Magnatone Twilighter 2020--MOD BUILD-Only Tremolo

[wsscott]

I'm hooked on Magnatone's sound, and after building the Sluckey M-10A without the power supply so its just Vibrato and Reverb, I thought I would like to try building just a Magnatone Amp with Tremolo.  But I wanted to base it on the new Magnatones that are being built.  I really like the sound of the Twilighter model with the 6V6's and the 12" speaker that I've only heard on the Magnatone USA website, and maybe some YouTube channels.  But since I have a Vibrato/Reverb effects unit, I wanted to simplify the circuit of the Twilighter, leave those parts out, but keep the Tremolo circuit in.  I also was thinking about changing the grounding system so that its what I think is called a "floating ground", which would hopefully eliminate any ground loop hum when another AC powered effects unit, like my Vibrato/Reverb, is plugged in and used.  Maybe that's not a good idea.  I requested Magnatone for a copy of their schematic for the Twilighter and they sent it to me with no restriction on sharing.  I told them what I was thinking of doing.  So at this stage, its still in the concept stage, but I really enjoyed my last scratch, modified, project so much and got such great help from the folks on this forum, I thought I would throw this out there, and see what your thoughts are.  Your input is greatly appreciated.  Thanks.

[sluckey]

Can we see the schematic?

[wsscott]

Sure, here you go.  Magnatone Tech Support said it was fine for me to share.

[sluckey]

The amp is a Fender AB763 Deluxe Reverb. Preamp and reverb are identical. PI and PA are 95% identical. The vibrato modulator is sandwiched between the preamp and PI. The oscillator signal is applied to the 6V6 grids when in Tremolo mode, very similar to the 6G12 trem that Hoffman uses.

[wsscott]

Sluckey-Any thoughts on my comments above on what I'd like to do with it.  In addition, I'd probably also want to add a Mid-Tone Control and a Master Volume and also maybe a line-input and also line-output.

[sluckey]

Build Hoffman's single channel AB763.

[wsscott]

I'll take a look at it.  Thanks.

[sluckey]

Print the two schematics and put them side by side. You'll see what I mean.

[wsscott]

I printed out both schematics and compared them as Sluckey suggested, and yes they probably are "95%" the same. I don't want the vibrato or the reverb from either build, but I do want the tremolo from the Magnatone.  As I said before, I have a vibrato/reverb Mag mod that I built, and that's what I would use.  So I would take those out of both circuits.  The Deluxe has the Mid which I would want to add to the Mag circuit.  The Mag circuit has the line-out which I want.  Neither have a Master Volume which I'd like to add.  That might be about it. Is there a DIYLC for the AB763?  I didn't see one on Hoffman's site, but I did see one on Rob Robinette's site.  So I could start from scratch doing a layout with DIYLC, or find a close match and then start modifying from there.  I know you probably think I'm spending a lot of unnecessary time not just going straight with the Hoffman/Sluckey design/layout, but I don't have anything else to do and I know I will learn something in the process!  Thoughts? 

[acheld]

You'll be better off in the long run by drawing up a schematic.  Either by hand or via whichever program you use. 

A schematic really helps the forum help you better. 

Once you have a stable schematic, the layout follows easily (on a fresh build), and you can have Doug drill your board on his CNC machine. 

[sluckey]

Here's your stripped down schematic. Have fun.

[wsscott]

Sluckey- Gee Thanks again--How did you do that? Is there a program you have that can do it quickly?  Wow!!

[sluckey]

Acrobat Pro and PaintShop Pro.

[wsscott]

Sluckey--I really appreciate it. You know you haven't heard the last of me!  HaHa!  Best.

[wsscott]

Now I'm trying to find the dimensions for the Twilighter chassis so I can get an idea on the size of the turret board(s) and ultimately cabinet.  I like the way the front of the amp is just grill cloth and no controls, and its easier to remove the chassis than from a Fender Blackface style.  I dealt with that Princeton Reverb and it was always a little "sticky" getting it out of the cabinet.

[acheld]

From:  https://www.magnatoneusa.com/twilighter

Twilighter (not the stereo version):  22.5" x 10" x 19.5"

[wsscott]

I think that’s the dimensions for the cabinet. I need the chassis dimensions for the circuit board and other guts.

[sluckey]

I could easily fit that MT Lite schematic into a 12 x 6.5 x 2.5 chassis, or even Hoffman's pre-punched Stout chassis. A 17 x 6.5 x 2.5 chassis would look spacious inside.

Look at my Dual Lite, or Plexi 6V6, or L'il Maggie, or Phoenix. All built in a 12 x 6.5 x 2.5 chassis. In fact, I would use the Phoenix layout because 90% of the layout would be identical to the MT Lite. Simply replace the Ampeg preamp with the MT Lite tremolo.

     http://sluckeyamps.com/dual_lite/dual_lite.htm

     http://sluckeyamps.com/6v6plexi/6v6plexi.htm

     http://sluckeyamps.com/lil_maggie/lil_maggie.htm

     http://sluckeyamps.com/phoenix/phoenix.htm

[wsscott]

Thanks, I'll do that.  While I've got you I have 2 questions about the Twilighter schematic symbols, if you could help.

First, in the tremolo section, near the Speed pot, to its right, is the label "LFO".  What is that for?  Is it a component or just identifying the part of the circuit?

Second, in various places in the schematic there are 2 *'s shown like this "**".  There are several shown in the tremolo and other parts of the schematic.  I can't find a reference, so I'm curious as to what they are indicating. 

Thanks.

[sluckey]

LFO = Low Frequency Oscillator. That's the name for the entire V5B oscillator circuit. LFO is probably stamped on the circuit board next to a test point (P$1).

The note (** DO NOT INSERT) can be seen on the left side of the schematic near the LFO circuit. It's not important so I'm not even gonna guess.

[wsscott]

Thanks.  I should have seen that ** note. 

If I wanted to reduce the power output for some reason, for ex. a friend likes the 14 Watts of his Princeton Reverb and wonders if 22 Watts is too much, could I just replace the 6V6's in this with a cooler tube, and then adjust the bias, or would there be more to it?

[terminalgs]

a friend likes the 14 Watts of his Princeton Reverb and wonders if 22 Watts is too much, could I just replace the 6V6's in this with a cooler tube, and then adjust the bias, or would there be more to it?

Then build a  Princeton Reverb.  or  a 6G2 Brown Princeton with the 125P1A power transformer.  lower B+, still two 6V6's.   heralded as perhaps the best princeton Fender made. Probably among the top five of all Fenders  (if there was/is such a ranking).

[shooter]

add a self biased/fixed bias setup, although the "perceived" difference probably isn't worth the effort.


I'd build it as is, then;
bias cool
add less efficient speaker
use pedals
turn MV down
walla, ~~~ 15W

[wsscott]

With a Master Volume added to the circuit, that might be a solution?

The Line-Out shown on the Twilghter Schematic and as on the modified MT-Lite that Sluckey did for me shows a cap, and I'm trying to confirm what it is.  Is it a 1 ohm 630V polypropelene capacitor, or is it something else?

As to the filter caps, its showing a 40 and then 20 20 20 8.  I was thinking of using a Cap Can for the 40 20 20 20 and then a separate 8 ohm.  How does that sound?

[wsscott]

Also, to clarify, in the Tremolo section is that cap identified as " C_220_10V_Radial" a 220 ohm 10 V radial electrolytic? 

Does it have to be radial?

[sluckey]

Is it a 1 ohm 630V polypropelene capacitor, or is it something else?

in the Tremolo section is that cap identified as " C_220_10V_Radial" a 220 ohm 10 V radial electrolytic?

Caps are not measured in ohms!

The line out cap is 1µF @ 63V and poly is stated rather than an electrolytic. The trem cap is a 220µF @ 10V electrolytic. Radial specified because it is small.

[wsscott]

I know, it must be slipping!  Thanks for the clarification.

[wsscott]

Sluckey, for this build, as to the filter caps, its showing a 40 and then 20 20 20 8.  I was thinking of using a Cap Can for the 40 20 20 20 and then a separate 8 ohm.  How does that sound?

[shooter]

separate 8 ohm

8uF not ohm
should be fine

[wsscott]

Right.  Thanks.

[sluckey]

Should be easier to look at...

EDIT... Preliminary drawing deleted. Final drawing posted in reply #50.

[wsscott]

Boy, that's great.  Much better.  Thanks again.

[wsscott]

Sluckey, to add a Master Volume pot, would I just wire it between the coupling cap going to the PI Grid pin2 and GND, or is there another/better way to add it?

[sluckey]

Just replace that 220K/270K voltage divider with a 500K-A pot.

[wsscott]

Thanks.  I also didn't know if it should come before or after the PI. 

[sluckey]

Magnatone didn't think the MV was worthwhile. Neither do I.

[wsscott]

One of the reasons I was thinking the Master Volume might be useful is that a friend who has a small recording studio might be using the amp some, and you could get the break-up or distortion, etc. but not at high volumes. He currently uses a Princeton Reverb which has been working fine for his needs. Otherwise, I wouldn't be adding it.  Would the Master Volume be effective for that purpose?

[shooter]

since it's a studio, he can use 12 amps, mike them, tweak them, blend/mix them. 
build your amp as designed.  If a MV becomes a "must", you can shoe-horn it inside an hour after the fact 

[wsscott]

Makes sense.  I just need to allow for a location for the pot.

[wsscott]

Sluckey-I'm going to start working on the DIYLC layout tomorrow. A couple of questions.

Is there one DIYLC already at Hoffman, or elsewhere that you know of, that I could use to get a quick start for this layout?

And, the schematics are designed from left to right, but the layout is designed from right to left.  Am I missing something.  That's the way I did the layout for the Mag10A/VibroVerb mod, and sometimes it was a bit confusing, but I wanted to make sure I wasn't missing a better way to do the layout.

Thanks.

[sluckey]

I know nothing about DIYLC.

My Visio layouts are from right to left, just like my amps look when flipped over. I'm pretty sure Hoffman did his Visio layouts from left to right.

Get your mind straight on this. Otherwise you risk designing a mirror image board. Usually shows up after you drill the chassis.   

[wsscott]

Yep, I know what you mean.

[wsscott]

Sluckey, I'm starting to work on the Layout today, I wanted to know where in the layout you think I should put the components for the Tremolo Oscillator.  I see where it ties into the circuit at the "MOD" point.  But I'm trying to figure out where so the circuit is easiest to follow.  Thanks.

[sluckey]

Here's a preliminary drawing. Still need to proofread a couple times...

EDIT... Preliminary drawing deleted. Final drawing posted in reply #50.

[wsscott]

Wow Sluckey-I really didn't expect you to do all this work on this for me!  Its really great. I certainly appreciate your time.  Thanks.  I'm sure there aren't any mistakes, but I've learned from you and others to review numerous times before thinking its done. 

[sluckey]

Haha. I've already fixed two component values. A lot of that layout came from my AB763 Lite. Mainly had to just shoehorn the trem circuit between the PI and PA. Very easy to do a drill guide template once I'm confident there are no more errors.

Are you really interested to build this project, either with my layout or your own? You know, everything that was Magnatone has been stripped away and all that's left is a single channel AB763 with bias vary tremolo. I was really surprised to see that Magnatone simply copied a Deluxe Reverb and added a single stage vibrato circuit. The original Magnatone amps had very unique circuits that were well thought out.
 

[wsscott]

Well I've only heard the sound on Magnatone's site and YouTube, but it sounds really nice to me.  So that's why I'm interested in it.  Have you compared this new circuit with their old Twilighter circuit?  I was just curious how much of a difference there is.  If there's a significant difference that you think might make this one better, I'm certainly open to other changes.  Anyway, I'm still good to go with this one unless you have some suggestions for changes to it, and I'll probably use your layout.  I'm sure it would be much better than mine.  But I may continue with mine just to do it.  Thanks again.

[sluckey]

I did put a couple of small changes in it. You mentioned a MID control so I added that, but used a 250K pot which operates as a mid control from 0 to 5 but acts as a raw control from 6 to 10. And I used a Volume pot with a push/pull switch to add a bright switch. And I did not include a STBY switch. None of this affects the board layout.

I'll proof the drawings again tomorrow morning over coffee and then post the final sometime tomorrow. It will include an updated schematic, layout, and drill guide.

You may want to visit this website. The creator is a current member here...

     http://www.magnatoneamps.com/

[wsscott]

Thanks!  Great, I really appreciate it. 

[wsscott]

Oh yes, I've visited that site many times over the last year or more!  Good stuff.