5F2a layout

King Fan

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Following a fun recent discussion here, I'm sharing this version. Warning: It may still contain errors, and it shows *many* choices others might make differently. I gratefully acknowledge @robrob for the original 5F1 drawing and many details; all errors and many matters of possible debate belong to me.

5F2a reference.png


If this won't enlarge enough, I'll attach a PDF below.
 

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King Fan

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Well, D'tar and I *are* very close. :)

No, seriously, if you're about to build an amp, you need to know that one. Ever been in the army? You need to KISS if don't want to end up with a bunch of SNAFUs and an amp that's FUBAR....
 

BigDaddy23

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Excellent job, mate!

I especially like the grounding and CT management. Great for the SE builder and easy to incorporate into other designs.
 

King Fan

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Thanks, all. This was a lot of fun, at least by covid standards. :)

This layout is especially aimed at new builders, to whom I often suggest the spacious, flexible 5F2a over the 5F1 Cramp. A layout alone can't contain all the *many* basic build tips you need. For folks planning a 5F2a, I highly recommend the group insights in the initial thread. The smart folks there discuss many build tips and other possible solutions.

And then there are so many other options, like the fact the bigger cab lets you run a 10" speaker (highly recommended). So also check out the many 5F2A build threads over the years -- one I especially liked for its detail and discussion was by our wise friend @theprofessor . Or recently, a nice thread by @Sylas Wojo has excellent insights about key choices, like the tricky hunt for the 'right' PT and OT. My choices in the layout are *far* from the only ones; but I decided to follow Rob's example and show an actual amp with actual parts so the actual details match up.

Another simple option would be to follow Rob's classic 5F1 layout and just add in the few 5F2a bits; tone stack, V1a bypass (which actually appeared in Champs if not on the layout), extra filtering, somewhat different OT, separate power switch....

More experienced builders will have lots of options; our smart friend @corliss1 is currently building one of these point-to-point (and sharing a great build thread).

If I drew a *more* modernized / optimized version, I'd switch both hot and neutral, install Rob's rectifier backup diodes, hang the 68Ks on the input jacks or better yet on the V1 grid pins, run shielded cable to both V1 grids, and maybe a few other things. On my actual 5F2a, I also have isolated (and explicitly grounded) input and speaker jacks, a mild NFB cut on a foot pedal, and a Weber 10A100T -- can you say '1957'?

Sonic mods? Not many. NFB tuning options are popular -- a total cut can work, but is a bit of a blunt instrument. A pot to adjust the stock resistance upwards is often suggested. Maybe an on-off switch for the V1a bypass? But this is a simple amp -- no need to go nuts.

Final thought for folks early in their build careers. Build stock, mod later. Sometimes it seems almost all of the problems we see in *so many* first builds come from mods included from the get-go. Just sayin'....
 
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theprofessor

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Thanks, all. This was a lot of fun, at least by covid standards. :)

This layout is especially aimed at new builders, to whom I often suggest the spacious, flexible 5F2a over the 5F1 Cramp. A layout alone can't contain all the *many* basic build tips you need. For folks planning a 5F2a, I highly recommend the group insights in the initial thread. The smart folks there discuss many build tips and other possible solutions.

And then there are so many other options, like the fact the bigger cab lets you run a 10" speaker (highly recommended). So also check out the many 5F2A build threads over the years -- one I especially liked for its detail and discussion was by our wise friend @theprofessor . Or recently, a nice thread by @Sylas Wojo has excellent insights about key choices, like the tricky hunt for the 'right' PT and OT. My choices in the layout are *far* from the only ones; but I decided to follow Rob's example and show an actual amp with actual parts so the actual details match up.

Another simple option would be to follow Rob's classic 5F1 layout and just add in the few 5F2a bits; tone stack, V1a bypass (which actually appeared in Champs if not on the layout), extra filtering, somewhat different OT, separate power switch....

More experienced builders will have lots of options; our smart friend @corliss1 is currently building one of these point-to-point (and sharing a great build thread).

If I drew a *more* modernized / optimized version, I'd switch both hot and neutral, install Rob's rectifier backup diodes, hang the 68Ks on the input jacks or better yet on the V1 grid pins, run shielded cable to both V1 grids, and maybe a few other things. On my actual 5F2a, I also have isolated (and explicitly grounded) input and speaker jacks, a mild NFB cut on a foot pedal, and a Weber 10A100T -- can you say '1957'?

Sonic mods? Not many. NFB tuning options are popular -- a total cut can work, but is a bit of a blunt instrument. A pot to adjust the stock resistance upwards is often suggested. Maybe an on-off switch for the V1a bypass? But this is a simple amp -- no need to go nuts.

Final thought for folks early in their build careers. Build stock, mod later. Sometimes it seems almost all of the problems we see in *so many* first builds come from mods included from the get-go. Just sayin'....
Thanks so much for doing this, @King Fan . Folks like you are the stuff of what TDPRI is made of. You share experience and knowledge freely in an attempt to make everyone's experience better. That's why I love this place.

I totally agree with what you say, too: Having built a few amps a couple of different ways and with a few different "mods," I say, "Keep it simple, stupid, and just update the power wiring (and possibly the grounding scheme)." Unfortunately for the world of amp kits that don't do this (ever?), I now believe that finding the appropriate voltage-adjusted power transformer and the correct output transformer is a critical move in getting the amp to sound right. If you do that, just build it out the Fender way, only updating the power wiring as necessary, and rock on.
 

Lowerleftcoast

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New guy question. What's the reason/benefit for the 5f2's extra filter cap vs the 5f1?
I'll take a crack at this question.
There are a couple reasons.

Two 16uF caps in parallel equal 32uF. Leo probably used two to save money. He had many 16uF in the bin. May as well put two together rather than put in a costlier 32uF cap.

Reasoning/benefit. 32uF will stiffen the bottom frequencies. I assume this was the main reason for the 32uF.
A second benefit. A 32uF cap will filter out more noise. Single ended amps are know to be noisier than PP amps so a little more filtering is good.
Two caps in parallel have lower ESR than one cap has. Again better for noise.

All good.;)
 

Timmay

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I'll take a crack at this question.
There are a couple reasons.

Two 16uF caps in parallel equal 32uF. Leo probably used two to save money. He had many 16uF in the bin. May as well put two together rather than put in a costlier 32uF cap.

Reasoning/benefit. 32uF will stiffen the bottom frequencies. I assume this was the main reason for the 32uF.
A second benefit. A 32uF cap will filter out more noise. Single ended amps are know to be noisier than PP amps so a little more filtering is good.
Two caps in parallel have lower ESR than one cap has. Again better for noise.

All good.;)

Thank you for the explanation.
 

Tom Kamphuys

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Two caps in parallel have lower ESR than one cap has. Again better for noise.

I've heard this before and I don't get it. Let's forget that for e.g. F&T caps a single 33uF has an ESR of 3.3R
and a single 15uF has an ESR of 7.4R.

Screenshot_20200907-230613.png

My real problem is that the 5Y3(GT) has an internal resistance of 50R, the OT is 8k and the dropping resistor is 10k. How can the ESR be the determining factor in the noise/hum?
 

Lowerleftcoast

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@Tom Kamphuys
IIRC, it is more about the internals of the capacitor. The resistance within the cap will heat the cap due to ripple current, causing the degradation of the cap. I think with the improvements in construction and materials this may be mute as you have pointed out. These days you can choose caps with low ESR. I am pretty sure it was more important in Leo's day.

IMO the ripple current is the determining factor but maybe the max ripple current is partially determined by the resistance within the cap.
With two caps there will be half the mA ripple being filtered by each cap so the resistance within the cap is just battling half the current resulting in half the heat.

I just took a look at a IC 10uF 450v. It is rated at 90mA ripple current at 120Hz. Many of our amps are near or past that threshold. Of course the 5F2A is not...
 
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Gazza74

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This is a great schematic! Question for @King Fan, what's the purpose of the 100k bleed resistor on the 16uf cap? Also, any possibility you have a version of this schematic with 4, 8, and 16ohm speaker jacks? Thanks Again! :D
 

King Fan

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This is a great schematic! Question for @King Fan, what's the purpose of the 100k bleed resistor on the 16uf cap? Also, any possibility you have a version of this schematic with 4, 8, and 16ohm speaker jacks? Thanks Again! :D

From Rob's amp textbook masquerading as his 5e3 mods page: “This is especially helpful when you test your amp with no tubes installed when the capacitor charge has no place to go and just sits there waiting to bite you. A bleeder resistor is a nice safety feature which can keep you from getting seriously zapped by your amp. Just solder a 100k 2watt or 220k 1watt resistor from the first filter cap's + lead (B+) to ground.”

And I don’t have a version with a multi-tap OT, but Rob’s same page may help. Me, I prefer to do this with a switch — rotary if possible — and lots of amps show that done. Rob’s 5e3 diagram shows the general idea; the NFB connects to the 8-ohm lug on the switch, and his photo there shows a rotary switch fitting in a crowded 5e3.

A5341E35-075E-4923-A33A-C957F5863D1A.jpeg

As a detail (see nice note by @Lowerleftcoast in post 14 above) that cap is part of a 2x16 = 32uF reservoir. No big deal, but something I didn’t see when I started out.
 
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Gazza74

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From Rob's amp textbook masquerading as his 5e3 mods page: “This is especially helpful when you test your amp with no tubes installed when the capacitor charge has no place to go and just sits there waiting to bite you. A bleeder resistor is a nice safety feature which can keep you from getting seriously zapped by your amp. Just solder a 100k 2watt or 220k 1watt resistor from the first filter cap's + lead (B+) to ground.”

And I don’t have a version with a multi-tap OT, but Rob’s same page may help. Me, I prefer to do this with a switch — rotary if possible — and lots of amps show that done. Rob’s 5e3 diagram shows the general idea; the NFB connects to the 8-ohm lug on the switch, and his photo there shows a rotary switch fitting in a crowded 5e3.

View attachment 899772

As a detail (see nice note by @Lowerleftcoast in post 14 above) that cap is part of a 2x16 = 32uF reservoir. No big deal, but something I didn’t see when I started out.
This is fantastic! Thanks for the information!
 




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