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    Adding bias meters to next ST-70 Build ?

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    4MarkNY

    Posts : 19
    Join date : 2009-10-13

    Adding bias meters to next ST-70 Build ?

    Post by 4MarkNY on Thu Oct 13, 2011 11:15 am

    has anyone used these for bias meters in a Dynaco rebuild? I'm planning my winter project, and have a suitable amp already, and a VTA-70 board, components and bias resistors, etc.

    http://www.mojotone.com/store/0-100-DC-mA-Meter

    I'll have to find the appropriate radio chassis punch to enlarge the hole to 1.34 ( 1 5/16 plus some dremel filing....)
    But I'm not a EE, so I need to know how to wire it up, if its a viable solution.....

    Thanks
    Mark

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: Adding bias meters to next ST-70 Build ?

    Post by tubes4hifi on Thu Oct 13, 2011 12:12 pm

    very nice little meters and a great price, but make sure you use fuses in series with them, otherwise if a tube goes over current, it will take out the meter also. You'll need a chassis punch, there is a 1-3/8" on ebay for $55, they are about twice that much new

    jhoak

    Posts : 46
    Join date : 2009-10-16
    Age : 58
    Location : Central Florida

    Re: Adding bias meters to next ST-70 Build ?

    Post by jhoak on Thu Oct 13, 2011 2:12 pm

    Interesting project. Can't wait to see the finished version

    Are you going to use 4 meters or some manner of selector switch?

    4MarkNY

    Posts : 19
    Join date : 2009-10-13

    Re: Adding bias meters to next ST-70 Build ?

    Post by 4MarkNY on Thu Oct 13, 2011 3:00 pm

    I borrowed a Greenlee round Radio Chassis punch from my buddy of 1 1/8 to cut the hole in a Leslie Combo Preamp pedal I built for Sonny's Leslie tone cabinet. They work neat!
    I suppose I'd want to have 2 meters, and a switch for each, to toggle between front and back tubes , for each side.
    I also ran into this set from Weber, with a choice of up to 200Ma dial, and a Voltage companion dial....Analog Bias and High Voltage Monitor Kit: https://taweber.powweb.com/store/modkits.htm#biasmeter
    The weber version has a switch included.
    How high is the heater supply on a Dynaco ST-70? I know on a '59 bassman, its 6.3 vac, and this lights off of a filament connection.
    I know how to check and adjust my bias on my current ST-70, but I thought it's be cool to switch back and forth between the KT-66s and El-34s , and have this in place hard wired.....and mostly because it'd look cool......


    jhoak

    Posts : 46
    Join date : 2009-10-16
    Age : 58
    Location : Central Florida

    Re: Adding bias meters to next ST-70 Build ?

    Post by jhoak on Thu Oct 13, 2011 3:59 pm

    A good friend of mine did essentially the same thing but he used a single mV meter (I think) and 2 toggle switches. One switch selects left and right banks and the other selects front and back tubes. Pretty cool actually. He installed them such that one switch was oriented horizontal and the other vertical.

    Unfortunately he passed away a little over a year ago. His kids sold off all of his "old audio crap" to the highest bidder on eBay. As I recall his ST-70 brought pretty good $$ for them. More than I could afford at the time. I really wanted his "as new" walnut Klipsch Belles but they ended up selling for really stupid money. Oh well...

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: Adding bias meters to next ST-70 Build ?

    Post by baddog1946 on Tue Nov 01, 2011 9:59 am

    I found this meter a couple of weeks ago and ordered one for my new amp project. I got it this morning direct from China to Costa Rica. It is an excellent product. quite small and very robust construction. It was made for an aviation panel and on testing it is spot on. They are cheap $18 plus shipping. Worth a look on eBay for those interested.

    http://www.ebay.com/itm/100mA-mA-meter-BIAS-300B-2A3-6L6-EL34-6550-Tube-Amp-/400229187733?_trksid=p3286.m7&_trkparms=algo%3DLVI%26itu%3DUCI%26otn%3D3%26po%3DLVI%26ps%3D63%26clkid%3D3888463526870752327

    4MarkNY

    Posts : 19
    Join date : 2009-10-13

    Does that meter light up ?

    Post by 4MarkNY on Tue Nov 01, 2011 10:35 am

    Nice looking....I couldn't find any reference to lights on it, so I'm presuming it does not light up ?
    The 34 mm comes out to 1.33858 inches in my language, so I'd have to ream out those existing holes a bit, but hey, its a custom job......

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: Adding bias meters to next ST-70 Build ?

    Post by baddog1946 on Tue Nov 01, 2011 9:54 pm

    Yep, you are right it has no light. But if it is only used for re-biasing then it could be done without a light.
    A light would look cool though. I thought that since you were using an original chassis which is quite shallow that it was small enough for the flange holes to fit neatly on a ST-70 chassis at 40 mm square and a little reaming of the existing octal hole should do it.
    That's why I made my new custom ST-120 chassis a little bigger. It also lets me mount the driver board parts on the bottom of the driver board except for the tubes and bias pots. Check out my chassis on photobucket at:

    http://s1212.photobucket.com/albums/cc446/baddog1946/
    http://i1212.photobucket.com/albums/cc446/baddog1946/NEWSCANST-120ALUMINUMTOPEMAIL.jpg

    bcinnamon

    Posts : 26
    Join date : 2011-05-07

    Re: Adding bias meters to next ST-70 Build ?

    Post by bcinnamon on Thu Nov 03, 2011 7:46 pm

    Is there a way to set these meters up as power meters -- display output wattage of each tube, or combination of tubes? You could also use a switch to toggle between bias and power.

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