill give you two hints....


    1.  The only pin without a DC voltage on the 7199 is PIN 7
    2.  The schematic shows PIN 2 and PIN9 of the 7199 tied together


    Last edited by jrubin on Tue Oct 27, 2015 10:56 pm; edited 1 time in total (Reason for editing : I cant spell)

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Wed Oct 28, 2015 11:04 pm


    Here is video 6 posted, discussing issues in video 5, retesting and matching 7199's for a balanced circuit.





    jrubin

    Posts : 50
    Join date : 2015-08-28

    Circuit board restored along with original components

    Post by jrubin on Sun Nov 15, 2015 10:32 pm

    Another movie is on the way.  The original board was taken apart, all components tested and cleaned and reassembled.
    Heres a few photos












    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Nov 17, 2015 9:50 am

    Movie 7 posted



    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Thu Nov 19, 2015 9:58 am

    Movie Number 8



    mazeeff

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

    Re: Barn find ST70

    Post by mazeeff on Thu Nov 19, 2015 1:57 pm

    These videos are fantastic! Do you have a list of the test equipment in your lab? A video dedicated to just test equipment would be extremely useful. Every time, I see your lab, I start dreaming about adding some equipment.

    Mike

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Thu Nov 19, 2015 6:03 pm

    sounds good.... ill make one up and post it

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Nov 22, 2015 10:21 pm


    Early Frequency response measurements in RMS and Watts.    Im going to go deeper to plot the lower end next time, from 1K to 20hz


    First group has a typo, second chart in first picture should say @ 1.1Volt input






    wrong data in right channel at 50hz, disregard





    50 - 20hz were distorted in the following....   20hz beyond any possible measurment



    Last edited by jrubin on Sun Dec 13, 2015 11:02 pm; edited 2 times in total

    jrubin

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    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Tue Dec 08, 2015 10:12 am

    My HP 331A looks to be repaired. Once I finish calibration I will be able to continue onto the next video




    Burnt cap




    Power supply fixed


    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Dec 13, 2015 10:58 pm

    The Hp 331A has been repaired with a few part replacements and calibrated. It will be used to test the amplifier.   Videos of the repair can be found at https://youtu.be/NJxPiqW5Ww4

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sat Dec 19, 2015 10:42 pm

    Ive gone to great lengths to repair and calibrate the 331A to ensure my values are true.  Ive decided to give the Amp a workout today
    on the 331A good bad or indifferent.   The 331A doesn't lie.  I should point out that the 47K resistors have not yet been replaced and cause
    the uneven outputs of the 7199.  

    Interesting point of reference, the yellow which is bigger than the blue, is the output of the 7199 to the rear tube.
    On FFT ch1/ch2, a slightly net negative will be displayed,   the biggest artifact if shown where the waves converge, and that artifact increases
    with power input.  Ill be using a decade resistor to test this theory shortly.




    the biggest artifact if shown where the waves converge, and that artifact increases with power input.  Ill be using a decade resistor to test this theory shortly.




    However in the spirit of testing, here are the results, obviously not optimal, but provides great room for improvement.


    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Sun Dec 20, 2015 2:52 pm

    Ive posted a 1 off video showing my issue of the amplitude change of the 7199 outputs in which
    the change increases with the frequency.  I found something, however even though it corrects the amplitude, the phase does not align
    Surely this is masking another imbalance somewhere that needs correction.

    The 47K resistors are only off by 1.1%, out of tolerance, but not by much.

    from an older video



    with mod

    jrubin

    Posts : 50
    Join date : 2015-08-28

    Re: Barn find ST70

    Post by jrubin on Mon Jan 04, 2016 9:34 pm

    This previous post is due to the usage of separate probes which load up the circuit differently and have slightly different capacitances.
    A ton of work and testing has been done on the AMP and now I need only replace the 2 47K resistors in the right channel.
    Shortly after, a new movie will be posted once I have the final readings

    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Re: Barn find ST70

    Post by wildiowa on Mon Jan 04, 2016 11:14 pm

    Thank God there are people like you who have the ability, knowledge and equipment to do these kinds of things. This is why we have seen the development of such great amps and speakers and other stuff that is just wonderful equipment, both vintage and new. But...I must say I have absolutely no idea what you are up to here....you guys got so far into the weeds you lost me a long time ago but I am thoroughly entertained by the videos of your rebuild and testing being done here. I am just so far behind your curve it makes me realize how much I really don't know. But I still enjoy poking around under the hood.

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