Thinking of building an amp kit - Page 4

Bluside · #46 01-02-2021, 05:57 AM

Quote:

Originally Posted by clintj

Sure do. The vintage one is via the internet (I lost a whole lot of pictures during a phone upgrade a few years back where I had gotten firsthand pics) and the modern board is where I picked up a build someone else started and got frustrated and had me finish it for him. On the vintage amp, it's the huge cap on the far right end.

Yes, I did a quick search and found some other pictures on the internet after you posted your first comment showing the capacitor. I wonder how and why this got eliminated? Thanks for posting.

clintj · #47 01-02-2021, 06:35 AM

Quote:

Originally Posted by Bluside

Yes, I did a quick search and found some other pictures on the internet after you posted your first comment showing the capacitor. I wonder how and why this got eliminated? Thanks for posting.

There's suspicion it was one of those factory floor mods that Leo Fender or Freddie Tavares made after production started, but no one has any evidence of who did it or why. No tech I know has seen an original without that part.

The 59 and 60 Bassman apparently also differ ever so slightly from the official documentation in a couple of minor ways, just a couple of component values here and there. It's still an amazing amp when built to plan.

redir · #48 01-02-2021, 10:01 AM

Clint can you explain what that cap actually does? I mean, I think I know what a cap does but what does it physically do in that circuit to cause the overdrive?

Could it be that they eliminated it in later models to clean the amp up then?

clintj · #49 01-02-2021, 02:06 PM

Quote:

Originally Posted by redir

Clint can you explain what that cap actually does? I mean, I think I know what a cap does but what does it physically do in that circuit to cause the overdrive?

Could it be that they eliminated it in later models to clean the amp up then?

That cap does a couple of things. One, it provides a way to send any noise in the cathode circuit to ground and reduce it (capacitors are transparent to AC signals like noise, but block DC). This helps reduce background noise and any heater hum a bit.

Second, it increases gain by stopping what's called cathode degeneration, which is basically a form of negative feedback. This roughly doubles the gain in that stage, which helps push the next stage and power tube harder. The amp still works, but you won't get to Joe Walsh or "Layla" levels of breakup without some assistance like humbuckers or a boost in front of the amp.

All evidence points to his being an error on the documentation or basically an "oops". The previous model Champ, the 5E1, has it. The later blackface Champ, AA764, has it. Every late tweed Champ that techs I chat with have seen has it, and it may very well be a "Hey, I forgot something" change by the designers once production started.

If you want to see what happens and if you like the change, you can clip one in or tack solder it in. Just watch polarity, it should be negative towards ground. If you like it, make it permanent. If not, pull it back out.

Bluside · #50 01-02-2021, 02:54 PM

Quote:

Originally Posted by clintj

That cap does a couple of things. One, it provides a way to send any noise in the cathode circuit to ground and reduce it (capacitors are transparent to AC signals like noise, but block DC). This helps reduce background noise and any heater hum a bit.

Second, it increases gain by stopping what's called cathode degeneration, which is basically a form of negative feedback. This roughly doubles the gain in that stage, which helps push the next stage and power tube harder. The amp still works, but you won't get to Joe Walsh or "Layla" levels of breakup without some assistance like humbuckers or a boost in front of the amp.

All evidence points to his being an error on the documentation or basically an "oops". The previous model Champ, the 5E1, has it. The later blackface Champ, AA764, has it. Every late tweed Champ that techs I chat with have seen has it, and it may very well be a "Hey, I forgot something" change by the designers once production started.

If you want to see what happens and if you like the change, you can clip one in or tack solder it in. Just watch polarity, it should be negative towards ground. If you like it, make it permanent. If not, pull it back out.

Great info. I may give it a try just for fun. Thanks.

redir · #51 01-03-2021, 03:05 PM

Thanks clintj. That's great. I'm still trying to figure this stuff out. I did learn some things building this kit but basically it's just following a recipe and in the end you have a loaf of bread but you don't really understand how gluten works or why you should let the dough rise and so on.

What I would love to see is a video or explanation of an amp circuit that is described from the input all the way through but I've not found anything like that. I suppose I'd have to go take a class or something. But with the kit's basically solder this thing to that thing and this one to that one but no real explanation of why. But it's still VERY cool though.

Mojo Tone has been known to be a bit sloppy with their layouts. I wonder though if the schematic actually had that cap in it?

acoustigoat · #52 01-04-2021, 11:19 AM

Quote:

Originally Posted by redir

What I would love to see is a video or explanation of an amp circuit that is described from the input all the way through but I've not found anything like that. I suppose I'd have to go take a class or something. But with the kit's basically solder this thing to that thing and this one to that one but no real explanation of why. But it's still VERY cool though.

Mojo Tone has been known to be a bit sloppy with their layouts. I wonder though if the schematic actually had that cap in it?

It's not in video format, but Rob Robinette's site describes how amps work and traces the signal path through the Champ 5F1 circuit: https://robrobinette.com/How_Amps_Work.htm

Also, most people on the TDPRI subforum for amp building rely on Rob's explanations, schematics, and wiring diagrams rather than just going by Mojotone or Stewmac instructions.

redir · #53 01-06-2021, 06:44 PM

Quote:

Originally Posted by acoustigoat

It's not in video format, but Rob Robinette's site describes how amps work and traces the signal path through the Champ 5F1 circuit: https://robrobinette.com/How_Amps_Work.htm

Also, most people on the TDPRI subforum for amp building rely on Rob's explanations, schematics, and wiring diagrams rather than just going by Mojotone or Stewmac instructions.

Yes that site is fantastic. I guess I am still interested in even more detail.

Bluside · #54 01-07-2021, 06:03 AM

I'm toying with the idea of putting my new 5F1 Champ build into a cabinet with a 12" speaker.

Has anyone done this or is it best to just leave it in the 8" cabinet?

clintj · #55 01-08-2021, 02:04 PM

Depending on the speaker it may get a bit boomy. The Champ circuit has fairly weak power supply filtering and no low end cut at all. I've played one as a head through a Marshall JCM800 series slant 4x12 and it was pretty impressive, though. Huge sounding combo. Worst you can do is not like it, right?

Bluside · #56 01-10-2021, 06:57 AM

[QUOTE=clintj;6599011]Depending on the speaker it may get a bit boomy. The Champ circuit has fairly weak power supply filtering and no low end cut at all. I've played one as a head through a Marshall JCM800 series slant 4x12 and it was pretty impressive, though. Huge sounding combo. Worst you can do is not like it, right?[/QUOTE]

I just ordered a Weber 4 ohm 12A125 speaker and a new cabinet to with it. We'll see.

sam.spoons · #57 01-10-2021, 09:01 AM

Once you've done the required reading you could probably mod the amp to better suit the 12" speaker

I keep meaning to try to build a 10" into my Epi VJ combo, it should just about fit with a couple of mods to the speaker chassis but I might just build a 110% copy of the cabinet to gain the extra space instead.

Bluside · #58 01-12-2021, 10:28 AM

EDIT...Disregard this question. I desoldered the old and put the new jack in today using the chassis as a ground and it works perfectly.

I just want to make sure I do this right. I want to change out the RCA speaker jack to a ¼" jack to be able to use different speakers a bit easier.

On this one, there are 2 yellow wires going to the jack tip and the ground is provided through the chassis. Should I do the same for the ¼" witchcraft jack and just wire up the tip or do I need to add a ground wire to the new jack instead of using the chassis?

redir · #59 01-13-2021, 09:34 AM

It seems to me that yes, you should wire it the same. The RCA jack is nothing but hot and ground as is your Switchcraft jack, no stereo switch or anything like that.

Maybe clintj will chime in, don't take my word on it

But it does seem to make sense.

printer2 · #60 01-13-2021, 09:34 PM

Quote:

Originally Posted by redir

It seems to me that yes, you should wire it the same. The RCA jack is nothing but hot and ground as is your Switchcraft jack, no stereo switch or anything like that.

Maybe clintj will chime in, don't take my word on it

But it does seem to make sense.

Before Rob started his website there was a forum that did an EL84 champ type amp. They also developed the first 12AU7 1W amplifier before everyone else. They have an amp that this documentation was written for but any basic single ended amp is kin to this documentation. You might find it instructional.

http://ax84.rru.com/media/ax84_m35.pdf

http://ax84.rru.com/p1.html

The AX84 project was more Marshall inspired and they put the tonestack behind the second stage.