Amp build for a first time builder?

[M Fowler]

Be careful triode MC transformers don't always fit the mojo chassis I found that out with the 5E3 Deluxe MC tranny set and the triode MC JTM45 tranny set didn't fit the mojo chassis.

The Mojo kit without cab and speaker is $570 you would have less headaches getting the mojo kit.

Mark

[martin manning]

Thanks for bringing the transformer fit up again, Mark. I would certainly contact Mojo and ask for the transformer mounting centers (all of them), and the dimensions of the PT cut-out. The data sheets for the MC transformers have those dimensions listed, so compatibility can be checked before ordering. The desire here is to have more than just a 2-ohm output, and that is the reason for going with the MC set. I don't know if that is an option with the Mojo kit. Another question for them, maybe.

Homieg, Skyboltone is right, swapping tubes is something that can be played with (endlessly) later. To me, 5881's offer little advantage over 6L6GC. Note the reissue Bassman uses 6L6GC. If you want 5881's you can assemble your own set of tubes, and maybe not spend any more than $100. See: http://tubedepot.com/ts-5881.html (and those stubby bottles do look cool). They have all the other tubes needed as well.

I've had one more idea cooking for modernizing the circuit, which is to add a voltage divider from the B+ to elevate the heater center tap 80V or so. This is to minimize stress on the heater-cathode insulation in the cathode follower stage (the one driving the tone controls). The cathode of that stage runs at 180V, and some 12AX7's are only rated at 100V max heater-cathode voltage Vh-k. I have a good plan for that now that only requires a small rework to the circuit board. The other useful purpose this serves is that it creates a bleeder that will drain the power supply caps when you turn the power off and leave the standby on. I'd like to see a scan of the stock board to finalize the layout.

[yoyohomieg5432]

Hey Guys,

I've been incredibly busy and haven't really had time to look at this anymore. Thanksgiving break is coming up soon and I definitely want to use some of that time to build this, which means I want to get it ordered like on Monday.

You guys have brought up an issue where the transformers might not fit the chassis. If they don't is this a major issue to fix? When I'm home I have access to a drill press so if it's just the matter of drilling a couple of holes it shouldn't be an issue unless there is major differences.

[M Fowler]

Most of the time the transformer is physically to large to drop into transformer cutout and the bolt holes are off.

That is why I don't mix Mojo chassis with Magnetic Component transformer even if they state they are for the same model such as JTM45 or Tweed Bassman.

If the chassis was using a standup mount rather then laydown with transformer chassis cutout you would simple drill new holes like you said with the drill press. You certainly can increase the cutout with a jigsaw.

Mark

[yoyohomieg5432]

are these the right datasheets?
http://www.classictone.net/40-18029.pdf
http://www.classictone.net/40-18000.pdf

I have a few questions

1) the dimensions are given as 3.5, 3.75, etc.. what units is this in?

2) i don't really understand which part of the transformer you don't think will fit on the chassis? which dimensions should I be checking when I call? Is it just the length and width of the base? (i.e. 2 and 3.5 for the OT?) so we know the screw holes will line up.

3) from the triode store http://store.triodestore.com/bassmanbundle.html it also mentions a choke "22699". i can't find anything online about this. is this also a MC part? Do I need to worry about the dimensions of this as well?

4) i'm also interested if anyone has any physical understanding of the what the choke does. I read on wikipedia that it's used to block high frequencies and I understand that due to its impedance. I'm curious how a choke differs from a low pass filter other than the obvious difference in components and construction (i.e. why not use a choke instead of an RC LPF?)

thanks

[martin manning]

Magnetic Components Tweed Bassman listing: http://www.classictone.net/Fender-Tweed ... rmers.html Click on the PT link and then scroll down, and you will see the various options for the PT and OT. Data sheets are there too; click the link to each part and then the link to the spec sheets. Dimensions on the data sheets are given in inches.

The power transformer is a "laydown" or "Z" mount with one bell (the outer cover) protruding through the chassis surface. On the MC site there is a separate data sheet showing the required cutout size and maximum corner radius. Since the chassis is chrome plated steel it will be hard to modify the opening size without making a mess of it, and you'll lose the rust protection. If a mounting hole location is less than one diameter off, it is more of a problem to correct it with a drill.

The choke 022699 is the old Fender part number, same as their later part number 125C1A. "Choke" is another term for an inductor. See: http://en.wikipedia.org/wiki/Inductor

BTW, I noticed that the Mojo small parts kit does not appear to contain shields for the preamp tubes or retainers for the power tubes and rectifier.

PM me your e-mail address and I'll send you some pics of a '59 Bassman.

[martin manning]

If the cut out and mounting center issues work out, you can get the MC PT, OT, and choke individually from Triode instead of buying their bundle. You probably don't need the 240V primary leads on the PT, so you can go with the 120V 40-18073 PT at $81.11, 40-18000 OT w/ 2/4/8 secondaries at $64.63, and 40-18003 at $17.60 for a total of $163.34.

[skyboltone]

Yoyo are you funning with us? A couple of times here in the last eight pages of back and forth you've come up with questions that stretch credulity to it's limits. Do you really not know what unit of measurement is used on those data sheets? Is Illinois still in the USA? Here in the US we use the English standard system of measurement. Those units are obviously in cubic yards aren't they? Just kidding.

Earlier I suggested that you get yourself a basic text on tube based electronics, like a tube manual or a Radio Amateurs Handbook from the late 1950s so that you can smarten up. You asked a question about what a choke does then you suggested that a person might just substitute a low pass filter in it's place. If you were to read up as I suggested you would know already that the choke in these circuits when in interaction with the power supply capacitors comprise the very thing; a low pass filter.

So here's the deal on this transformer mounting. Transformers are made by winding coils of wire around an iron core. For a variety of reasons spelled out for you in the texts I suggested, the iron core is sliced up into thin sheets called laminations. These laminations can be oriented either vertically or horizontally. If they are oriented vertically they are generally contained in a sheet metal retainer that has little feet on it. These feet attach the transformer to the chassis so that the entire transformer is all above the chassis. In this orientation there will generally be a couple of bushed holes drilled in the chassis top that the wires pass through so that they can be soldered into the circuit on the underside of the chassis. When the laminations are horizontally oriented it is necessary to make a big hole in the top of the chassis so that one half of the wire coil can recess below the chassis top. All of the coil wires on this type exit the transformer below the chassis. The reason this is done is to lower the physical profile of the transformer and to reduce the magnetic interaction between the coils and other components of the amp. There are thousands of photographs on the site here of transformers mounted this way.

Now, for many of us here, or perhaps most of us, adapting a transformer to a chassis for which it wasn't designed might be child's play. It never the less requires access to various tools and a certain proficiency in using them that comes from time and practice. Most of us started by tearing stuff apart when we were kids and so have a pretty good understanding about how this old stuff was built. If you don't have that experience doesn't mean you can't gain it. It just means you'll need to use your head and acquire the knowledge. It's not rocket science; any idiot can do it. I can.
On the other hand, your lack of understanding of basic units of measurement and how to convert them to something you can read off a tape measure is frightening. Do you remember working with something called fractions when you were in primary school? An English Standard tape measure or ruler will not have any decimal marks on it. Try as you might you will not find a place where it says 3.5 something or other on there. If you think in metric perhaps you can get a ruler that has both markings on it? That may help. I know that I have to use a calculator to convert metric to inches because my mindset got frozen early in life. Perhaps you'll have to work that problem in reverse. I don't know.

Sincerely
Dan

[martin manning]

Dan, you might be surprised at how little exposure young people get to such practical matters today. I regularly teach to new grads from a variety of engineering disciplines (Mechanical, Aero, Electrical) and find that they have had little experience of English units. They don't do the kinds of things you and I did for fun anymore, either. Every high school used to have a shop, but that is no longer the case. I was really lucky- my high school had a drafting room, an electronics lab, a wood shop, a machine shop, and a foundry... and it wasn't even a vocational school.

[skyboltone]

Dan, you might be surprised at how little exposure young people get to such practical matters today. I regularly teach to new grads from a variety of engineering disciplines (Mechanical, Aero, Electrical) and find that they have had little experience of English units. They don't do the kinds of things you and I did for fun anymore, either. Every high school used to have a shop, but that is no longer the case. I was really lucky- my high school had a drafting room, an electronics lab, a wood shop, a machine shop, and a foundry... and it wasn't even a vocational school.

Thanks for that Martin. I was trying to be gentle but my gosh, sometimes my age becomes a painful thing I guess. Our High School had a similar set up to yours though we didn't have a foundry. So you did some pattern making? Cool. It's easy to stand back and lament at what the world has become. At least this young man is taking a shot at this. Electrical Engineering encompasses so many different disciplines these days that it's hard to tell what Yoyo is studying. I spent my life in relay and PLC logic process control for the most part. Power distribution too. All big scale stuff you know? Today there is a whole new world at the micro and nano level that goes way beyond my meager resources. Carry on. And Yoyo, have you got a name other than your sign in?

Dan

[bcmatt]

It is sure nice when chassis holes are all pre-punched and lined up perfectly, but for me that is pretty rare. I always need to drill something, and it is no big deal. Actually, my last few projects were either using a blank hammond chassis or converting crappy solid state amps into something useful by trying to use as many of the existing holes in a new build and then drilling the rest and extra visible holes get covered up by wooden face-plates. A few extra holes in the top of a chassis are no big deal... these things aren't meant to be fish tanks.

All this is to say that even though most of my build time these days is spent trying to plan a layout and drilling the chassis accordingly. Even though it would be nice to cut that time down, I doubt I can avoid drilling altogether...
and it sure is nice not using lay-down transformers because it sure is a pain trying to file a round uni-bit hole into a square that fits a transformer. I feel like it is good training for trying to bust myself out of prison.

at least I finally splurged for the bigger unibit that can do octal socket holes.

[yoyohomieg5432]

Yoyo are you funning with us? A couple of times here in the last eight pages of back and forth you've come up with questions that stretch credulity to it's limits. Do you really not know what unit of measurement is used on those data sheets? Is Illinois still in the USA? Here in the US we use the English standard system of measurement. Those units are obviously in cubic yards aren't they? Just kidding.

Earlier I suggested that you get yourself a basic text on tube based electronics, like a tube manual or a Radio Amateurs Handbook from the late 1950s so that you can smarten up. You asked a question about what a choke does then you suggested that a person might just substitute a low pass filter in it's place. If you were to read up as I suggested you would know already that the choke in these circuits when in interaction with the power supply capacitors comprise the very thing; a low pass filter.

So here's the deal on this transformer mounting. Transformers are made by winding coils of wire around an iron core. For a variety of reasons spelled out for you in the texts I suggested, the iron core is sliced up into thin sheets called laminations. These laminations can be oriented either vertically or horizontally. If they are oriented vertically they are generally contained in a sheet metal retainer that has little feet on it. These feet attach the transformer to the chassis so that the entire transformer is all above the chassis. In this orientation there will generally be a couple of bushed holes drilled in the chassis top that the wires pass through so that they can be soldered into the circuit on the underside of the chassis. When the laminations are horizontally oriented it is necessary to make a big hole in the top of the chassis so that one half of the wire coil can recess below the chassis top. All of the coil wires on this type exit the transformer below the chassis. The reason this is done is to lower the physical profile of the transformer and to reduce the magnetic interaction between the coils and other components of the amp. There are thousands of photographs on the site here of transformers mounted this way.

Now, for many of us here, or perhaps most of us, adapting a transformer to a chassis for which it wasn't designed might be child's play. It never the less requires access to various tools and a certain proficiency in using them that comes from time and practice. Most of us started by tearing stuff apart when we were kids and so have a pretty good understanding about how this old stuff was built. If you don't have that experience doesn't mean you can't gain it. It just means you'll need to use your head and acquire the knowledge. It's not rocket science; any idiot can do it. I can.
On the other hand, your lack of understanding of basic units of measurement and how to convert them to something you can read off a tape measure is frightening. Do you remember working with something called fractions when you were in primary school? An English Standard tape measure or ruler will not have any decimal marks on it. Try as you might you will not find a place where it says 3.5 something or other on there. If you think in metric perhaps you can get a ruler that has both markings on it? That may help. I know that I have to use a calculator to convert metric to inches because my mindset got frozen early in life. Perhaps you'll have to work that problem in reverse. I don't know.

Sincerely
Dan

Dan, I found your post incredibly condescending. If you don't want to help please don't post in here. I simply asked what units it was in..... Every calculation or measurement I've ever made in college is in the metric system since that is the way the rest of the world thinks and I also think it's far superior to US system. The datasheet could have just as easily been in metrics for all i know.. A lot of documents in the US can be found in metrics because not everyone is ignorant enough to think that only americans will be reading the document.

Analog design is not covered much anymore since practically everything is digital these days. I have one class on analog circuits... I know many of my questions are probably stupid but I don't have much more knowledge than the average joe on analog components.

My name is Ryan.

[Phil_S]

1) the dimensions are given as 3.5, 3.75, etc.. what units is this in?

We take a lot for granted, particularly those of us who may have been born before 1960. Times have changed. Let's not be hard on someone just because the question may seem outlandish to us. All questions are good questions. Maybe Yoyo has never actually seen a transformer or his experience with them is so "light" that 3.5mm could be plausible measurement in his mind? We who have repeated exposure to the physical stuff know right away, it's got to be 3.5" x 3.75". Yet, we typically don't express inch measurements in decimal fractions. We know metric measurements are always expressed in fractions. Just saying, I can see where this question arises.

Besides, before the digital age, it was nothing to write fractions like one-quarter that appeared like ¼. Now you have to know it's ALT+0188 or leave that messy 1 1/4 for one and a fourth in your writing. So we've moved to decimal fractions on the English system. It can be confusing.

Yoyo, you deserve a break. Keep trying. People will help.

And give the other guys a break, too. Generally speaking, the tone here is quite civil and I was rather surprised to see it veer off this time. Just take the high road.

[skyboltone]

Ryan; It did sound condescending and I apologize for that. It's not uncommon for folks to take a forum for a ride with a lot of silliness. Again, sorry. Ask away and I'll try to be nice. I do suggest though that you take my suggestion and get an old RCA tube manual. There is a tremendous amount of information in there that will make your journey more enjoyable.

Dan

[skyboltone]

Here you go Ryan. And anyone else who doesn't have one. This is a good edition.

http://www.amazon.com/gp/offer-listing/ ... dition=new