Help: Voltage Check on Speedball Install - values OK?

[Paul Birkeland]

I would suggest clipping your black meter probe to the chassis, then measure the DC voltage at terminal 3, then terminal 12, then terminal 14, then terminal 20.  These should all be 0V like terminal 3, but one of them is likely to show some weird high voltage, and then you know that on one end (or both) of that wire, you have your issue.

 

[mchuckp]

Well the saga continues....  I check all of those spots with the black lead of the meter on the chassis.  I got 0 for all of them. I moved the red lead around at that joint to different spots and also checked the lower one just in case something lit up but nothing.

While I had it grounded to the chassis I checked OA, OB, B+, & G on the circuit board.  G was 0 and the other three were "1" meaning off the charts.  Is this a clue to anything or what you would expect?

Also, just to be 100% sure that I'm reading my meter correctly, when I get the 400ish reading on B+ it is on "ACV" and set to 600.  So that would be 400, correct?

Thanks,
Mike

 

[Doc B]

There's at least part of your problem. You measure DC Volts on the DCV setting, not ACV.

 

[mchuckp]

Well geez...I feel stupid.  Can't believe I wasted your time plus my time re-soldering all that stuff.  Sorry man!  I'm working on another project that had some recent testing in AC and guessing that was on my brain.  Sorry again.

Anyway, I went ahead and retested both boards with the black lead of the meter at 12U.  As you'll see my B+ now checks out.  However my "O"s on each board still come out high and mismatched to each other.  Should I check anything concerning these or go with it?

Here are my results.

1st Board:
OA=92
OB=81
IA=185
IB=185
BAB=0

2nd Board:
OA=116
OB=122
G=0
B+=185

 

[Paul Birkeland]

Oh that's totally reasonable.  With a tube gainstage like this, you can get those plate voltages to match each other if you use a cathode bias resistor and resistor plate load.  As you add a CCS like the Speedball, then the plate voltages will start to wander a bit more, and if the cathode bias resistor is replaced with an LED, that further allows the plate voltages to wander more than expected.  In the Crack with the Speedball, the correct currents will be nailed down no matter what, so 10V of mismatch is not a problem.

-PB

 

[mchuckp]

Paul,

I got the amp up and running.  Sounds great!  However, I am getting some sounds in the right channel that come and go and mainly heard when music isn't playing or a song is fading out with low volume.  I'm assuming this is what is called microphonics?  Changes pitch and comes and go. I don't hear any hum, buzz, or anything like that.  When there is sounds like this, is it a tube?  If so, is it usually the power tube or input tube? 

Thanks.

 

[Paul Birkeland]

You could give this a try:
https://forum.bottlehead.com/index.php?topic=11676.0

 

[mchuckp]

Thanks.  Just read the whole thread.  Seems I might as well do it no matter what.

BTW, after reading your response, I took my high quality power cleaner from my main speaker rig and put it on my headphone rig.  It has been 99% dead silent.  I heard an ever so slight noise one time in about an hour of listening that was barely detectable inbetween tracks.  So it is definitely noise from my line.  I also notice my sound is much "blacker".  I must have some dirty power.  I'm convinced this started happening when we had a radon abatement system installed.

Anyway, I'll likely just leave this power cleaner on my can set up but still go ahead and do your mod.

Thanks for pointing that out.