Has anyone built a Weber Maggie?

[Guitarzan]

Hiya!


This looks like a nice, easy little project. I was wondering if anyone has built one or if you know of anyone who has. I was thinking of doing one without the "voicing/boost" switch and maybe an octal preamp tube. For what it has in it, the kit looks really expensive. I have a lot of the stuff I'd need laying around & think I'd just buy the odds & ends I don't have.
Any thoughts?


https://taweber.powweb.com/store/maggie_schem.jpg

[simonallaway]

I'd suspect the boost/voice mod is the most interesting part. Otherwise it looks similar to a Champ (or an AX84 P1extreme, which I know well).

[Guitarzan]

Yeah, after I posted I sort of reconsidered that part. It's simple enough to do and I think you're probably right about it being the interesting part. I don't think I like the output jack setup tho and would probably just wire it for whatever speaker(s) I'm using. I'm thinking about a 6SL7 input as well. Can I just use the same values for the caps/resistors as the 12AX7 if I go with the 6SL7?

[Guitarzan]

I'd suspect the boost/voice mod is the most interesting part. Otherwise it looks similar to a Champ (or an AX84 P1extreme, which I know well).

I just checked out the ax84 online. There's some good readin' there for a noob like me.

[hesamadman]

I agree that the cathode bypass switch would be a very nice add on to the amp. Also the price to do so is so minute. I have built a couple amps similar to this one. I never buy anything in kit form. I piece it together and like you said, you have most of the stuff lying around anyway.

[hesamadman]

You could even throw a tone control right after C4. Give you a little versatility in the tone. If youre interested in doing so, I will add it to the schematic for you if you dont already know how to do so.

:edit: looked at wrong schematic sorry. Thought that was volume only and no tone knob

[Guitarzan]

You could even throw a tone control right after C4. Give you a little versatility in the tone. If youre interested in doing so, I will add it to the schematic for you if you dont already know how to do so.

I'm open to almost anything since this is still in the thinking stage, but what would be the advantage to having another one in that spot?

[HotBluePlates]

Otherwise it looks similar to a Champ ...

It's a tweed Fender Princeton (aka, Champ with a 1-knob tone control), but with a 6L6 output tube and adjusted negative feedback and power supply. Plus the voicing switch.

I'd keep the voicing switch. Tubenit has used similar on a lot of his amps, it works well and is useful. If you typically play only 1 electric guitar (or type of pickup) you might find you have a favorite setting and leave it there. But if you switch guitars/pickup-types, or if you want a brighter setting for recording the neck pickup of your guitar but a fuller setting for recording your bridge pickup, it's very handy.

Notice also the switch is center-off; the center position will have flat voicing as well as about half-gain. Flip one way and the highs get boosted to full-gain, flip the other and everything gets full gain.

You can tinker the cap values. Use something smaller than 25uF if it sounds muddy with your guitar, something bigger if the 0.47uF bright setting is too much. I suspect the 0.47uF cap will make it brighter-than-typical on purpose, so you can boost treble & upper mids but then use the tone control to take of the harsh top end to have a unique punchy & cutting sound. This is a guess based on the circuit, as I haven't heard this amp, though I used to own a '54 Princeton (with a 6SL7).

I'm thinking about a 6SL7 input as well. Can I just use the same values for the caps/resistors as the 12AX7 if I go with the 6SL7?

Yep, keep the same circuit values and just use the octal socket and appropriate wiring.

[HotBluePlates]

You could even throw a tone control right after C4.

I wouldn't suggest this, at least not without bread-boarding it first (so you don't have drilled holes and extra pots which may not get used).

First, the added tone controls will be inside a feedback loop, which can make their operation be very counter-intuitive.

Second, the 6L6 is harder to drive than the 6V6, which is at least part of the reason the feedback resistor was raised from 22kΩ (original Princeton) to 68k (Maggie). The feedback loop sets the gain from input to 2nd stage to speaker at ~46. It takes 13.3v peak across the terminals of an 8Ω to make 11w (which seems a reasonable target output power), so you need 0.29v peak at the input to the 2nd stage. 1st stage gain will be around 60 giving sensitivity of 5mV at max volume and guaranteed distortion.

But half-volume is also 1/10th voltage output to 2nd gain stage, so you'll need 50mV at the input jack for max output. You'll get some hair on the notes unless you're in the center-off flat position of the voicing switch. That leaves the cathode resistor unbypassed, and cuts 1st stage gain to ~30. Max-volume sensitivity drops to ~10mV (still distorts full-volume), while half-volume give full power with 100mV at the input jack (clean unless you have humbuckers or *very* hot single coils).

So the amp's gain is structured right for what it is now. If you add a tone control more-elaborate than the single-knob circuit, insertion loss will the amp seem hard to distort, which is probably counter to why you'd build a SE amp in the first place. More knobs, more adjustment seems like a good thing until you actually use it, when things tend to be set & forget. I think the Maggie has the right mix of stuff for what is intended to be a simpler small amp where you fiddle less and play more.

[Guitarzan]

You could even throw a tone control right after C4.

I wouldn't suggest this, at least not without bread-boarding it first (so you don't have drilled holes and extra pots which may not get used).

First, the added tone controls will be inside a feedback loop, which can make their operation be very counter-intuitive.

Second, the 6L6 is harder to drive than the 6V6, which is at least part of the reason the feedback resistor was raised from 22kΩ (original Princeton) to 68k (Maggie). The feedback loop sets the gain from input to 2nd stage to speaker at ~46. It takes 13.3v peak across the terminals of an 8Ω to make 11w (which seems a reasonable target output power), so you need 0.29v peak at the input to the 2nd stage. 1st stage gain will be around 60 giving sensitivity of 5mV at max volume and guaranteed distortion.

But half-volume is also 1/10th voltage output to 2nd gain stage, so you'll need 50mV at the input jack for max output. You'll get some hair on the notes unless you're in the center-off flat position of the voicing switch. That leaves the cathode resistor unbypassed, and cuts 1st stage gain to ~30. Max-volume sensitivity drops to ~10mV (still distorts full-volume), while half-volume give full power with 100mV at the input jack (clean unless you have humbuckers or *very* hot single coils).

So the amp's gain is structured right for what it is now. If you add a tone control more-elaborate than the single-knob circuit, insertion loss will the amp seem hard to distort, which is probably counter to why you'd build a SE amp in the first place. More knobs, more adjustment seems like a good thing until you actually use it, when things tend to be set & forget. I think the Maggie has the right mix of stuff for what is intended to be a simpler small amp where you fiddle less and play more.

I gotta tellya, you lost me with the technical explanation BUT I tend to agree from the gut. That was why I asked the poster what the advantage to such a control might be. I was thinking a bass/treble type of setup would make sense, but have never seen a second tone control like that.


As to your previous post, I didn't even notice that the switch actually works 3 ways. Cool! I may even have the appropriate switch in my piles of stuff not that it's an expensive item. I know for sure I don't have that 10 watt resistor tho, and maybe a few other odds & ends. I wonder if a 16 uF filter cap would be OK to use in place of that 20? Although, if I'm ordering the stuff I don't have I might as well order that as well. I'm happy to know about the 6SL7 values now and will have to remember that for future reference. Thanks much for all of the input. I truly appreciate it.

[hesamadman]

Leave it to me to miss somthing. I looked at wrong schematic. Thought the circuit was volume only. Also i may have clicked "report to moderator" on someone's comment by accident. I need to stay off this forum on my busted iPhone. :(

[Guitarzan]

Leave it to me to miss somthing. I looked at wrong schematic. Thought the circuit was volume only. Also i may have clicked "report to moderator" on someone's comment by accident. I need to stay off this forum on my busted iPhone. :(

No harm done

[Guitarzan]

I was watching this Stromberg Carlson AU-29 on eBay for weeks. The guy wanted 40 bux for it which wasn't all that bad but then the shipping was gonna be another 20 and I thought that was a tad high so I never pulled the trigger. Now of course it's gone. It would have been the perfect platform to build this amp from. Three octal sockets, a PT and OT already sized for roughly 10 watts, I can't remember now what all controls it had but IIRC there were 3 pots. So that would have taken care of the Vol, Tone, and 3-way boost switch. The thing was filthy, but not burnt looking. So after gutting, cleaning, and a few extra holes for inputs, speaker jacks, ect. it would have been just the ticket. I'm kicking myself now for not going for it.
I have an appropriate PT but the Maggie OT is prolly gonna cost me at least half of the 60 bux that I balked at in the first place. 


If anyone has something similar in your "never gonna get around to" pile, I'd be interested in taking it off your hands.

[Guitarzan]

Hiya!
Well, I got the Stromberg Carlson AU-29 that I was dreaming of using for the Maggie project.
http://www.ebay.com/itm/Stromberg-Carlson-Model-29-Mono-Tube-Amplifier-/151441260756?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2047675.l2557&nma=true&si=eLuga95hLYVcdyNmWfits92u3ZY%253D&orig_cvip=true&rt=nc
It's grimey, but if the transformers are good, I'm way ahead of the game as it pertains to ordering new iron, chassis, etc. Plus I shouldn't have to drill very many holes. I like that too.
I also have a SAMS Photofact complete with schemo coming. So my winter project is on the slate. Rather than follow the Weber design to the letter, I wanna put one 6Sl7 input, and one 6SN7 input in the mix. I also got an appropriate cap can for the Maggie specs since the one in the eBay amp is obviously a quitter. I guess it'll be about a week 'til I have the eBay stuff in hand.  I'm looking forward to stripping it all down to bare bones and cleaning it up. As always, any and all input is encouraged and definitely appreciated.