Magnatone Vibrato/Reverb build Project

[sluckey]

They're called dual concentric pots. You also need dual concentric knobs. Maybe you just need to build the simpler M4?

[wsscott]

I'm trying to adjust my layout to make the components flow a little better with fewer jumpers, and I have a question about a connection in the attached schematic.  If you look at the attached part of the circuit where the Vibrato section is starting on the V5 tube, Pins 3 and 8 from the cathodes show their 47K resistors R-24 and R-30 connecting on their GND sides which leads to GND from R-24.  Is there any reason I couldn't run them separately to GND, maybe even different GND points, or do they need to connect and then go to GND?

[wsscott]

Now this is more of an issue of layout image.  Attached is a pdf of my layout without the hookup wires showing. I took them off just so I could better see how the components lineup.  I'm 95% sure that the components on the circuit are properly connected, and of course before I start assembling anything I'll recheck all the connections 2-3 more times.  My question is whether you see anything about the layout itself that I should strive to change.  The images that Sluckey and Shooter have posted of their projects show all the components on the board running parallel to each other in nice, neat rows, and mine seem to go all over the place.  I guess it doesn't matter if its not an issue electronically, but would appreciate your comment.  Thanks.

[sluckey]

I'm trying to adjust my layout to make the components flow a little better with fewer jumpers, and I have a question about a connection in the attached schematic.  If you look at the attached part of the circuit where the Vibrato section is starting on the V5 tube, Pins 3 and 8 from the cathodes show their 47K resistors R-24 and R-30 connecting on their GND sides which leads to GND from R-24.  Is there any reason I couldn't run them separately to GND, maybe even different GND points, or do they need to connect and then go to GND?

All that's important is that each resistor connects to ground. I don't see any attachment?

[wsscott]

Let me try the attachment again.

[sluckey]

I predict a lot of trouble with that layout. Nothing is drawn to scale. Actual components will likely not fit. It looks like you just stuck a bunch of components on the board without any consideration for signal flow. Getting a good workable layout from a schematic is probably the hardest and most important phase of a complicated project such as this. Study some of Hoffman's layouts as well as some of mine. Our styles are slightly different but both follow good proven guidelines.

I suggest something similar to the attached layout of the preamp only. (Ran out of steam before getting to the effects). This layout is drawn full scale. Components will fit. For example, here's a pic that shows a board layout and an actual board. Pretty close match...

     http://sluckeyamps.com/supro/board.jpg

And here's the partial layout for your project that I started. Might help you get your layout more logical and practical.

[wsscott]

Thanks Sluckey-I really appreciate all your help.  Yes, I started with the schematic, and the board dimensions that I want to use if possible, then started with the DIY program, and just added the components as I read through the schematic starting with the input jack.  So this will give me a fresh lead on getting going.  I appreciate your time.  Stephen

[wsscott]

Sluckey-I was looking at the 2 layouts you made for me, and was thinking of deleting the tone boost part of the circuit, since this is not really going to serve as an amp for me.  In looking at the layout without the tone boost circuit in it, am I correct that I eliminate the V3 tube altogether and go from Pin 6 on V1 to Pin 2 (grid) on V5, and not Pin 1 (plate) on V5 as you have shown on the layout?

[sluckey]

In looking at the layout without the tone boost circuit in it, am I correct that I eliminate the V3 tube altogether and go from Pin 6 on V1 to Pin 2 (grid) on V5, and not Pin 1 (plate) on V5 as you have shown on the layout?

You are correct. I made an error on the drawing. I'll see about correcting that.

[wsscott]

Well I've been working on a revised layout with the helpful start from Sluckey, and I've attached it for comments. It covers the preamp and vibrato stages.  The vibrato circuit is the toughest part of the layout for me so far, and hopefully I've got it all wired as the original schematic.  I decided to leave out the Tone Boost part of the circuit since this is going to be an effects box and not an amp. 

[sluckey]

VR1, VR3, and VR4 are shorted, ie, both ends are connected together. I didn't look at the rest of the layout after seeing that.

[wsscott]

Thanks for the catch on the Varistor shorts Sluckey.  I'll check it out.  I understand what Greenwichpaul is saying.  Being grumpy is not necessarily a bad thing.  I'm an old, grumpy retired lawyer, and having done commercial litigation for a long time, I'm used to being surprised by "clients" and spending a long time trying to solve problems.  That's what makes this project/client interesting for me--even if it may have a lot of land mines waiting for me. Sometimes you win, sometimes you lose, and a lot of the times you compromise. Thanks to everyone for their input.  Stephen

[shooter]

grumpy old men try to scare new builders off

I've been teched in pubic, private, corporate, and Military schools, the grumpy cranky teachers that won't hold your hand, make you use your own brain, tell you to read the F*ckin book,  are hands down the best of best!

[wsscott]

I've looked at my layout again, and don't see the short connection where V1, V3 and V4 are tied together.  I've moved the turrets a little bit in the attached image to better show the connections, but this is how I put them together. Maybe this updated pdf is better. My read of the schematic is that the front ends of V1 and V2 connect to each other, as well as the front ends of V3 and V4.  The back ends of V1 and V2 connect together through two .047uf caps, and the same for the back ends of V3 and V4. Then the back ends of V1 and V3 connect together through two 100K resistors, and the same thing for V2 and V4.  I apologize if I'm being dense on this.  Thanks. Stephen

[PRR]

> don't see the short connection where V1, V3 and V4 are tied together

"V" = "VR"??

I see VR1 and VR4 shorted. What am I mis-reading?

[wsscott]

PRR-Thanks, I've probably been looking at this toooooo long.  Yes "V" = "VR".  I understand what I did, and that it created the short, but don't know "why" I made the mistake.  I'm going through the circuit again, and hope there's nothing else that I screwed up. If you see anything else in the updated file that I've attached, that you want to pass on for me, that would be great.  I appreciate your help.  Stephen

[sluckey]

(and sorry, it always irritates me when grumpy old men try to scare new builders off, I'd like to see this build work, but I'm a science teacher and I'm concerned you need a bit more scaffolding on this project!)

Do I irritate you? Seems I've done more to encourage this project than anyone participating in this thread.

[wsscott]

Sluckey, no not at all.  You don't irritate me at all.  I feel you're being helpful.  I'm someone with a hobbyist skill level, and it seems like everyone else is much more experienced, so I appreciate the input.  That's why I came to this forum with this project. I want this project to work when its finished, and if I'm missing something because I'm inexperienced I honestly want to know so I can learn from my mistakes.  Stephen

[wsscott]

I'm still working with your layout that you made for me, and I'm using the one without the boost circuit in it.  One thing I've done was to add a .022 coupling cap coming off of the 220K resistor coming off the plate of V1, and then connecting that cap from the plate on V1 to the grid pin 2 on V5.  I hope that was correct.

[sluckey]

Sluckey, no not at all.  You don't irritate me at all.

I've never felt that from you. My question was directed to greenwichpaul in response to his statement.

then connecting that cap from the plate on V1 to the grid pin 2 on V5.  I hope that was correct.

No. Remove the cap. Put the wire back. A direct connection is required.

[wsscott]

Thanks.  Can I ask why you don't need a coupling cap in this instance to connect the plate from V1 to the grid on V5?

[sluckey]

That's a good question.

V5 is a cathodyne phase splitter. By design, it's cathode will have a large dc voltage on it. This means the grid will also need a large dc voltage that will be just a bit less than the cathode. This is necessary for proper biasing of V5. The direct coupling from the plate of the previous stage is the easiest way to obtain the large dc voltage for the grid without having to use any components for bias. The Magnatone engineers decided to use the direct coupled cathodyne.

You can use an AC coupled cathodyne by using a coupling cap, but now you have to get the high dc voltage for the grid by some other means. You could use a fixed bias circuit which requires a 2 resistor voltage divider from a B+ source, or you could bootstrap to the cathode, which also requires 2 resistors.

When you run across stuff like this that doesn't make sense to you, please ask. Sometimes there is a logical answer. Other times it may just be an error in the schematic/layout (like my last drawing     )

[shooter]

+1

for a little deeper read (that I keep going back to over and over..:)
near the bottom you'll find the DC coupled version.
http://www.valvewizard.co.uk/cathodyne.html

[wsscott]

Wow Sluckey!  Obviously way over my head.  At this point in my education I had thought a coupling cap was needed in order to get the AC Signal passed on to the next tube/section of the amp, and if you didn't use a coupling cap the DC would pass through instead and would mess up the signal.  So that's why I assumed a cap was needed to get the signal from V1 to V5.  I'll read the article that shooter attached.  Thanks.

[wsscott]

Sluckey, I've finished the section of the vibrato where the signal leaves the varistors and is headed to the tremolo section for V6, but I need to do the V4 wiring to complete the vibrato.  As to layout, should I just continue with the vibrato circuit in the layout until it is completed, or wire the reverb layout next and put the V4 section of the vibrato circuit at the end of the layout.  I'm assuming that it would be better to finish up the vibrato circuit, and then move on to the balance even though the schematic shows the V4 section pushed off to the side.

[sluckey]

I would put V6 and V7 next to V5. And put V4 at the end of the line.

[Greenwichpaul]

(and sorry, it always irritates me when grumpy old men try to scare new builders off, I'd like to see this build work, but I'm a science teacher and I'm concerned you need a bit more scaffolding on this project!)

Do I irritate you? Seems I've done more to encourage this project than anyone participating in this thread.

Not at all. You're amazing, I'm constantly impressed by how helpful you are. Sorry if I sounded patronising to the OP, I was trying to be helpful as I have suffered from being over-ambitious.

(and btw grumpy old men is referring to me... and also a german amp forum where you can get told off for asking naive questions. )

[sluckey]

(and sorry, it always irritates me when grumpy old men try to scare new builders off, I'd like to see this build work, but I'm a science teacher and I'm concerned you need a bit more scaffolding on this project!)

Do I irritate you? Seems I've done more to encourage this project than anyone participating in this thread.

Not at all. You're amazing, I'm constantly impressed by how helpful you are. Sorry if I sounded patronising to the OP, I was trying to be helpful as I have suffered from being over-ambitious.

(and btw grumpy old men is referring to me... and also a german amp forum where you can get told off for asking naive questions. )

So how the hell is that comment even related to this thread? If you aint got anything nice (or helpful) to say why say anything at all?

[PRR]

> if you didn't use a coupling cap the DC would pass through instead and would mess up the signal.

Well, usually.

"MOST" tube stages set the grid near zero V with a large resistor, stand the cathode up a few volts with a resistor, and a plate resistor set the plate at 40% to 70% of B+.

BUT there are cathode followers. The idealistic CF has plate at full B+, grid at half of B+, and cathode "following" grid a bit above half B+. For 300V B+ we want a source of 150V bias.

And then there is the Cathodyne which has equal resistors in both plate and cathode. The idealistic grid bias here looks like 1/4 of B+, putting 1/4 of B+ on cathode and 1/4 of B+ across plate resistor (plate sits near 3/4 of B+). For 300V B+ we want a source of 75V bias.

If the stage before this stage has its output at appropriate voltage, or we can cleverly contrive a good voltage without much performance sacrifice, why not omit a cap and direct connect it?

There's reasons not to. But usually, if you are not sure (or off the edge of your scaffolding), go with what is on the plan.

[wsscott]

I've done some more work on the layout and the updated pdf is attached, and in this work I have added 1/2 of the Vibrato circuit, the Reverb to Output circuit, and the Power Supply part of the circuit. Once this layout is resolved, then I just have the last 1/2 of the Vibrato to layout. There are 2 areas that I would like to ask for special input on.  First, and perhaps easiest, I used the ground on the RCA Reverb Input jack as a point to connect several grounds to, primarily for convenience based upon the way things are laid out currently.  Any issues/concerns with that?  Second, and more of concern for me, is the power wiring for D and E on the board.  Have I got that correct?  It was hard for me to think "in reverse", ie. going from the end of the power supply to the beginning.  The resistors and caps in that part of the circuit were making me dizzy!  So once again, thanks for your help.  Stephen

[shooter]

fwiw;
take a look at my board example Pg 2 reply #68.  putting the filter caps closer to where they do work is a better approach, usually   

[sluckey]

Second, and more of concern for me, is the power wiring for D and E on the board.  Have I got that correct?

No you don't. You have D and E shorted together! Remove that jumper. And the long wire that you have connected to E does not go to V7-1. It only goes to that 270K resistor on the left end of the board. Then the other side of that 270K goes to V7-1.

I had to stop looking at that point.   

[wsscott]

Thanks Sluckey- I was trying to figure out how to get B+ from D to E, and didn't realize that the B+ feed for E could come from the 10K resistor off of D.  I re-checked the B+ feed for C on the V7, and it looks correct to me-for what that's worth (Ha Ha).  Whenever you get a chance, let me know if you see any other "issues"!  Thanks.  Stephen

[wsscott]

Shooter, thanks for reference to the Silvertone layout.  I put the caps where I did because I thought I needed to isolate all of them from the preamp part of the circuit.  Does it matter in a build like this one where this is an effects box and not an amplifier, or is my assumption incorrect? Thanks.  Stephen

[shooter]

Does it matter in a build like this

I'm ripping off my garage roof so i'll hunt down your schematics later;
typically I put the caps where the "taps" are, so if V1 gets "tap D", I try and put cap close to V1.  (That "assumes" you still don't have to fly a 12' ground wire)

[sluckey]

I re-checked the B+ feed for C on the V7, and it looks correct to me-for what that's worth (Ha Ha).

I never said anything about that. I said, "... the long wire that you have connected to E does not go to V7-1."

[terminalgs]

If you are looking to simplify your build, get rid of the 1st gain stage and tone stack.  you said you want this to be effectively a vibe stomp box (maybe to sit on top of your amp)., three control knobs needed: speed, intensity, level(volume). 


you do need some gain before the effect so there is one gain stage (you don't need two),  and you can reduce it with the level pot right before the output jack.  See attached.

[wsscott]

Sluckey, I was just commenting about the other B+ connection for C on V7.  I understood what you were saying about the short and the mis-wiring to Pin 1.  Thanks. Stephen

[wsscott]

Terminalgs, thanks for the info and the layout.  I'll probably continue as is since I think I've gotten through most of the layout.  But I'm going to keep this as an option if I get into trouble with the build.

[shooter]

if I get into trouble with the build.

   That's why we do this design stuff with computers n paper, to avoid trouble,  mayhem, and gremlins during the build 

[wsscott]

I was wondering if someone would be willing to educate me a little bit on some theory.  The schematic that Sluckey provided and from which I'm working, shows the Power Transformer connecting to a rectifier and then out to the capacitors.  I think the transformer that is being used has an initial output voltage of 250 VAC at the secondary windings.  Then it goes through the reciifier and at Cap A it's voltage is 410 VDC, and at B it's 338 VDC and gradually decreases as it goes through the caps.  I'd like to understand what is happening that boosts the voltage initially and then gradually decreases, and how do you calculate the voltages you want in a circuit like this.  I assume it has something to do with the value of the caps that are used and the resistors, but if anyone could give me a simple explanation, it would be appreciated.  Thanks. Stephen

[sluckey]

Read this...

     http://www.valvewizard.co.uk/

Scroll down to the power supply section where you will find several links to everything you need to know about power supplies. Easy reading.

PS... My PT puts out 290-0-290VAC. 290 x 1.414 = 410VDC.

[wsscott]

Thanks, I’ll check it out.

[wsscott]

I read the article on the power supplies, but I guess a better way to phrase my question is "how do you know what DC voltage you need for a particular circuit and for the different parts of the circuit?"  In other words how did the Magnatone designer determine that for this M10 circuit there needed to be 410 volts at A, and 338 at B cap, etc., and then secondly how do you know what sized resistor to put between the caps to reduce the voltage to what's needed at that point?

[shooter]

it's NOT an easy explanation;
what I've learned, start with the output, work to the input.
so;
I want a 20W amp
I want to use PP 6L6
I get tube datasheet, find ALL the cool data, look up OT, walla, I need 350vdc at plate
I go to PT, deside type of rectification, filtering, ohms law for series Resistors, boom tap A is 350vdc.
I need to drive the tubes...........
I need to provide a killer preamp sound to drive the driver.......

I give up, steal from everyone else that did the hard work, put it together and rock out 

[Willabe]

I give up, steal from everyone else that did the hard work, put it together and rock out 

   

[wsscott]

Sluckey-I'm finishing up the layout, and was working on the Power Supply section, and noticed that B+ A no longer has a connection in the revised circuit you sent me and that I'm working from.  Looks like it originally powered the OT.  So, is there any need to have the first A Cap 40uf 450 V in the circuit at all anymore?  Could I just take it out, then I'd be starting with the B+ B cap.  Thanks.  Stephen

[sluckey]

Node A is not used since there is no power amp, but it is still necessary to provide the smooth dc voltages needed by the other circuits. Don't take it out.

[shooter]

was typing anyway 

is there any need to have the first A Cap

fwiw;
a lot of times, especially in SE or small signal builds, I put a cap/resistor filter "up front"
typically that takes the AC "ripple" from 2vac to <1vac on the backside (Tap B)

[wsscott]

Thanks to both of you guys.  I just wanted to make sure that I hadn't missed something, or that it would be okay just leaving it in the circuit.  If it were taken out, then in order to get the voltages down to the level needed coming out of B+ B cap, wouldn't you need to increase the resistor between B and C?