Magnatone Vibrato/Reverb build Project

[wsscott]

Ok, its at 5HZ and reading 2.78 VPP with a Vrms of 882mv.  Attached is an image of the scope screen in pdf format.

[sluckey]

I don't understand. Now you have less than 3V_pp. Earlier you had 42Vpp, then a range of 20 to 78vpp. What are you doing wrong?

[wsscott]

Well before the Intensity Pot was turned up about mid-way, so you said to turn it down to 0, so I turned it down as low as I could go and still get a 5HZ signal, before it got to a flat line.  So the VPP varies with the setting of the Vibrato Intensity.

[sluckey]

Not if you have the scope connected to the TOP lug of the pot as I requested.

[wsscott]

I've got it on the middle lug.  Is that not the TOP lug?

[sluckey]

No. The middle lug is the middle lug. The top lug is the top lug. LOOK AT THE SCHEMATIC. It'll make sense then.

[wsscott]

I moved it to the input lug, set it to 5HZ with Vib. Speed pot, and it reads 224Vpp and Vrms of 71.6 V.

[wsscott]

Here's an image of the Sine Wave.

[sluckey]

I moved it to the input lug, set it to 5HZ with Vib. Speed pot, and it reads 224Vpp and Vrms of 71.6 V.

That's what I'm talkin' about!   

[sluckey]

Here's the updated schematic with voltages.

[wsscott]

Sluckey-This is great.  Thanks so much for all of your help.  Best.  Stephen

[sluckey]

Having the voltages on the schematic will be very useful if you ever need to troubleshoot the unit if it fails. And having a visual scope pic of the vibrato signal helps to understand exactly what is going on in the oscillator circuit.

FYI, that oscillator is called a phase shift oscillator, sometimes called RC phase shift oscillator. Lots of google theory info if you want to read about how it actually works.

The Gretsch 6163 amp even uses this same oscillator (higher frequency though) as a built-in  "tuner" signal.

     https://el34world.com/charts/Schematics/files/Gretsch/Gretsch_6163.pdf

[wsscott]

Does my scope image look good, ie. the way its supposed to be when working properly?  Thanks for the Gretsch schematic.  I have the Gretsch 6165 Supro Amp which I think is pretty much the same as the 6163.  Its a really nice amp.  I've had it since new in 1966, but the actual number of On Time/Playing hours is pretty small.  I got it in high school, and then went off to college etc., so my playing days were numbered.  Its nice to be able to use it, and still with the same tubes.

[sluckey]

Scope looks good. More than adequate for this stuff. One slight discrepancy... Display graticules showed frequency to be 5Hz and 5.2Hz on the two pics using the old fart analog method of counting graticules, multiplying times time/division, then getting reciprocal . Your FREQ numeric readout showed 5Hz on one pic and 4Hz on the other. Maybe the counter can only resolve to one Hz? Not a big deal unless you're gonna fly this amp to the moon.   

There are way more features on that scope than you'll ever find a need for working on a tube amp. Kinda like using a scientific calculator to balance your check book.   

[wsscott]

I tried my scope today to see what the signal at the Output jack looks like when both the Vibrato and Reverb are Off, and then when the Vibrato is Off and the Reverb is On.  Attached the 2 files. I don't know if this looks normal or not, but thought I would ask for your comments.

[shooter]

do you have 2 probes?


If so;
use ch 1 connect to input jack and leave it
use ch 2 to "move along the path"


the "hitch" shouldn't be there with both off, (i think)
what were your tone n vol set at?
when doing "spec", you want to be able to repeat those conditions EVERY time you measure, otherwise you add variables that can n will mess up your "spec"

[wsscott]

Shooter--Attached is a scope image showing both channels.  Ch.1 is at the input jack where the 1KHZ tone is coming in, and Ch.2 is at the output jack.  I have a guitar cable plugged into the Output jack, but the other end of the cable is not plugged into anything. Both are set to AC and 1x on the probes. Both are showing 200mV/div.  Both VIb and Rev. are off.  All pots are at Noon position.  Both Vppp are around 300mv, and both Vrms are 99mv.  I did this same check a month or so ago and they were both smooth.  Any idea where to probe to find the source of the hitch? 

[wsscott]

If I lower the Input Volume Pot from Noon position where it gets me 99-100 Vrms, to about 8 o'clock position where it reads 50Vrms, then it smooths out, but of course the amplitude or heigth of the signal is much shorter.

[shooter]

find the scope setting were you read volts AC RMS, that should be a "standard" when measuring signals.
so the input should be 50-100mV rms
leave that as your "baseline"
now measure the output, IF it's "clean" (all knobs @ 5..reb, trem OFF) then that will be your "spec".  note all the settings on your schematic.  then 12yrs from now you can "see" where things went wonky.
all these are arbitrary settings, it's just a means so you can compare apples to apples in the future

[wsscott]

Thanks.  When you say "5", do you mean "5 o'clock" position (basically the pot is turned all the way up), or 50%, or what exactly?

[sluckey]

half way

[wsscott]

Sluckey-I've been looking at some schematics of the M-9 and the M-10 models, and in the oscillation section  they have a .02uf cap coming off plate 6 of the 12DW7 tube which then connects to the .068uf cap that then connects to the top of the 1M Vibrato Intensity pot.  Looks like the m-10A changed that to delete the .02 cap.  Do you know why that was done, and what the effect would be on the pot and circuit?

[sluckey]

The bean counters said you don't really need that cap so it was removed. Savings = $0.25. Pure profit! 

That cap was a smoothing cap to make the sine wave look smoother. Why don't you temporarily put a .02 (or .05) cap from plate to ground and look at the top side of the INT pot with your scope. Does the cap make the waveform look smoother, more like a sinewave? Post a pic for all to see. BTW, it's fine to post an image file rather than convert to pdf. Easier/quicker to view.

[wsscott]

Will do. I noticed on your drawing that the M2 used 2 .02uf caps rather than a .02 and a .068. Sometimes my image files were too big to post so I converted to PDF.

[PRR]

...my image files were too big to post so I converted to PDF.

Irfanview
http://www.irfanview.com/
https://el34world.com/Forum/index.php?topic=10974.msg100982#msg100982

[wsscott]

I attached a .02uf cap at the input of the .068uf cap that it's other lead goes to the Top side of the Vib Int pot, and connected the other end of the .02 cap to the BUSS gnd.  I then injected a 1KHZ signal and probed it at the TOP side of the Vib Int pot.  Attached is that image.  They look pretty similar to me. For some reason though it makes a horrible buzzing sound after adding that cap.  So that's definitely coming off.  Any ideas why?  Sorry, but it was a .bmp image that this website doesn't accept so I had to change it to a pdf.

[sluckey]

Don't know why.

[wsscott]

Any reason it should be grounded somewhere other than the Buss bar ground where the Vib. Int. Pot is also grounded?

[wsscott]

Well I moved the Ground side of the cap off of the Buss bar and onto the ground bar where the filter caps are grounded, and the hiss/noise cleared up!  Can't imagine why!  Does it matter on which side of the .068 cap I connect the .02 cap, ie. on the side where the Plate signal comes in or on the other side of the cap?

[sluckey]

Just follow the schematic.

[PRR]

....it was a .bmp image that this website doesn't accept so I had to change it to a pdf.

https://el34world.com/Forum/index.php?topic=25746.msg293111#msg293111

[wsscott]

Well adding that .02uf cap to the front end of the .068 cap and grounding the other end of it to the filter cap ground buss really solved everything.  No more of the mild hum, buzz or oscillation of any kind when the reverb is turned on with the vibrato.  I had been getting some background oscillation I think from the LFO section that was bleeding through the reverb circuit when the reverb was on.  Now its as quiet as when the effects are on as when it bypasses them. 

[wsscott]

Well guys, I'm getting restless and need another project.  Any ideas/suggestions you have would be greatly appreciated.  Happy Holidays to All.  Best.  Stephen

[sluckey]

This simple project is my favorite...

     http://sluckeyamps.com/dual_lite/dual_lite.htm

[wsscott]

Looks like a possibility to keep me busy for awhile if I decide to pursue it.  What is it about this project that makes it a favorite?  So does this "amp" give you the "sounds" of a Vox and a Marshall that you can use to expand your playing style?  Since I have my Vibrato/Reverb project as an effects unit, it could be plugged into this to give these 2 amps the benefit of vibrato and reverb.  I wonder if I could change this project to have a "floating ground" (I think that's what someone in this forum called it) system like the ReVibe circuit uses in order to eliminate any hum caused by ground loops?  Thoughts?

[sluckey]

What is it about this project that makes it a favorite?

Simple, small, great sound.

[shooter]

this is my simple;
the schematic posted "will be" the 4th iteration.
should yield ~ 8-10W clean and ~ 17W full on.
it's a tinker'rs amp, I typically add 6-12 extra turrets for changes, takes pedals well.

[wsscott]

Thanks Shooter.  I'll check it out!  How do you think it will do with my VibroVerb build?  Any ground loops issues since there would be 2 devices, ie. the amp and the VibroVerb, running off of separate power supplies and grounds?

[shooter]

not sure, only had pedals with batteries in front. SE amps are hard to keep/get hum low as is. 
you did a basic "mechanical" ground earlier, I wouldn't be surprised if a grounding strap was needed
maybe a differential line out on the VIBVEB 

[wsscott]

What are "SE" amps?  How would you do a different line out?

[shooter]

SE = single ended
PP = pushpull
SE amps operate in class A, A1
PP can operate there, but they can get to class AB2
Differential, not different
look up audio differential line drivers.

[wsscott]

Sluckey, Shooter et al, I just started a new topic and hoped maybe you can help:  https://el34world.com/Forum/index.php?topic=26944.msg295588#msg295588

Thanks.  Stephen

[wsscott]

Sluckey/Shooter-I've noticed that my VibroVerb has some mild "fuzzy" sound that increases only with the Volume or the Output Level pot. I don't know if you'd call it gain/breakup or what. Its not affected by any other control.  Its probably always been there, but I have just recently started to focus on it.  I was wondering if it could be that the Input resistor is a 4.7M resistor and not a 1M as is commonly used?  Why would Magnatone use such a large resistor here?  I haven't put my scope on it yet, but was looking for some insight.  Thanks.  Stephen

[shooter]

not understanding
you say your vibroverb, but then you insert the magnatone
so without a schematic I'm going with the 4.7M as "grid biased" vs the 1M "typical" being cathode biased

[sluckey]

If you suspect the 4.7M just change it. Much quicker than typing this confusing message.   

[wsscott]

Sorry for creating confusing.  I've referred to this build that you guys helped me with last year as a "VibroVerb", since I was wanting to build a ReVibe type unit but it was based on Sluckey's M10A, ie. Magnatone, schematic that he modded. 

[wsscott]

Here's my issue.  My Magnatone 10-A Mod (which I've referenced at times as a VibroVerb ((Not realizing that Fender also used that name)) ) that I built based on Sluckey's changes as shown on the attached schematic of the finished product, works great except that the Treble starts to get "fizz" in the sound if its turned higher than about 9:00 p.m., and if I want more output from this unit in order to be at a similar level to the amplifier its going through so that I don't have to increase the amp's volume, I get what I'll call "breakup" starting at the Noon position on the Volume level pot.  I can increase or decrease the output of the unit by increasing either the Volume Level or the Output Level, and the effect is the same.  So I'd like to be able to get more Treble without the "fizz", and more volume to match the volume of the amp, without breakup.  I have scope images if interested.

[sluckey]

What amp is this plugged into? Do you have an amp with a power input jack such that you could connect this unit directly into the other amp's power amp?

[wsscott]

Yes, I'm using it with the build I just completed with your Magnatone Twilighter Mod.  I added an aux. power source so I could plug directly into the amp.  I've also used it with by 5F1 and my Gretsch 6165, neither of which have aux. power inputs.

[sluckey]

So what does it sound like when plugged directly into the power amp?