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    It's ALIVE!!!

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    crashb4

    Posts : 13
    Join date : 2016-01-07
    Age : 55
    Location : Monson, Ma.

    It's ALIVE!!!

    Post by crashb4 on Fri Jan 15, 2016 11:08 am

    I received this unit on Saturday 1/9. Got a great deal on it and have wanted to go to tubes for some time. It was reported as having been removed from service and shipped. If I had a dime for every time I heard that. I tested all components and all measured in spec. The power supply and bias circuit have been upgraded. I found a toasty 15.6 ohm resister at V7 pin one to ground, but it tested on the nose. Poor connectivity at the connector and tube pin seems to be the cause of the heat. A new VTA board will eliminate this resister. I rounded the oblong connectors as best as I could to improve current flow, plugged in the tubes and crossed my fingers.

    No source or load applied as yet. I just wanted to check for operation minus smoke. I will set the bias and see if there is sound.
    Here is a pictorial of the amp. If anyone can ID the board I would appreciate it. The PO had no information on it.


    corndog71

    Posts : 444
    Join date : 2013-03-19
    Location : It can get windy here

    Re: It's ALIVE!!!

    Post by corndog71 on Fri Jan 15, 2016 12:22 pm

    I could be wrong but the driver board looks similar to one that came with my used ST70. Sound Values if I remember correctly. I believe it's the original driver circuit but using 6GH8 tubes instead of 7199. I wasn't too thrilled by the sound of it although I was just dipping my toes into tube amp land back then. I had a lot to learn. I ended up replacing it with Joe Curcio's driver circuit which did sound better. Now I think VTA circuit is the best one out there for the ST70.

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: It's ALIVE!!!

    Post by deepee99 on Fri Jan 15, 2016 12:55 pm

    Nice! Be careful about running it without a load, though. Any old thrashed-out speaker will work, but I'm told you can hurt a tube amp if there isn't a load across the speaker terminals.

    crashb4

    Posts : 13
    Join date : 2016-01-07
    Age : 55
    Location : Monson, Ma.

    Re: It's ALIVE!!!

    Post by crashb4 on Fri Jan 15, 2016 1:07 pm

    The VTA will be the board for me. Already determined that. Thanks for the info on the current board corndog71.

    deepee99, I ran the unit for about 90 seconds to see if the candle would light. Very pleased that it did. I had plug in hand as I turned it on for fear of disaster. The wife thought I was nuts. I guess she's not too far off.

    Doctor Hugocat

    Posts : 75
    Join date : 2015-08-19
    Location : Los Angeles CA

    Re: It's ALIVE!!!

    Post by Doctor Hugocat on Fri Jan 15, 2016 1:18 pm

    crashb4 wrote:I had plug in hand as I turned it on for fear of disaster. The wife thought I was nuts. I guess she's not too far off.

    When I fire mine up for the first time, you can BET I'll be doing the same! With a fire extinguisher in the other hand most likely... Laughing

    I like the pics and the slideshow, glad you got a good deal! Keep us updated with progress.......

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: It's ALIVE!!!

    Post by j beede on Fri Jan 15, 2016 10:48 pm

    Dim bulb!

    Doctor Hugocat

    Posts : 75
    Join date : 2015-08-19
    Location : Los Angeles CA

    Re: It's ALIVE!!!

    Post by Doctor Hugocat on Fri Jan 15, 2016 11:21 pm

    BTW this is a factory-built unit, right? Looks like it had the cage over the board originally and riveted tube sockets......experts please correct me!

    crashb4

    Posts : 13
    Join date : 2016-01-07
    Age : 55
    Location : Monson, Ma.

    Re: It's ALIVE!!!

    Post by crashb4 on Sat Jan 16, 2016 6:27 am

    Doctor Hugocat wrote:BTW this is a factory-built unit, right? Looks like it had the cage over the board originally and riveted tube sockets......experts please correct me!

    Yes. This is factory wired. All rivets where they are supposed to be and model designation of 70A on the front apron of the chassis. This leaves me in a quandary. The tube sockets need to be replaced; but I don't want to alter the factory wired look. Has anyone replaced the terminals in the existing sockets? Is it even possible?

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: It's ALIVE!!!

    Post by deepee99 on Sat Jan 16, 2016 12:12 pm

    crashb4 wrote:
    Doctor Hugocat wrote:BTW this is a factory-built unit, right? Looks like it had the cage over the board originally and riveted tube sockets......experts please correct me!

    Yes. This is factory wired. All rivets where they are supposed to be and model designation of 70A on the front apron of the chassis. This leaves me in a quandary. The tube sockets need to be replaced; but I don't want to alter the factory wired look. Has anyone replaced the terminals in the existing sockets? Is it even possible?

    You might try socket-savers that are used by tube-rollers and on testers. There are plenty on FleaBay:
    http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1311.R1.TR3.TRC2.A0.H3.Xtube+socket+saver.TRS0&_nkw=tube+socket+saver&_sacat=0
    That would preserve the wiring below-decks and your original sockets. You'd probably have to lose the cage as these raise the tubes up an inch or two.

    crashb4

    Posts : 13
    Join date : 2016-01-07
    Age : 55
    Location : Monson, Ma.

    Re: It's ALIVE!!!

    Post by crashb4 on Sat Jan 16, 2016 2:35 pm

    That's a great idea. I don't care about the cage. I prefer the look without it anyway. Hooked up my old crappy BIC speakers this morning. Set the bias from 2.7 left and 3.1 right, to 1.5 each and cranked her up. Got a little power switch noise and hiss. Then a most pleasant sound came from those junky old speakers. Heaven!

    Thanks for the suggestion. I really appreciate all input. Lots to learn.

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: It's ALIVE!!!

    Post by deepee99 on Sat Jan 16, 2016 2:54 pm

    crashb4 wrote:That's a great idea. I don't care about the cage. I prefer the look without it anyway. Hooked up my old crappy BIC speakers this morning. Set the bias from 2.7 left and 3.1 right, to 1.5 each and cranked her up. Got a little power switch noise and hiss. Then a most pleasant sound came from those junky old speakers. Heaven!

    Thanks for the suggestion. I really appreciate all input. Lots to learn.

    No worries. If we don't know the right answer to your question we'll make something up that sounds plausible Smile
    I like the socket-savers idea, gets some of the tube heat away from direct contact with the chassis.
    Sounds like you're making great headway. Congrats!

    You might try a squirt of tuner-cleaner or DeOxit (from any hardware store) on that power switch, give it a good blast then cycle the power switch a few times with your amp un-plugged. Likely just a dirty contact and that should fix it. Tuner-cleaner and DeOxit are harmless to electronics and work wonders if you've got a funky contact.
    I always fire my preamp up first, let it go through the old-tube spitting and hissing warm-up act, then power the amps up.
    Cheers,
    dave

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