Magnatone Twilighter 2020--MOD BUILD-Only Tremolo

[wsscott]

One other grounding clarification, in your Layout where you show the Preamp Ground and the Power Ground connections, there's shown a Chassis Gnd coming off each of the connecting points.  Is that just to indicate that the Ground Ring, or whatever type of connector is used, is grounding to the chassis directly at that point with the screw holding the Ground Ring down?  I"m sure this is obvious but just want to be sure its not intended to connect somewhere else.  I think your layout shows only 2 wired ground points in the circuit (one for Power and the other for Preamp so probably no ground loop issues), other than the Power supply ground at the receptacle. Thanks.

[sluckey]

yes

[Glenn]

Steve, you have such patience. People here have a real fountain of information.

[wsscott]

I know they do.  I've gotten a lot of help on both of my projects from a lot of folks out there.  Good to know where to go for the answers when you're new at this.  I wish I knew more (maybe "something" is a better word) about the theory behind the design, etc., like how to know what size cap or resistor to put in a circuit and why.  Thanks.

[Ed_Chambley]

I know they do.  I've gotten a lot of help on both of my projects from a lot of folks out there.  Good to know where to go for the answers when you're new at this.  I wish I knew more (maybe "something" is a better word) about the theory behind the design, etc., like how to know what size cap or resistor to put in a circuit and why.  Thanks.

Learn about frequency and how to use a resistor/cap to make a low pass or high pass filters.


Read tube charts and if you do not understand what it all means, you are not alone!


Nice contributions from Sluckey, but should have the More Tremolo and more cowbell.  Half of a tube not used is the sin of the Bassman, but makes a pretty nice gain stage.


I had fun, I hope y'all did too!

[wsscott]

Sluckey-in the schematic/layout for the Line Out, they show a 1 uf 63 V Poly cap.  I have a 1 uf 650V Poly cap, but its pretty big.  I also have a 1uf 50V Electrolytic cap.  Can either of these be used?

[sluckey]

DO NOT USE ELECTROLYTIC! Of course you can use that huge 650v cap but that seems a bit silly. Wouldn't you rather have a proper $0.25 cap that's the size of the fingernail on your pinky? I know I would.

     http://smallbear-electronics.mybigcommerce.com/topmay-tmc05-15-f-1-f/

Might even be able to find one at RadioShack.

[wsscott]

Thanks for the source.  I've had a hard time finding them elsewhere.  I was hoping to get it finished this weekend but I also don't have the .01uf 2KV cap for the standby switch.  So it looks like I'll have to wait until I can place an order for this stuff.

[sluckey]

You can find all these caps at Mouser. May take some extra time to figure out how to apply filters to their cap listings. That STBY switch will work just fine without that .01 cap. Do you really even need a STBY switch with a 6V6 amp? I suspect the line out will work without that 1µF too.

[wsscott]

Thanks.  As to the Standby, I wouldn't have put it in except it was part of the schematic.  So the hole is drilled, and I may as well use it.  It seems like 2kv is pretty big.  I can't imagine that much voltage going through this system.  I might have  a .01uf film cap that's rated at 630V just to deal with putting in something there, or might skip it.  They don't have it at Small Bear.  Options?

[PRR]

....Might even be able to find one at RadioShack. ...

I dunno your town; here the Shack closed a couple years ago.

https://el34world.com/Forum/index.php?action=dlattach;topic=27260.0;attach=90518

The 1uFd is not -too- essential? Just prevents ice-pick?

Anyway 470/47 is real generous. 4k7/470 would be fine even sending signal across the stadium. Then the cap could be 0.1uFd, which is a common value in tube amps. Or 10k/1k and 0.05uFd.

[wsscott]

If I put a .01uf 630VDC film cap on the Standby Switch, just to make me feel better until I get a proper size, will it get blown the first time I turn on the amp? Should I just leave it out for the time being?

[sluckey]

The cap across the STBY switch is there to deal with arcing/sparking of the switch contacts. An arc can contain voltages much higher than the voltages you can measure with your DMM. Those caps were never used in the old '50s/'60s vintage amps we love. As I said, you don't need it. You could just put a pin in it , build the amp, and order the cap whenever it's more convenient. Or never.   

You're building a clone of a modern recreation of an old amp. They are using modern components. Gonna have to turn to modern sources for those items. Hoffman ain't gonna have them.

But an old radar tech just might have them.     Want these? It'll cost you a beer. Nickle not included. Send me a pm with your mailing address if interested in this new old stock.

[sluckey]

If I put a .01uf 630VDC film cap on the Standby Switch, just to make me feel better until I get a proper size, will it get blown the first time I turn on the amp? Should I just leave it out for the time being?

Maybe not the first time, or even the second, but some day... A 630V cap ain't gonna make you feel better. I know you. It's gonna worry you to death!   

[wsscott]

You’ve got me figured out alright!

[wsscott]

While I'm waiting for the cap, I guess I could avoid the arcing by leaving the Standby turned OFF, ie. in the Play position. 

[sluckey]

Not interested in my offer? OK, back in the parts bins.

[wsscott]

Sorry Sluckey, I thought you were just kidding.  Sure, I'd appreciate them if the offer is still outstanding.  You were up mighty late last night.  I"ll pm you my address and let me know what I owe you.  Thanks.

[wsscott]

Sluckey, I finished the wiring today--WHEW--, and started going over it with my DMM.  I have a question about the readings I'm getting on the grounding on the Footswitch, Int. Speaker and Ext. Speaker jacks.  When I connect the meter's black wire to the chassis, and probe the jacks with the Red wire, I get a continuity reading when its touching the GND pole on the jack as well as when I touch the Tip pole of the jack, for both the internal and external jacks.  Is this because the OT Secondary Black wire is wired to the GND pole of the Internal Speaker Jack and so it shows up as being in continuity on both of the GND poles and the Tip poles.  I am using a Switched Jack as you show in your Layout drawing, and nothing is connected to any of the Switch Poles of the jacks.  OR, have I wired something incorrectly?  Thanks.

[sluckey]

Hmm, I wonder what it could be?     I bet all your other amps read just like this one.  What could cause that?   

[wsscott]

Well, I first thought it was because it’s a switched type jack. So I put a plug in the jack to release the tip from the ground switch, but it still read as grounded. Does it relate to the NFB looping back in the circuit to ground?

[shooter]

I get a continuity reading

can your meter read milli-ohms?
think about what's "in-circuit" that produces "continuity"

[sluckey]

Come on buddy. You can do this.

[tubeswell]

Line up a whole bunch of output transformers. Measure the secondary winding DC resistances.


Or if you haven't got a bunch of output transformers to measure, try googling: 'DC resistance across output transformer secondary winding' or something like that


Or better still, try this: https://letmegooglethat.com/?q=DC+resistance+across+output+transformer+secondary+winding&l=1

[wsscott]

So what I’m actually seeing when I have the meter in continuity beeping mode, is not resistance, but impedance?

[tubeswell]

So what I’m actually seeing when I have the meter in continuity beeping mode, is not resistance, but impedance?

Not impedance. Impedance is ‘AC resistance’ and you need to measure the pr:sec VAC ratio to calculate impedance.


When you meter a winding for continuity, it’s using the closed circuit resistance between one end of the winding and the other, i.e. DC resistance.

[sluckey]

...I have the meter in continuity beeping mode...

Continuity testing is rarely useful when working with any electronics equipment. There is a resistance threshold that triggers the beep. In some cheap meters, that threshold may even be as high as 1KΩ! This means that everything above 1K is considered "no continuity" and anything below 1K is considered "continuity". Better meters have a lower threshold but it will still be some none-zero number. Some meter manuals will state what the threshold resistance is. You can discover what your threshold is by connecting progressively larger resistors to your meter. As long as you are below the threshold the meter will beep, but as soon as you connect a resistor that's greater than the threshold, the meter will not beep.

I'm sot saying the beep is useless. It can be very useful, just not inside an amp chassis. Knowing actual resistance is much more valuable inside an amp.

[wsscott]

I guess I was in the neighborhood in my Reply #168 above when I posed whether the OT was the culprit.  So it is, but not completely for the reason I was suggesting.  Thanks as always for the learning experience.  As you have observed, I'm not satisfied with a "just do it" approach, but rather "why am I doing it". Thanks.  I realized that the reason I hadn't seen this in my last build of the Mag 10A-Revibe Mod of Sluckey is because it doesn't have an OT!  It just goes to the output jack which connects to the input of my Gretsch or Champ power amp that I have.  I'll be going over the circuit a couple times today to make sure everything is wired up correctly before plugging it in and starting it up.  I may way to do that tomorrow.

Any suggestions on startup testing, etc?

[Ed_Chambley]

Continuity simply means you have a closed circuit, therefore if you have no continuity you have no circuit.


MY METER READS CONTINUITY FANTASTIC, SLuckey.  Invest in a Fluke why don't ya.  Doesn't tell you much tho, but it does have a piezo speaker and it is annoying.  Use you resistance settings and if it changes in ANY way when you probe, you have continuity.


That capacitor is way too important to the tone of the amp.  Go and get one rolled on the inner thigh of a polynesian 18 year old chic.


Yes, Start it with no tubes on a Light Bulb Limiter if you do not have a variac (Autoformer) and if you do not have one, consider finding one used.  Take voltage readings and they will be high, no load from the tubes.  It will still let you know if it is wired correctly.  Post your voltages.  Let's see them.


Then maybe we willl give you a break to watch a movie!

[sluckey]

In addition to Ed's advice, while connected to a light bulb limiter and with no tubes plugged in, adjust the bias pot for maximum negative voltage on pin 5 of both 6V6 sockets. Then connect an appropriate speaker and put the tubes in. If the light bulb is not glowing very bright, then unplug the amp and put the light bulb limiter under the bench (because you no longer need it). Now plug the amp directly into the wall and turn it on. Be prepared for a loud squeal. If you get the squeal unplug the amp from the wall and swap the OT plate leads. If no squeal measure the voltages on every filter cap. Then play some guitar.

[wsscott]

Thanks everyone.  I've got a Variac and I also built a Light Bulb limiter.  So good to go there.  Also, I found I did have the .01 uf 3KV cap that I purchased for this build.  It wasn't labeled, but Sluckey's photo told me what I should be looking for and I checked out my invoices and found it.  So that's now installed on the Standby Switch.  Sluckey should I have a dummy load plugged into the Speaker Jack when I'm setting the initial bias you describe?

[sluckey]

Sluckey should I have a dummy load plugged into the Speaker Jack when I'm setting the initial bias you describe?

Well, if you follow my instructions you will not have any tubes plugged in. So what do you think?   

When you finally plug in the tubes you want a real speaker connected so you can listen for odd sounds like a squeal.

[wsscott]

DUhhhhhhh-Ok, that was a brain fart!

[shooter]

I like to use my meter to measure Resistance of the power supply WITH NO POWER applied, NO tubes reqired
Ground the Neg lead, put the lead on The 1st Tap, usually A and watch, the resistance will start going up real fast and hopefully settle somewhere in the > 100k, howfully Meg range




NOTE:  you will put a charge on the caps doing this, but not enough to bite...hard 


This essentially the same as the limiter without volts, anything < 100k might indicate a problem

[wsscott]

Shooter what are saying here: “ howfully Meg range” ?

[sluckey]

Shooter what are saying here: “ howfully Meg range” ?

That's weed speak for "hopefully Megohm range".   

[shooter]

 
that happens at 6:20pm local time, I'm old 

[wsscott]

Shooter, where are you? In the UK?

[shooter]

 
I'm a long-haired country boy in farm country MI, passed 30yrs, plus 0-18. Indiana, Virginia, and a year north of the bay bridge in San Francisco

[wsscott]

Shooter it peaked at about 40 Meg Ohms.

[wsscott]

Sluckey, the only amp I've had to deal with the bias was the Tweed Champ which is Cathode fixed bias as you know.  So no pot involved.  Could you walk me through how to bias this one?  I have access to a Eurotube single socket bias probe that measures DC Milliamps if that's helpful. 

[sluckey]

All you need is a meter and calculator. No fancy bias probes needed. I already told you how to initially set the bias pot. Now read this. Maybe even save it in your favorites...

     http://www.duncanamps.com/technical/lvbias.html

[wsscott]

Will do.  Thanks.

[wsscott]

Power problem. I’ve got 120 VAC to the on/off switch showing on my DMM at the 2 tabs of the SPST switch in the OFF position, and when I turn the switch to ON, the Power Lite comes on but there’s NO voltage at the switch tabs. If I disconnect both wires, and put the Ohm meter on the tabs, it shows OL when off, and 0.000 when On. What have I messed up? All the wiring looks correct.

[wsscott]

Sorry, ignore that post. I am getting AC power out of the switch and int the PT. I don’t have any tubes installed. I’m not getting any DCV readings at any of the nodes on the cap can regardless of whether the Standby is on or off.

[sluckey]

I am getting AC power out of the switch and int the PT. I don’t have any tubes installed. I’m not getting any DCV readings at any of the nodes on the cap can regardless of whether the Standby is on or off.

Hmm, I wonder why. What would it take to rectify that problem?

More important... set the bias pot for max negative voltage at pin 5 of the output tubes. Both of them.

[wsscott]

I don’t know what happened to an update I sent about 45 minutes ago, but I’ll recreate it.

I checked voltages at the rectifier socket, and I’m getting voltage at all pins except for the yellow wire at pin 8. The other yellow wire at pin 2 is showing 5 VAC, and each of the red wires show 350 VAC. So no power is getting to the OT node on the cap can. Is it a bad PT winding?

[shooter]

I checked voltages at the rectifier socket,

with OR without a tube?

[BrassElephant]

I am getting AC power out of the switch and int the PT. I don’t have any tubes installed. I’m not getting any DCV readings at any of the nodes on the cap can regardless of whether the Standby is on or off.

Hmm, I wonder why. What would it take to rectify that problem?

More important... set the bias pot for max negative voltage at pin 5 of the output tubes. Both of them.

Haha this is just the best thing I've ever read pertaining to tube amp troubleshooting

Gotdang, gotta pop open a cold one after that

[wsscott]

Shooter—No tube—

As to Sluckey”s “rectify” hint, I actually went there before seeing his hint-for what that’s worth! Still, my question is whether there’s a problem with the PT since I’m getting voltage on one yellow wire going from the PT to the rectifier socket, but not the other yellow wire.  And it’s the “other” wire that’s feeding the OT at the cap can.