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    What's the first thing to do when you restore an old Dynaco amp ?

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    Rick
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    What's the first thing to do when you restore an old Dynaco amp ?

    Post by Rick on Mon Dec 01, 2008 5:05 pm

    I inherited an old Dynaco stereo-70 amp from my father. It sort of plays but it hums a little. I don't want to spend a whole lot of money but is there something easy or simple to do to fix it up ? Rick

    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

    Re: What's the first thing to do when you restore an old Dynaco amp ?

    Post by Bob Latino on Mon Dec 01, 2008 5:23 pm

    Hi Rick,

      The real achilles heal of the old Dynaco amps is the quad cap which is the cylindrical "can" near the right output transformer. More than likely your hum issue is caused by a weak or leaking quad cap. The ST-70 was introduced in 1959 and some of the old ones out there are almost 50 years old. If there is one thing you should do it's replace the quad cap. The original one was a 30, 20, 20, 20 Mfd cap and was a little under capacity for that amp. You should replace it with a new one. One of the best replacement quad caps is the 80, 40, 30, 20 cap that is sold by www.dynakitparts.com. It's $35 and the cap is easy to replace. Check it out at > http://www.dynakitparts.com/store/product.aspx?id=269

     For a more extensive update of your amp you could replace the four coupling caps on the driver board and the tube sockets. If you really want to update the amp then replace the driver board completely with the VTA driver board from the Tubes4hifi web site at > http://www.tubes4hifi.com/st70.htm  This is probably the best upgrade replacement driver board for the ST-70. There is more information on the board at that link.

    If you have any other questions about restoring your ST-70 amp just post them here ..

    Bob


    Last edited by Bob Latino on Thu Oct 02, 2014 12:31 pm; edited 3 times in total

    tubes4hifi
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    Re: What's the first thing to do when you restore an old Dynaco amp ?

    Post by tubes4hifi on Thu Dec 04, 2008 12:53 am

    Bob hit the answer right on, the old quad cap is the thing to replace.
    Next are the filter caps for the filament circuit (the 2 - 100uf caps on the terminal strip, about a $1 each) and if you still have an old selenium rectifier (the square item bolted to the exact center of the chassis) that needs to be replaced with a new terminal strip and new diode (about $2 total cost).
    The other problem with the original circuit is the 7199 tubes.
    I've probably owned or rebuilt nearly 100 of these amps over the past 20 years, and 90% of them have 7199 tubes that are bad, even though they still "work". Since a new pair will run you about $60 IF you can find any, my VTA driver board is a bargain for $90 plus $25 for the 3 tubes.
    This also replaces the other items mentioned above, except the quad cap, your #1 item to replace. You wind up with an amp that sounds as good or better than almost anything available under $2500.
    Roy www.tubes4hifi.com/st70.htm

    scott6058

    Posts : 37
    Join date : 2009-01-21

    Re: What's the first thing to do when you restore an old Dynaco amp ?

    Post by scott6058 on Fri Jan 30, 2009 2:57 am

    I am working on a few projects and found this forum to be a breath of fresh air.
    Valuable experts with expertise.
    Thanks for sharing.
    1 standard ST-70 and a pair
    Standard MKIII's for rebuild.
    Best regards Scott

    Bob Latino
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    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: What's the first thing to do when you restore an old Dynaco amp ?

    Post by Bob Latino on Fri Jan 30, 2009 1:35 pm

    Replacement of the quad cap and the driver board are the two most important upgrade mods that have been mentioned.

    Another thing that has been mentioned in another post is the replacement of the old selenium rectifier with a diode. It's an easy fix and makes for a much more reliable bias system. An explanation is at the link below.

    http://dynacotubeaudio.forumotion.com/basket-f2/selenium-rectifier-t43.htm

    Old octal tube sockets in Dynaco amps should also be replaced. Do you get funny intermittant noises now and then ? Do they only happen after the amp is warmed up ? The original tube sockets were an inexpensive black phenolic plastic. The fiberglass reinforced Celanex PBT octal tube sockets carried by Dynakitparts look exactly like the original tube sockets but are much more heat resistant than the original sockets. Many people like ceramic sockets but two things to remember about ceramics. One - they are WHITE in color and won't look "original" on your Dynaco amp. Two - they have no give and if you are not careful removing a tube you can break the plastic center guide pin. Yes - ceramics are highly heat resistant but but many ceramic sockets also have "knife-edge" pins and don't contact the tube pin all the way around the pin. You are more likely to get an intermittent contact with a socket with knife-edge pins. Whatever socket you do get make sure that the socket is BOTTOM MOUNT that will fit a 1 1/8" hole and has 1 1/2" mounting centers. Some of the newer sockets do not have a built in ground connection that you can bend back like the original Dynaco sockets. No problem though - just pick up some grounding lugs and attach them under one of the two tube socket mounting screws. On an amp with an original driver board and bias system you will need a ground lug for each channel to reground the bias resistor.

    Replacing tube sockets is very straightforward. Unsolder all the connections, pull out the wires and write down or mark where the wires go. Remove the old socket. Install your new sockets and make sure that the key way points in the same direction as the old socket. Reattach and solder your wires to the new sockets.

    Bob

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