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    ST-70 pico cap question

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    Elrick

    Posts : 71
    Join date : 2012-09-02
    Location : Bellingham WA

    ST-70 pico cap question

    Post by Elrick on Sat Sep 28, 2013 3:28 pm

    I still have the stock ST-70 driver board from a mid sixties rig that I replaced the paper caps a couple of years ago and never got around to changing out the smaller 390 and 82 pF caps. I would like to do that today but the closest values I have in my stash would be to use either 510 pF or 330 pF for the 390 pF. For the 82 pF, closest I have are 100 pF or 50 pF.

    I have already upgraded the power transformer, selenium rectifier, quad cap, bias caps etc. a couple years ago. Amp play great but... one channel plays a little softer than the other. I'll also go through the resistors and see if they are in good shape.


    Should I just order the proper ones and be patient or can I make do with what I have.

    Analog Man

    Posts : 56
    Join date : 2013-09-01
    Age : 54
    Location : St. Louis, MO - On The Hill

    Re: ST-70 pico cap question

    Post by Analog Man on Sat Sep 28, 2013 5:40 pm

    You can always use a little higher pf rating but make sure they are silver - mica type.

    Bob Latino
    Admin

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

    Re: ST-70 pico cap question

    Post by Bob Latino on Sat Sep 28, 2013 6:26 pm

    Go to Antique Electronic Supply online. They have the correct 82 pF and 390 pF caps that you need. See link below ..

    Silver Mica caps at Antique Electronic Supply

    Personally, if you have the original driver board and you want to keep the original driver circuit, I would get a new board and parts kit from Dynakitparts. The original boards were not made that well. The boards from Dynakitparts are much better quality. If one channel plays louder than the other, that is usually caused by the original carbon composition resistors than have drifted in value over the years.

    You could also use an "upgrade" board like the VTA driver board in your ST-70. If you do that then you would also need three new 12AU7 driver tubes.

    Bob

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: ST-70 pico cap question

    Post by arledgsc on Sun Sep 29, 2013 12:37 pm

    The pF caps from that era are highly sought after so if you try others save the originals.  And the pF caps are very stable and more than likely just fine (but check out of the circuit).   PIO and electrolytic caps can degrade severely over time.

    Flyquail56

    Posts : 24
    Join date : 2009-05-04

    Re: ST-70 pico cap question

    Post by Flyquail56 on Sun Sep 29, 2013 1:09 pm

    The silver mica caps at Antique Electronic Supply appear to be rated for 500 volts. The 390 pF is rated for 1000 volts. Mouser sells a 390pF/1000v silver mica cap, but the original domino caps are usually fine.

    Mike

    Tiziano73

    Posts : 29
    Join date : 2012-09-11
    Location : Italy

    ST-70 pico cap question

    Post by Tiziano73 on Tue Oct 08, 2013 10:30 am

    ST-70 pico caps in stock board was placed there by dynaco in matched pairs. I repaired many stock Dynakit amplifiers and if you test them with a real capacitor tester usually they doesn't drift or change their value. I suggest to keep in place and forget them.
    Check resistors as Bob suggest.

    Ps: (imho) 7199 are still good and really matched pairs? Pentode section of 7199 give all the amplifier voltage amplification and should be matched for gm and Ia.

    I hope this help.

    Regards

    Elrick

    Posts : 71
    Join date : 2012-09-02
    Location : Bellingham WA

    Re: ST-70 pico cap question

    Post by Elrick on Wed Oct 09, 2013 9:40 pm

    Just to resolve this issue, the channel imbalance had to do with the pre-amp all along. That problem is behind me but I have however ordered a new resistors and caps for the ST70's original driver board but will probably just leave the pico caps alone for now.  I ordered increased coupling caps of 0.22mF for what that's worth and choose the best match for the resistors.

    Tiziano73

    Posts : 29
    Join date : 2012-09-11
    Location : Italy

    Re: ST-70 pico cap question

    Post by Tiziano73 on Thu Oct 10, 2013 9:19 am

    It's a good idea if new capacitors are in matched pairs also, as suggested by Frank Van Alstine many years ago about an ST-70 upgrades.

    About the preamp, if it works well, sometimes a bad tube matching can generate an imbalanced sound.
    In 12ax7 triode matching is important match mu and Ra. Mu is the gain and Ra (Mu/gm) affect directly output impedence.

    For Example:

    if you have two tubes with this values:


    even if the matching of the gain is almost perfect, the two Ra are very unbalanced between them.
    The difference between the two values ​​is about 10,000 ohms. This difference is a difference of 20% (50,000 out of 10,000 is a difference of 20%) and will negatively affect the output impedance of your preamp, creating an audible imbalance between the left and right channels of your system (in substance when you hear the music a speaker will sound louder than the other).
    So is Very important when you buy Double Triodes for your preamplifier that they are matched for Mu and Ra. Max with a tolerance max respectively 10% and 5%.

    Hope this helps.

    Tiziano

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