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    Custom Chassis?

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    jjones3318

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    Re: Custom Chassis?

    Post by jjones3318 on Sat Oct 12, 2013 9:33 pm

    I think Dave missed that you were switching the meter between two circuits. An mA meter has to have the current go through it to measure anything. When you switch that meter out of the circuit, you also lift the cathode from ground.

    The easiest way for you to fix this is to use mV meters and not mA meters. You can then hook the meters up to measure across the cathode resistor, and switching the meter out of the circuit won't disrupt anything.


    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Sun Oct 13, 2013 12:13 am

    I'm getting confused here jumping boards ... woosie, even. Rolling Eyes 

    It's been suggested that with the current setup, I'm opening a cathode here, shutting off one (or more) tubes. That would definitely cause distortion on a push/pull ... yah ... definitely.

    And just to be sure, I not only have to have constant current with an mA meter, I also have to keep the +/- sides of the meter isolated for them to work on a two tube set? Can't get there from here, can I ...

    I did find these over on da Bay ...



    They're narrower than what I got, but the height is good ... that's the important part with the sloped panel. I might have to dremel out the holes a bit, but hopefully not that they'd show, and even then, I could always add a bezel plate behind them if necessary.

    Think it'd be a simple meter swap then? I hope I hope I hope ...

    Bob Latino
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    Re: Custom Chassis?

    Post by Bob Latino on Sun Oct 13, 2013 8:49 am

    sKiZo,

    I like measuring bias with a voltmeter. When you do that you are measuring the voltage drop across a resistor of a known value and the voltmeter itself is not really in the circuit. An ampmeter has to be "ïn the circuit" to really measure current flow. The ampmeter itself could then possibly affect the circuit OR because of the internal resistance of the ampmeter, give an inaccurate reading.

    Personally, I would change out your ampmeters to the millivolt meters you show in your post.

    Bob

    baddog1946

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    Re: Custom Chassis?

    Post by baddog1946 on Sun Oct 13, 2013 9:17 am

    Right on Bob!
    Mv. metrs are the way to go IMHO. The swap should be relatively simple. Hope this solves your issues Skizo.
    I know you will Keep us posted (with maybe a shematic of the circuit that finally works)?
    I for one am curious.
    cheers
    Baddog

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Sun Oct 13, 2013 4:40 pm

    So, the consensus of opinion is I should be able to use the same circuit currently installed with a meter swap? That'd be too easy ...

    I did order up a couple of the mV meters, so that gives me a break in the action while they wend their way to my front step ...

    Dug up an old schematic of an HK Citation II and traced out what I figure to be the important connections for just a simple bias check. The Citation I'm told used an mA meter ...



    It looks like they used an additional layer on their fancy switch to kill the meter to ground via a resistor when the amp's in use. Substitute an SPDT switch to select front and rear on a bank, and the kill switch would need to be a DPST to keep the cathode taps isolated from each other?

    Other than that, it doesn't look any different from my first approach though, which is where I was getting the 15mA discrepancy between the bias test points and meter, unless it's just a difference in where the test points are located in the circuit ...

    Are we having fun yet? tongue

    jjones3318

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    Re: Custom Chassis?

    Post by jjones3318 on Sun Oct 13, 2013 4:46 pm

    No. The meter in that schematic is measuring voltage across the cathode resistor - it's a volt meter. It may say mA on the front, but it's measuring voltage.


    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Sun Oct 13, 2013 4:54 pm

    If I'm reading this right, the major difference between the meter types is that an mV would provide a constant connection in the primary circuit and just provide a reading based on a sample ...

    Brings me back to my original question ... would a simple swap from an mA to an mV meter in the circuit I'm currently using prevent the cathode breaks that are causing the distortion? I'm getting good readings at both the meter and VOM test points with the current connections. I'd also want to swap out the center OFF DPDT's for some that keep continuity when cycling to prevent the popping from the speakers.

    Sorry if I'm going in circles here, but I'm like ... going in circles here.

    jjones3318

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    Re: Custom Chassis?

    Post by jjones3318 on Sun Oct 13, 2013 5:14 pm

    I think you're over thinking it.

    You measure current by measuring the voltage drop across a resistance. For the current through the tube, you measure the voltage drop across the cathode resistor (ohms law) - this is what you're doing with your DVM and the test points in an 'out of the box' ST-120.

    An mA meter will typically have the resistance (shunt) inside the meter. When using an mA meter, it has to be in the circuit - the current has to go THROUGH the meter. This is what Dave on AK told you: pin 8 to meter +, meter - to one side of the cathode resistor, and the other side of the cathode resistor to ground. The meter is in series with the cathode resistor, so if you switch the meter out like you were the cathode resistor, and thus the tube, are no longer connected to ground - no current can flow.

    If you use an mA meter as you first did, you have the cathode resistor, and the shunt (resistor) in the meter in parallel. This is why you got one reading with the meter connected to a tube, and another without it. Your DVM reading would only have been accurate with the meter out of the circuit, and the meter was never accurate as the cathode resistor shunted it.

    So, you need to:

    A) figure out a switching mechanism that maintains the current flow with your mA meter not in the circuit as Dave explained
    B) use an mV meter and go back to how you wired it the first time
    C) go back to how you wired it the first time and remove the internal shunt from your existing mA meters, AND add a series resistor so the meter measures voltage correctly.

    The simplest solution is get new mV meters and go back to how you wired it the first time.

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Sun Oct 13, 2013 6:21 pm

    Totally confused at this point ... Been thru a few versions now.

    So ... THIS ...



    Only with mV instead of mA meters?

    That's basically the original stock circuit with the meters piggybacked onto the test points ... that once again, didn't have the distortion, but is what was giving me the 15mA (or mV, or whatever ... just semantics in this case as it's simply describing the needle position) discrepancy between the test points and meters ... which is what started all this.

    So ... same as above, BUT with the addition of a VR coming into the meter to correct the meter to match the test points?

    jjones3318

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    Re: Custom Chassis?

    Post by jjones3318 on Sun Oct 13, 2013 7:02 pm

    sKiZo wrote:Totally confused at this point ... Been thru a few versions now.

    So ... THIS ...



    Only with mV instead of mA meters?
    Yes.

    sKiZo wrote:That's basically the original stock circuit with the meters piggybacked onto the test points ... that once again, didn't have the distortion, but is what was giving me the 15mA (or mV, or whatever ... just semantics in this case as it's simply describing the needle position) discrepancy between the test points and meters ... which is what started all this.
    Not just semantics.  It gave you discrepancy because you had mA meters in parallel with the cathode resistor, and the mA meter has a shunt (resistor) in it.  What happens when you put two resistors in parallel?

    sKiZo wrote:So ... same as above, BUT with the addition of a VR coming into the meter to correct the meter to match the test points?
    Same as above with NEW meters that are mV meters, with no VR. This is the easiest thing to do.

    The series R or VR comes in to play if you modify your EXISTING meters to remove the shunt.

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Sun Oct 13, 2013 7:26 pm

    Well, hell's bells ... that'd be TOO easy ...

    ... couldn't be right ... or could it?? What a Face 

    If this works, that puts me almost back to where I was ... and what a long strange trip it's been ...

    Got the mV meters on the way ... in the meantime, I can rewire the system to the original piggyback circuit, and just not hook up the meters until the new ones get in ...

    One last question that's probably already been answered a hundred times, but bear with me ... any issues with turning the meters off with this circuit? Seems I'll still have a pop when switching between tubes in a set, but that's really not all that bad and is unaffected by load.

    Also, it's been suggested I use SP3T switches instead of the SPDT's ... same thing, only "make before break" ... those "should" eliminate any pop or noise when switching ...

    jjones3318

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    Re: Custom Chassis?

    Post by jjones3318 on Sun Oct 13, 2013 7:51 pm

    sKiZo wrote:Well, hell's bells ... that'd be TOO easy ...

    ... couldn't be right ... or could it?? What a Face 

    If this works, that puts me almost back to where I was ... and what a long strange trip it's been ...

    Got the mV meters on the way ... in the meantime, I can rewire the system to the original piggyback circuit, and just not hook up the meters until the new ones get in ...

    One last question that's probably already been answered a hundred times, but bear with me ... any issues with turning the meters off with this circuit? Seems I'll still have a pop when switching between tubes in a set, but that's really not all that bad and is unaffected by load.
    No.  I'd recommend turning them off when you're not checking the bias.

    sKiZo wrote:Also, it's been suggested I use SP3T switches instead of the SPDT's ... same thing, only "make before break" ... those "should" eliminate any pop or noise when switching ...
    1) an SP3T switch is not the same as a SPDT - double throw vs. triple throw. Make before break, or a shorting switch, is an independent spec.
    2) this was recommend to you if you were going to use mA meters, which you're not.

    Get the mV meters, and put it back to exactly how you had it originally.

    And, just for clarity sake, the meter you posted a picture of on your AK thread is an AC Volts meter - not going to work, as the scale is wrong and it most likely has a rectifier and voltage divider in it.  You want something like THIS..

    Bob Latino
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    Re: Custom Chassis?

    Post by Bob Latino on Sun Oct 13, 2013 8:11 pm

    sKiZo wrote:I'd also want to swap out the center OFF DPDT's for some that keep continuity when cycling to prevent the popping from the speakers.
    sKiZo - where did you get these switches ? The DPDT triode/ultralinear switches supplied with the VTA ST-120 do not have a "center off" ?

    Bob

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Sun Oct 13, 2013 8:38 pm

    jjones3318 wrote:No.  I'd recommend turning them off when you're not checking the bias.

    1) an SP3T switch is not the same as a SPDT - double throw vs. triple throw. Make before break, or a shorting switch, is an independent spec.
    2) this was recommend to you if you were going to use mA meters, which you're not.

    Get the mV meters, and put it back to exactly how you had it originally.
    OK then ... same as before ... with ONE minor change then. SPDT's I used before didn't have a center off ... I could still use the SP3T's if, like ... for instance ... I clicked the buy button before you stopped me? clown 

    And, just for clarity sake, the meter you posted a picture of on your AK thread is an AC Volts meter - not going to work, as the scale is wrong and it most likely has a rectifier and voltage divider in it.  You want something like THIS..
    That meter was more for some room for the bias to bounce under load, but that won't be an issue if I turn the meters off during play. I did fire off an email requesting the actual range of the 0-300 meter I posted just to make sure, so should be interesting to see what the range really is.

    Thanx ... but your meter recommendation won't work for me due to size, but I did find these ...



    The mA meters I'm using now are 60x47 and these are 55x47. Not as pretty, but sized to fit the front panel. Only case mods might be having to do a little dremel work on the holes as the back is different. Once again ... I'm amazed at how few options there are for mV meters until you get into the uber expensive stuff, and those tend towards large too.

    Bob Latino wrote:sKiZo - where did you get these switches ? The DPDT triode/ultralinear switches supplied with the VTA ST-120 do not have a "center off" ?

    Bob
    Glad I'm not the only one confused ... but I do have that affect on people ... drunken 

    The switches I was referring to are those added to allow me to switch the meters between front and back tubes. These ...


    jjones3318

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    Re: Custom Chassis?

    Post by jjones3318 on Sun Oct 13, 2013 9:09 pm

    My only caution on getting 300mV meters is the scale resolution. At the end of the day, the important part is dialing in your current within a couple mA of where you want it. That won't be easy if each graduation is 4-5mA instead of 1-2.

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Mon Oct 14, 2013 1:54 am

    I'll still use the test points and VOM regularly. Where I really appreciated the meters (when they were working ... sort of) was being able to turn on the amp, go get a cuppa, then just flip the switches for a quick check prior to getting comfy. And more importantly of course ... BLING!!

    I did order the 100mV meters I pic'd earlier - those should do the trick and won't be banging the stops as long as I turn them off when listening. Probably won't see them before the end of the week at the earliest, so I'll have to suffer with the McIntosh ss amp in the meantime ... the sacrifices we make ... tongue 

    PS ... that will be my next project. The MC2205 is due for some serious TLC ... 35 years old and starting to show it's age. Hard part will be getting it down in the shop ... maybe a crane ...

    ValleySounds

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    Re: Custom Chassis?

    Post by ValleySounds on Wed Oct 23, 2013 3:15 am

    Is a 100 milivolt meter going to work? I thought there was 1000 milivolts in a volt and we are setting the bias at 550 milivolts. Way off the scale of the meter.

    I may be confused by these 2 pictures

    and

    anbitet66

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    Re: Custom Chassis?

    Post by anbitet66 on Wed Oct 23, 2013 8:45 am

    ValleySounds,

    The meters in the photo are mA (milliamp) meters.  They are tied into the circuit in series with the output tube.  Open the cathode circuit and insert the meter.  The mV (millivolt) meter is different as it must be tied in parallel to the component to be measured.  This would be the cathode resistor (in this case a 10 ohm resistor).  Normally the cathode resistor would be connected between the cathode and ground, and a wire would be attached to the cathode and a tie point so a direct reading would happen with an external meter connected to the tie point and ground.

    Now, if you were to purchase a meter to use internally, a mV meter would work provided a resistor, when connected in series with the meter, is used to drop the excess voltage to allow the meter to read a different voltage at its full scale.  What I mean is lets say the meter purchased reads at full scale 100mV or 0.10 volts.  The meter won't work unless a resistor is placed in series with the meter movement to drop another 900mV or 0.90 volts.  This allows the meter to read to 1 volt full scale, and by multiplying the reading in your head by 10 allows the use of the original scale.

    To do this, the meter should have a spec sheet detailing what the full scale voltage is and what the meters internal resistance is, or sometimes its ohms-per-volt (or sensitivity) rating.  I'd calculate the required resistor, then use the nearest lower value, and take up the slack with a small trimmer resistor to allow a fine adjustment to allow the meter to read exactly what an accurate multimeter would read.

    I'm sorry if this is a bit technical.  Hopefully this clarifies why some prefer the convenience of an internal meter.  With a little tech savvy, anything is possible study .

    If you want to get fancy, the mV meter can be used as is, just reduce the cathode resistor to 1 ohm and the voltage drop across it becomes 55mV or 0.055 volts.  Well within the maximum deflection of the meter. Cool   Tony

    ValleySounds

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    Re: Custom Chassis?

    Post by ValleySounds on Wed Oct 23, 2013 1:02 pm

    Thanks anbitet66
    I don't want to hijack the thread just wanted to make sure the OP's new meters are going to work. In the pics above I was looking at the DMM. The decimal point moves over a place in each pic and that was confusing me.

    So to make the 100 mv meters work you can put a divider resistor in series with the meter OR change the cathode bias resistors to 1 ohm and wire pin 8 to + of the meter?

    tubes4hifi
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    Re: Custom Chassis?

    Post by tubes4hifi on Wed Oct 23, 2013 1:11 pm

    in post 116 you say you are getting the 100mV meters as shown in post 114.
    That's the way I would go, then you just need a switch to connect the meter across a ONE OHM cathode resistor, so the meter will read 55mv which will be 55ma of tube current.
    Personally I would never use a mA meter in this situation, if the tube current ever exceeds the meter max (100mA) then the meter is toast.

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Wed Oct 23, 2013 1:50 pm

    All good stuff ... this keeps up, I'll have the knowledge to put a meter on most anything, including my toaster. tongue 

    Meter circuit v3.2 (beta)

    Didn't like the mV meters ... they're made to mount behind the panel and aren't going to work unless I add an extra bezel and space that out another half inch up from the main panel. Yuk. So ... back to one of the original ideas of using the original mA meters, but converting them to mV ...

    Snip!



    Easy so far ... the clear covers pop right off and two screws to remove the scale. Just bent the shunt resistor lead out of the way and that's the only internal mod. I went with a VR on the plus terminal instead to adjust the meter to match the circuit. They're log scale, and I'll have them turned all the way down prior to powering up the first time.



    Had some nice 50k laying about - those might be a bit twitchy to adjust, but no worries about not having enough range to work with. Mounting them was simple enough - soldered them in, then hot glued to the back of the meter is all.

    NOTE >> There IS another resistor already in place on the hot lead ... 0.75 ohm ... I left that in place for now. The external VR should still be able to match the meter, unless I end up with a negative value. Those HopSun meters actually have quite a bit of room to work in, so be easy enough to remove if it's an issue.

    That's as far as I got last night - it was getting late, and I want to fire it up without the meter wiring in place just to make sure things are good as is. All that's left then is to wire them into the amp. The amp's been rewired to the original schematic otherwise. The meter feeds will just be piggybacked onto the same test points I use for the VOM readings and be selectable with the SPDT mini switches which now have a center off position.

    Short version ... I'm pretty much back where I was, only with the modded meters w/VR and the center off switches ...

    Anyway ... that will leave me with this as the final(?) circuit ...


    anbitet66

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    Re: Custom Chassis?

    Post by anbitet66 on Wed Oct 23, 2013 11:18 pm

    Huh,

    Shows how much I pay attention to the pictures.  Embarassed Anyhoo... glad to be of service.

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Thu Oct 24, 2013 12:13 am

    Progress report ... got the wiring done and the distortion is gone.

    However ... the meters still aren't right.

    I can match the VOM test points and the meter using the 50K VRs on the back of the meters. It's very twitchy as I expected ... have to head over to ratshack for a couple 10k linear pots maybe. Problem though ... the reading at the test points drops by 4mV when I switch the meter out of the circuit. Drat!

    Thing is, with the meter switch off, the meter itself is completely out of the circuit. Would the wire lengths be the culprit? Those are twisted 22 gauge, approx a foot long. Also wondering now about that internal 0.75 ohm resistor on the plus lead ...

    On the bright side, the amp's working fine now, so I disconnected the meter circuit from the test points once again to eliminate any error potential when setting the bias, but I'd really like to eventually track this down.

    dOH! One thing I forgot to check is whether the 4mV drop is at all the test points whenever a meter is switched on.

    tubes4hifi
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    Re: Custom Chassis?

    Post by tubes4hifi on Thu Oct 24, 2013 11:48 am

    I think that 4mv difference is the difference across your 0.75 ohm meter resistor

    sKiZo

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    Re: Custom Chassis?

    Post by sKiZo on Thu Oct 24, 2013 6:20 pm

    Seems reasonable, As they say, I had a concern that would be the case.

    It's been pointed out that what I should be doing is adjusting the VOM test points for the proper bias with the meters ON, and just leaving them on. That also eliminates having to pop the internal resistor in the meter that's in series with the plus post. That looks tricky, and my hands aren't as steady as they once were. The center off thing was to eliminate the meters pegging the stops under load, and that's no longer an issue. Now, using the original piggyback circuit, with the converted meters and VRs to adjust the range, I'm only getting a point or two variation when the music's playing, so that will work.

    The current 50k VRs I'm using to adjust the meters are ... tweaky. VERY narrow range from stop to stop. Made a trip to the local ham/cb radio repair, said howdy and 73's, and scored a couple terminal strips and four 22k VRs. I plan to use one VR per tube now so I can match some variance due to the wire lengths to each test point (keeping in mind mine are a LOT longer than with the standard dynaco chassis). If the 22k pots are still tweaky, I got some 20k resistors to bridge them and take them down to around 10k range. I should be able to get an exact match to each test point this way with a lot more turn to set the range.

    I suppose I should also replace the center off toggles with the original On/On SPDTs I was using to eliminate the lag when selecting tubes. That will keep the circuit resistance constant, with a slight peak when switching tubes in a bank. Not a biggie, as we're only talking 4mV when the switch breaks ...

    Getting close ...

    Right now, I got it back in the stereo room and am just groovin' on da muzak ... this thing is ... what's the technical term ... Freakin' Amazin' !!

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