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    newbie with some questions about ST70

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    mazeeff

    Posts : 122
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: newbie with some questions about ST70

    Post by mazeeff on Wed Apr 02, 2014 7:28 pm

    colin86325 wrote:
    mazeeff wrote:
    Are you saying that pin 1&8 of the front left el34 goes to pin 8 of both the left & right bias ports? If so, this explains the need for the switch! Also, can you confirm that each of the EL34's has a 15.6 ohm resistor? When it comes time to bias, you will need to set the bias to 1/2 of the normal 1.56v. .78v for 50ma, or .62v for 40ma. I have a similar setup, but no front/back switch.

    On the front left EL34, pins 1&8 go to pin 8 on the left bias port only. (It does not extend to the right side bias port.)

    Each EL34 has actually two 15.6k resistors in series (so 31.2k resistance).  Will this change my ideal bias voltage?

    Thanks for the help!  Once I get this business figured out I think I will be ordering the VTA driver board.

    15.6K????. it should be 15.6 ohms. Two of them in series would yield 31.2 ohms, and would put a 50ma bias back at 1.56V! The pictures, do indeed look like they are in series.

    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: newbie with some questions about ST70

    Post by colin86325 on Wed Apr 02, 2014 7:51 pm

    My mistake.  Two 15.6 ohm resistors in series on each EL34, for a total of 8 resistors.

    mazeeff

    Posts : 122
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: newbie with some questions about ST70

    Post by mazeeff on Wed Apr 02, 2014 8:17 pm

    colin86325 wrote:My mistake.  Two 15.6 ohm resistors in series on each EL34, for a total of 8 resistors.

    At this point, I would consider powering it up. Make sure to attach speakers first. Be ready to check bias on both sides (after the 30s delay). Use the switch to check both tubes. If a bias change is needed, you will need to figure out which pot goes to which tube. It does not take a lot of movement on the bias pots to make a big change. Go slow. Good luck!


    Last edited by mazeeff on Wed Apr 02, 2014 8:56 pm; edited 1 time in total

    MontanaWay

    Posts : 781
    Join date : 2014-02-06
    Age : 58
    Location : Brookings, Oregon

    Re: newbie with some questions about ST70

    Post by MontanaWay on Wed Apr 02, 2014 8:20 pm

    drum roll please!!!!!  Very Happy 

    deluxmon

    Posts : 17
    Join date : 2013-03-20
    Age : 62
    Location : DENVER

    I have a used high gain PCB with tubes you may want or need

    Post by deluxmon on Thu Apr 03, 2014 1:49 pm

    Hi there colin86325 and welcome to the wonderful world of tubes!  If you are going to strip it and upgrade the PCB. I have a used VTA / Latino high gain PCB with 3 12at7's that is perfectly functional that I'd make available to you at a good price...you can get details in the For Sale section.
    http://dynacotubeaudio.forumotion.com/t115-place-for-sale-or-wanted-audio-related-items-in-this-sticky-thread#15088

    Good luck with the project

    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: newbie with some questions about ST70

    Post by colin86325 on Sun Apr 06, 2014 6:46 pm

    Sorry for the delay.  I brought the amp over to a friend's house yesterday and we set it up (I eliminated most of my stereo system when i moved across the country, unfortunately, and am now building back up. So I had to "borrow" a system to insert the amp into). Anyway, we finally got it set up to his preamp and I promptly blew a fuse.  A trip to Home Depot and it was playing!  Albeit the radio broadcast of the local game...  
    It was a very strange set-up we had to cobble together and was not a very good audition by any stretch of the imagination, but it works!  And it did sound fine, though it would be nice to have heard music instead of a sportscaster. LOL

    Sorry for the weak review; I do intend to have a better review once I actually have my own system to use and abuse.

    I spent this afternoon stripping the paint off the perforated tube cover as the paint was blistering and the metal was rusted underneath.  This was a big mess and a PITA (pain in the neck) as the build-up kept collecting in the perforations. I think I'm just going to get the stainless chassis from Bob and a new cover.  I have a really nice spraypaint that I want to use.  It's actually Harley Davidson wrinkle coat paint and it should look great. Oh and I saw some nice pics of a copper chasis on the web, so I think the chassis will be plated in copper.

    I hope you guys won't hate me too much if I deviate from the stock look, but I gotta convice the woman it looks great.  LOL

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: newbie with some questions about ST70

    Post by sKiZo on Sun Apr 06, 2014 7:19 pm

    There's a few folk here who've felt the need to modify a perfectly good design for whatever reason ... we don't hold it against them too much.  clown 

    For the paint clogging the holes, use a scraper to get most of the old paint off, then once it's good and dry, attack it with a wire brush. One of those brass ones for cleaning barbecue grilles work real nice for that.

    Another way to texture paint is to get a small foam corner roller from the local hardware. Let the paint tack up, then lightly pull the roller across, blending as you go. Might take a couple coats to get just what you want, but doing it that way, you can go light coats and not worry about getting too much paint plugging the holes.

    Have fun with it!

    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: newbie with some questions about ST70

    Post by colin86325 on Sun Apr 06, 2014 7:43 pm

    sKiZo, I wish I would have taken your advice. I thought I was doing pretty well until the fiance started asking why she smelled chemicals inside the house. I was stripping it all outside, but had hosed it off and was going to dry it in the oven. LMAO.

    I haven't tried that method of texturizing paint, but it sounds like a nice way of saving some money over the $25 they want for a can!

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: newbie with some questions about ST70

    Post by sKiZo on Sun Apr 06, 2014 7:51 pm

    Indoor chemicals can be fun!

    My favorite was bug bombing a chassis in the basement during the winter. Project went nicely until the furnace kicked on and started pulling fumes into the air intake on the oil burner. Started getting this sic looking green smoke coming out of that.  Shocked Can't imagine it was all that healthy to breathe.

    Didna stop me from painting in the basement - but it did get me to turning the furnace off first.


    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: newbie with some questions about ST70

    Post by colin86325 on Sun Apr 06, 2014 8:03 pm

    So F'in  Funny!!!  Smile

    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: newbie with some questions about ST70

    Post by colin86325 on Sun Jul 06, 2014 4:56 pm

    Ok, I just checked my bias for the first time and it is at 0.470V.
    If you look back a few posts we discovered that this amp has a unique biasing scheme, but we concluded that I should still set to 1.56V. (There was concern that I only had one 15.5 ohm resistor, but in fact I have 2 on each tube)

    Any explanations for this low bias? Will I damage the amp by keeping it at .470V?

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: newbie with some questions about ST70

    Post by tubes4hifi on Sun Jul 06, 2014 8:24 pm

    bias setting is going to depend on what the bias measurement resistor is, and whether there is one one tube or one that is shared by a pair
    (one 15.6 ohm resistor is shared by a pair on an original Dyna ST70, and bias should be set to 1.25v to equal 40ma per tube, Dyna ran them too hot at 1.56v or 50ma per tube).
    On a VTA modified ST70, there is a 10 ohm resistor on EACH output tube, and bias voltage should measure 0.40v (on each tube) to get 40ma per tube.

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