Amp build for a first time builder?

[yoyohomieg5432]

I agree it doesn't look like they are compatible. Here's another idea: Hoffman Amps http://www.hoffmanamps.com/ has a 5F6-A chassis which I believe is Mojo's for $80, and the cap pan for $22. He has transformers including an OT with 2/4/8 secondaries for $75, a PT for $90, and the choke for $18. If he will confirm that the transformers he sells will fit in the chassis he sells, you would be in business. Its kind of funny but there are people selling Mojo stuff for both a bit more and a bit less than what Mojo charges for it.

what brand are these transformers? i liked the other ones since they were MC and were supposedly quality made parts.

[martin manning]

I don't know. The MC may still work... see if Mojo can clear up the question on mounting centers for the PT.

[yoyohomieg5432]

I don't know. The MC may still work... see if Mojo can clear up the question on mounting centers for the PT.

centers refers to the distance between where the screws would go, right?

and isn't that only part of the issue? the cutout dimensions i listed were smaller than the dimensions from the datasheet, assuming i used the correct dimensions. I'm pretty sure I did though.

[martin manning]

Yes the "mounting centers" referrs to the dimensions of a rectangle with its verticies at the centers of the mounting bolts/holes. The cut-out dimensions you listed (3.25 x 2.5) are larger than the MC drawing's 3.13 x 2.46. MC's OT is 3.5 x 2 vs 3.25 x 2 on the chassis, so there is a quarter inch miss-match there. You could probably make that work pretty easily with a small round file, ovalizing the holes by 1/8" each.

[yoyohomieg5432]

Yes the "mounting centers" referrs to the dimensions of a rectangle with its verticies at the centers of the mounting bolts/holes. The cut-out dimensions you listed (3.25 x 2.5) are larger than the MC drawing's 3.13 x 2.46. MC's OT is 3.5 x 2 vs 3.25 x 2 on the chassis, so there is a quarter inch miss-match there. You could probably make that work pretty easily with a small round file, ovalizing the holes by 1/8" each.

ok. so i'll call about the centers for the PT and if those are pretty close i'm thinking i'll just pull the trigger on that. i'll be home so I have access to all the tools I should need if i need to make any modifications.

[martin manning]

The only thing still bothering me slightly is the footprint of the OT. You don't want the mounting feet to overhang the 4-1/8" (outside) width of the chassis on the top (control) side since it has to butt up against the inside of the cabinet. With mounting holes 3.5" on center that is cutting it close, and the OT will have to be centered on the chassis because there is also the need for some clearance for either the nut or the head of the atachment bolt on the inside. It might be better to file a 1/8" notch in the slots in the transformer feet to get the 3.5" dimension and leave the chassis holes alone. Unfortunately MC doesn't provide the outside dimension of the mounting feet, either. Mojo's OT http://www.mojotone.com/amp-parts/trans ... nformation is likely made by Heyboer, a very good brand, but it is $20 more than the MC and has only 2-ohm output. Anybody know who makes Hoffman's transformers?

[yoyohomieg5432]

i emailed the guy from hoffman today, here's what he said:

Hi I get my transformers from a few sources
Mainly new sensor and Mojo

You would have to ask them who makes them as they get them made from several sources.

The power tranny drops down into a hole on the bassman chassis

There are screw holes for several types of outputs and chokes but you may have to drill a small hole or two if needed

i don't know.. this doesn't seem like a good route either.. the transformers probably aren't as good of quality as the MC and there will still be extra work involved.

should i just pull the trigger on the MC transformers and do some modifications? i am starting to reconsider the weber package although the OT only has a 2 ohm output, i'm probably going to just end up getting a 4x10 cab down the road anyways..

[martin manning]

I'm going to say the MC PT will fit in the Mojo chassis since the cut-out dimensions look good, and the mounting centers are the same as the PT Mojo sells.

The MC OT is 1/4" too big in the long dimension of the mounting centers, but as long as the outside overall dimension is 4-1/8" or less, it will fit on the chassis without any overhang. Based on the picture below, and knowing that the slots are 3-1/2" on center and 3/16" wide, it looks like it will fit. You can e-mail MC and ask them for that outside dimension, and they have responded quickly for me before. It also looks like you can easily remove the mounting brackets and file a notch in the slots to get the bolts through using the existing chassis holes.

Edit: In fact the OT could overhang the bottom edge of the chassis a bit- there is plenty of room on the transformer brackets (feet). I just would rather it didn't for aesthetic reasons...

The choke is 1/16" off at most and should work, or can be made to.

So, my put would be:
MC 40-18000 OT (4k pri, 2/4/8 sec), 40-18073 120V PT, and 40-18003 choke, all from Triode; chassis, cap cover, and small parts kit from Mojo; tubes from Tube Depot (1 GZ34, 2 Tung-Sol 5881, 2 EH 12AX7, 1 EH 12AY7. And, get 3 Octal clip retainers and 3 9-pin tube shields from Triode too, since the Mojo small parts kit doesn't seem to include those parts.

[yoyohomieg5432]

Martin,

Why did you choose not to go with the 18029 PT with the 240V line? Why is that the PT for the bassman package if the 240V line isn't needed?

[Firestorm]

Martin,

Why did you choose not to go with the 18029 PT with the 240V line? Why is that the PT for the bassman package if the 240V line isn't needed?

Golly. You're in Illinois. Where are you going to plug into 240VAC? Trust Martin; he has devoted a lot of thought (and time) to your project. He's one of the smartest guy's here.

[martin manning]

Unless you see a need to power the amp from a 240V mains supply, you don't need the dual-voltage primary. It's a few bucks cheaper and you will have one less pair of leads to deal with.

Here's a link for you: https://sites.google.com/site/stringsan ... me/bassman This is a Weber 5F6-A kit built in a senior EE class. Not a particularly neat job in my opinion... see if you can do better! I don't think the dual bias adjust is needed, nor the shielded cable runs. The chassis is so compact that the leads are short anyway, and the grid resistors would really be better placed on the tube sockets to be most effective at keeping RF out. I would recommend losing the four RCA jacks as used on the original and using three Switchcraft phone jacks, one for each of the 2/4/8 speaker outputs.

[yoyohomieg5432]

wow.... the shipping costs are astronomical. triode charged me 22 and since im in illinois i had to pay an additional 14 in tax.

im at the checkout in mojo and they want $48 for the cheapest shipping option... does this seem absolutely ridiculous to anyone else? the chassis only ways like 4 pounds... the parts kid can't take up much space either. i don't believe it could be that much

edit: i just found another option with ups that's $25.. still seems a bit high but certainly much better than $48.

[martin manning]

Transformers are heavy. The Mojo shipping is stupid, though. Look for another carrier option. Let's hope it all works out. You need a few more parts which you can get from Mouser: A few eyelets, a few ground lugs, a few resistors, e-caps, and a trimmer. I'll have a look for the part numbers.

Be patient as you proceed. Maybe you won't be playing through it until Christmas break.

Thanks FIrestorm. I have a soft spot for this amp... Glad to help out, and maybe I'll build one for myself someday.

[yoyohomieg5432]

Transformers are heavy. The Mojo shipping is stupid, though. Look for another carrier option. Let's hope it all works out. You need a few more parts which you can get from Mouser: A few eyelets, a few ground lugs, a few resistors, e-caps, and a trimmer. I'll have a look for the part numbers.

Be patient as you proceed. Maybe you won't be playing through it until Christmas break.

Thanks FIrestorm. I have a soft spot for this amp... Glad to help out, and maybe I'll build one for myself someday.

yea, Martin I really appreciate your help through this.

I am being patient. I probably won't receive my order until the end of the week. Once school starts again finals will be coming up so I won't have much free time again until break. It's going to be a while before this is done.

[Firestorm]

Unless you want to build 100+ amps, shipping is what it is. If you start moving large quantities of things, you have options. But if you're building an amp just for yourself, it will cost you more than if you bought it off the shelf. It can also sound way better, and you'll have bragging rights. Bite the bullet. And don't chintz on things to save a shekel or two.