Magnatone Vibrato/Reverb build Project

[sluckey]

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 usually make some power on checks of the power supply before early on. But as for the rest of the amp, it's best to just get it wired up, then compare your schematic, layout, and actual wiring to be sure they are all in agreement. This is why it's so important to spend so much time on the layout drawing. Finally, just plug it in, check voltages, and hope it works. If not, it's time to improve your troubleshooting skills.

[wsscott]

Well as the old song of The Temptations goes "Its Growing"!  See attached photo.  Seems to be moving along smoothly.  I think the best advice I've gotten from you guys on this project, so far, was to take the time to create a Layout--don't wing it!  It did take a lot of time to do the layout, and hopefully I read the schematic correctly-with your assistance in some instances, but I find that not only does the layout speed up the installation of the components, but if I had not created it, the installation by just reading from the schematic would have been filled with problems and errors.  So I'll keep you posted. 

[wsscott]

Sluckey--Catastrophe (maybe) averted!!  I was installing the last resistor in the circuit, a 270K that connects to the plate on the V4 tube, and I couldn't find the resistor among the packets that I got from Mouser.  I checked the circuit and saw that there are 2 other instances of the 270K that connect to the 2 plates in the V7 tube, and also just a small 270 ohm (not a 270K) that connects to the V6 cathode pin.  I kept looking through the packets and saw a packet of 270 ohm resistors, and also a packet of 2.7K resistors, but not 270K?  So I thought, maybe I screwed up the order and the installation.  Sure enough, the 2 resistors on the V7 were in fact 270 ohm resistors and not 270K ohms. I didn't order 270k's, but ordered more 270's than I needed. Lucky for me SLuckey!  So I disconnected them, and ordered the correct resistors from Mouser!  Just out of morbid curiosity what would have happened if I had not caught my error?  Thanks.  Stephen

[Dave]

It wouldn't have fried anything. Just wouldn't have worked and you've been digging around trying to figure out why you had such high voltages on the plates of that tube and no signal.

Dave

[wsscott]

Dave-whew!  That's good to know. 

[wsscott]

Sluckey-while I'm waiting for the correct resistors to arrive, I thought I would wire the jacks for the Reverb input and output.  In looking at the schematic attached below, am I correct that both of the jacks are grounded?  The way I'm reading the schematic, the negative end of the 1uf cap coming off of Plates 1 and 6 that are jumpered, lead to the "tip" connection on the input jack going to the tank and the shield lead connects to ground.  And then the "tip" lead on the reverb return jack connects to the Grid 2 pin on V7, with its shield also connected to ground.  Am I correct?  If both jacks are grounded, then won't both of the jacks on the Reverb Tank also need to be grounded?  The folks at MOD said that the 4FB3A1B tank that I was planning on getting say that the tank has jumpers at the jacks that can be unsoldered and changed to both grounding--That's what I understand I would need to do for this tank, assuming I'm correct in the wiring.  Thoughts?

[sluckey]

Both reverb jacks on the original M10A are grounded to the chassis. I have no idea about the jacks on the original reverb tank. I suggest you plug into your tank and try it without messing with any ground jumpers. If you have a hum that cannot be resolved inside the chassis, then play with the jumpers.

[wsscott]

Thanks.

[sluckey]

Both reverb jacks on the original M10A are grounded to the chassis.

Let me back up and retract that statement. There are no reverb jacks on the chassis. The reverb cables are hard wired on the chassis with rca plugs on the end that connect to the reverb tank. My pics are not detailed enough to see if the shielding is connected to the chassis but I really suspect that's the case. Not gonna open the amp to verify. Sorry.

[wsscott]

Is there any reason not to install RCA jacks on the chassis, and wire from the circuit board to them? Then I would just plug the RCA cables into those jacks and the jacks on the reverb tank.

[wsscott]

I've got a fitment problem.  The 2K 10W wire coated cement resistor connecting between 40uf Cap 1 and 40uf Cap 2 is way too big physically to fit!  Remember I won't be using Cap 1 in the circuit because it would have been feeding a part of the circuit that's been eliminated.  So is there a substitute I can use to connect those 2 caps?

[sluckey]

The 2K 10W wire coated cement resistor connecting between 40uf Cap 1 and 40uf Cap 2 is way too big physically to fit!

That happens when you don't use properly scaled components on the layout. Looks like you used a 1/2w resistor on the layout.

Maybe the 10 watt resistor is way oversized. You could try a 2K/3W and when it's up and running measure the voltage across the resistor and calculate the actual power being dissipated. Double that number for a safety margin. If it comes in under 3 watts then you're fine. Otherwise you'll have to figure out a way to mount the 10 watt resistor.

[shooter]

fwiw;
sorry for poor detail;
this is how I "shoe-horned" in a 10w'r in a 5w space

[wsscott]

Fair point Sluckey.  Is the 10 W resistor going to be dissipating a lot of heat so that it shouldn't touch other components?  Shooter, thanks for the image.  I might be able to get that to work. I just am concerned about the heat issue--is it an issue?

[wsscott]

Does the fact this circuit has been redesigned to become, as Sluckey said, just a pre-amp and not an amp powering speakers, impact this?  Could I connect 2 resistors in parallel with each being 2K ohms, but being 2 or 3 watts resulting in 4 or 6 ohms by wiring in parallel, be an option?

[sluckey]

Fair point Sluckey.  Is the 10 W resistor going to be dissipating a lot of heat so that it shouldn't touch other components?

Yes. Heat is an issue, otherwise Magnatone would have used a smaller/cheaper resistor. All power resistors should be given enough air space to dissipate heat.

Shooter's method is a good one.

[sluckey]

Does the fact this circuit has been redesigned to become, as Sluckey said, just a pre-amp and not an amp powering speakers, impact this?

In this case, not enough to really matter.

Could I connect 2 resistors in parallel with each being 2K ohms, but being 2 or 3 watts resulting in 4 or 6 ohms by wiring in parallel, be an option?

You would need two 4K in parallel to equal 2K.

I would mount the resistor as shooter suggested.

[wsscott]

I think I solved it.  I moved the #2 40uf cap to where #3 30uf cap is, and put #3 in the #2 position.  This also gave me more space with the pilot light housing.  Then I mount the 2K/10W resistor at positions #1 and #3.  And by mounting it up rather than at the base of the caps, I get a lot more air circulation. Then I switched the wiring from the + sides of those 2 caps, and switched the position of the 2 resistors.  #4 Cap stayed where it always was.  I haven't soldered anything, but take a look at the photo.  Issues?

[sluckey]

[wsscott]

Sluckey, I'm finalizing how to ground the footswitch, and the input/output reverb jacks, and the cathode pin 8 on V4, and would appreciate your thoughts.  The input/output jacks are grounded to the chassis through their shields as a result of screwing them onto the chassis.  Those jacks also have their shields jumpered.  Should I run a jumper from pin 8 to the shield tab on one of those jacks and use it as the ground, or should I install a small terminal strip which is screwed to the chassis and run all of the wires to that?  If so, should I use the terminal strip's connection to the chassis as the ground connection, or run a wire from the strip to the ground bus where everything else attaches?  Thanks.

[sluckey]

I would ground V4-8 to the same place that V4-3 cathode resistor is grounded.

[wsscott]

Thanks.

[wsscott]

I'm at the stage where the board is finished and I'm about to install it in the chassis and wire up the tube sockets.  Does anyone have any thoughts on how to test the work I've done on the board so far?  It will be a real pain if I have to remove the board after wiring it up to the tube sockets (they're surface mounted because of the tube covers) in order to remove and work on the board.  Thanks.

[sluckey]

About all you can do is verify your schematic, layout, and actual board all agree. Correct components, correct jumpers. Verify that any underboard jumpers are securely soldered such there is no chance of one coming loose when you solder on top of the board.

[wsscott]

Thanks.  Well its good to know that I've done all of those things, so hopefully I haven't missed something and  things will go smoothly.

[Dave]

Don't worry do much. Chances are high that you screwed something up. It happens regularly. Half the fun of building stuff like this is figuring out what's wrong when it doesn't work. By half the fun, what I really mean is super frustrating and annoying learning moments. Where you make a discovery that translates to one mistake you will never make again. Once you have made 100 mistakes that you will never make again, you will be a guru.


Dave

[wsscott]

Most likely true!!

[wsscott]

Sluckey-I wired the input cable from the input jack as shown in the attached layout that you did for me, with the lead of the shielded cable connecting to the switch lug (not the tip lug), and with the shield connected to a separate ground at the chassis.  The switch lug and the ground lug are jumpered, and will go to the ground buss bar, but of course the ground lug is also grounded via the chassis because of the jack's attachment to the chassis.  Should the "lead" be connected to the switch as you show in your layout?  I thought maybe it should be connected to the tip at the input jack.  The reason is that when I ohm out the lead at the other end where it will connect to the V1 tube, it shows continuity--so its grounded.  That doesn't seem right.  It's connection to the 47K R is supposed to be isolated from ground.  Thoughts?

[sluckey]

Should the "lead" be connected to the switch as you show in your layout?

No. That's a layout error. Connect the lead to the tip as shown in the schematic you attached.

[wsscott]

Thanks.  I was thinking maybe the shield was touching its lead somewhere-like maybe the insulation on the lead had melted, etc. 

[wsscott]

Well the wiring is complete except for running the main ground buss and attaching the grounds from the pots!!  Looks really good.  Let's hope it works really well too.  I've got the tubes and reverb tank ordered, so it probably won't be in until later next week, so I've got time to go back over the circuit and check everything and the connections.  When someone gets a chance, I would appreciate your thoughts on starting it up and what readings to check, etc.  Thanks.  Stephen

[wsscott]

I've got all the wiring to the buss finished, and so I'm just waiting for the tubes and reverb tank.  I've checked the wiring and the circuit seems fine.  Here's what I did.  I set my DMM to the continuity setting where it beeps when there's continuity.  Then I went to each tube socket on the side where the tube would be inserted, and put 1 probe in one pin and the other probe in the adjoining pin, and went around the circle from one pin to the next.  No beeps.  Pins 4&5 did beep but they're jumpered, and they also beeped when each was connected with Pin 9. I also confirmed each connection to the buss beeped where it was supposed to, and checked the other components with 1 probe grounded to chassis, and none of them beeped unless they had a ground connection off the buss.  All of the grounds for the jacks beeped.  Any flaws with my methodology, or anything else I should check out?  Thanks for everyone's help on this project.

[shooter]

I set my DMM

besides what you've done;
I also like to set it to resistance NOT continuity, then put one lead in the tube socket
plate pin,(tube side), the other on it's corresponding "B+" tap, verify the R value.  repeat for cathode socket pin to ground. repeat

[wsscott]

Thanks.  I just tried that and they checked out fine too.

[wsscott]

Still waiting for the tubes and tank.  I'd really like to plug it in and test it. Should I ? What can I check out?

[wsscott]

Sluckey, Shooter, et al, my tubes and reverb tank came in yesterday, so I guess I'm ready to light this candle up tomorrow. Any tips, procedures, plate voltage readings or otherwise that I should follow? Should I turn it on initially without the tubes installed? Thanks to everyone  for your help on this project.

[shooter]

Should I turn it on initially without

fwiw;
I typically do, I measure all my volts at the tube side socket, that way I "catch" mis-wires, (B+ on filament kinda thing) the volts will be high-ish

[wsscott]

Well, I guess I should let you know how it went.  I got it setup with no tubes and turned it on and nothing blew up.  So I decided to go ahead and added the tubes and plugged in the Champ and my Strat and turned everything on.  The tubes lit up but not very brightly.  And I gave it a minute to warm up.  AND IT WORKS!!!!!!  The Vibrato and the Reverb sound great!!  I just can't believe it.  I just can't believe it!!!  I'll send you some meter readings and a video or sound clip later.  Just want to thank all of you again for your help and support!!  WOOOOOOOOOOOO

[sluckey]

Congrats! You certainly put in a lot of time and effort and planning into this project. And it shows. Gotta feel very rewarding. Enjoy

[Dave]

Hooray, congratulations!!!!!

Dave

[wsscott]

Thanks everyone.  I've been running it some today to see if things need to "settle in". I'm running this through my 5F1 Champ kit that I built a year or so ago. I'm going to share some meter readings in a moment, but here are my initial reactions:  There is no hum coming from the unit.  When the output volume is turned all the way up you can barely hear the vibrato oscillation in the background, assuming it is switched on from the foot switch--but very minor and only if you aren't playing.  The reverb has great range and plenty of volume/intensity.  The vibrato effect is really unique.  The Intensity of the vibrato could be stronger.  The Vibrato speed is very noticeable when adjusting it, and has a really good range of speed.  I've poked around the tube pins and hear crackling on 1 or 2 of the plates at tubes V5 and V4.  I think its probably a bad solder joint.  You only hear it when you poke around.

So I did some plate voltage measurements and they all seem low vs. what the schematic shows.  I connected the red lead of my DMM set to DCV to the plates and the black lead to chassis.

Readings taken at each plate are as follows:
V1-Pin1-E Node-212v schematic   Actual: 122.0
     Pin6-D Node-256v               128.7

V5-P1-C Node--316v                 188.3
     P6-C Node--316v                 195.6

V6-P1&P6 jumped-B Node-338v  174.6

V7-P1-E Node-212v                   100.5
     P6-C Node-316v                   143.4

V4-P1 B Node-338v                    190.9
     P6  B Node-338v                   Fluctuating-maybe lose solder

It seems like these actual readings are each about 1/2 of what they show on the schematic.  Is that correct?

The readings at each node of the caps with no tubes installed/tubes installed:
A--410 schematic         Actual 357.0/320.1--Note that A node is not being used because circuit eliminated
B--338                                  350.0/263.6
C--316                                  347.8/249.9
D--256                                  332.9/224.1
E--212                                  318.4/202.8

For the filaments, I've got pins 4&5 jumpered, and then pin 9.  Pins 4&5 are daisy chained and Pins 9 are daisy chained together.  Pins 4 read 3.3 VAC, Pins 5 read 0.1 VAC, and Pins 9 read 3.3 VAC for all of the tubes.  Is this ok?

So any comments you have about these readings would be appreciated.

[shooter]

Pins 4 read 3.3 VAC, Pins 5 read 0.1 VAC, and Pins 9 read 3.3 VAC for all of the tubes.  Is this ok?

not clear here;
4 & 5 are typically shorted together and get one filament wire, pin 9 get's the other

I typically measure cathode vdc, use the cathode R to calc I, ballpark ~~~ 1-5mA idle

[sluckey]

I fully expected to have to fine tune the B+ rail. Change that 2K/10W to 100Ω/1W. Remeasure and post voltages for B+ nodes A through E and let's see where you are.

[wsscott]

Sluckey, thanks.  Yes pins 4 & 5 were twisted to face each other and then 1 filament wire ran through them and on to the next socket, and the other filament wire came off of 9.

[sluckey]

If you don't have a 100Ω or other low value resistor, just replace the 2K/10W with a jumper wire and recheck voltages.

[shooter]

re-check fil also, they're probably good, but you're indicating 3.2v across a short

[wsscott]

As to the filament wiring, this transformer doesn't have a center tap for the filaments, so I put a 100 ohm resistor on each of the filament wires from the transformer at the pilot light, twisted the other ends of the 2 resistors together, and then grounded that connection on the ground buss.  Then ran the filament wires first to V4, since it was closest, and on down the line and ended up with V1.  Does this sound correct?

[sluckey]

The only way to accurately measure filament voltage is to put one probe on one pin of the socket and put the other probe on the other pin of the socket. Ex., for a 12AX7 tube wired for 6.3vac operation, put one probe on pins 4/5 and place the other probe on pin 9.

Did you increase the B+ yet? And?

[wsscott]

Thanks.  I'm just getting started in a few minutes.  I'll let you know what I read.  I realize now that when I read the filament voltages yesterday, I had the black probe grounded to chassis, and only read the filament with the red probe.  My mistake.  So hope that checks out ok.  If so, then I'll remove the 2W resistor and replace with a jumper between Caps A & B. 

[sluckey]

If so, then I'll remove the 2W resistor and replace with a jumper between Caps A & B.

You don't have to actually remove that 2K/10W resistor. Just temporarily solder a jumper across it's easily accessible leads. The real fix is to replace that resistor with a much smaller value, something around 100Ω and 2 watts. Having a resistor between caps A and B will provide better filtering than a wire jumper.