alamo overhaul, magnatone in Iraq?

[drlowlow]

Ok, here is the deal. I am a guy that wears a green suit and goes and repairs radars in far away lands while people try to make me dead. I am in Afghanistan at the moment but will get a short break in Europe before heading out to Iraq for a one year vacation. As my responsibilities entail operating and maintaining a system and not much else I end up with an amount of time on my hands. I have back at my parents house a certain Alamo 1-10 tube combo that is in the truest sense a complete death trap in a hot ground kind of way. As I was thinking about what it would take to transform from death trap to usable piece of musical equipment I remembered that when I was brave enough to forgo safety and plug it in the frequency response was poor and the fact that making it safe would mean new power supply I didn’t see the expense being worthwhile unless I changed the amplifier circuits as well. I think that turning the 3 tube (if I remember correctly it was 3 tube) into a bare bones 6v6 push pull set up would be the best bet. I looked in my schematics and found one for a Magnatone 213 combo and that is what I am basing my design after. I wish to do away with the vibrato circuit and the 2nd channel part of the preamp circuit. I would like to put a level pot right before the phase inverter driver amp and put in a more sophisticated tone circuit with bass, mid, and treble. Put the new transformer on the bottom of the cabinet (because the chassis wont support it). My questions about the plan are as follows:

Can I use the free section of the first preamp stage 12AX7 as a direct replacement of the section of the 12AU7 that goes before the phase inverter driver, should I attenuate the signal with a voltage dividing network because12AX7s should have more gain?

Any mods for setting up a bias control for the power tubes.

Any reasons why any of my ideas are poorly conceived, doomed to failure, or unsubstantiated.

Any links that have good information for figuring capacitance and inductance values for frequency cutoff.

I have about 6 months before I am scheduled to get over to that particular sandbox so I have plenty of time to my research order the supplies so that when my parents send it over to me it’s all in one shipment (well you that’s a misconception). I would appreciate any ideas or concerns that I haven’t addressed here. Thanks in advance for any insight into this project.
Benjamin

here is a schematic of the magnatone 213. http://www.freeinfosociety.com/elect...gnatone213.pdf

[stokes]

"Any mods for setting up a bias control for the power tubes."
I assume you want to convert from cathode bias to an adjustable fixed bias?If you want to keep it cathode biased but adjustable you could use a 500ohm adjustable wirewound resistor,if you can find one.I have a 500ohm 25watt rheostat that would work,it is kind of large,but I used one in a build I did.I'll send it to you if you like.The 12AU7 in that circuit is part of your vibrato,so if you are doing away with the vibrato there is no need to use
the free half of the 12AX7 as you describe,I would just connect the plate from the half of the 12AX7 after the tone control right to the grid of the PI with a .02 cap."Any links that have good information for figuring capacitance and inductance values for frequency cutoff." I assume you are refering to a tone stack here?Try www.duncanamps.com he has a free download for a tone stack calculator.I would also suggest looking at a Fender 5e3 schem,it is a fairly easy circuit for a first build,fairly similar to the power section of the Magnatone,but I think it will give you better tone.And there is a lot of info on this amp right here.

[drlowlow]

Thanks for the great link, really awesome information. I looked at the schematics for the 5E3 and I am digging that idea. I own a lo duca licensed magnatone 213 copy so that is why I had the schematic but it also makes a reason not to build another. I had also picked it because it was simple but not really any simpler then the 5e3 and with the 5e3 I can buy a kit and not worry about forgetting to order the odd cap or whatever. I have looked online for a bit and there are several companies that make kits for it, is there some feature I should look for in it?

[Enzo]

And to drift back to the initial point. All it takes to safety-ize a hot chassis is an isolation transformer at the mains input. You can certainly make it more complex than that and take care of circuit shortcomings in the bargain, but minimally, that is all you need.

Not in the slightest trying to talk you out of any of these ideas you have presented, but some of the tonal problems you found might be due to aging parts.

You might like to visit www.schematicheaven.com and explore the world of schematics. You might find it informative to look at all the variations of the Fender Deluxe over the years for example. And between his Post 70s amps and the Bargain Bin amps sections, you can find many unusual circuits.

And isn't that tone stack calculator a great thing?

[Ray Ivers]

Enzo,

All it takes to safety-ize a hot chassis is an isolation transformer at the mains input.

I was thinking about this last week... if an amp has its neutral power conductor - normally connected to earth ground at the distribution panel - connected to the chassis, won't an isolation transformer (120V primary, isolated 120V secondary) "float" the chassis, removing the 50/50 chance of making an earth-ground connection? I'm talking about just plugging the unmodified amp - with a 2-prong, non-polarized power cord - into the transformer.

Ray

[Mars Amp Repair]

Ray,
I think I'm understanding you correctly...Yes I agree..the only way to a/c ground the chassis in that scenario (plugging the alamo into an isloation xformer) would be to add a death cap from the a/c line side of the isolation xformer to the chassis ground...still better than no xformer, but not by much. Best way would of course be to also ground the chassis by the 3rd prong. -glen

[Enzo]

Hot chassis means one side of the line is either directly wired to chasis or the line is rectified and one side of THAT is wired to chassis. With a 2 wire you get 50/50 the chassis is grounded.

When you plug this 2 wire into an iso, then the whole circuit floats, yes. But it is also completely not referenced to ground. Touch any one point in it you like at a time. Might as well run on batteries.

You want the chassis AC grounded? Go ahead and wire the third prong power cord to chassis, after all, the circuit floats. The 120VAC in your curcuit is NOT the same 120VAC of the mains. With a floating circuit, you can ground any single point in it to the rest of the world. In our tube amps, if we felt like it, we could connect all the B+ to ground and run the cathodes on negative 400v rails. The amp would not know the difference.

If it makes anyone feel more clear, don't call it an isolation tranny, just call it the power tranny. We ground parts of various windings on transformers every day and think nothing of it. No difference here.

I keep an iso on my bench - anything comes in with a hot chassis, I plug it into the iso and now my scope ground can connect right to the chasis of the unit.

I work in another industry and all our RGB video displays are hot chassis. Must have an iso for the power to the thing. I also work on switching power supplies, which have finally found their way into audio too. The switcher directly rectifies the AC mains so we have 330VDC to play with. The control circuits for the switching might run on 12VDC, but that 12v has its common as the -165v side of the rectified mains. Good luck grounding your scope to that without an iso. But once it IS on the iso, than I can feel free to ground my scope to the -165VDC to look at the 12v waveforms I need.

[Steve Conner]

I always keep an isolation transformer around for the same reasons. Somebody gave me one that they were going to throw out, and later when I was messing around with hot chassis equipment, I found how useful it was.

Newbies seem to have trouble understanding the concept of hot chassis, isolation and grounding, but they always seem to learn real quick the first time they try to clip a scope ground clip to a hot chassis and the alligator clip disappears with a huge bang and shower of sparks.

However, even after this lesson, Americans always seem to have terrible trouble telling the difference between neutral and ground. If you "ground" the chassis to neutral, which is what the "Death cap" does, you might hardly notice, even though neutral can have something like 10V AC on it relative to "Real" ground.

Our wiring code enforces the distinction strictly (so does the US code nowadays, to be fair) since we ditched two-prong plugs some time between the two world wars. We use neutral only for returning the current we drew from hot, and the third prong only for safety ground.

[drlowlow]

with such an easy fix to the danger problem, it would make no sense to rip appart this amp for a less then ideal platform for a more powerful amp. I was refusing to sell it to a friend because of the hot chassis but now no such weight on my concious I can easily part with it. I have never seen a piece like this for sale on a market, what would be reasonable for a low power I am guessing 3 to 7 watt tube combo with a hot chassis? I am certainly going to invest in a isolation transformer for my arsenal of tools.

[Enzo]

To be fair to hot chassis products, when I work on hot chassis stuff like CRTs, the chassis is Serious hot - the mains are wired right to it. But in things like record players and 2 wire tiny guitar amps, the mains are connected to the chassis metal through relatively high resistances and by caps. SO if you guess wrong on the mains plug, you are not really wired direct.

I have a number of isos, most of them are plain old raw transformers, but I do have a couple nice RCA VIZ bench isos. Actually just one now, I sent one to KB a while back. You can select the mains voltage with a switch - 105, 110, 115, 120, 130 etc or something like that. Then on top are three iso outlets - 2 prong - one right at the mains voltage and the other two are one a little high and the other a little low. There are also three outlets that are direct connect to the mains as a convenience. TV guys used them in their shops. You can find these around for not a lot. Amateur radio gatherings are usually great places to find cheap test gear.

HAve we cleared up any confusion about using an iso? That plugging a 2 wire hot chassis thing into one means you can now connect the chassis of the hot chassis item directly to earth ground.

[hasserl]

I've picked up small iso trannies from Antique Electronics Supply and mounted them directly insde the cabinet of these little hot chassis guitar amps, with the amp hard wired right to the iso trans. Works nice.

The biggest problem is going to be how to get a decent sound out of that amp. The power is so low that headroom is almost nonexistent. The distortion can get pretty grainy, okay for bluesy stuff, but not really much for any kind of hard rock. Still, they can be fun. I prefer the circuits that use a 12AX7 in a champ or Princeton type circuit over the ones that use a 12AU6 or other pentode. I think the twin triodes give you a little more flexibility in tuning the circuit than the single pentode.

[Regis]

Drlowlow, I don't have anything to add to the thread but I want to thank you and bless you for your service to our country and for doing your part to take care of us and protect us. Like all service men and women overseas you are an American hero.

Regis