Magnatone Vibrato/Reverb build Project

[wsscott]

So I guess what was confusing me was the type of rectifier in the circuit schematic.  I saw 4 diodes so I thought it was a full wave rectifier, not realizing the difference between that and a full wave bridge rectifier.  So I learned about the use of a PT center tap with the full wave rectifier but not with the bridge rectifier.  Also, can I ask, why does this circuit use 4 diodes for the rectification rather than just 2, which seems to be the way they are usually designed--at least based on some of the Youtube videos I've seen?  Thanks.

[shooter]

PIV 
typically when you see 4 it's because of large "spike" potential that could cause a diode to fail.

[wsscott]

So does it make sense for me to install the remaining filter caps and voltage dropping resistors now to be sure each node meets the output voltage values called for in the schematic before doing anything else on the build?

[Willabe]

No, there's no current draw, so the B+ dropping R's wont drop any dcv.

[Willabe]

Also, can I ask, why does this circuit use 4 diodes for the rectification rather than just 2, which seems to be the way they are usually designed--at least based on some of the Youtube videos I've seen?

Like shooter said, PIV = peak inverse voltage. Look it up and read up on it. 

From Merlin's web page;

http://www.valvewizard.co.uk/bridge.html

http://www.valvewizard.co.uk/smoothing.html

http://www.valvewizard.co.uk/psu.html

[wsscott]

Thanks for the lead on the articles.  I'll check them out today. 

[wsscott]

Sluckey-I woke up at 4:30 this morning thinking about this.  Since I've taken the Power Amp out of the circuit, is there any need for the 100K Volume Pot at the output?  Is the signal supposed to be at the same level going out as it is at the input?  Also, what is the purpose of the Volume Pot after the pre-amp stage? Thanks. Stephen

[sluckey]

Sluckey-I woke up at 4:30 this morning thinking about this.  Since I've taken the Power Amp out of the circuit, is there any need for the 100K Volume Pot at the output?  Is the signal supposed to be at the same level going out as it is at the input?

The signal level at the output of V7 is BIG, big enough to drive a power amp. But since you will be connecting this unit to the instrument input of another amp, you need a way to knock that BIG signal back down to instrument level. So, I added a 1M plus the 100K pot to make a 10:1 voltage divider just to knock that BIG signal down. The 100K pot allows fine adjustment of the output level.

Also, what is the purpose of the Volume Pot after the pre-amp stage? Thanks. Stephen

It's the volume pot from the original M10A. Turns the volume up and down.

IMO, both pots are desirable and even necessary. In fact, I had considered putting a switch across the 1M resistor that feeds the output level pot. This would provide a "line level" output as well as an "instrument level" output. This would allow you to also plug this unit directly into a power amp if desired. If I were to build this I would put the switch in.

[wsscott]

Yes, I woke up wondering about the size of the signal at V7. I assumed the level would need to be reduced, but I was concerned that with the pot, a user could overdrive the output signal.  So I assume the 1M resistor reduces the level so that won't happen. I was thinking you might just recommend a resistor rather than a pot.  But I understand the flexibility the pot gives.  I intend to run a guitar through this device and then out and through the Fender Champ I built, but it could be used with other amps too.  What type of setup are you thinking of for using it with just a power amp?  As to the switch, would it just be a SPST?  Thanks.

[wsscott]

I'm starting cutting out the holes for the chassis, and need to decide about the foot pedal(s) for the vibrato and reverb.  I'm mounting the vibrato/reverb jack(s) on the bottom of the chassis where the tubes will be, along with the RCA jacks for the reverb tank.  I would like to use 1 jack controlled by a single foot pedal that has 2 switches--like the Fenders.  I assume it terminates in a 1/4" stereo jack. But are there issues to consider that would go in favor of 2 separate foot pedals?   If just one jack is used, would it be a stereo 1/4" jack or something different?  Thoughts?

[wsscott]

Sluckey/Shooter/et al:  Do any of you see any issues with the tube/reverb tank jacks/or reverb-vibrato foot switch jack placement shown in my attached layout?

[sluckey]

Your layout is fine. No problems using a 1/4" stereo jack. The cable must be shielded for the reverb but vibrato wire does not need to be shielded. I suggest using a Fender reissue footswitch...

     https://www.tubesandmore.com/products/footswitch-box-fender-two-button-vibrato-reverb-vintage

[wsscott]

Thanks Sluckey.  I really appreciate your help with this project.  Stephen

[wsscott]

Does the wiring into the RCA Input/Output Jacks for the Reverb Tank connections also need to be shielded?

[shooter]

The cable must be shielded for the reverb

[sluckey]

Does the wiring into the RCA Input/Output Jacks for the Reverb Tank connections also need to be shielded?

Not needed if the wires are only a couple inches long. Otherwise use shielded cable. Same for wires between reverb pot and circuitry.

[wsscott]

Thanks.  The folks at Antique Electronic Supply got me the info on the wiring of the plug for the foot pedal: The tip is the vibrato and the ring is the reverb. I"m finishing up drilling the holes for the chassis today or tomorrow, and then will start the chassis wiring. 

Two questions on wiring the filaments.  First, do I need to install a 100 ohm resistor (for a total of 2 resistors) on each filament lead from the transformer secondary to the Fender style pilot lamp I'm using?  My Hammond 269 JX transformer doesn't have a center tap for the filament leads, just for the main leads.  Second, do I use pin 9 on these tubes, or just run both the leads to filament pins 4 and 5? 

[Willabe]

Also, can I ask, why does this circuit use 4 diodes for the rectification rather than just 2, which seems to be the way they are usually designed--at least based on some of the Youtube videos I've seen?

Like shooter said, PIV = peak inverse voltage. Look it up and read up on it. 

From Merlin's web page;

http://www.valvewizard.co.uk/bridge.html

http://www.valvewizard.co.uk/smoothing.html

http://www.valvewizard.co.uk/psu.html

Did you read this and search out PIV?

[sluckey]

Two questions on wiring the filaments.  First, do I need to install a 100 ohm resistor (for a total of 2 resistors) on each filament lead from the transformer secondary to the Fender style pilot lamp I'm using?  My Hammond 269 JX transformer doesn't have a center tap for the filament leads, just for the main leads.

The resistors are necessary to reduce hum. Each resistor connects from a filament lead to chassis. Fender was fond of physically connecting them between the pilot lamp and chassis but you can place them anywhere that's convenient.

Second, do I use pin 9 on these tubes, or just run both the leads to filament pins 4 and 5?

Do it the same as you did in your champ. For all these tubes... connect one filament lead to pin 9. Connect the other filament lead to pins 4 and 5 (jumpered together)

[wsscott]

Gotcha-So they're wired in parallel keeping it at 6.3 V, rather than running them in series from the 12.6 V pin 9.

[Willabe]

Gotcha-So they're wired in parallel keeping it at 6.3 V, rather than running them in series from the 12.6 V pin 9.

That's backwards.

You use pins 4 and 5 to run them in series.

Pin 9 is the center tap point between the 2 filament heaters. 

[wsscott]

Slukey-I was thinking of installing a grounding lug to the chassis in an open area between tubes V6 and V7, and move the 2.7K and 270 ohm cathode resistors from the ground bus wire running along the top edge of the board and connecting those resistors directly to those tubes and then ground them to chassis on the added lug.  In fact I could also do it with the 2.7K cathode resistor on V1. Thoughts?  Is this likely to create a ground loop problem?  I"ve attached a current version of the layout for you. Thanks.

[Willabe]

Is this likely to create a ground loop problem?

Link for grounding;

http://www.valvewizard.co.uk/Grounding.html

[sluckey]

I would just copy the ground scheme on the original. 

     http://sluckeyamps.com/magnatone/M10A_guts.jpg

[wsscott]

Wow, what a mess!

[Williamblake]

Running the wire for reverb- and tremolo footswitch inside the same shielding may induce tremolo into the reverb-recovery.

[sluckey]

Running the wire for reverb- and tremolo footswitch inside the same shielding may induce tremolo into the reverb-recovery.

Not a problem for the Fender footswitch cable.

[wsscott]

Do I need to use this type of cabling to connect the vibrato and reverb circuits to the stereo output jack, or just regular hookup wire?

[sluckey]

Do I need to use this type of cabling to connect the vibrato and reverb circuits to the stereo output jack, or just regular hookup wire?

No. This cable is only used for the footswitch. Use regular hookup wire inside the chassis for the Vibrato. If reverb wire is more than a couple inches long use regular shielded cable.

[wsscott]

Sluckey--I"m starting to install the hardware in the chassis.  Do the input or output jacks need to be insulated from the chassis? Is the shielded input jack wiring grounded at the main ground near the jack, but not grounded at the other end? Finally, for now, is the connection of the shielded input wiring that connects to a resistor and a cap, that then connects to the grid on V1, as shown in the attached layout image, attached to a lug that is insulated from the chassis? Thanks

[wsscott]

As to the two 100 ohm resistors needed for the filament leads, I only have 1/4 watt 100 ohm resistors (unless I order 1/2 watt), but I do have 270 ohm 1/2 watt resistors.  Will either of these work okay, or should I just go ahead and order the 1/2 watt 100 ohm resistors?  I don't have a store within 70 miles of where I'm located, so its a real pain to get anything unless ordered online.

[sluckey]

Do the input or output jacks need to be insulated from the chassis?  no

Is the shielded input jack wiring grounded at the main ground near the jack, but not grounded at the other end?  yes

Finally, for now, is the connection of the shielded input wiring that connects to a resistor and a cap, that then connects to the grid on V1, as shown in the attached layout image, attached to a lug that is insulated from the chassis?  The wire and resistor connect to an insulated standoff turret, but the cap connects to a ground lug that is secured to chassis with the same screw that secures the standoff turret. You may wish to use a more common terminal lug.

As to the two 100 ohm resistors needed for the filament leads, I only have 1/4 watt 100 ohm resistors  Each resistor only dissipates 0.1W so 1/4W resistors will be fine.

[wsscott]

Thanks.

[wsscott]

Should the RCA jacks going to the Reverb Tank be insulated from the chassis?  I have the Switchcraft 3501FR RCA jacks, like is shown in the image below.  I also got some fiber washers with shoulders that I could put between the rim on the jack and the ground lug that follows, and then the nut.  Is this overkill? 

[sluckey]

Should the RCA jacks going to the Reverb Tank be insulated from the chassis?

no

[wsscott]

Well I've pretty much completed the infrastructure and did the filament wiring, as you can see.  Pretty tedious, but very satisfied.  I'll work on some more of the off turret board wiring tomorrow. 

[Dave]

Not to cause you to do extra work or anything, but I see two things about your filament wiring that I would change.


1. Looking at the picture, it looks like your filament leads from the transformer come inside the chassis and then head straight north, go under some of the control pots and then go west toward the lamp. I would reroute those and have them go west straight out of the hole and keep them away from those control pots. Might be fine as-is, but filament wires are one of the places where noise can come from, so keeping them away from those pots would be an anti-noise precaution.


2. You can run the filaments as you did, twisting them and running them along the chassis (many commercial amps were done this way), but I think you'll find that the common (again noise reducing) scheme is to raise them above the tube sockets. Look up ab763 gutshots on yahoo or something and look at the pictures. You'll see what I am talking about. I went ahead and attached a picture. I would do it neater and tighter than the picture, but you get the idea.


Dave

[wsscott]

Thanks Dave for your insight and suggestions--The filament leads from the PT go under the Vibrato Speed and Intensity Pots. I was concerned that having them go under the turret board would expose components to possible noise, but as I look at the layout, the wiring as you suggest would be going under the reservoir and filter caps.  So I'm guessing its not an issue. Unless someone has another perspective on this, I'll rewire as you suggest.  As to the filament wiring on the tube sockets, I'm going to let that stay as is.  It was pretty tedious, plus I'm out of filament wire!  I appreciate everyone's input to my project.  Stephen

[Dave]

You are very welcome. I think you'll note that there are a lot of folks rooting for you because not only do we want you to succeed in this build, but many of us are excited to see how it turns out because (at least in my case) a future project may be germinating in the recesses of my mind.


Dave

[wsscott]

Sluckey-Do I use shielded RG-174 cable to connect the #2 lug on the Main Volume Pot to Grid pin 7 on V1 tube socket, and leave the ground in the shield disconnected at both ends?  Also use the same type of cable for the input jack to Grid Pin 2 on V1?

[sluckey]

Connect the shield to the ground lug of the volume pot. Use the same type cable for the input jack.

[wsscott]

Sluckey-I"m starting to work on installing components to the board, and wanted your thoughts on where/how to ground the 4 filter caps as shown in my layout attached.  I assume they shouldn't be grounded to the ground bus that I'm connecting the pots and pre-amp, etc. to, but should use a different location.  Thoughts? Thanks.

[sluckey]

I don't think it really matters since this whole unit is a preamp.

[wsscott]

Thanks.

[wsscott]

Well I started loading the board today, and made pretty good progress.  Its pretty tedious, but that's part of what its all about.  I got through the pre-amp and the first pair of varistors installed, and called it a day.  I'm pleased with it.  Not as pretty as Sluckey's or Shooter's, but just hope it works. Now its time for a bourbon!

[Dave]

You should see some of my builds. Sluckey would probably slap me if he saw how I go about building an amp. My method is as follows, come up with an idea. Very little prior planning except an idea of how big the board needs to be based on what a old Fender might look like with similar features. A power transformer that will provide what I need for the basic plan. Start building it so that it looks similar to what a typical fender layout might look like. Try to leave a little room here and there on the circuit board in case I need to make revisions or I want to add something later on. Build it. Go back and redo the stuff that wouldn't fit.


As long as my amps look better than a Silver Face Fender I'm pretty happy. I usually shoot for Black Face, but my amps tend to be a little more busy, so I settle for a pretty Silver Face. Hey, but all my amps have worked well. Except the one I'm currently working on. More to come on that.


Dave

[shooter]

Now its time for a bourbon!

I'll smoke to that!
want tedious AND pretty;
one the "frayed" end, put an inch or so of shrink tube, after you pushed back to expose wire, then heat to hold.  (practice while sippin :)

[shooter]

Beer nuts for thought;
Don't rush this part
your solder looks solid on the cap, my recommendation, top of yellow cap, wrap the lead around the jumper, I like 3 wraps, but have been known to just do 1 

the bottom, good flow, kinda cool imagery, again I would leave the R on left "up some" then wrap the cap.

the reason, solder is a poor conductor so we want a good mechanical bond.

put on the tunes, sip while "thinking", once you "unlock" the turret n friends, work it like origami, or bonze, save bull dozer driver for yard flatting   
edit attachment 

[wsscott]

Thanks everyone--your comments are really helpful, and I'll try to incorporate them when I pick up tomorrow.  I may go back and make some adjustments.  Here's the question:  Is there some way that I can test the circuit at its various stages to make sure things are doing what they should be?  I know I can check for grounding, but I'd hate to get to the end of this and find a problem--if there was some way I could have done testing at each phase of the circuit.  Thanks again for your interest and support.  Stephen

[Dave]

I think it would be pretty hard to do that unless you have the entirety of the amp connected and running. If that is the case, you can search Doug's library for the "listening amp" which is a little amp that will let you tap into your amp at different places to see if it is function up to that point.
Before you have voltages applied, I don't know how you could do it if at all.


Dave