Magnatone Vibrato/Reverb build Project

[wsscott]

Just as I described. Also some minor hum that increases a little when reverb is active.

[sluckey]

I want you to connect V1 pin 6 directly to V5 pin 2. This requires you to remove the .047 cap and the wire between V1-1 and V5-2 indicated in the attached schematic.

This will totally bypass the volume and tone controls. Evaluate the sound and report back.

[wsscott]

Ok, so NO pots are functioning for Volume, Treble or Bass Pots.
Output Level pot controls volume.
Vibrato not functioning.
Reverb working as normal.

Sound seems unaffected.  When scope is connected, see image attached, shows "dirty" signal, but the "dirt" appears the same regardless of the Output Level setting.  The amplitude,ie. the VRMS and the VPP, of the signal increases/decreases with changes to Output Level pot, but the shape of the "dirt" at the bottom remains the same regardless of the Output Level setting.

[sluckey]

Where is channel 1 connected? Where is channel 2 connected? If channel 1 is connected to the input jack, then turn the signal level down to 100mV_PP.

The simple changes I asked you to do have should not have interferred with the vibrato working.

[wsscott]

Sorry, I had only the input signal going into the input jack and no probe was connected to #1, so I don't know why there was an image.  Anyway, I ran the scope again and the image is attached from Probe #2 and set for VRMS 100mv.  Only Probe 2 is connected.

[wsscott]

Ok, I can get the Vibrato to work, but the Intensity has to be at 99% of the way Clockwise, ie. turned up.  Back if off just a hair, and its not audible.  Works fine with the circuit wired as before.

[sluckey]

I'm out of ideas. Good luck.

[wsscott]

Thanks for your help Sluckey. 

[Willabe]

Where is channel 1 connected? Where is channel 2 connected? If channel 1 is connected to the input jack, then turn the signal level down to 100mV_PP.

Sorry, I had only the input signal going into the input jack and no probe was connected to #1, so I don't know why there was an image.  Anyway, I ran the scope again and the image is attached from Probe #2 and set for VRMS 100mv.  Only Probe 2 is connected.

Where did you connect probe #2? 

[wsscott]

Probe 2 is connected at the Output Jack.  Probe 1 was not connected in this last photo. The shape of the signal as shown in the last image posted, stays the same regardless of the Output Level setting.  Does the shape of the signal tell you anything about the cause?  I'm a real novice, particularly with scope stuff.

[wsscott]

How would increasing the size of the 470pf cap going into the Treble pot affect the symptoms I've tried to describe?

[Willabe]

I don't know what the scope pic means other than it's a little distorted on the bottom 1/2 of the sine wave. (I have no experience with scopes.)

I think you have to verify that the test tone is clean 1st. So put your probe on the input jack and run your test tone in to check that 1st. If the tone signal is clean then.....

Then you have to 'divide and conquer'.

You have to find exactly where that signal is getting distorted. That FX unit is 4 separate stages, 5 if you include the power supply (PSU). You put your probe on the output and that doesn't tell you where it's distorting. 

You want to see if the 1st section/block, the preamp V1-A/V1-B, is clean. I think Sluckey had you take the tone controls and volume control out to eliminate them as the problem. So do that but put your probe on the plate of V1-A. If you clip in a small cap, 0.02, 0.047, what ever you have handy, to V1-A's plate that will block the dcv and you can read the ac sine wave.

If the wave is clean, then hook up the tone/volume controls again. Now with the probe in the same place, far end of coupling cap hooked to V1-A's plate, see if the sine wave is still clean. If clean, it's not the preamp V1-A/V1-B section.

Stay focused on the sine wave 1st. If it's really distorting, and not the signal generator, if you can find where it's distorting, which section, then you can find out why it's distorting. Then your treble fizz might go away.

If the preamp sine wave looks clean, then reconnect the 1st section/preamp to the 2nd section/vibrato modulator V5 and the vibrato oscillator/driver V4. Disconnect the 0.0022 cap from pin 2 of V6. Hook up your probe to the open end of that cap. See if the sine wave is clean there.

Stay focused, don't worry about the 470pF treble cap, that's not the problem, unless the cap got cooked when you put it in.

Output volume can be worked on later.

[Willabe]

Do you have a layout drawing?

Long thread, or I'd look for it. 

[wsscott]

Yes I have a layout I can post tomorrow.

[wsscott]

The input signal is clean. No issue with that. With Probe 2 on output jack, the signal is clean until the Treble pot is turned past 9:00. Then you start seeing the distortion on the bottom of the signal. Changing Volume pot or Output level pot doesn’t affect it. It’s solely controlled by the Treble pot. If you increase Treble, the Amplitude and the VPP/VRMS increase but the signal still has the same shape at the bottom. So it seems to me that the source of the problem is in that part of the circuit. That’s why I was thinking that changing the cap may be a solution. I know some will say “Change the cap and see what happens “.  Yes I can do that, but I figure that the less I solder and unsolder a joint the better. Why take something apart if there’s no reason to. Maybe stupid, but that’s my thinking. Hope this makes sense.  I’ll try some more probes on the circuit tomorrow and post my findings.

[Willabe]

The input signal is clean. No issue with that.

How do you know that?

[Willabe]

With Probe 2 on output jack, the signal is clean until the Treble pot is turned past 9:00. Then you start seeing the distortion on the bottom of the signal. Changing Volume pot or Output level pot doesn’t affect it. It’s solely controlled by the Treble pot. If you increase Treble, the Amplitude and the VPP/VRMS increase but the signal still has the same shape at the bottom.

The tone stack in your build with treb/bass set to 12:00 is flat. So treb set to 9:00 treble is still cut not boosted. Adding treble causes the problem, seems to be parasitic, or that cap is bad, or the solder connection is bad.   

So it seems to me that the source of the problem is in that part of the circuit. That’s why I was thinking that changing the cap may be a solution. I know some will say “Change the cap and see what happens “.  Yes I can do that, but I figure that the less I solder and unsolder a joint the better.

Why take something apart if there’s no reason to.

There is a reason according to your explanation.

Change the cap.

[wsscott]

As to the input signal, I’ve posted images of it on probe 1, yellow, connected at the V-1 pin from the input jack, along with probe 2, blue, at the output jack showing the issue. I think the file is named 14.pdf.  As a reminder, changing the Bass pot has no effect on the output signal. So in replacing the cap, do you have a thought on value? Thanks.

[wsscott]

See post 652 for the file image 14.pdf.

[Willabe]

So in replacing the cap, do you have a thought on value?

Same value.

[wsscott]

Here's the Layout.

[Willabe]

Thank you, I found it yesterday.

[wsscott]

Well, I used Probe 1 to check the signal coming from the Input test tone, and probed it at V-1 Pins 1, 2 and 6, and V-5 Pins 2 and 6, both sides of the .047uf cap and the 470pf cap, and at terminals 1 and 2 on the 1M Treble pot (I think its a linear and not an audio pot), and other than changes in the VRMS/VPP the signal stayed the same and clean if the Treble Pot was around 9:00 or less.  I could get good amplitude at the Output probe, and a clean signal, even if I increased the Volume or Output Level pots, as long as the Treble pot stayed at that setting.  So maybe its something with the pot. But I guess since I feel like I've found where the clean sound is located, and I can get the Output level, then why should I worry with the amount of treble in the signal.  Interesting though that the Bass pot doesn't have the issue.  Maybe its time for me to just let it go.

[Willabe]

Your not being clear. Does the signal get distorted when you turn up the treble pot past 9:00 at pin 6 of V1-B, the input triode? If it stays clean, then it's happening after the tone stack/volume pot.

I thought you were going to change that 470pF treble cap? When you change that cap, use a heat sink on the leads when you solder them. There have been many builds here that it ended up being a small cap causing problems like yours. You spent all this time on this build and you wont change a cap?

Forget about the output. If you get the signal cleaned up where it's coming from then the output will be fine. The output volume can be increased be changing the voltage divider at the output and maybe by adding a bypass K cap on V1-A.

That series 1M R in the voltage divider will kill off highs. The 1st thing to go with a high value series R is the hi end, that's why Fender used a 10pF cap across the 1M R in the BF reverb amps, to restore the highs. Hi gain amps use them too across the series R in the voltage dividers. Very common. You could try bypassing it with a small cap, start with 10pF. But that might make your fizz problem worse. So you have to find/fix that 1st.

This sounds like;

1. That 470pF cap is bad or a bad solder joint.

2. Parasitic problem, only happens when you turn up the treble. Grounding/lead dress.

3. Treble pot could be bad, or bad solder joint.

4. Grounding problem, causing symptoms like/close to #2. There's a few things I'd try if changing that cap and maybe the pot didn't fix it.   

If those things don't fix it there is a band aide trick you can try, that should fix it.   

[wsscott]

Thanks for the encouragement.  I'll do as you suggest and get back later today or tomorrow.

[Willabe]

I think you can fix it. 

I think it's just fine tuning it a little now.

[Willabe]

You have tried different tubes for V1?

Sometimes a tube will sound fizzy. 

[sluckey]

Set both scope channels to measure VPP, not VRMS.

Connect your test tone signal to the input jack. Connect scope CH1 probe to the input jack also and set CH1 for AC 1X 50mV/div. Adjust the level of the test tone to be 100mVPP (***NOT 100mVRMS***) as seen on CH1 display. You should have a clean sine wave that is two vertical divisions tall. Don't change any of this throughout your testing.

NOTE! You may have to change the mV/div vertical sensitivity to keep the sine wave on the display while making the following checks with CH2 probe.

Now set CH2 for AC 1X 200mV/div and connect the probe to V1 pin 6.  Save the scope image to a jpg or png file with the name V1-6.jpg or V1-6.png.  ***PLEASE, NO MORE PDF IMAGES!***

Next, move the scope probe to V1 pin 2. Adjust the BASS, TREBLE, and VOLUME to mid position. While monitoring V1 pin 2, adjust the TREBLE pot to see if the distortion appears. Save the scope image to a jpg or png file with the name V1-2.jpg or V1-2.png.  ***PLEASE, NO MORE PDF IMAGES!***

Finally, move scope probe to V1 pin 1. While monitoring V1 pin 1, adjust the TREBLE pot to see if the distortion appears. Save the scope image to a jpg or png file with the name V1-1.jpg or V1-1.png.  ***PLEASE, NO MORE PDF IMAGES!***

Post your findings with these three image files attached.

[wsscott]

Here are the images.  My scope just does auto-setting for the images. 

[sluckey]

Here are the images.  My scope just does auto-setting for the images.

jpg is just fine. Much better and easier/faster to view than your converted pdf.

No sign of that kinked/crooked waveform on the bottom of the signal. In fact, V1 pin 2 looks exactly like the input signal. So I'd quit suspecting anything before V1-2. That includes the 470pf cap. V1 pin 1 is a little slanted. May be due to loading from V5. How about pulling V5 tube and put CH2 probe back on V1 pin 1. Save the display pic as V1-1b.jpg.

Next, put V5 back in and move CH2 probe to the output jack. Adjust the output level for 100mVPP. Save the display pic as output.jpg.

Report your findings and attach these two pics to your message.

[wsscott]

Thanks.  Will do, but the cocktail hour has arrived, and so my Manhattan is awaiting me.  I'll report back in the morning.  Thanks again for everyone's time. 

[sluckey]

Question about your layout. In the attached pic I have circled three groups of turrets. Is the center turret in each group connected to the left and right turrets?

[wsscott]

The dotted lines are for jumpers going under the board.  The only instance where there is a connection is in the lower group where the 47K resistor is jumpered to the .1uf cap on the right.  That cap is also jumpered to the other .1uf cap on the left under the board.

[wsscott]

Actually its the .1uf cap on the left, jumpered to the 47K resistor, which is then jumped to the other .1uf on the right.

[wsscott]

I did switch the 12ax7 from V-1 to V-7 positions as Wallabe suggested, in case it might be a tube issue.  Then I did as Sluckey requested. The Input on Probe 1 is set at about 100 mvPP, the Volume, Treble and Bass pots are all midway at Noon position, V-5 tube is removed, and then Probe 2 is connected on V1-1 and the image is attached.  Then I installed the tube, and moved Probe 2 to the output jack and adjusted Output Level pot until it read about 100mvPP and the image is attached.

[sluckey]

So, the output looks fine when the input signal is 100mVPP. But when the input signal is 431mVPP the output gets the kink in the negative portion of the waveform.

431mVPP is much hotter than a typical guitar and is just overdriving the revibe. 100mVPP is much closer to the output of a typical guitar and is the level you should set your signal generator output whether you're connecting to the input of your revibe or any guitar amplifier.

[wsscott]

So what I understand that you're saying is that I just need to tone down the output of the unit, and find some treble/bass settings that I like the sound of and then adjust the volume/output pot levels to keep it to an output level at the output jack of about 100mVPP.

[wsscott]

I guess part of my problem was that I thought I had read that I should use a setting of 150 VRMS for the typical output of a guitar, and maybe that should have been 150 VPP.  The VRMS reading of 150 results in a much higher VPP figure.  So, am I correct that I should be using an input Test Tone Signal of 100-150 VPP when scoping any guitar amp or effects unit like this one? 

[sluckey]

No. What I was saying is don't overdrive the input of the revibe because doing so will kink the output.

[wsscott]

Yes, I understand what you're saying.

[shooter]

if you have a 1/4" jack laying around;
plug your guitar into it, clip a scope probe to the 2 tabs on the jack and play.


what I've found;
hot pickups played real hard come in ~~~~~~  200mV rms
that's when you're going for the Jimmi in you 

[Willabe]

No. What I was saying is don't overdrive the input of the revibe because doing so will kink the output.

So what's the bottom line here?

There's nothing wrong with the build? He was getting a false scope reading because he was inputting to much signal and/or setting the scope for vrms, not vpp and/or turning up the input gain/volume too much? 

So if that's correct ^^^^, then if he doesn't turn up the input volume too much, the hi end fizz won't be there?   

[sluckey]

Nothing has changed, except he should know the proper signal level to inject into an amp.

[wsscott]

I guess some confusion for me now exists because Sluckey says use an input tone signal of 100mVPP and Shooter says the output of a guitar is around 200mVRMS.  And they each result in significantly different strengths for an input.

[sluckey]

I guess some confusion for me now exists because Sluckey says use an input tone signal of 100mVPP and Shooter says the output of a guitar is around 200mVRMS.  And they each result in significantly different strengths for an input.

Sigh!

[shooter]

^^^^^


I said;
that is the MAX
what I was INFERING;
measure your own guitar, find out WHAT "normal IS" for your equipment
Also;
what has already been pointed out
volts RMS
and
volts PP
are different, pick one, DON'T mix them, they cause extreme heartburn when mixed 

[wsscott]

Now that makes sense.

[Willabe]

hot pickups played real hard come in ~~~~~~  200mV rms

You probably don't have high output pickups and don't play real hard.

Adjust the level of the test tone to be 100mVPP (***NOT 100mVRMS***) as seen on CH1 display.[/u] You should have a clean sine wave that is two vertical divisions tall. Don't change any of this throughout your testing.

[shooter]

In the novels I read;
a lawyer will not ask a question they don't already know the answer to
As a tech, i don't probe willy nilly without knowing "what to expect"
sometimes, both professionals are surprised with "the reply", that's where the fun begins 


EDIT:
I did most of the testing with a musician, playing a early 60's Les Paul, stock.
I found spikes as high as 2+ volts  but the mean average over time was 200 - 250.
once I had "that number" I "tuned"  amps around that value, so ya, not a big sample set

[wsscott]

As a retired attorney who did some litigation, Shooter is right unless you really want to know “If the glove fits, or if you must acquit.” I’ll get some readings on my Strat and my Gretsch Country Gentleman, and I’ll let you know what I find.