Magnatone Twilighter 2020--MOD BUILD-Only Tremolo

[Willabe]

What part of the country are you in?

[wsscott]

Charlotteville VA

[wsscott]

Great news!!  Everything checked out OK.  No squeals, pops, smoke, etc.  Here are the VDC readings on the Cap Nodes:

A:  437
B:  383.1
C:  359.9
D:  431
OT:  441 (PLAY)/ 499 (STANDBY)

Looks like B & C are a little higher than on the Mag Twilighter Schematic, which I understand would be different than mine:

A:  422
B:  297
C:  269
D:  424

I'm taking a lunch break and then see what the guitar sounds like and will report back.

Any comments?

[sluckey]

What is the voltage on pin 8 of each 6V6? Looking for two numbers.

[PRR]

.... OT:  441 (PLAY)/ 499 (STANDBY)...

Is that first cap good for 500V??

In fact: in "standby" there should not be ANY voltage at the OT? Or have we moved on to another plan?

[sluckey]

He's talking about how I have the labeling on the cap can. My layout has the first node cap labeled as "OT". My original layout did not have a STBY switch so I connected the OT ( and choke, and the rectifier) directly to the first node cap. Later when he decided he needed a STBY switch, I installed it in the usual manner, but I did not change the "OT" label on the cap can. It's all my fault! The actual OT lead is no longer connected to the cap labeled "OT".

[wsscott]

Sluckey I didn’t have any 1 ohm resistors for the bias test points, so I wired pin 8 on each 6v6 to a insulated post that I can use when I get the resistors, and then ran that connection to Power Ground point. So in reading Pins 8, with Standby Off, black probe on chassis, and red probe on Pin 8, I read 0.4 MV DC on each one.

As to the cap can, it’s rated for 525 V.

[wsscott]

I’m also using right now a dummy load resistor instead of the speaker.

[sluckey]

You need a speaker to tell if it's squealing. What does the amp sound like with guitar?

[wsscott]

Ok. Guitar sounds great. No hum, hiss, noise or anything. Quiet as a church mouse. Mid seems to have greater effect than Bass. Only obvious issue is NO TREMOLO.

[Williamblake]

Sounds like progress. This is just to let you know you are not the only one who took Ohms for kOhms. Keep on keeping on.

[shooter]

using a foot switch?

[wsscott]

Yes using a foot switch. It came with a RCA jack and I am using an adapter to 1/4”.

[sluckey]

Don't need a footswitch. Tremolo is enabled by default.

[wsscott]

The Intensity seems to slightly increase the sound, so it’s probably working, but the SPEED doesn’t. I also tried a different foot switch but no different.

[wsscott]

The Layout shows  no wire connection on the Far Right tab on the Speed pot-looking from behind. Is that correct?

[sluckey]

The Layout shows  no wire connection on the Far Right tab on the Speed pot-looking from behind. Is that correct?

yes

Measure voltage on V3, pins 1, 2, and 3. When measuring pin 1 watch it for several seconds. If the tube is working, pin 1 voltage will fluctuate at the speed of the tremolo. This fluctuation will just appear as an erratic reading on a digital meter.

Post a hi-rez pic showing the area of the board that has the tremolo components and the tube socket and the INT and SPEED pots.

[wsscott]

Pins 1, 2 & 3 respectively. No fluctuating on any of them. Pictures attached. May have to send separately.

202.3 VDC
1.3 mv DC
1.83 VDC

[wsscott]

Another photo.

[wsscott]

I switched tubes, but no change.

[Willabe]

And you have 2 RCA jacks by the trem tube that are not wired up.

[sluckey]

See that 47K that's parallel to the yellow .01 cap? Remove that resistor. Got trem now?

[wsscott]

Yep! Yea!

[wsscott]

Sorry, so is it too big, or too small or not needed at all?

As to the unwired RCA jacks, that’s for future expansion for a reverb if I decide to add one.

[sluckey]

Sorry, so is it too big, or too small or not needed at all?

Not needed at all. I've seen a lot of tremolo circuits, including lots of Maggies, and not one ever had a resistor across that cap. I wonder if that resistor is really in the original?

LMK if you find you need a stronger tremolo. I have a simple mod to make it so.

[Willabe]

Not needed at all. I've seen a lot of tremolo circuits, including lots of Maggies, and not one ever had a resistor across that cap. I wonder if that resistor is really in the original?

I was wondering why that R was there too. I was going to ask you about. 

[wsscott]

Sluckey thanks for all your help. I’ll have to get used to the sound of the Trem as it currently exists, and may want to change it. I definitely like the Mag Vibrato better. The speed dial goes from Fast to Slow Trem on this amp. Can I just leave the center wire on the Speed pot where it is, and reverse the position of the other wire in order to have it go from Slow to Fast? It just seems more intuitive to me.

Also, how did my voltages look on the Nodes? Any issues on B and C that were high vs. Mag. schematic?

[dude]

This has to be the greatest post in a long, long time, l still can’t stop laughing. Fantastic, post.  

[sluckey]

The speed dial goes from Fast to Slow Trem on this amp. Can I just leave the center wire on the Speed pot where it is, and reverse the position of the other wire in order to have it go from Slow to Fast?

Yes, except the other wire is a resistor. Just move the resistor from the left side of the pot to the right side of the pot.

I'm happy with the voltages.

[wsscott]

Willabe attached is a copy of the schematic that Magnatone gave me. The 47k is definitely in the circuit, and the wiring looks a little different than Sluckey’s-but what do I know as you all can tell by my posts. It looks like the grounding may be a little different. I’m sure someone here can appreciate the subtleties.

[wsscott]

Sluckey, thanks again for your help and time. Much appreciated. Best.

[shooter]

when you swap the resistor, WRAP it or at least bend it 180.  a good solder joint relies on a good physical bond.

[sluckey]

The 47k is definitely in the circuit

Of course it's on the ***SCHEMATIC***. That's why it was included in my drawings. My question is the resistor actually inside a real amp.

[PRR]

I notice several other, more essential, parts have the "**" notation for "do not insert".

I think this part may have been drawn after happy hour.

[sluckey]

parts have the "**" notation for "do not insert"

I never understood the significance of that notation. Didn't think too hard on it.

[sluckey]

I'd like to see a pic showing the entire top side of the chassis and another pic showing the entire inside of the chassis.

[wsscott]

Sluckey, I was looking at Rob Robinette’s bias calculator for 6V6’s and it shows a bias setting of around 25 mA. I used a friend’s bias probe on 1 of the 6V6’s this morning just to see how it works, and it reads a bias of 37.61 mA for that tube. Is my bias set too high for the 6V6’s I’m using?

[sluckey]

Looks like it's too hot. Adjust the bias pot for 25mA using the bias probe. Then switch to the other tube and see what the current is. If it's higher than 25mA readjust the bias pot for 25mA on this tube.

This is only half of the process. Now you have to measure the voltage on pin 3 and multiply times the .025 current reading to get the static dissipation in watts. This is a good time to read that bias link you bookmarked.

[wsscott]

I bought a set of matched tubes, and used the bias probe on them, and got them arranged so that they were reading 25.14/25.6 mV! So I used Robinette’s calculator, and based on the plate to cathode voltage it has of 447 VDC, I could increase the bias. So settled on 29.65 mA. The pin 3 plate voltage read 445, so I get a little over 13 watts. So seems good to me.

[sluckey]

I bought a set of matched tubes, and used the bias probe on them, and got them arranged so that they were reading 25.14/25.6 mV! So I used Robinette’s calculator, and based on the plate to cathode voltage it has of 447 VDC, I could increase the bias. So settled on 29.65 mA. The pin 3 plate voltage read 445, so I get a little over 13 watts. So seems good to me.

What is the voltage on pin 5 of the 6V6s with this setup?

[wsscott]

V-4 is -39.79
V-5 is -39.7

[wsscott]

I've been enjoying playing the new amp!  Has a really clear tone to it.  Here are some first impressions after a couple days of playing it:

1.  The Treble tone control has great responsiveness as does the Mid tone control pot.

2.  It seems like the Bass tone control doesn't have much affect on the sound.  You have to turn it up really high to get any noticeable change in tone, and then there's very little range or bass added to the sound.  Sluckey added the Mid Pot for me, and in his schematic he shows a 10K Pot for the Mid, but makes a notation "Use 100K-A for Mid-Raw".  I don't know what this distinction means. His layout shows a 100K-A pot, so that's what I installed.  What's the tonal difference in using the 10K vs. the 100K?  Would that be the reason for the limited response on the Bass tone control?

3.  The Bright switch, which I added in the form of a toggle switch, really takes the upper frequencies to another level.  Sometimes more than I need, but its a good option to have.

4.  After using the Tremolo for a couple of days, I've gotten to really like it.  It has a sound different than any other tremolo I've ever heard.  I some ways its similar to the Mag Vibrato.  A friend has an Original 1965 Deluxe Reverb and a 1978 Princeton Reverb with no pull pot on the Volume knob, and he's into recording in his studio, and he says this Tremolo is nothing like any Fender he's ever had or heard and really has a neat distinctive sound. Yes its definitely a keeper and is great to have as an option to my Vibrato.

5.  The amp definitely has plenty of Volume, so no worries there.  I can keep Volume pot at around 8 o'clock and have plenty of sound at home.

6.  The 2 input jacks have a noticeable difference in hi frequencies.  So its another great option that is meaningful and may be useful to a player.

7.  I haven't been able to test out the Line Out yet since I need to order a cap and a resistor that I didn't have.  I also will install the Bias test point 1 ohm resistors after I order those parts.

8.  I guess a final comment is how quiet and noise free the amp is.  No hum, buzz, static, etc.  I think maybe that has to do with the grounding scheme Sluckey shows on his Layout.  There are really only 2 ground points, other than the power source line input to chassis.  He has a Power Ground and a Preamp Ground.  So seems to me that's going to keep it quiet.

I had a lot of stops and starts in doing this build.  I had to resolve a chassis build design that took time and created a delay.  Then because of the way I wanted the front of the chassis to look in the cabinet, I had a long delay in working out the specs on the cabinet dimensions and then the build.  I couldn't have the cabinet built until I got the chassis so I could send it to the cabinet guy.  Building the turret board was pretty much a breeze.  Sluckey included a turret layout which I modified slightly, in part to include empty space on the turret board to add future turrets for a Reverb addition.  Hoffman turned that around in a day, and I loaded that up in a couple days, but then it sat waiting on everything else.  I also had ankle surgery that left me out of commission for a couple weeks.  So with all the starts and stops, I felt a bit disorganized in comparison to the Mag10 Mod-Revibe build that I did last year, and I'd like to think that's part of the reason for those stupid mistakes I made with some of the wiring. But I'm very pleased with the final results.  Maybe a few tweeks left to try, with the assistance of you guys.

Thanks again for all your input.  Best.

[sluckey]

Good review. Glad you're enjoying the amp.

The mid/raw pot works like this... The first half of rotation (0-5) behaves exactly like a standard Fender 10K mid pot. The second half of rotation (5-10) actually begins to "lift" the tone stack out of the circuit. Not entirely like a switch, but the effect is very noticeable. You should hear the amp getting louder (because the tone stack is not attenuating the signal as much) and the sound should take on a raw sound (not as refined as the standard Fender tone stack). Useful if you like a little hairy growl to your sound.

To properly evaluate the operation of the Bass and Treble controls, you should keep the Mid control under half rotation (out of the raw control area). The Bass may have more effect with the mid set to about 2 or 3.

If you find you don't care for the raw sound, just replace the pot with a 10K-A pot. This will spread the mid control over the entire sweep of the pot, just like a Fender TR.

[sluckey]

Are you gonna show us some pics?

[wsscott]

Sure-I've got it installed in the cabinet, so you can't see the chassis, but I"ll send a cabinet view now, and will send the chassis when I pull it to install the Line Out components.  I still have to label the front panel and maybe get one made for it.

[wsscott]

Still waiting for some parts to finish the Line Out and add the Bias test points. Maybe they'll come this weekend.

Sluckey, if I decide at some point to add on-board reverb to this amp, can I take the section I've outlined in yellow on the attached schematic of the Mag Twilighter that you used for this Mod, and just insert it in the circuit where it comes off the V-1 Tube?  Are there any changes needed?  I know you changed a cap and a couple resistors when you did the Mod, but didn't know if some more changes to the circuit would be needed to add back in the Reverb section from the Mag Twi.

And this is just a general question about Schematics and Layouts.  Over the last year or so, as I've been working on these projects, and looking at schematics and layouts, it seems that the Schematics are always drawn from left to right, input to output.  But when the Layout is drawn, its right to left, input to output.  It seems like it would make more sense to draw the Layout in the same direction as the Schematic.  I'm sure the answer is obvious to someone experienced in this, but I'm not there.  Can you give me your thoughts?  Thanks.

[shooter]

But when the Layout is drawn,

when the board is installed, then the chassis gets flipped upright, which way is it "drawn" 

[sluckey]

if I decide at some point to add on-board reverb to this amp, can I take the section I've outlined in yellow

You also need the section outlined in red. See attached.

But when the Layout is drawn, its right to left, input to output.  It seems like it would make more sense to draw the Layout in the same direction as the Schematic.

It's a matter of personal preference. I draw my layouts the same as Fender did. Hoffman draws his layouts left to right, input to output. You can rotate my layouts 180° if you prefer, but the writing will be upside down.   

[kagliostro]

I draw my layouts the same as Fender did

I've find something interesting to be read

https://www.ampbooks.com/mobile/amp-technology/layout/

Franco

[wsscott]

Sluckey when I built your Mag 10A mod last year for my Revibe Mod, that circuit used a diode rectifier and this amp uses a tube rectifier.  Why the difference?