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    Dumb Question on Setting Bias

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    mkcarnut

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    Dumb Question on Setting Bias

    Post by mkcarnut on Fri Nov 18, 2016 8:15 am

    I've searched the forum so forgive me if this has been answered a dozen time as I could not find an answer. But, I am looking into building a pair of M125's, and I want to know - when adjusting the bias, I see lots of info on what the settings would be, etc - but nothing on where the adjustment is actually done. Do I have to take off the bottom plate to get to it?

    I am looking to build a custom case and just want to make sure in my planning process that I make sure I provide easy access to wherever the adjustment is made.

    Also, the bias test points are on the front, but for aesthetic reasons - in the case I am designing I would like to move them (and the power switch) to the side. From what I can see of the images on the tubes4hifi site, I'd have to run longer wires and would do so around the perimeter of the output tubes over to the side. Would that cause any issue? Any special watchouts for that?

    Thanks, Mark
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    Bob Latino
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    Re: Dumb Question on Setting Bias

    Post by Bob Latino on Fri Nov 18, 2016 9:14 am

    mkcarnut wrote:I've searched the forum so forgive me if this has been answered a dozen time as I could not find an answer.  But, I am looking into building a pair of M125's, and I want to know - when adjusting the bias, I see lots of info on what the settings would be, etc - but nothing on where the adjustment is actually done.  Do I have to take off the bottom plate to get to it?

    I am looking to build a custom case and just want to make sure in my planning process that I make sure I provide easy access to wherever the adjustment is made.

    Also, the bias test points are on the front, but for aesthetic reasons - in the case I am designing I would like to move them (and the power switch) to the side.  From what I can see of the images on the tubes4hifi site, I'd have to run longer wires and would do so around the perimeter of the output tubes over to the side.  Would that cause any issue?  Any special watchouts for that?

    Thanks, Mark

    Hi Mark.

    There is a detailed paragraph in the VTA M-125 manual on how to set the bias for your amp.

    1. If you use just TWO output tubes, the recommended bias setting is .50 volts DC per each side. If you use FOUR output tubes, the recommended bias setting is 1.00 volts DC per each side. You do not have to take the bottom cover off. You use a multimeter set for the 0 to 2 volt DC scale. You place the red probe into the left bias test point and the black probe anywhere on the chassis. You turn the left bias potentiometer on the driver board until the meter reads the appropriate value. You then repeat for the right side tube(s) with the red probe in the right bias test point and adjust with the right bias potentiometer on the driver board.

    2. You can place the bias test points anywhere on the amp. It is more convenient to have them on the front for easy access, but you can place them on the top or sides of the amp if you want to ..

    Bob

    mkcarnut

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    Re: Dumb Question on Setting Bias

    Post by mkcarnut on Fri Nov 18, 2016 10:22 am

    Thanks Bob - I am trying to see the bias potentiometer from the pic on the website, but the image angle makes it a little hard to see. Are those the little blue blocks with the silver flat blade screw in them?

    Thanks! Mark
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    Bob Latino
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    Re: Dumb Question on Setting Bias

    Post by Bob Latino on Fri Nov 18, 2016 10:30 am

    mkcarnut wrote:Thanks Bob - I am trying to see the bias potentiometer from the pic on the website, but the image angle makes it a little hard to see.  Are those the little blue blocks with the silver flat blade screw in them?

    Thanks!  Mark

    Yes - Those two small blue blocks are the two bias pots. You just use a small flat bladed screwdriver to adjust them .. On the newer M-125's they are tan in color ..

    Bob

    mkcarnut

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    Re: Dumb Question on Setting Bias

    Post by mkcarnut on Fri Nov 18, 2016 10:36 am

    Great - thanks! As I design my custom case, that really helps me make sure I keep those readily accessible, and some options I was debating would have made them harder to reach.

    I was going to build some standard M125's, but those custom wooden cases shown on the main page got me thinking about doing my own custom case from wood and smoked glass (to match my custom wood cases on my nCore Hypex and to match my custom stereo stand).

    Mark
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    deepee99

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    Re: Dumb Question on Setting Bias

    Post by deepee99 on Sat Nov 19, 2016 8:49 pm

    Mark,
    Several people on this Forum install the driver boards upside-down when building a custom chassis, which keeps those big hurkin' Russian PIO caps underground. But you still have to put the bias pots on top for access purposes, which reverses their turn. In other words, a clockwise turn will lower the bias to the power tubes; and an anti-clockwise turn will increase it. So when installing new output tubes, start with the little blue or brown pots all the way to the right, then back them to the left as the amp heats up.

    mkcarnut

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    Re: Dumb Question on Setting Bias

    Post by mkcarnut on Sun Nov 20, 2016 7:50 am

    Interesting! Thanks for the info!

    Mark
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    bluemeanies

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    Re: Dumb Question on Setting Bias

    Post by bluemeanies on Tue Nov 22, 2016 12:45 pm

    mkcarnut wrote:Interesting!  Thanks for the info!

    Mark


    Also, a recommendation about the screwdriver to adjust bias.
    Purchase a ceramic screwdriver to avoid any shock or potential problems.
    They can be had online "CHEAP"
    Also it is wise to purchase a better than cheap voltmeter like a "FLUKE"
    I purchased a cheap meter from a TUBE out fit whose name I won't mention and it was the worse.

    Frank

    mkcarnut

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    Re: Dumb Question on Setting Bias

    Post by mkcarnut on Tue Nov 22, 2016 1:12 pm

    Excellent idea! Thanks!
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    Maintarget

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    Re: Dumb Question on Setting Bias

    Post by Maintarget on Tue Nov 22, 2016 3:27 pm

    You can make it easy on yourself and NOT flip the CB and just mount the caps underneath as long as polarity is observed shouldn't be a problem I did this on my ST-120 build with no issues.
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    deepee99

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    Re: Dumb Question on Setting Bias

    Post by deepee99 on Tue Nov 22, 2016 5:00 pm

    Maintarget wrote:You can make it easy on yourself and NOT flip the CB and just mount the caps underneath as long as polarity is observed shouldn't be a problem I did this on my ST-120 build with no issues.
    Aw, that would be too easy!
    As to meters, there are only two ways to go. Junk or Fluke.
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    Peter W.

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    Re: Dumb Question on Setting Bias

    Post by Peter W. on Tue Nov 22, 2016 5:17 pm

    deepee99 wrote:
    Maintarget wrote:You can make it easy on yourself and NOT flip the CB and just mount the caps underneath as long as polarity is observed shouldn't be a problem I did this on my ST-120 build with no issues.
    Aw, that would be too easy!
    As to meters, there are only two ways to go. Junk or Fluke.

    Oh, I dunno... I have a B&K LCR meter, made in USA, that is no slouch at all.
    And, if you like analog meters, Simpson is still in business, and still makes some very good stuff.

    I also do keep a Fluke, of course.

    GP49

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    Re: Dumb Question on Setting Bias

    Post by GP49 on Wed Nov 23, 2016 6:53 pm

    Peter:

    Among the stuff that came out of that basement, along with the hundreds of tubes you picked up, were a Simpson 260 VOM and an RCA Senior VoltOhmyst VTVM. I couldn't resist keeping them, though I will probably never use them, beyond putting them back to original performance.

    I drew the line at the Tektronics scope dating from 1948, though. Could hardly lift it and its bandwidth would not have been enough to align the RF and IF of the Pilot FM tuner I salvaged.
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    Tube Nube

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    Re: Dumb Question on Setting Bias

    Post by Tube Nube on Wed Nov 23, 2016 9:37 pm

    I regret I didn't build mine with those massive Rusky paper-in-oil-skis standing big an proud atop the circuit board. Instead they were made to hide out underneath, as if ashamed of themselves.
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    deepee99

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    Re: Dumb Question on Setting Bias

    Post by deepee99 on Thu Nov 24, 2016 8:22 am

    Tube Nube wrote:I regret I didn't build mine with those massive Rusky paper-in-oil-skis standing big an proud atop the circuit board. Instead they were made to hide out underneath, as if ashamed of themselves.
    Just be sure you take off your wristwatch and other metal gimckracks before tweaking the bias pots. A friend gave me, a couple of years ago, an all-wooden screwdriver that just fits the bias pots. Even the blade is wood. So no matter how wobbly you are, you're reasonably safe from a heart-starting voltage blast.
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    Peter W.

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    Re: Dumb Question on Setting Bias

    Post by Peter W. on Thu Nov 24, 2016 9:04 am

    Just be sure you take off your wristwatch and other metal gimckracks before tweaking the bias pots. A friend gave me, a couple of years ago, an all-wooden screwdriver that just fits the bias pots. Even the blade is wood. So no matter how wobbly you are, you're reasonably safe from a heart-starting voltage blast.[/quote]

    Hmmm..... These are the same people who do not 'cage' their amps?   tongue
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    deepee99

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    Re: Dumb Question on Setting Bias

    Post by deepee99 on Thu Nov 24, 2016 10:18 am

    Ever caught your wedding ring on a timing belt while the engine was running? That was interesting.
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    Bob Latino
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    Re: Dumb Question on Setting Bias

    Post by Bob Latino on Thu Nov 24, 2016 10:26 am

    Tube Nube wrote:I regret I didn't build mine with those massive Rusky paper-in-oil-skis standing big an proud atop the circuit board. Instead they were made to hide out underneath, as if ashamed of themselves.

    Keep in mind that the VTA driver board is larger than the opening in the chassis and must be underhung below the chassis. The Russian PIO caps won't really fit on the top of the VTA driver board. The outside lead on all four caps would hit the chassis if you placed them on the top of the board. Yes - You could insulate the outer lead that would rub against the chassis but it is still not a good idea.

    As to "wooden screwdrivers" .. I use a small METAL screwdriver to adjust the bias pots which has an insulated plastic handle.. Yes - it is not a good idea to use a "jeweler's screwdriver" that is all metal. You could, however, wrap the handle in electrical tape on a jewelers screwdriver and get the job done ..

    Bob
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    Dave_in_Va

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    Re: Dumb Question on Setting Bias

    Post by Dave_in_Va on Thu Nov 24, 2016 11:03 am

    I've got a VTA ST 70. Should I be worried about my cat jumping up on the amp and electrocuting herself?
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    Peter W.

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    Re: Dumb Question on Setting Bias

    Post by Peter W. on Thu Nov 24, 2016 11:18 am

    Dave_in_Va wrote:I've got a VTA ST 70. Should I be worried about my cat jumping up on the amp and electrocuting herself?


    As unlikely as that may seem, it is possible. I do not let our cats, dogs or grandkids near an operating uncaged amp. Between hot tubes and high voltage, it simply isn't worth it.

    They put tops on these things for a reason back in the day.
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    peterh

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    Re: Dumb Question on Setting Bias

    Post by peterh on Thu Nov 24, 2016 11:22 am

    Dave_in_Va wrote:I've got a VTA ST 70. Should I be worried about my cat jumping up on the amp and electrocuting herself?
    Not really. Cats are cheap. But they might damage some tubes that is expensive.
    Get a st-70 cage , that will protect from cats, but they will hum ...

    The best is probably to make sure the amp is located in a way that the cat is not tempted to
    go there.

    You have the same concern with kids. Noone knows what a little kid might poke with.
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    Bob Latino
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    Re: Dumb Question on Setting Bias

    Post by Bob Latino on Thu Nov 24, 2016 11:23 am

    Dave_in_Va wrote:I've got a VTA ST 70. Should I be worried about my cat jumping up on the amp and electrocuting herself?

    I have a cat here also .. My cat has never shown any interest in the amp and does not go near the amp whether it is on or off. If you do worry that your cat, dog or a small child may touch your amp, then either buy a tube cage or place the amp up high enough on a table so they won't be able to touch the amp.

    Bob
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    Dave_in_Va

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    Re: Dumb Question on Setting Bias

    Post by Dave_in_Va on Thu Nov 24, 2016 11:27 am

    Is there anywhere to buy a cover for the board that's ready to go? I just dislike the look of those cages.

    I've seen pics here where the board was covered with a piece of glass or something (with holes for the three tubes, bias pots, etc.

    Thanks.
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    deepee99

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    Re: Dumb Question on Setting Bias

    Post by deepee99 on Thu Nov 24, 2016 11:30 am

    Dave_in_Va wrote:I've got a VTA ST 70. Should I be worried about my cat jumping up on the amp and electrocuting herself?

    Depends on how you like your cat. Is she worth a $100 output tube?
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    Dave_in_Va

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    Re: Dumb Question on Setting Bias

    Post by Dave_in_Va on Thu Nov 24, 2016 11:47 am

    Yes, I like my cat more than a freakin' tube.
    I just ordered a cover for the ST70 (in black).

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