Magnatone Vibrato/Reverb build Project

[wsscott]

Here are the VDC with the Vib disconnected and the 100 ohm R installed at Node A-B:
V-6:
P1/6-217.1
P2/7-37.5 mv
P3/8-5.92

V-7:
P1-121.3
P2-.2 mv
P3-1.284
P6-186.7
P7-1.3 mv
P8-1.940

I'm checking on the wiring.

[wsscott]

Ok, got it fixed.  The problem was at the reverb output jack.  I had a shielded cable going from V7-P2 to the output jack for the reverb but had not connected the cable's shield to ground.  So I tried to fix that by connecting the ground shield in the cable to the ground lug on the output jack.  The problem is that I have the 47K R connecting the ground lug with the tip lug of the jack and that created a short.  So I disconnected the shield, and now it works.  My question though is how do I ground the shield?  Wouldn't it cause the same problem even if I connected it to another ground point?

[sluckey]

The problem is that I have the 47K R connecting the ground lug with the tip lug of the jack and that created a short.

That's not the problem. I suspect the shield is shorted to the center conductor, probably at one end of the cable. Check the cable with your ohm meter. Looks like you are using RG-174 cable? It's very easy to melt the insulation of the center conductor when soldering the shield. Just use another piece of cable and be careful. I quit using 174 for that very reason.

[wsscott]

You were correct.  Should I use shielded cable for this connection, and do it over, or just use regular wire for now?

[sluckey]

Try it with a regular wire. A shielded cable would be better. I would use shielded it it's over 2-3 inches long.

[wsscott]

It was about 4" so I ran another shielded cable and seems to have worked this time with no short.  So now I've got reverb, and I've got vibrato oscillation that will stop if I disconnect the jumper from the chassis, and then it leaves me with a scratchy background sound with reverb still operational.  I guess I'm ready for the next step.

[sluckey]

Reconnect the stuff to the FS jack and test the footswitch.

And recheck the voltages on V5 pins 1, 2, 3, and 6, 7, 8. Also recheck the voltages on all the filter caps. Post your findings. And give me the model number for your PT. I've put together a one page schematic for you, complete with your voltages. Makes it easier to see the flow when it's all on one page.

[wsscott]

I reconnected the foot pedal wiring.  I did not connect the original wire going from the 47K to Vibrato since you have the new wire that was added to control it.  The foot pedal responds as it should.  Let me know if you want me to reconnect that other wire to Vibrato.  Below are the voltages.

V-5
P1-234.2
P2-155.0
P3-159.0
P6-241.6
P7-211.3
P8-208.9

The Caps VDC readings are as follows:
A-329.4
B-325.8
C-308.8
D-276.7
E-250.0

[wsscott]

The PT is the Hammond 269JX.

[wsscott]

In comparing the voltages of my V-5 with yours, mine are significantly higher for both sets of grids and cathodes.  That tube's output leads to the V-6 for the reverb.  Do you think the background oscillation could be due to the fact that my voltages are so much higher for some reason than yours?  Is there a way to drop the voltages on my V-5 grids and cathodes?  Would a change in the 47K resistors on the V-5 have any effect?

[sluckey]

V5 voltages are not an issue. You cannot compare my V5 voltages to yours because you removed V3 from the preamp. V5-2 is direct coupled to the V3-6 plate in my amp, but it is directly coupled to V1-1 plate in your amp. V1 plate is higher than V3 plate. And that higher voltage directly couples to V5-2 and that higher voltage causes the other V5 voltages to be higher. No further action needed for V5. It's OKAY.

Do you think the background oscillation could be due to the fact that my voltages are so much higher for some reason than yours?

I hope you don't have any background oscillation now. The only time you have oscillation now is when you turn on the vibrato footswitch and that should put the oscillation in the foreground because you want to hear it.

[wsscott]

I guess what I'm trying to describe is that if you have the vibrato on with no reverb, and you're not playing anything, everything is quiet.  However, if you have the vibrato on and you turn the reverb on, and you're not playing anything, you hear oscillation.  Seems to me that it should be quiet too.  That's why I was thinking that if the V-5 is getting more voltage maybe that's emphasizing the background oscillation.  If I turn down the reverb intensity pot then the background oscillation is quieter, but then of course there's less reverb.  Is it that way it is in your M-10A amp too?  If so, then that's fine.  And of course its not noticeable when you're playing.

[sluckey]

I haven't had my amp plugged in for over 5 years. Don't remember.

[wsscott]

Why not plug it in and give it a test? It probably misses you! Haha!

[sluckey]

It's sitting in a room with about a dozen other amps that don't get played these days. They have to entertain each other. 

[wsscott]

Sluckey-Well its sounding great.  Your fix worked perfectly.  Thanks again.  I got the 1M Linear Pot and installed it for the Vibrato Intensity to replace the Audio pot I had used, and I like the Linear pot much better. I have more control over the vibrato effect volume.  I've taken off the test gnd lead that you had me add, and also removed the original wire going from the 47K to Vibrato since you have the new wire that was added to control it.  I guess the only other thing that came up was putting in a larger Node E cap to replace the 1uf 450V that's presently installed.  What are your thoughts, and what would the result have on the sound, etc? 

If I can figure out how to upload a video or sound file that's small enough to upload to this thread, I'd like to share the results with everyone. Looks like this thread is winding down with a successful outcome.  Thanks again.

[wsscott]

I thought I would upload an As Built layout in pdf and DIY files of my Sluckey M10-Reverb/Vibrato modified build that I successfully completed, along with his schematic.  Really pleased with it.  I having a cabinet built by Guitar Cabinets Direct and hopefully that should arrive this week, and I'll send photos when the chassis is installed.  I also am working on a sound recording so you can hear that this device actually does work, and sounds great.  Thanks again for everyone's help with this project.  Stephen

[wsscott]

Attached are photos of the final build in the cabinet that was made by Guitar Cabinets Direct.  They did a great job and were very prompt.

[wsscott]

I've got 2 more photos.  Here's one of them.

[wsscott]

Ok, here's the last photo.

[wsscott]

Ok, well I made a short recording of a few bars of Ray Charles' song "What'd I Say".  I certainly haven't done him or the song justice, but I'm just starting to play again. Anyway, enough excuses, I recorded it without any effects, then added the Vibrato, and finally the Vibrato and Reverb so you can see how it sounds.  Vibrato Speed is at about 10 o'clock position, and Reverb Intensity is at about 8 o'clock position.  Volume and Treble are at 12 o'clock, and Bass is at 2 o'clock.  Thanks for everyone's help.

[sluckey]

Sounds good. Now you need a big amp.   

[wsscott]

Yea, I was using my 5F1 amp.  I should have used my 1966 Gretsch Variety Plus amp with 35 watts output. It’s about ready for a re-cap job. Thanks again.

[sluckey]

Would you say this project has met your expectations from your original post?

I've attached a reformatted schematic that you may find useful. It can be printed on one sheet of legal size paper.

Edit... Removed attachment and added it below.

[wsscott]

Sluckey--Thanks so much-This is great--Yes, there is no doubt for me that this project came out exactly as I had hoped for.  I've never heard any Magnatone in person, and I don't think I've ever heard a Mag 10A even on YouTube or elsewhere, so I don't have anything to compare mine to, but it certainly has the distinctive Vibrato effect that I was looking for, and the reverb is a bonus.  I have read a little about "reverb dwell" control, but I'm not sure I know what that does, so I don't know if that would be anything I would want to add.  I think I've seen folks running a 1M pot off the Grid of the V7 tube in place of the 47K resistor.  And if that's correct, it would be easy to do, but I don't know what the result would be by way of sound.  In looking at your schematic I noticed that you have a date of "2010" in case you want to change that to "2020".  Thanks so much for all your time and help.  Best.  Stephen

[sluckey]

I don't see any viable way to add a dwell pot to the maggie circuit. Dwell is used to control the level going into the tank. It is usually implemented by replacing the 1M grid resistor on the driver with a pot. But that would also affect the dry signal in that circuit and that's not good. I've updated the new drawing (pun intended).

[wsscott]

I haven't seen many amps with dwell pots, so its probably something that doesn't make much impact on the sound.  I'm happy just the way mine is!  Best.

[PRR]

> add a dwell pot

Split the 12AU7. This *will* need tinkering to get the relative gains acceptable. (I suspect this is way too much gain on the drive side; a AUDio taper dwell pot may be best for initial testing so you don't have to turn-down to the 0-1 crack.)

[shooter]

Split the 12AU7.

He's still in the "I can't quit playing, or believe it works faze"


Maybe in a month

[wsscott]

Shooter my exact words to my wife who was with me when I fired it up were "F*** I can't believe it works!"  And I said it again!!  So yes I'm still in that stage but will keep this in mind if I want to pursue it. 

[shooter]

 


You ignored "time" and focused  on the task at hand
my last build I gave Pam $100 and said, I'm gonna be making noise for awhile today 

[wsscott]

Here's a question for you guys.  I've been playing the VibroVerb (not yet Trademarked) through my 5F1 Champ (5 watts), and I set its Output level which controls the "output" at the output jack, to about 9 o'clock on the pot.  Sound volume is fine.  Today I decided to run it through my Gretsch/Valco amp which has 35 watts output.  When I did that I had to turn up the Output Level on the VibroVerb to about 3 or 4 o'clock to get equivalent volume.  So for some reason it seems that the input sensitivity on the Gretsch is not as high as on the Champ.  Am I interpreting this correctly, or does anyone know the reason why I have to increase the Output Level so much more.  Thanks.

[sluckey]

The 5F1 hi input has no voltage divider at the input but the Gretsch amps often have a voltage divider at the input jacks. Show us your schematic and we can be more specific.

[wsscott]

Thanks.  Here you go.  I have the 6165 model. I'm pretty sure this is the schematic for the preamp section of the amp. There's another schematic for the power amp.  I'm plugged into the "dual input".

[sluckey]

You need a stereo plug to use the dual input jack properly. Plug into the channel 1 input.

But really, it's just a matter of different input voltage dividers. Nothing wrong. That's why there's an output level knob. 

[wsscott]

Sorry, I see I posted the wrong schematic last night--I posted the Power Amp.  Attached is the schematic for the pre-amp.  I don't know if this helps.

[wsscott]

Yes, it looks like when I use the Dual jack input on the Gretsch, as I always have, with a regular phone plug, which I always have, the input from the guitar/Vibro-Verb is going through the 470K Resistor to GND rather than the 1M Resistor if I had used the Input 1 or had a Stereo plug.  The 5F1 Champ's input goes to a 1M resistor, so that probably explains it.  I'll switch inputs and let you know the result.  Thanks.

[sluckey]

The idea of the dual input jack is to send the guitar signal through both channels. But if you use a standard plug channel 1 input gets grounded out and the signal only goes through channel 2. Easy fix. Just put a stereo plug on your input cable and jumper the tip and ring together. (The stereo plug will still work with a normal input jack.) Then you can use both channels and all the knobs. The M-10A does the same thing but calls that jack a stereo input.

[wsscott]

Well I think I figured it out, and the result is that the Output in decibels is in fact the same.  I download a Decibel App to my Ipad.  Then I plugged the guitar directly into the Gretsch Channel 1 jack and measured the Decibel level at around 64 when I plucked the E string.  Then I plugged the guitar directly into the 5F1 #1 jack, plucked the E string and adjusted the volume on the amp until it also showed a level of about 64 Decibels.  So now the amps are set at the same output level.  Obviously the Gretsch that's 35 watts with 2 12" Jensen speakers has a lot fuller sound than the 5F1 with 5 watts and a 8" speaker.  So then I plugged the guitar into the Vibro-Verb, and then connected it to the Gretsch.  I turned the Output Level all the way up on the VV and after adjusting the Treble and Bass on the VV to get a similar, but richer, sound as the 5F1, I got a reading of about 64 Decibles.  To finish the test, I then plugged the VV output into the 5F1 without changing any setting on the VV, and also read about 64 Decibles.  Although the sound was not as full and a bit brighter/tinnier on the 5F1, the actual sound levels were pretty much the same whether I played directly through either amp, or through the VV--as long as its Output level was turned all the way up.  When I plugged the guitar into the dual input jack on the Gretsch the decibles were just a little bit lower, around 60 or so. If you lowered the Output Level setting, then of course the decibel level would be less than if it went straight from the guitar to the amp.  Although my 1965 Gretsch amp that I purchased new has some years on it, it is still original except for 1 filter cap replacement, and a replacement to a 3 prong plug. Still original tubes.  Thanks again for the help.

[wsscott]

Question for you about my Gretsch pre-amp schematic shown in my reply #485 above:  What are the VR1, VR2 and VR3 components shown on the V2 and V3 tubes, and what are their values?  Since I've been playing the Gretsch amp more its really begun picking up more hum than normal.  I guess components are starting to fail--it is over 50 years old!  When I have the guitar plugged into Input 1 it has the hum which goes away when I unplug the guitar or turn the volume down/off.  Ideas on where to look?  Could it be the 35uf cap coming off the cathode on V2 Pin 8?  Thanks.

[sluckey]

What are the VR1, VR2 and VR3 components shown on the V2 and V3 tubes, and what are their values?

Those are voltage dependent resistors, AKA, varistors. Their resistance changes depending on the applied voltage. You need a part number if you're considering replacing. Not likely to find one. I'd leave them be.

When I have the guitar plugged into Input 1 it has the hum which goes away when I unplug the guitar or turn the volume down/off.

Probably nothing wrong with the amp. Can you kill the hum with the volume control on the guitar?

[wsscott]

I thought they might be varistors like I'm using in the Mag 10 A vibrato circuit.  Neat.  Yes, I'll leave them alone.  But now I know what they are.  The volume control on the guitar has no effect on the hum from the amp. With the guitar unplugged, I hear a higher pitched tone or hum coming from the amp.  When I plug the guitar in, I still here the higher tone, but then I also hear a low hum so I'm getting both types of noise when the guitar is plugged in, and the level of the noise varies with the Volume control on the amp and will disappear when the volume is totally down to 0 level.  I hear it from both of the Channel 1 inputs, but a bit lower on the "dual" jack (I'm still using the mono plug).  Same effect on the Channel 2 Bass channel, but a bit lower in volume.

[sluckey]

Have you ever changed filter caps in that amp?

[wsscott]

Nope.

[wsscott]

I decided to check the caps in the power supply and wanted your thoughts on the results.

I have a Blue ESR Tester, and all of the results seem to appear fine.  I connected 1 lead to chassis ground and the other to the + side of the caps and took the readings.

The can cap w/ 20-10-10 at 450 v read ESR's of .80,.98, and 2.4.
The single 20/500v which was replaced a few years ago read ESR of .95
The single 35/50v which was replaced at the same time with a 47/160 read a ESR of .57.

So with these ESR readings is there any reason to replace these, or is there another test I should do?

Thanks.

[sluckey]

I've never measured cap ESR. I usually replace filter caps based on hum, decoupling issues, or ripple voltage measurements. If this is an amp that you rely on for gigging, I would replace the filter caps based on age just for reliability, but for a home amp, I would not bother.

[shooter]

+1


never done esr either, ripple voltage and age are 1 n 2, if you're bored, watching each tap on a scope + meter while driving amp hard/soft/hard/soft can be fun 

[wsscott]

Yes, I'm a bit bored right now, but I don't think I want to take it to the next level and learn how to use a scope!  I'll rely on you guys!

Speaking of which, I was going through the Gretsch's pre-amp to ID the caps to replace, and it has two caps that I don't know if they're electrolytic or not--I'm assuming they're not, but I'd like your input.

They're 1uf 400V, they have the name "Standard" on them, and they have a black ring at one end.  They appear to be paper, and not a metal shell, and don't have an indent in them like the other larger electrolytics in the amp. One of them is connected on one end to the Vibrato footswitch tip, and the other end connects to the Plate on the 6DR7.  The schematic I have isn't real clear, but I think the other one is coming off the Cathode, pin 8, on the 12AX7 for the Reverb recovery and going to GND.

[shooter]

They're 1uf 400V

"most" of the time when a cap =>1uf it's an Ecap.  There are non-polarized but typically in HIFI not guitar

[sluckey]

They are most likely electrolytic although they don't have to be. If your tremolo and reverb are working fine I suggest you leave them alone.