5F2A-type amp build in pictures

Strategizer

Strat-Talk Member
Feb 13, 2021
99
PNW
I say "5F2A-type" because it's a 5F2A Princeton preamp, but as you'll see it's not an extremely faithful reproduction of the Fender original. Rather, it's my twist on it in design and style.

The short summary is that I made everything from raw components, from designing and milling the PCBs to chassis and cabinet construction and finishing. It was a fun, longer-term project that I started around February and finished in September of 2022.

It's a ~1W amp that uses the dual triodes of a 12AU7 in PP arrangement as the power section, Rob Robinette-style.


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Board design in KiCad.

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Copper-side view of the PCB.

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Component-side view of the PCB.

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Chassis and PCB.

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Full-circuit test. Everything looks good, on to cabinet construction!

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The cab is built of 5/8" hickory. The only pine I could find locally was... undesirable. All I could find was either warped boards or overpriced stock.

If you really peer at the control cutout above, you'll see some added material there. I made a stupid move at the router table which caused the top panel to rip out of my hands, break into three pieces and fly across the garage. A hard lesson learned. Thankfully my education didn't require bodily injury.

At any rate, I was able to glue the pieces back together and pack the tearout in that area with glue and sawdust. I don't have a picture to show, but the end result is a hardly noticeable repair.

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More cab assembly and shaping.

To be continued..!
 

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Strategizer

Strat-Talk Member
Feb 13, 2021
99
PNW
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Edges rounded over. After test-fitting a makeshift baffle and amp chassis I could tell that the clearance between xfmrs and speaker chassis would be tight. So at this point I decided to cut the curves into the top and bottom face panels. It turned out to be a very good choice.

Applied a couple coats of ebony stain to everything, then sanded it back to make the grain pop. Hickory has open grain and this got me the same visual effect grain filling would without as much work. And I liked the notion of the open-grained texture for this piece rather than a universally super-smooth finish.

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Dye was applied and here it is getting its first few coats of lacquer.

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Quite good for a first attempt at covering a speaker baffle! The speaker is an old Heppner. Date code says 1963 IIRC.

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It's all coming together.

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Finished product. Photos have been doctored to keep my name and location anonymous, but the subscript on the badge says "Crafted by [name] - [city], WA". To me, the badge is the thing that really sets it off!
 
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Strategizer

Strat-Talk Member
Feb 13, 2021
99
PNW
Anyone whom likes to go the extra mile with joinery has got my attention. Very nice work.

Did you CNC the copper or etch it for the PCB?
Thanks for the compliment! Building the cab was a lot of very fun work. I made many mistakes but learned how to hide them well, so I think that makes me a true woodworker now! 😜

I'm not so skilled that I dared attempt 1/4" fingers like most guitar cabinets have, though...

I CNC'd the PCB. I had spare copper stock and access to a PCB mill, so I figured I'd try a new build approach. But this will be my first and last PCB-based amp. If I ever have to service this circuit will not be easy or fun. 🤦‍♂️ Lesson learned!
 

cometazzi

New Member!
Silver Member
I CNC'd the PCB. I had spare copper stock and access to a PCB mill, so I figured I'd try a new build approach. But this will be my first and last PCB-based amp. If I ever have to service this circuit will not be easy or fun. 🤦‍♂️ Lesson learned!

Good to know, as I was thinking of doing a PCB amp at some point. As many have said, that's some beautiful cabinetry work!

Question: Have you actually calculated the output of the amp at 1W? Mine isn't PP, but SE with both sides of the 12AU7 going in parallel. I calculated 2.6W into an 8-ohm load. I know Robinette's site says "about one watt" but maybe there's some wiggle room there. That, or I don't know what I'm doing (which is possible).
 

Strategizer

Strat-Talk Member
Feb 13, 2021
99
PNW
Question: Have you actually calculated the output of the amp at 1W?
No, I haven't calculated it. Might be fun to do sometime! I was just repeating that spec from Rob's page. I've read a fair amount of his material and I trust the guy.

If you're seeking to understand why there such a difference in output between the two modes of operation, my understanding with these circuits is that in parallel SE both triodes are constantly driving the speaker, while in PP each triode is only operating half of the time. But I'm not an authority on amp building, so that statement merits scrutiny.
 

cometazzi

New Member!
Silver Member
No, I haven't calculated it. Might be fun to do sometime! I was just repeating that spec from Rob's page. I've read a fair amount of his material and I trust the guy.

If you're seeking to understand why there such a difference in output between the two modes of operation, my understanding with these circuits is that in parallel SE both triodes are constantly driving the speaker, while in PP each triode is only operating half of the time. But I'm not an authority on amp building, so that statement merits scrutiny.

Nope, you're completely right about the difference in output. And of course usually PP has higher output because it's got a larger voltage swing from end-to end.

I too trust Rob and think he's a totally swell guy, and his site taught me most I know about tube amps. I'm no authority either and I figure that I'm getting a different number because I'm doing it wrong. 😁
 

Geoff06

Strat-O-Master
Nov 15, 2021
557
Wisconsin
It looks like you've achieved a VERY nice final result!

Looking through this thread, I literally just told my wife that I want to build another amp cabinet. I was so happy (well, I still am) with how my last project came out. These things are so fun!
 


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