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    Finally picked up my new (to me) ST-70-Pics

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    j4570

    Posts : 141
    Join date : 2010-08-30

    Finally picked up my new (to me) ST-70-Pics

    Post by j4570 on Mon Nov 15, 2010 8:16 pm

    I finally picked up my ST-70 after waiting several months as a friend got it from another friend of mine. Here are the pics:










    The guy my friend got it from said he was the original owner/builder. He gave it or sold it to him for very little and said one channel was bad. Well, probably right, I tested the tubes on my emission tester:

    Left Front 6CA7 Realistic Japan -Marginally Good
    Left Rear 6CA7 Realistic Germany -Bad
    Right Front 6CA7 RCA USA -Good
    Right Rear 6CA7 RCA USA - Good
    5U4 GE Good
    Left 7199 Dynaco Bad (EIA code 274=RCA, Data 67-52)
    Right 7199 Dynaco Marginally Good (EIA code 274=RCA, Data 67-52)

    The unit looks really clean and neatly wired. Now to get with Bob and Kevin and order some parts from both to make it what I want.

    Later, I'll inspect the PAS-3

    Jason

    ejc

    Posts : 25
    Join date : 2010-10-16
    Age : 65
    Location : Connecticut

    Re: Finally picked up my new (to me) ST-70-Pics

    Post by ejc on Mon Nov 15, 2010 9:24 pm

    Have fun with the project

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Finally picked up my new (to me) ST-70-Pics

    Post by Bob Latino on Mon Nov 15, 2010 9:36 pm

    Hi Jason,

    That amp appears to be in bone stock condition and in pretty good shape. If you intend to play it on a daily basis though, just replacing the quad cap and the driver board IMHO would give you an amp that you can play every day. The VTA driver board is my favorite ST-70 board and the F&T 550 volt quad cap has a higher voltage rating and greater DC power storage than the stock cap. Both are a good bet if you want to upgrade the amp.

    One other thing. A 5U4 rectifier will work just fine in an ST-70 but it has a higher voltage drop than a GZ34 and your B+ high voltage will be lower. The lower B+ voltage will restrict the overall power of the amp. Dynaco even mentioned the slight loss of power when using a 5U4 in their original ST-70 manual. Get a GZ34 rectifier to get full power out of the amp.

    Bob

    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: Finally picked up my new (to me) ST-70-Pics

    Post by Sal on Tue Nov 16, 2010 12:07 am

    Nice all original ST-70. I would replace the quad capacitor, bias selenium rectifier with a 1N4003 or greater diode and the 2 bias electrolytics before you use it on a daily basis. You should also replace the caps on the printed circuit board or replace it with the upgraded board Bob mentioned.

    Sal

    quadaptor

    Posts : 59
    Join date : 2009-05-04

    Re: Finally picked up my new (to me) ST-70-Pics

    Post by quadaptor on Wed Nov 17, 2010 4:13 pm

    I installed Dynakit's Balance Control Upgrade Kit in a stock ST-70. The kit allows you to bias the output tubes from side to side, and front to back per side. In theory, you don't need a matched quad of EL-34s.

    Also, does anybody have a reccomendation for replacement caps on a stock board?

    Luddite

    Posts : 235
    Join date : 2009-02-04
    Age : 66
    Location : Texas

    Re: Finally picked up my new (to me) ST-70-Pics

    Post by Luddite on Wed Nov 17, 2010 6:09 pm

    quadaptor wrote:I installed Dynakit's Balance Control Upgrade Kit in a stock ST-70. The kit allows you to bias the output tubes from side to side, and front to back per side. In theory, you don't need a matched quad of EL-34s.

    Also, does anybody have a reccomendation for replacement caps on a stock board?

    This sounds intriguing, but wouldn't there be the possibility of unmatched tubes (even with identical bias settings afforded by their individual controls) operating on different portions of the gain curve? If that were true then wouldn't there be a negative effect to the desired mirror image of the two push-pull halves of a given channel? This is not my area of expertise, I was just wondering.

    If you are looking for some good quality standard poly caps, try Dynakitparts.com. They should have some modern "drop-in" replacements.

    Charlie

    quadaptor

    Posts : 59
    Join date : 2009-05-04

    Re: Finally picked up my new (to me) ST-70-Pics

    Post by quadaptor on Wed Nov 17, 2010 8:40 pm

    Luddite,

    I'm afraid I'm more of a "luddite" than you. In theory, the upgrade sounded like a good idea to me. I'm currently using a matched quad of Winged SEDs on a stock ST-70 with an upgraded quad cap. Maybe somebody else could offer a better technical description of Dynakit's Balance Control Upgrade Kit. I'll be sure to check out Dynakitparts caps - Thanks

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Finally picked up my new (to me) ST-70-Pics

    Post by Bob Latino on Wed Nov 17, 2010 9:08 pm

    This is not meant to knock the Dynakitparts Balance Control Upgrade kit. It's a good product for ST-70 amps with the stock driver board and stock bias circuit (with just two bias pots) because it allows balancing between all four ouput tubes. If your ST-70 amp has a stock or a stock replacement driver circuit with 7199 (or 6GH8) tubes it's a good upgrade. It isn't needed, however, if you have a VTA driver board which has FOUR bias pots which allow the balancing of each output tube separately. The VTA driver board also has an AC balance control. The AC balance control allows precise setting of the phase splitter/phase inverter section of the driver circuit so that precisely half the audio signal goes to each one of the pair of output tubes. Most driver boards use regular resistors. If these resistors are not matched to within 1%, each individual tube of the output pair won't get exactly half the signal.

    Bob

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