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    ST70 7199 board component change question

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    camaroguy62

    Posts : 4
    Join date : 2015-01-02
    Location : Mid-Michigan

    ST70 7199 board component change question

    Post by camaroguy62 on Sun Jan 11, 2015 9:15 pm

    Hi, I'm new to the forum, lots of great information.  I have a ST70 that I'm restoring and the original 7199 board has had a few changes by a previous owner.  The 4 coupling caps have been changed from .1uf to .2 uf. and the .05 uf caps to .1 uf.  I understand that is not an issue but a recommended change.  However, in addition the two 390 pf caps have been removed and it looks like the adjacent 1K resistors replaced with 2.2K ohm units.  To my ears, the amp sounds good and performs well.
    My question:  Were the elimination of the 390 uf caps and resistor's replacement an OK change for this amp?  I would just as soon keep the 7199 board.
    Thanks in advance, see pic.

    http://s1169.photobucket.com/user/camaroguy62/media/IMG_2240.jpg.html?sort=3&o=0

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: ST70 7199 board component change question

    Post by PeterCapo on Sun Jan 11, 2015 10:49 pm

    The 7199 board is still a very desirable choice for some folks, myself included.  Was the 390pf, or substitutions, moved to underneath the board?  I'd want to get a complete copy of the original Dynaco manual including the schematic and both pictorial diagrams and go through the whole amp making a list of things that have been changed away from the original design. If the 390pf is indeed missing with no other changes made to compensate for its absence - well it's nothing I've ever heard of doing and does not strike me as being quite right. But, what do I know anyway?

    camaroguy62

    Posts : 4
    Join date : 2015-01-02
    Location : Mid-Michigan

    Re: ST70 7199 board component change question

    Post by camaroguy62 on Sun Jan 11, 2015 11:12 pm

    No, nothing under the board, no additional caps on the amp, looks like they were just removed and the resistors changed.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: ST70 7199 board component change question

    Post by PeterCapo on Mon Jan 12, 2015 12:10 am

    Well, that’s a new one on me.  The 390pf comprised part of the original feedback scheme, and my guess is that it may have been removed in concert with the changes in the coupling cap values as part of an effort to "improve" on a commonly believed issue with the Stereo 70’s bandwidth inconsistencies specifically in relation to the feedback loop/coupling caps.

    The thing is, our respected friend, Dave Gillespie, has recently done an extensive study of the original Stereo 70 design and finds that some of the commonly held beliefs about deficiencies in the original design are unfounded.  He also addressed the matter of the coupling cap values and the feedback loop, and I believe he felt that the original parts’ values were reasonable and that increasing the coupling cap values could possibly result in instability under some circumstances, or so I understand.

    Here are a couple of links, the first to his overall study and the second to a discussion of the feedback loop and the coupling cap values, both of which seem relevant to your situation:

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

    http://www.audiokarma.org/forums/showthread.php?p=8400580#post8400580

    Unless I could find something validating the rationale for removal of the 390pf, and if everything else in the amp is configured according to the original design, I’d be inclined to put the 390pf back in and change the resistor back to whatever the original value and power rating was.

    BTW, it has been my understanding that the 390pf needs to be a 1000VDC part: http://www.audioasylum.com/forums/dynaco/messages/6416.html I found Joe's comments to be true when the corresponding part in my Stereo 35 went. It was quite a show.

    Mouser Part # 598-CDV16FF391JO3

    MontanaWay

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    Age : 58
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    Re: ST70 7199 board component change question

    Post by MontanaWay on Mon Jan 12, 2015 2:09 am

    camaroguy62 wrote:Hi, I'm new to the forum, lots of great information.  I have a ST70 that I'm restoring and the original 7199 board has had a few changes by a previous owner.  The 4 coupling caps have been changed from .1uf to .2 uf. and the .05 uf caps to .1 uf.  I understand that is not an issue but a recommended change.  However, in addition the two 390 pf caps have been removed and it looks like the adjacent 1K resistors replaced with 2.2K ohm units.  To my ears, the amp sounds good and performs well.
    My question:  Were the elimination of the 390 uf caps and resistor's replacement an OK change for this amp?  I would just as soon keep the 7199 board.
    Thanks in advance, see pic.

    http://s1169.photobucket.com/user/camaroguy62/media/IMG_2240.jpg.html?sort=3&o=0

    I'll start off by saying that sound is extremely subjective.
    However, I do recommend, that you perhaps consider updating to the VTA driver board. Apart from it sounding better, (many here can attest to that), one really good benefit is that with the new VTA driver, you are going from BIAS adjustment of two output tubes together to individual output tube BIAS adjustment. This enables you to 'match the performance' of each output tube as closely as possible. Some additional mods will be required, such as adding two BIAS resistors to two output tunes.
    Another benefit is that you would open up the possibility of using a number of different more commonly available driver tubes, 12AU7, 12BH7, 5963, etc., mix and match them to get the best sound for your ears, or even going with the octal VTA using 6SN7 tubes.

    camaroguy62

    Posts : 4
    Join date : 2015-01-02
    Location : Mid-Michigan

    Re: ST70 7199 board component change question

    Post by camaroguy62 on Mon Jan 12, 2015 3:11 pm

    Thanks for all the good advice, I am leaning toward making this amp as it was originally designed, with the exception of replacing the selenium rectifier. I'm not sure when the Sprague Vitamin Q caps were installed, but I would expect it was decades ago, and I'm concerned about their longevity--I have had good success in other amps (not Dynaco) with Sprague/Vishay 716 series orange drops, and may go that route, going back to the original values, .1uf and .05uf. (I also have available Russian PIO K40's in the proper values). I already have replaced the can cap with a new, same value unit, as well as two new 50uf bias caps.  I then would reinstall new 390 uf caps (1000V) and replace the two resistors with original value units, essentially making the amp as designed except for the diode.  I do appreciate the value of a replacement board, both in terms of tube availability and also individual bias adjustment, but I guess I'm more of a restorer at heart than a 'modder'.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: ST70 7199 board component change question

    Post by PeterCapo on Mon Jan 12, 2015 4:08 pm

    Sounds like a good plan of action.  One note about replacing the selenium stack with a diode...  The replacement diode will likely have a considerably lower voltage drop than a selenium stack.  After making the replacement, I'd definitely measure the voltages in the bias circuit per the chart in the original Dynaco manual.  Of particular interest is the voltage level the two electrolytics in the bias supply will see.  With the replacement diode, and with an AC wall voltage above 117VAC, you might be pushing up very close to the voltage rating for the two bias supply electrolytics.  The originals were 50uF @75VDC.  You might want to consider replacing those with 100VDC parts, if need be.  You could also install a thermistor in the power transformer primary to drop the voltage there a bit.

    I’d also suggest proceeding delicately with changing parts on the brown, phenolic PC board.  The pads and traces delaminate easily.  Since it has already had parts changed, the likelihood of running into this difficulty is greater.  It is still manageable even if some circuit foil lifts, as you can just run the component leads a bit farther down the trace to a secure spot.  So, maybe refrain from trimming the leads of any new parts until you see what’s what.

    camaroguy62

    Posts : 4
    Join date : 2015-01-02
    Location : Mid-Michigan

    Re: ST70 7199 board component change question

    Post by camaroguy62 on Mon Jan 12, 2015 5:12 pm

    I have replaced the bias supply caps with ones I had on hand and they are rated at 160V, so that should not be a problem.  I did read on a thread in this forum by Bob Latino that the silicon based bias voltage may be a little too high to control with the pots, and if so, to parallel the two bias 10K resistors, one with a 10K and parallel the other one with a 20K.


    "The only minor thing that could happen is that in some amps after a change from the stock selenium rectifier to a diode the bias range will change somewhat. You won't get your "1.56 volts DC" bias voltage near the center of the bias control anymore. The standard thing to do if this does happen is to alter the values of the two 10K resistors on the 7 lug terminal strip. The usual cure is to add another 10K resistor in parallel with the 10K resistor between lugs 1 and 2 (to give a 5K effective resistance) and a larger 20K or so resistor in parallel with the 10K resistor that is now between lugs 3 and 4. A 20K resitor here will give you an effective 6.66K resistance between lugs 3 and 4. This should now give you the correct bias somewhere near the center of the bias control. The reason for the necessity of the resistor alterations is due to the greater forward flow of current by a diode as opposed to a selenium rectifier.

    Bob"

    I have a variac which is a great help on slowly bringing up the amp voltage when checking the bias settings.  Also, thanks for the caution on the pc board, I'll be careful.
    Tom

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