Proofread another 5F2a layout, please

[King Fan]

So in March (which seems like an age ago -- what a long strange trip it's been) I ran a thread where I tried to draw my first-build 5F2a as it ended up, mods, mistakes and all. Humorous but humbling to see how much I couldn't remember and couldn't reconstruct from photos.

I decided the layout might be more useful to someone someday if I drew it to be more generic -- no sonic mods, but some circuit modernization.

So after a few more hours in DIYLC, here's a first draft. First draft. Not final. Not correct. Not for saving, sharing, or building amps from...

If the image doesn't blow up enough, here's a Dropbox link to the full-size JPG.

As always, thanks to @robrob , whose kind sharing of his great DIYLC files gives me a framework and lots of ideas.

Notes:

I've moved the HT CT to the reservoir negative pole as Rob has now started doing (more info here), and furthermore now return the ground wire from the second cap, which I *think* was suggested as an incremental improvement in a recent thread (see posts 11 and 14 here).

I kept my shielded inputs to the V1 grids. Not necessary, though. I kept my 68K grid stoppers on the input jacks for the same reason. Not necessary. I show an elevated heater CT, because it's so easy in this amp. I kept my MF resistors in the input chain because it does really reduce hiss. Not necessary. I show a 10W bias resistor -- 5W or less can be used -- and made mine wirewound because quiet and robust. Not necessary.

Questions:

Any chance I actually got the grounds right from the output tube bias resistor and the NFB sector? As you'll know, this is not how Fender did it, but I'm trying to keep the output tube grounds together and the preamp grounds together.

And what did I screw up?

 

[James Knox]

Thank you Sir for doing this (my next build)!

#watching

I like the shielded cables running underneath the board and the elevated Heater CT to Pin 8 of the 6V6.

For your consideration, I recently mined a nugget from the archives where @hackworth1 shared a tip he learned from @muchxs. The Red Lead from the OT goes to Pin 8 of V3 (5Y3) to avoid the crowded Eyelet/turret of the first filter cap. I used that hack on my 5E3 and my last 5F1.

 

[screefer]

subscribed.
I was just reading your 2019 thread regarding switching both hot and common with a dpst to avoid the 'splice'.

I see you are grounding the power tube cathode r8/c6 to the HT/B+1 like Rob has it here...
https://robrobinette.com/images/Guitar/HowAmpsWork/5F1_Layout.png
and yet in his signal path image it is connected to the preamp bus..
https://robrobinette.com/images/Guitar/HowAmpsWork/Signal_Path_5F1.png

Both Mojotone's kits for 5f1 and 5f2a have it going to the preamp ...
https://923962.app.netsuite.com/cor...uM9mQR7hrcIb211;1598494456;622281000&gc=clear
https://923962.app.netsuite.com/cor...2lnAMc0D8zXBTZD;1598475600;693285000&gc=clear

This confuses me but I'm pretty green yet.

 

[monkeybanana]

I will have to try the CT to the filter caps. Thanks for sharing. Your wiring is so neato!

 

[King Fan]

subscribed.
I was just reading your 2019 thread regarding switching both hot and common with a dpst to avoid the 'splice'.

I see you are grounding the power tube cathode r8/c6 to the HT/B+1 like Rob has it here...
and yet in his signal path image it is connected to the preamp bus....

Both Mojotone's kits for 5f1 and 5f2a have it going to the preamp ...
.....
This confuses me but I'm pretty green yet.

You're right. I didn't include hot/neutral switching here, but it is the way I'm building 'em now -- safer, more UL, easier (no splice), prevents mistakenly switching only the neutral on SPST, and the Carling DPST switches are more robust. They're a bit bigger, a factor in a tweed chassis, but the smallish PT here should allow clearance, so it's an option to consider for sure.

As for reference layouts, Rob improves his here and there over time, so they can't be 100% consistent. Mojo and other kit makers are good companies, but they're not amp design experts; kit layouts have shown everything from undocumented mods to outdated solutions to plain mistakes; mostly it's a matter of less ideal solutions.

Not something to worry about, just to think about. Our beloved Fender originals did a lot of things totally 'wrong' in modern safety and sonic terms, and they sounded great and only killed people on rare occasions....

 

[King Fan]

I'm working on possible revisions around the gathering of the power amp grounds (like a gathering of the clans, you don't want to seat Campbells next to MacDonalds). But it's slow going -- read some Merlin, go for a walk, re-read the same Merlin...

In the meantime, would it be better to keep it simple? This is a small vintage amp, ideal for first builders, and will sound much the same with any reasonable ground scheme. If we keep the HT-to-reservoir negative a la Rob, is this right? Or should the 6V6 grid ground come back to the first node, since it's also the cathode ground?

 

[Nickfl]

If you're going to move the input grid stoppers off the board why not move them onto the socket instead of the inputs? Either way you've got them hanging on one end, so why not place them where they will do their job the best?

 

[andrewRneumann]

Or should the 6V6 grid ground come back to the first node, since it's also the cathode ground?

I tried to follow Merlin and I grounded the power tube grid leak and cathode to the screen supply filter ground just like you have shown. If you’re reading Merlin, check out fig 15.7 on pg 268.

There’s a great sentence in the preface btw, “...this book is mainly biased towards designing new preamps, rather than venerating old ones...” (emphasis his)

 

[King Fan]

If you're going to move the input grid stoppers off the board why not move them onto the socket instead of the inputs? Either way you've got them hanging on one end, so why not place them where they will do their job the best?

Absolutely. Great question. I've always wanted to do that, in fact; by chance, it was pointed out to me as best practice before my first build. Didn't know about isolated terminal posts and such back then, didn't want to hang them in space. Hmm, would you even need to use shielded cable? Two shielded cables? Quad cable? Plain wires? I'd need to think about mounting Fender's paired 68Ks down there, but it couldn't be harder than fitting them between jack and cable. Hmmm, now I think of it, I don't recall ever seeing a Fender build that actually did it. Musta seen it, don't recall seeing it.

Which makes me switch gears a bit. For *this* project, which was intended to be a general-purpose, bog-standard 5F2a layout, shielded cable and displaced grid stoppers might just add confusion. The idea here would be a generic, standard 5F2a layout for new builders. Why? I'm one of the guys who always say, "Don't do a cramped 5F1, spread out in the roomy (OK, less cramped) chassis of a 5F2a" -- and then feel bad when folks start to puzzle over the changes to layout and wiring. Not that many changes, but we're talking about new builders. I remember just how long it took me to 'grow' my first 5F1 kit into a 5F2a.

 

[King Fan]

I tried to follow Merlin and I grounded the power tube grid leak and cathode to the screen supply filter ground just like you have shown. If you’re reading Merlin, check out fig 15.7 on pg 268.

There’s a great sentence in the preface btw, “...this book is mainly biased towards designing new preamps, rather than venerating old ones...” (emphasis his)

Heh, forget veneration; even plain, safe re-creation is a good goal. Right in step with where I'm going these last few posts; just build a safe, simple modern 5F2a, not a perfect one.

Thanks for the validation on the power tube grid-to-ground anchor. And thanks for the quote; I'm gonna remember that whenever Merlin's perfect becomes the enemy of my good.

 

[Lowerleftcoast]

I know what you are thinking when you show a layout with big red letters that say DRAFT right next to the word *light*. lol.

For *this* project, which was intended to be a general-purpose, bog-standard 5F2a layout, shielded cable and displaced grid stoppers might just add confusion. The idea here would be a generic, standard 5F2a layout for new builders.

If you mean what you say, get rid of the ground bus and put the 68k resistors back on the board. I can see leaving the center taps. The preamp grounds can be serviced with two terminals. No need for some two terminal bus the noob would have to scrounge up. Just let him/her use a wire.

You can include an optimum version or three showing the fancy changes as optional design choices.

 

[Lowerleftcoast]

 

[FenderLover]

LLC is on the right track, let's see how simple we can actually get it. Go one step farther and loose the Power Amp "Ground". The power supply return for the power amp is the (-) terminal of the reservoir cap, not the chassis (ground loop). The chassis currently only connects the (-) terminals of the filter caps (the long way around, from the input jack). Loose the Power Amp "ground" to the chassis and connect the filter cap (-) terminals with a short wire instead. Isolate the speaker jack and fix that ground loop too.

I know I sound like a broken record, but it would be fun to see a (Merlin certified) layout like that.

 

[Lowerleftcoast]

Go one step farther and loose the Power Amp "Ground".

Are you suggesting a *one star ground scheme*?

 

[FenderLover]

Star ground or bus, the point is that the chassis is not used as an electrical connection between preamp and power amp. The chassis is referenced to the circuit ground in only one location, at the input, using Merlin parlance. The circuit "ground" is the power supply return, not the chassis. The chassis is for shielding and safety ground.

 

[King Fan]

If you mean what you say, get rid of the ground bus and put the 68k resistors back on the board

I read you loud and clear. In fact I drew a version with the 68Ks on the board this morning, which also lost the (hard for new builders) shielded cables.

It's shocking, perhaps shameful, but I've never built an amp with the V1 grid stoppers on the board, or without shielded cables, or without a big hunk of tinned square ground bus (LOL I think I ordered 10' on my first order).

let's see how simple we can actually get it. Go one step farther and loose the Power Amp "Ground". The power supply return for the power amp is the (-) terminal of the reservoir cap, not the chassis (ground loop). The chassis currently only connects the (-) terminals of the filter caps (the long way around, from the input jack). Loose the Power Amp "ground" to the chassis and connect the filter cap (-) terminals with a short wire instead. Isolate the speaker jack and fix that ground loop too.

I know I sound like a broken record, but it would be fun to see a (Merlin certified) layout like that.

A how-simple-can-you-get-but-also-Merlin-certified amp? Military intelligence? Jumbo shrimp?

No, seriously, I know what you mean. In fact, my first build 5F2a had quite a bit of Merlin-y goodness: isolated input jacks, isolated speaker jack, a unified bus.... Well, unified, but grounded at both ends. Weirdly, it worked fine. I always remind myself we're competing against a boatload of local, random chassis grounds soldered to a brass plate.

As I say, my first build; I'd read every possible article about advanced grounding, and understood barely a word. In fact, FL, IIRC, LOL again, you were the one who pointed that this year it should be grounded only at the input end, which I went back and did. So, in many ways the build I'm simplifying here is moderately Merlin-ized. As you say, fun.

But I'm starting to realize if I want to draw a 5F2a for first-time builders, many of whom come to this thinking they're gonna build a 5F1 and reading Rob's 'how an amp works' about a 5F1 and studying his drawings of a 5F1, then in addition to the KISS thing, I sorta want to stay in the 'Robrob lane.'

 

[Lowerleftcoast]

...I've never built an amp with the V1 grid stoppers on the board, or without shielded cables...

Neither have I. I have included grid stoppers right on the socket of PCB amps I have repaired as well.

Like I said, there should also be <other versions> of the layout so a noob could try to see why it would be preferable to change to an alternate layout.

 

[James Knox]

But I'm starting to realize if I want to draw a 5F2a for first-time builders, many of whom come to this thinking they're gonna build a 5F1 and reading Rob's 'how an amp works' about a 5F1 and studying his drawings of a 5F1, then in addition to the KISS thing, I sorta want to stay in the 'Robrob lane.'

I think you are on to something here. You are kinda the 5F2A Evangelist around here and guys who decide to “move on up” to the “better” 5F2A will not find a good “RobRob” like layout. As you say, the closest is Robs 5F1 Layout. So first-timers try to build a 5F2A off of a 5F1 Layout. Or, an incorrect And/or confusing Mojo, Weber, etc.

I like @Lowerleftcoast idea of putting up the “basic but correct” version, then follow up with a “modded” version!

 

[andrewRneumann]

My two cents...
How about the reference to the outer foil on the CC's? Is that an unnecessary complication for "noobs"? (Seriously, I'm still a noob...) I agree with whoever suggested getting rid of the ground bus. I think Merlin's grounding is actually the simplest. No chassis lugs... just connect one end to the HT- and the other to the input jack. It doesn't seem much easier than that. For the sake of simplicity, I think I would just leave the speaker jack grounded on the chassis. Not perfect, but simple and has it ever caused a problem in this application, especially if we get the major sources of current out of the chassis?

The only pain I've run into with Merlin's scheme is that it can get crowded at certain nodes. My MC10 driver ground has 2 stone stack grounds, 1 volume pot ground, the speaker ground, NFB pot filter ground, and the cathode resistor. In this case, I was glad to have a bus wire. I haven't done eyelet construction, so y'all know better than I how many leads you can get into an eyelet. Don't think it will be a problem on the Princeton.

 

[King Fan]

Hmmm, you're right, a single bus terminating on an input jack (isolated jacks may also be a bridge too far for brand new builders) would certainly be simple. We could do the 'right way' as noted by @FenderLover but also keep it simple. Simpler. Maybe we can look beyond Rob's (very functional) 5F1 split grounds. After all, he goes all unified on his 'optimized 5E3 layout'.

But, hmm, would I build a unified bus without using bus bar? (Shakes head, looks deeply pained....) No. Maybe time for one more bit of Merlin:

"To make the bus bar, tinned-copper wire can be bought on the roll, but a piece of stripped 24A or 32A solid-core mains wire is a cheap alternative in Europe. In the US, 14AWG solid-core may also be readily available..." (think Romex).