Belated cool NAD - Magnatone 613 Custom

[Tortuga]

Pretty sure I haven't posted this here - apologies if I already did. Was given this awesome early 60s tweed-like combo amp from a good friend who'd only used it for running an electrified accordion

...little did she know that it's a really killer vintage guitar amp in disguise

Gonna be doing some basic maintenance on it pretty soon, but it's in stellar condition, and about as original as could possibly be.

This is the amp, after I picked it up last September:









Pretty sure this amp is very similar to the Tonemaster that PhilX has been using for years, and the one they used to recreate into the Evil Robot amps that he collaborated on.

[Tortuga]

The amp has a little hiss/crackle going on (likely due to old filter caps), but far less than the 66 Bassman had when I picked that up in 2020. The 2-prong cord and death cap will be changed out, and I'm considering pulling the original tubes and at least putting the speaker on a switch so I'm not pushing it during loud jams. I can tell you that it does sound stellar through my greenback 4x12, so no need to destroy that cute little alnico speaker.

[Yarbicus]

Badass!

[Tortuga]

Unfortunately, schematics for these are pretty scarce. Whereas there are tons of variations on the vintage Fender and Marshall diagrams (not to mention updated artwork from amp/electronic books) available all over the place, I've really only been able to locate one diagram - and I already know it's not exactly the same as my amp (there's an additional tone mod switch in the diagram that doesn't have a corresponding switch on the physical amp). That diagram is extremely faded and crusty, so I decided to redraw it for historical as well as my purposes in working on it.

I haven't completely proofread my draft version, but will be doing so asap. Will then do some actual component comparisons to the amp and then start spec'ing out replacement components that I'll need.

Please feel free to look these over and tell me if you see anything that that jumps out as being wrong.

Original schematic:



First draft of updated schematic. Note: there is a slight difference in the cathode wiring for the tube in the vibrato section in the lower left. I drew the cathode resistor on the right of the other leads, due to the way it works in the designer software. I think it's right, though.

[mortatone]

Oh man I’m jelly!

Recently had a trade worked out for a Magnatone M7 that sounded killer but at the last second the guy flaked out on the trade and tried offering me something different instead.

[Beef]

That’s a super cool little amp. Hope the repair goes smoothly.

[Tortuga]

Thanks dudes - I'll get some clips posted at some point. The overall tone is super-warm, almost too dark, and I wind up the tone knob quite a bit to compensate. Wondering what changing out the worn components will do to that. I'm also pondering how to hack-in a tone switch like the schematic shows without bunging up the control panel - maybe I'll pull a jack and put a 3-way switch in its place. We'll see

Mort - would love for you, RiffRaff, and others here who have done the amp stuff to take a look at my schematic and let me know if anything is jumping out at you. Planning to release that drawing on a couple Magnatone groups I'm on to see what they think, too.

[mortatone]

I'm very unfamiliar with magnatone amps so I can't really help circuitwise. I hope to learn more about them though. What's that varistor thing? Issat a MV?

[Tortuga]

I’ve read the varistors are a “magic” part of the vibrato circuit that makes the Magnatones unique from Fender and other designs, which aren’t actually vibrato.

[long standing member]

Dayum!!!!!!!!
HOT!!!!!!!!!!!

AND, it's got "High Fidelity"!!

Even old Marshall's and Fender's don't claim that!

[Tortuga]

I’ve had it in the same room with Greg’s (itchyfingers) bona-fide tweed amps (deluxe and another I can’t remember), and while the Maggie has its own thing going on, it’s definitely more like those than my blackface Bassman or any other amp I’ve owned. Poor man’s tweed… I’ll take it, all day long

[Tortuga]

Did some reading on the particular phase inverter this thing runs (cathodyne, rather than the long-tailed pair used on most tube amps, making this more of a Princeton than the rest of the Fender-types out there.

Supposed to be a cleaner / less aggressive push to the output tubes, from what I’m reading. Maybe that’s contributing to how dark the amp seems. Regardless, it deals beautifully with my treble booster and gain pedals - especially when the tone knob is cranked to the brighter side.

I’m wondering how the amp would do with a master volume, and how that might be implemented in this type of PI

[itchyfingers]

Dude, don't fuck this amp up with a bunch of mods. Maintenance and replacement of burned out parts, sure that comes with the territory, but its a unique and killer sounding old amp. It's small enough to sound great at bedroom levels, it really shines when (almost) dimed, and you can stick a mic in front of it for anything where it needs to get louder. Get familiar with the sweep of the second volume knob unjumpered and jumpered - tones for days.

[mortatone]

I agree with Itchy. Maintain it, swap out the speaker at most, and if you want to tweak it more than the controls allow, get an EQ pedal for it.

[Tortuga]

y'all are no fun

...not ignoring the advice - I will definitely stick to maintenance for now and probably get back to modding the Bassman and Pro Reverb instead of mucking with this thing. Started looking into master volumes, and the cathodyne PI definitely makes things more experimental/harder to implement cleanly - so, yeah... thinking I need to work with this thing like the tweed brethren it really is.

Apparently this amp uses a 4 x 20uF cap can (just like the blackface Princeton did) for filtering. I've found two possible candidates:

CE Manufacturing


JJ - note: one of them is 40uF, and I'm wondering if that'll have any difference in tone and where the 40 goes (first? last?)

[mortatone]

You have to be careful raising the filtering on tube rectified power supplies based on the rating of the tube used. Diode rectification can easily handle an increase in filtering.

One difference in raising the filtering would be in feel. More filtering = tighter response. Upping by 20uF would be pretty negligible.

[Tortuga]

Just heard back from a guy at Eurotubes. He agreed with what you're saying - that using 40uF right after the rectifier is going to cause too big a strain for that application. Would it be ok, however, to use it further downstream, as circled below?

[fretless]

Dude, don't fuck this amp up with a bunch of mods. Maintenance and replacement of burned out parts, sure that comes with the territory, but its a unique and killer sounding old amp. It's small enough to sound great at bedroom levels, it really shines when (almost) dimed, and you can stick a mic in front of it for anything where it needs to get louder. Get familiar with the sweep of the second volume knob unjumpered and jumpered - tones for days.

QFMFT

[mortatone]

I'm not really sure how much down path filtering effects load on the rectifier tube vs the input filter, but given that a 5y3 shows 10uF as max input filter and this design is already using 20, I would err on the side of caution.

Plus the CE is made in the USA

[Tortuga]

I'm not really sure how much down path filtering effects load on the rectifier tube vs the input filter, but given that a 5y3 shows 10uF as max input filter and this design is already using 20, I would err on the side of caution.

Plus the CE is made in the USA

Good to know - I think I'll go that route, then. I guess I was put off because I'd never heard of the brand until now.

Been doing some comparisons of the schematic to actual components from the pics I took, and I'm seeing some differences that tell me my amp is already at least somewhat different from the schematic that Magnatone has posted on their site for this particular model. The big thing I hadn't realized was that the tone knob on my amp isn't even present in the schematic - I'm guessing it was put in there to give more variety than the switch shown. Lots more discovery work to do...

[Tortuga]

Just found this... wondering if it's a closer match

From what I'm reading, the Magnatone 213 evolved into the 413 (which is what mine is labeled as, on the control panel) in 1961. Looking at the transformer code, it does look like mine is from that year. Maybe they had transitional amps like Fender did during the 60s.

[mortatone]

I think just about every amp manufacturer back then continuously evolved their circuits to some degree. Seems to be a reoccurring theme when I start worm-holing about the latest amp I've picked up.

Some of the older techs I read will tell folks to draw their own schematic of the amp. They say the schematic is already there right in front of you

[Tortuga]

Did some sleuthing last night... wanted to see if I can get closer to pinning down the circuit.

There was a piece of the actual schematic from the factory stapled in the bottom of the cabinet, but it got torn long ago. This piece is all that's left of the original staple-job:



I did, however, find these three additional fragments tucked up near/around/under the speaker baffle:



...and after a bit, was able to determine they all fit together



Definitely different enough (particularly in the tone control area in the upper-left) from the published 413 Custom schematic:



<note: before someone spots it - yes, I do see that I'd left out the tone knob and the cathode hookup altogether from that 2nd stage >

...a lot more like the 213A that I suspect this particular amp to be:



Here's that schematic with my pieces superimposed:

[Tortuga]

All that said, I've now found a bug in the rub...

The tube used in the vibrato section is definitely NOT a 12AX7 - it's a 6AV6.

Here's the two sections (413 on top, 213a on bottom) compared:



...and the tube in question:



Interestingly, the component values and circuitry seem to be mostly same (only difference I'm seeing beside the tube is the extra pins on the 6AV6 that are wired to ground). Even the connection point for the plate (A) is same, meaning they're sending 12V to a 6V tube???

So, I'd say they were definitely moving to an updated design when this amp was built, but hadn't worked out all the differences yet. I'd love to find someone with an actual 413 Custom and do a side-by-side

[mortatone]

Just looked up the tube data for 6av6 and it's calling it a duplex diode triode, where as a 12ax7 is a dual triode. The 6av6 looks kinda like a single triode with 3 grids. No idea how that works. The two tubes have the same amplification factor though.

It's a good habit to reference tube data regularly. I don't though so if I ever learned anything it's been forgotten. But I know where to go to remember

https://frank.pocnet.net/sheets/093/6/6AV6.pdf

https://frank.pocnet.net/sheets/049/1/12AX7A.pdf