Magnatone Vibrato/Reverb build Project

[shooter]

an experiment;
find a nice length of 12 gauge solid copper wire, a couple of small clamping "things" (small vise grips, hemostats, c-clamps).  clamp one end of the copper wire to the VV chassis, clamp the other end to your Gretch (bare metal, not pained surface), repeat your plug in test (Using a cheater plug on the VV)

[wsscott]

Thanks.  I'll give it a try.

[wsscott]

I wanted to also note that I have the same symptoms using my 5F1 Champ that's plugged into a different receptacle than the Gretsch amp, but they're probably on the same circuit. So its not just an issue with the Gretsch.

[wsscott]

Shooter-I did your test and it was a vast improvement. I used the cheater adapter also as you suggested.  So I also tried just using the cheater adapter by itself after disconnecting the copper chassis ground wire, and it seemed to also clear up the signal. Maybe I had it inserted better on the plug than when I first tried it a couple days ago. If I remove the adapter as soon as the ground pin is inserted into the receptacle, and the other pins aren't inserted, and the VV turned off, I can start hearing the hum.  With the adapter plugged in I don't hear the hum since there's no power amplifying the signal to the amp.  If I turn the amp on, then there's some hum but substantially reduced.  Its volume is controlled by the Output Level on th the VV.

So I also tested this with the copper wire connected to the 2 chassis, but without the cheater plug, and the hum was even better than when the cheater plug is used.  Its practically gone!

So what does that tell us? Looks like a solution but obviously not something I can use! 

[sluckey]

I would use a braided ground strap to bond the two chassis together. Permanently bolt one end to your VV chassis. Install a stud bolt to the amp chassis. Use flat washers and a wing nut to secure the other end of the strap to the amp.

Amazon has a variety of "ground straps" available. Or build your own with a length of RG-59 or RG-6 cable. Just strip the outer jacket and remove the center conductor. Make it as long as you want.

https://www.amazon.com/Allstar-Performance-ALL76330-Copper-Ground/dp/B00F0CIO60/ref=rtpb_1/138-3958134-1216332?_encoding=UTF8&pd_rd_i=B00F0CILM2&pd_rd_r=5b891574-39de-43a5-b4a2-80194fb81558&pd_rd_w=ZokbT&pd_rd_wg=Ev0FG&pf_rd_p=49740592-2805-416d-896c-b825ad91c2cf&pf_rd_r=N85EH3ZW4V3Y0QDRENY6&refRID=N85EH3ZW4V3Y0QDRENY6&th=1

[wsscott]

Is this something that a balanced 1/4" cable with a balanced Output Jack would fix?  Any idea what causes this?  I'd like to avoid having to strap the 2 units together.

[sluckey]

Fender used a floating ground in the reissue reverb unit (see attached link). You would have to lift every ground connection in the unit from the chassis and float. This means, jacks pots, everything. The only thing connected to the chassis would be the green wire in the power cord. Then add CR5, CR6, and R23 connected ***BETWEEN*** your floating ground and chassis. This is probably a huge task. Makes the ground strap look like a good solution.

     https://el34world.com/charts/Schematics/files/Fender/Fender_63_reverb_manual.pdf

[sluckey]

Is this something that a balanced 1/4" cable with a balanced Output Jack would fix?  Any idea what causes this?  I'd like to avoid having to strap the 2 units together.

Your VV has an unbalanced output and your amp has an unbalanced input. I don't see how a balanced cable will help.

[wsscott]

Yea, I figured the jacks would also have to be balanced.

Would an isolation transformer be a solution?

[shooter]

possible, you still need to use isolated jacks in n out of the VV I believe.


What Steve said ^the "proper" fix is to "float" the VV, the alternative is to bond them both.  I've done it twice for other ppl's equipment.  I did Like Sluckey stated; used 400W speaker screw terminals, chassis mounted, and made up the braided jumper with soldered lugs.  takes an extra minute to cable up, not a big deal

[wsscott]

Shooter-I was finally able to get a chance to try out my little oscilloscope as you suggested and was able to get the 1K signal showing just fine.  I used the test signal from the scope and then I used the signal generator on my iPad to create a 1K signal and turned the output all the way up as you suggested.  It worked fine.

So I guess I'm ready to try some simple stuff with the VV unit.  One thing I'm not sure about is the AC vs. DC coupling and when to set it to which one.  The scope can be set to 1X, 10X or 100X, and the probes have the 1X/10X switch.  The manual gave some setups for various tests. One was to measure the AC voltage from a receptacle--which I have not done.  The scope can handle up to 400 V, but it said I needed to set it to the 100X position and use a 100X probe-which I don't have.  Seems like I could just set everything to 10X/AC.  Is that correct?  Thanks.

[shooter]

1st, what's the manual say about max input volts (not 10x 100x), IIRC it was 40v 


assume 40, then a probe set for 10X should "allow" you to measure  400v, there's a catch, some 1x/10x can ONLY measure 300v, so verify what you probes max V is


for signal path, keep it on AC coupling for now, once you get past the learning curve you can play with DC coupling.


You AC signal shouldn't be more than 100Vac so I'd feel comfortable saying a 10x probe, AC coupled will get you input to output just fine.


couple tips not to get "odd" readings, stay out of feedback circuits, TS, NFB.
probe grids on the "left" side of the grid R NOT at the grid, probe plates on the "left" side of the coupling caps, typically the plate R coupling cap junction.


I use both channels, I set CH1 at the input jack and leave it
I probe with ch2
that way you can "see" phase shifts, you can insure your input signal amplitude is ALWAYS constant (some tweaking may be required as you change frequencies)


before you power on, PRACTICE clipping the probe, do it many times, the last thing you want is a smoked probe-scope-amp error   


Make sure you're scope ground wire is secure, might need an extra gator clip to cover more distance, I always use chassis ground for everything


have fun

[wsscott]

Interesting development on the hum/noise issue.  I wanted a shorter cable to connect the amp to the VV, and so I ordered a pair of Neutrik 1/4" plugs + 6' of Mogami guitar cabling.  I had been using a name brand guitar cable that I had bought a few years ago, and it is 15' long.  I made up the new cable, and not really thinking much about it connected up the amp and the VV, and WOW! the hum and noise are dramatically reduced--maybe by 75%.  So maybe there is such a thing to using quality cable and connectors.

[sluckey]

Mogami cable has a substantial shield. Kinda like using a braided ground strap to bond the two chassis together.

[wsscott]

Shooter-When you say "probe grids on the "left" side of the grid R NOT at the grid, probe plates on the "left" side of the coupling caps, typically the plate R coupling cap junction", do you mean on the side of the resistor/cap opposite to the side that connects to the grid/plate, ie. before the signal goes through the cap/resistor?

[shooter]

you can probe plates, you can even probe grids, but, your scope might "change" the signal some so I just probe the grid on the left side of the series grid resistor.  that might simply be "each side of the coupling cap"


once you get a "feel" for what you expect and what you "see" and can reconcile them, you're set to probe anywhere, except PA tube plates - for now. 

[wsscott]

Sorry, but when you say "left" do you mean the side of the resistor before the signal goes through the resistor and then to the grid?

[shooter]

yep
scope>resistor>grid


for fun, once you do that, probe the grid (AC coupled), see IF there is a change on signal, that way you know for future reference

[wsscott]

I finally got a chance to get back to working with the OSC.  I connected it up to the VV at the input jack and the output jack to see what balancing the input and output signals would look like.  I got the output voltage set to the same as the input, and the results are shown in the attached pdf file.  Looks pretty good?  I turned on and off the vibrato and reverb and got reactions on the signal too.  Any thoughts you'd like to share?  Thanks.

[shooter]

pop quiz;
1. what are the 2 signals phase relationship?


2. when you plug the output signal into your Amp, what effect does it have on the signal?


3.  You like your scope don't you ? 

[wsscott]

I'll have to take this quiz tomorrow!  The first question has got me stumped right now!

[shooter]

Extra credit;


Channel 1 is ~ 100mV rms
channel 2 is ~ 300mV PP
WITHOUT using the scope, are the 2 signals ~ = ?


(calculators and computers are allowed :)

[wsscott]

I would say yes, they are about equal:  0.3535 x 300 = 106!

[shooter]

I pointed that out because as a "general" rule you want all your measurements the same reference. 


that way you got 2 apples instead of an apple and a banana

[wsscott]

Shooter, in answer to your reply 569 above, attached is an image pdf of the 2 signals overlaid.  I don't know how to describe it except to say they seem to be like a helix, or something.  I guess you would say they are in phase, but maybe they're actually 180 degrees out of phase.  As to the sound when connected to the amp, its a high pitched whine, very solid, and oscillates when I turn on the vibrato.  Signal seems very stable.  Thoughts?

[shooter]

they're actually 180 degrees out of phase

If they were in phase they would overlay so you couldn't tell one from the other.
phase is anything "shifted" from original, so a 10 degree phase shift might look "in-phase" until you overlay.  your scope probably has a channel add, channel subtract feature.  that comes in handy when you want to see really small differences.


what I was "looking for" is you scoping the output of the VV when you plugged it into your amp.  If there is a "mis-match" ,(VV to amp), the VV's output signal will decrease some, maybe small mV, the greater the mis-match the more the signal gets loaded down.

[wsscott]

Ok, so I set it up and before plugging the cable from the VV output to the amp it read 108 VRMS on Channel 2 Output from VV.  Then when I plugged it in to the amp, it read 103 VRMS, and didn't change when I turned on the power.  So looks like a difference of 5 VRMS.  Is that reasonable?

[sluckey]

Surely you mean millivolts? If so, that's good.

[wsscott]

Yes, mv.  Sorry.

[shooter]

without a calculator that "looks" like 5% drop in signal, I'd call that good, 20%, you might need a calculator to "fix" 

[wsscott]

In trying to learn more about using the OSC, can you all give me some pointers on things NOT to do so as to prevent "smokin'" the OSC or the VV?  Thanks.

[wsscott]

I guess I'd like to learn how to trace a signal through the circuit and see what the signal looks like along the way, and how it reacts to the VIB or Reverb being turned on and off.

[shooter]

know what you're "testing", so;
If i'm gonna probe a preamp plate, answer "all" the ?'s
how much volts
what kinda volts
do those values exceed my scope's capabilities


what do I "expect" to see
what "things" effect the outcome


have a trust and confidence in yourself, the equipment, know your limits


ask, I got more 


Oh, go through "your check-list" EVERY time you make a measurement, document well enough to decode tomorrow

[wsscott]

I wanted to trace the signal from input to output on the VV using my little oscilloscope to see how it changes as it goes through the various stages, and wondered if someone could show me on the Schematic that Sluckey did for me of my build, the path of the signal as it moves through the circuit.  You know showing lines and arrows, or something like that.  Just hand drawn and not elaborate unless you are so inclined. I'm attaching a copy of the schematic. Thanks. 

[shooter]

the signal path, in it's simple form is all the plates.  set all the pots to 5, turn verb n trem off, inject a 50-100mV RMS signal. Note Steve already indicated the dry signal "past" the reverb.


repeat with volume at max



now repeat with trem on and note the changes.
again with reverb on dialed 5 on knobs, wet signal will be at the plates


the modulator is a complex, same with TS, when you "interact with scope", you alter the results, kinda like modern day voting 

[wsscott]

Shooter, thanks.  What is TS?  What really confuses me is figuring out the path of the signal into and out of the Vibrato Oscillator/Driver section.  Can you describe that path for me?  Also, do I set the oscilloscope to AC or DC?  I assume AC, but you know what they say about assuming anything!

[shooter]

TS = tone stack
 
AC coupled, I haven’t got a chance to color the pdf.
Start small, T B n Vol at 5 ch1 on input jack, ch 2 trace probe
Inject signal, measure V1A plate, then V1B plate.
Now set Vol to 10
Repeat
Now set vol back to 5, set treble to 10
Repeat
Set treble to 5 and Bass to 10
Repeat
 
Move probe to the “white ” point between V5 and V6
Repeat above knob settings
Now turn on trem, all knobs 5 repeat again.
screen shot for each condition, note the "conditions"
set up a video loop of the conditions and "grasp" why things changed
 

[sluckey]

What really confuses me is figuring out the path of the signal into and out of the Vibrato Oscillator/Driver section.

The guitar signal never enters the osc/drv block. The first half of the 12DW7 is an oscillator, aka a LFO (low frequency oscillator) or signal generator. It generates a low frequency sine wave that can be viewed on the scope just like any other sine wave. But the frequency is very slow, about 3Hz to 10Hz, so you must slow down the time base speed of your scope, or just press an auto button if you have one. A very convenient place to view this sine wave signal is at the top of the INT pot. This will be a strong signal, but it's slow. The speed pot will change the frequency of this signal. You can use ac or dc coupling because there is no dc voltage present at this point.

The LFO signal from the INT pot is applied to the second half of the 12DW7, which is a standard cathodyne phase inverter, aka a phase splitter. The phase splitter produces an output at the plate and also at the cathode. These two signals will be exactly the same as the signal from the INT pot but they will be 180° out of phase with each other. You can view these signals at pins 1 and 3. Use AC coupling since these signals will be riding on a healthy DC voltage. These two signals exit the osc/drv block and head up to the modulator block where they do their magic to the guitar signal.

That's about all there is to the osc/drv block. Once again, the guitar signal never enters the osc/drv block.

If you will measure all the tube pin voltages (don't care about filament pins), power supply nodes A, B, C, D, and E, and line voltage, I'll put those on the schematic for you. They can come in handy if you ever find yourself troubleshooting a broken unit. I also like to include the LFO signal at the top of INT pot. I like to set the speed for 5Hz and measure the peak to peak amplitude. This can be handy when troubleshooting a weak or dead oscillator.

[wsscott]

Thanks to both Shooter and Sluckey- Its no wonder I couldn't figure out how the signal goes through the Oscillator--it doesn't!  I'll take some voltage readings and send them up to you Sluckey.  Thanks again.

[wsscott]

Sluckey, do I need to have a signal going into the amp just to test the voltages?

[sluckey]

no

[wsscott]

Here are the voltage readings. Thanks. Stephen

[sluckey]

Can you measure this one also?

...I also like to include the LFO signal at the top of INT pot. I like to set the speed for 5Hz and use a scope to measure the peak to peak amplitude. This can be handy when troubleshooting a weak or dead oscillator.

[wsscott]

Could you tell me how to hook up my scope's connections so I can do that?  I'm just trying to learn on this device.

[sluckey]

Connect a probe to the top lug (not the grounded lug) of the INT pot. Connect the probe's ground clip to chassis. That's all.

[wsscott]

Ok, so I hooked it up, and ran autoset, and then adjusted the Vibrato Intensity pot until I got a reading of 5HZ, and the VPP is reading 42.5V on the screen.  Does this sound correct or reasonable?

[sluckey]

Ok, so I hooked it up, and ran autoset, and then adjusted the Vibrato Intensity pot until I got a reading of 5HZ, and the VPP is reading 42.5V on the screen.  Does this sound correct or reasonable?

Looks good but surely you adjusted the speed pot for a reading of 5Hz? Are you sure the 42.5V is peak to peak?

Also, what is the frequency with the speed pot set max CCW? and max CW?

You may notice the voltage reading changes as you turn the speed pot. That's normal. It's also why I specified to measure the voltage with the speed set to 5Hz.

[wsscott]

I used the Speed Pot to set it to 5HZ.  The Vibrato Intensity Pot affects the VPP reading depending upon how low or high the pot is set.  Where should it be set to get the VPP for this test?

Speed Pot Max is 7HZ
Speed Pot Min is 4HZ

The VPP goes from around 20V at a minimum setting of the Intensity Pot to a high of about 78V at its maximum setting.

[wsscott]

Actually the VPP has a much wider Voltage range that I indicated when the Vib Intensity pot is adjusted from minimum to max.

I did make a change in the Intensity pot type.  I changed it from an audio to linear taper.  It gives me better control over the intensity.

[sluckey]

All good info. But I'm trying to get a set of numbers to put on the schematic that you can refer to later on. So, set the INT to zero and adjust the speed pot for 5Hz. Now measure the peak to peak LFO signal at the top of the INT pot. What have you? I'll put this number on the schematic with a note of how the pots were set.

Here's another little scope exercise for you... While observing the LFO sine wave with the pots set as above, adjust the time setting of your scope to show two (or more) complete cycles. Now save (or print) the screen to an image file (usually jpg or png format). Attach the file to your next message. I want to see it and may even put it on the schematic.