Bassman Build

[Chicken fingers]

Hello everyone,

I recently finished my AB763 and it is super fun to play. The build process was very fun and brought me back to better times when I worked on production equipment (been out of work many years with cancer...). Building the amp was a much welcomed distraction to my frustrating health issues. The next few months will continue to be stressful with health crap and I really need another distraction so a bassman clone popped into my mind... I spent quite a bit of time studying mojotone's schmatic/layout relative to Weber's. I prefer the Weber's grounding scheme as it avoids the back-of-the-pot ground bus and I don't have to make my own grounding scheme. Weber's purchase menu is nice so that I can buy better parts rather than pay for a second set of the cheapest components. I also like that they handmake speakers in the US.

Weber was kind enough to send me their bill of materials so that I could source all components on my own easily. That list is done and set aside. My question now comes to the PT, OT, and choke. I almost hate to bring this topic up as a quick search shows many many strong opinions - I'm not searching for special secret sauce tones, I simply want a reliable amp that sounds similar to vintage ones. I read some negative reviews about Weber transformers and chokes - not sure if they are valid so I wanted to ask folks here their thoughts (they are the cheapest money can buy it seems...). If folks think they are good quality parts, I am happy to use them as trying to determine which brand's product will fit the needs of the Weber circuit are a little beyond my headlights. Sure, I could figure out which components to swamp in, but I might not be certain about end results. Just to give a quick example - weber's choke is listed at 9H 120mA. Now, if i look at mercury magnetics tone clone, they list 3.5H 200mA. I don't have the experience to know if switching from 9 to 3.5 will have undesirable or maybe even desirable effects. As I look at OTs and PTs, slight differences crop up between other brands and what weber offers so i would need to figure that all out.

So, all this being said - do you all think my best bet is to simply use weber parts considering that's how it was designed? or do you think it could be worth the time to figure out better/different products for these? Like i said, I'm looking for a happy distraction so if you all think I can figure this out (maybe with a little of your assistance) that would be wonderful!!

Thanks!
Frank

[sluckey]

There are several different Bassmans from different eras. I'll assume you want the 5F6A tweed bassman?

One big consideration to keep in mind is will the PT fit the chassis cutout.

[Stevem]

H

[Chicken fingers]

Thanks, yes i forgot to mention its the 5F6A i'm looking at. Ah yes - hole sizes should definitely be considered for PT. Thanks for reminding me!!

[martin manning]

I have no recent experience, but Weber transformers were imported, and not as good in quality as some. Mojotone's transformers are AFAIK still made in US by Heyboer, and are very good. A 90 mA Twin choke will be more than adequate.

[M Fowler]

I would use mojotone Heyboer transformers. Pacific is good but cost more. Hammond are good as well.

[Stevem]

Does the kits OT have multiple secondary taps?

That’s a good thing to have with a 4-10” Bassman because it’s a loud amp with all that SPL it can pump out.


With multi taps you can just run 2-10” drivers at a time for home usage.

[Chicken fingers]

Does the kits OT have multiple secondary taps?

That’s a good thing to have with a 4-10” Bassman because it’s a loud amp with all that SPL it can pump out.


With multi taps you can just run 2-10” drivers at a time for home usage.

It looks a little slim on the secondaries. I've attached the drawing of the weber OT that comes standard.

I agree that the 4-10s should put out some serious sound. I told my wife i'd only ever play it in the barn haha

That being said, how exactly could you modify the circuit to run just two of the speakers without impedance mismatch? After reading your note, i checked Rob Robinette's page to see if he had any mod's and saw he has this method: https://robrobinette.com/5F6A_Modificat ... wer_Switch - is that kind of what you are talking about?

[Chicken fingers]

I forgot to attach the weber schematic if you wanted to see that.

[LOUDthud]

What I don't like about the Weber layout is the four electrolytic filter caps on the little standup board. Where does it ground ? Not sure. Tying all the grounds of the four caps together is bad practice.

[Chicken fingers]

What I don't like about the Weber layout is the four electrolytic filter caps on the little standup board. Where does it ground ? Not sure. Tying all the grounds of the four caps together is bad practice.

It’s hard to see in the picture I attached but they have a wire going from the last cap to eyelet 5 on the top row which is grounded to the chassis. MojoTone also has all caps grounded but they then go through the chassis to the bus bar that’s mounted to all the pots. I noticed robotrobinette has the first three grounds going to HV CT and the last goes to his board mounted bus bar. Are there benefits to this versus all four going directly to same ground connection?

[LOUDthud]

I noticed robotrobinette has the first three grounds going to HV CT and the last goes to his board mounted bus bar. Are there benefits to this versus all four going directly to same ground connection?

Yes. If a cap's minus is going to a noisy ground, the B+ for that cap will be just as noisy. A quiet ground lets the noise be removed from B+ by the resistor in the B+ line. This is how Fender got their seemingly random scatter ground to work

If you don't like the pot-buss type ground, just run the ground from that cap to cathode resistor in the preamp.

[Stevem]

I did not look at the layout drawing, but there's the way it should go.

The proper way to hook up the power supply grounds is to split open the ground connection between the 2nd and 3rd filters, then run the red / yellow wire right to the ground end of filters 1 and 2.

Next run a wire from filters 1 and 2 to the chassis and then run a ground wire from the last 3 filters to ground.

For a easy to manage grounding location I like to drill a hole for a long 8/32 machine screw that's mounts firmly to the chassis with a nut and washer.

Then I crimp on wire rings to each ground wire to get stacked on the 8/32" screw.

The green yellow wire from the PT should go to ground at a separate point from this D.C. Power supply ground.

That OT does not have multiple secondary taps unfortunately.

[Chicken fingers]

Thank you guys for providing this info. This morning I’m a little distracted as I’m having a procedure today and will be hospitalized for a day or so. This weekend I’ll be back to my self and deep dive into your comments. I definitely want to make sure I understand your points fully.

[johnnyreece]

Hey, there! Just wanted to chime in and say that, while Weber's iron comes from overseas, I've not had any issues with it not being robust enough for any of my projects. Although, the prices aren't nearly as attractive as they used to be. If buying a kit, though, you don't really save that much by not getting them ($20/each). Shoot...I'd buy them from you for that if you want to use something else! Seriously, though, if you were to buy just the OT from Weber, they want $70 just for that. So, my advice would be to go ahead and include them, then sell to someone else. As long as it's more than $40, you'll come out ahead.

I hope all goes well with your procedure (and eventually, your project)!