Magnatone Vibrato/Reverb build Project

[wsscott]

Thanks.  Good point.

[wsscott]

Well I've satisfied myself that the caps are all okay even though they're old.  It looks like I've got an issue with my vibro/verb creating some distortion.  It has just started, and it doesn't matter if the signal is processed or if its clean when it goes through either of my 2 amps.  I notice it worse on the G string.  Really sounds buzzy.  I thought my strings were bad, or they were hitting on the fretboard, but I plugged them, ie. the Gretsch CG and the Strat guitars, directly into the amp, and the sound is clear.  So any ideas on what could be causing this?  I tried adjusting the Output Level, as well as the Treble, Bass and Volume, but they didn't have any effect on the buzz.  Thoughts?

[sluckey]

No ideas.

[wsscott]

Looks like it was a tube.  The V6, 12AU7 was making intermittent noise when I tapped it.  I replaced that with one a friend had, and it seems to have cleared things up.

[wsscott]

As I've played with the effects unit, I've got a couple questions for someone.  I notice that if I turn on the reverb and the vibrato, and then disconnect the stereo footswitch that controls these effects, that the Vibrato stops but the Reverb stays on.  If I then reconnect the footswitch, then the Vibrato comes back too.  Why wouldn't they both deactivate when the plug is removed from the jack?

Second, I get a lot more treble than I would like, such that if the treble control is set much more than at about the 8 o'clock position, that it starts to get gritty.  I'm using 1M Audio/Log pot per the schematic. Would a different pot or cap/resistor setup be an option? Thanks.

[sluckey]

The Vibrato needs a ground to enable the oscillator. Since the footswitch provides that ground, it must stay plugged in. The reverb needs a ground to disable the effect, so it will still work with the switch unplugged. This is the same behavior that all the Fender AB763 amps exhibit.

As long as you can get a usable tone from the unit I would not be concerned with what number the dial lands on.

You probably have the only one of this Vib/Rev unit. That makes you the expert. 

[wsscott]

Thanks.  After I posted my question about the reverb I noticed that it didn't matter whether or not I had the reverb on or off, and that in either situation when I unplugged the footswitch plug the reverb is ON.  So  I guess its working properly.  Thanks again. Stephen

[sluckey]

If you want to be able to use vibrato and/or reverb without the footswitch, then replace the jack with this one...

[wsscott]

I'm trying to figure out how to set the VV so that the signal strength going out and on to my amp, is at the same level coming in.  I assume that would give me a flat response.  Right now the VV is unplugged and the caps are drained, and the switch is OFF.  I plugged the guitar into the Input jack, and have my DMM set to AC and clipped leads to the tip and ground of the input jack and strummed a bit.  It varied of course but read on the average around 0.035mv AC. If what I did was correct, how do I read the value at the Output Jack?  Do I do the same thing but plug it in and turn it on?  And of course only touching the guitar?  I assume by adjusting the Output Level Pot, I could adjust the reading until they are the same.  I'm not totally crazy, I promise. Thanks.

[shooter]

 
crazy has many forms
I do the same, only with a scope.  you're looking for relative so put a jack in the output, clip your meter and check.


.035mVac is pretty small for a signal, 20mVac is about as small as most amps can "see" at the input.
I use a signal source (with volume), I pick an amplitude, verify, then jack in

[sluckey]

Guitar is a terrible way to try to match input/output voltages.

Connect a signal generator to the input. Don't have a sig gen? Download a sig gen app for your phone. Set the sig gen output for 500Hz and 200mV RMS as measured with your DMM at the input jack. Now connect your DMM to the output jack. and adjust the volume and output knobs for 200mV RMS. Once you have it set to unity gain, plug your guitar in and readjust everything until you like the sound.   


If you really need it to be set for unity gain, I think you may need another project.   

[wsscott]

Thanks everyone.  I'll give these methods a try. 

[wsscott]

Am I correct that I have the VV plugged in and turned on so it has power?

[sluckey]

Doesn't have to be turned on to measure input, but it would be better to turn it on. Absolutely must be turned on to measure output.

[wsscott]

I was wondering if this oscilloscope that I saw on eBay would be worth the investment for a hobbyist like me, or do you think its too limited in what it could do:  http://vi.raptor.ebaydesc.com/ws/eBayISAPI.dll?ViewItemDescV4&item=392828722788&category=104247&pm=1&ds=0&t=1591502285000&ver=0

Thanks.

[shooter]

looks fine if our eyes are good with small screen

Max. Input Voltage: 50Vpk (100Vpp)

you'll need a 10:1 or 100:1 probe for PA stuff, but for normal signal path and frequency stuff looks ok

[wsscott]

I assume "PA" means Power Amp?  I assume this scope would accept either of those 2 types of probes?

You can see I don't know anything about this!

[shooter]

yep, power amp.  97% of the time I don't scope the plates any more, if it's good at the grid, bad at the speaker, there are just not enough parts to bust out a scope, meter, yes..

2 * MCX-clip Cables

scope probes use a BNC style connector, looks like that use's some type of sm? rf screw type connector so you'll have to "adapt".  most scope jockey's have a bag full of gender benders, connection adapters and a dozen specialty cables
it is kinda a "hobby" in itself scoping signals, so jump in the deep end and learn to tread water

[sluckey]

Most versions of that scope kit include a smc to bnc adapter cable. I still don't like the 50Vpk max input level. Fine for tinkering on low voltage ss or digital circuits but too fragile for tube stuff. One ah shit moment and the scope dies.

[wsscott]

I decided the screen was too small, and I purchased this model: https://www.amazon.com/Oscilloscope-Digital-Bandwidth-Functional-Portable/dp/B087TBFB3K/ref=sr_1_2?dchild=1&keywords=ADS1013D+oscilloscope&qid=1599670028&sr=8-2 on Amazon. 

Looks like its more useful.

[shooter]

bigger is better 


even if it comes with probes, pick up 1 quality 10X probe. 
I very seldom use a scope for anything DC, that's what my fluke is for, that said, the signal might be riding on big DC so put a giant Note to self "ALWAYS HAVE IT SET FOR AC Coupling".


pretty soon you'll be showing all your friends cool waveforms   

[wsscott]

You're over my head already!  Can you briefly explain the last sentence?  I am familiar with the probe issue though.  Thanks.

[sluckey]

pretty soon you'll be showing all your friends cool waveforms

You're over my head already!  Can you briefly explain the last sentence?  I am familiar with the probe issue though.  Thanks.

He means now that you are entering the visual world of electrons, you can invite your friends over to show them what your guitar really looks like. Better yet, use the USB port to save your screens as images and post them on spacebook, or even this forum.

[wsscott]

Sorry, I understood that sentence, I actually meant the Paragraph's last sentence preceding that one.  HaHa!

[sluckey]

Oh, you mean this one...

I very seldom use a scope for anything DC, that's what my fluke is for, that said, the signal might be riding on big DC so put a giant Note to self "ALWAYS HAVE IT SET FOR AC Coupling".

He means always set the input coupling to AC.

But I disagree with that statement. It's often necessary to view signal waveforms with dc coupling. Many times you need to measure the dc baseline on a signal. Can't do that if the scope input is set to AC coupling. Here's one example...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

Look at page 9. You could not analyze that circuit if your scope was set to AC coupling. Or look at page 16. Need DC coupling for that also.

But if you need to check the amplitude of a 1v signal that's riding on a 300v dc level, you must use AC coupling because the signal and +dc level would be deflected so far off the top of the screen that you most likely will never find it.

My point is... know when it's appropriate to use ac or dc coupling. That will become more meaningful as you gain experience. I suspect one of the first things you'll want to do is look at the input and out waveforms of your unit and set for unity gain. In this case, either AC or DC coupling would be appropriate since the input and output waveforms both have a zero volt baseline. But if you try to trace the signal through the amp you'll find that usually grid signals are usually at zero volt baselines and plate signals will be at big positive DC baselines. In this case, AC coupling is a better choice.

Don't worry. You'll get it if you play with it enough. Then you can move on to proper triggering so you can actually see the phase reversals as you trace the signal through a device.   

I highly recommend a simple sine wave signal generator.

[shooter]

+1
DC coupling is real handy, after you learn not to burn up a probe or a scope 


In my old life we had to measure AC riding on 1500 - 3500vdc, more than 1 newbie didn't take notes while the teachers demonstrated floating the scope, thankfully only scopes bit the dust.  Engineers finally got around to designing voltage dividers for HighVolts 


put some ice in a glass, your favorite single malt, watch some video of scope functions and operations.  within a week you have your amp "spec'd" out, with amplitude, phase, frequency using a sine wave.  then try looking at a complex guitar wave and give up   

[wsscott]

Thanks.  I'm sure I'll have more questions, but will first try to understand the basics before hitting you guys with my questions. 

[PRR]

http://tubebooks.org/technical_books_online.htm#Test%20&%20Measurement%20(equipment,%20instructions...)

[wsscott]

This is a great resource.  Thanks so much.  Stephen

[wsscott]

Hi guys.  I wanted to ask a favor of your thoughts and time.  I got my tablet oscilloscope today, and am trying to figure out how to use it and what kind of simple tests I could do to start my learning process.  It came with 2 probes that have 1X and 10X switches, its DC lithium battery supplied, and has a 400V protection circuit built in.  So I'd like some guidance on some simple tests I can run just to see what it does and how to start using it.  I have 2 things I'd like to check with it, but these may be more complicated than I should start with, but I'll tell you what I'd like to do.

First, I'd like to be able to balance the Output Level so that it is the same as the Input Level.  It may not be useful for anything, but I'd like to see how to do that.  I do have an iPad that can generate signals that I can connect to the VV through the earphone jack with a 1/4" phone jack.

Second, it has a low hum, maybe 60hz, that I'd like to figure out the cause.  The VV Output runs into the Gretsch amp's input, and if I unplug the guitar input into the VV there's no difference so it seems like the hum doesn't arise from the input source, but if I vary the Output Level the loudness of the hum varies.  I don't know if its something in the VV circuitry that I need to fix, like a ground somewhere, or if its because I've got 2 grounded amps, ie. the Gretsch and the VV, that are creating a ground loop or something.  So I'm hoping that the scope could help me find the cause.

Any input from you is always appreciated.  Thanks.

[shooter]

I do have an iPad that can generate signals

start there;
create a 1khz sine wave with an amplitude of ~~ 100mVAC_RMS
read through the manual, find RMS vs peak to peak, I personally use RMS for everything audio, go so far as convert (math) tube data sheets "27Vac peak" to rms, then measure.


once you're happy with that, vary volts (amplitude) and frequency (period time) till you have a good handle on those 2.


If you can't get it to "sync" (signal keeps scrolling), read up on triggering, play with that til you're happy
then maybe you're ready to jack in and follow the signal through the amp

[sluckey]

On the VV unit, disconnect the green wire in the power cord from chassis. Does the hum go away?

[wsscott]

Sluckey, I haven't tried disconnecting the green wire yet because it's connection to the chassis is hard to get too.  But, I saw that some people will use a 3 prong to 2 prong plug adapter to isolate the ground wire.  So I did that, and plugged it into the wall receptacle, but no difference.  So if this does the same thing as disconnecting the ground wire from the chassis, it must be something else.

I did note that I grounded the 4 big filter caps to a separate ground lug going to the chassis, rather than grounding them to the ground bus that most everything else is grounded to.  I thought doing that would isolate them from the grounds of the rest of the circuit and avoid hum.  Could that be the issue?

[wsscott]

Shooter-Thanks for the startup.  I'll give it a try.

[sluckey]

I did note that I grounded the 4 big filter caps to a separate ground lug going to the chassis, rather than grounding them to the ground bus that most everything else is grounded to.  I thought doing that would isolate them from the grounds of the rest of the circuit and avoid hum.  Could that be the issue?

I doubt it, but temporarily connect the ground buss to the filter cap ground to see if it helps.

[wsscott]

I just realized that the E Node 1uf cap which is not with the others but over near the pre-amp section is Grounded on the Ground Bus, and not at the same location where the others are grounded, and of course its feed is coming from the others. Could this be it?

[sluckey]

I just realized that the E Node 1uf cap which is not with the others but over near the pre-amp section is Grounded on the Ground Bus, and not at the same location where the others are grounded, and of course its feed is coming from the others. Could this be it?

Doubt it but see my last post. Also, temporarily jumper a 20µF cap across that 1µF cap. Any better?

[wsscott]

The closest electrolytic cap I have is a 10uf 25V cap.  Then I have a 100uf 16V.  Would either of them work for this?

[shooter]

PS nodes are typically high volts, so low volt caps only show their inerds if used 


don't have the schematic handy to verify though

[sluckey]

The closest electrolytic cap I have is a 10uf 25V cap.  Then I have a 100uf 16V.  Would either of them work for this?

no

[wsscott]

Yea, I didn't think they would work.  I need 450V caps.  I guess another purchase from Mouser or someone online.  Its really frustrating that there isn't a electrical parts supplier within 100 miles of Charlottesville.  Wow.  So I'm in pause on this part of the test.  Thanks.

[shooter]

welcome to the new world, same as the old world except worse 


play with your scope, once you are confident that what you see, is what you expect, you can still trace out a signal with hum.  that's lesson 2, .....by messing with scope settings you can see both the signal and hum, you can calculate the S/N (signal to noise).  then you have a "baseline". that way when the hums gone you can re-measure, re-calc, celebrate that your S/N went way up 

[wsscott]

Sluckey, this morning I was going to start working on some of this stuff.  The VV was unplugged.  I haven't done anything to it yet.  I turned on the Gretsch amp and let it warm up, and then when I was plugging the VV into the wall receptacle I started hearing the hum, but at a much lower volume level.  I turned the VV on and it came up much louder to the level I normally hear.  I turned it off, and then removed the plug from the receptacle.  I noticed that just as the Ground pin on the plug was going into the receptacle, and before the Hot and Neutral pins entered the socket, that the hum would start.  I then used the plug adapter that doesn't have the ground pin, and noticed that just as I was plugging it in, I could start to hear the hum at the same low level.  Does this help your analysis? I guess its really quiet in the house this morning so I'm hearing stuff better than the other day.  Thanks.

[wsscott]

I checked my wiring (I have photos) on the Ground lug and I have the green ground wire from the mains connected to the lug that is screwed to the chassis, and I also have the center tap from the transformer secondary (this transformer doesn't have a center tap for the filament wires) connected to the same lug--if that has any impact.

[sluckey]

this transformer doesn't have a center tap for the filament wires

In that case you need to make an artificial center tap using two 100Ω resistors. Connect one resistor between a filament wire and chassis. Do the same for the other filament wire.

[wsscott]

Yes, I did that with the resistors.

[shooter]

I turned on the Gretsch amp and let it warm up, and then when I was plugging the VV into the wall receptacle I started hearing the hum, 

so the Gretch is picking up the "extra" noise since the VV is off.  ("I turned the VV on and it came up much louder to the level I normally hear.")


have you jacked them together by then or just powered up?

[wsscott]

They are jacked together with the Gretsch amp on, and the VV off and unplugged.  When I start plugging in the VV into the receptacle, I can hear the hum starting on the Gretsch but at a very low volume.  When I turn the VV on, and it warms up, then the hum is at the same frequency just much louder.

[wsscott]

Sluckey-attached is the initial drawing you gave me to get me started.  I was wondering if I wired the connection at the input jack correctly. I ask in part because even when the VV is turned off and the signal is not going through the circuit, but the VV's output is plugged into the Gretsch amp, I get the hum when just the ground pin on the plug is inserted into the receptacle.  So I was thinking the hum gets started early on.

I attached a lug with a bolt to the chassis right below the input jack as shown in the drawing.  It obviously connects to ground through the chassis.  And I also wired a jumper from the lug to the circuit board where all the components are leading to a ground, and then ran a separate jumper from the lug to the Ground Buss. Finally I ran the shielded input lead to the input jack "tip", not the "sleeve" as shown in the drawing, and connected the shield to the same lug as shown in your drawing. One side of the 1M R goes to the "tip" of the input jack, and the other side goes directly to the Ground Buss and not through the GND leg on the input jack. Finally the "sleeve" and the "GND" legs of the jack are jumpered and connected to the Ground Buss from the GND leg of the jack.

Assuming this is all correct, could the connection of the Ground Lug to the chassis where a lot of connections wind up, and then go on to another ground on the Ground Buss be the issue?  Why did you use a lug to the chassis for those connections rather than just wiring them to the Buss?  That lug is not isolated from the chassis is it?  Thanks as always for your time and patience.

[wsscott]

One more point, my Ground Buss is bolted on one end to the chassis and is floating on the other end.  Its stability is due to the gauge of the wire and the jumpers connecting to it.  All of the ground legs of the pots, as well as the ground and sleeve of the Output Jack, connect to this Ground Buss.