5F2-A with EL84 build, minor changes

[Lingwendil]

Hey y'all. I'm a hi-fi guy primarily, and a terrible musician but I build folk instruments and work on guitar amps and gear from time to time. Figured it was time to start on a simple practice amp for home use and testing. I settled on a 5F2-A for its relative simplicity and ability to be semi-portable and flexible for testing cabs/speakers, gear, etc. Plan is to make it into a very simple head style cab with walnut for a nice simple look. I'll also be making a simple companion speaker cab for it with a Celestion Eight 15 that I have on hand.

As it's about halfway junk/parts box and a few new components I made a few thoughtful changes to avoid going to crazy on out of pocket cost, with what I think are minimal compromises.

Chief changes to the original circuit-

-repurposed 7.5k to 4/8 Epifone Valve Jr. output transformer, I have a couple I've had in the bin for years to finally use up.
-Separate B+ and Filament transformers (Triad N68x, and a Fry's 6 volt)
-Solid state diode bridge rectification
-higher capacitance in power supply filtering- 47uF for first capacitor as a start. I hate hum
-changes to bright and tone capacitors (3.3nF and 1.5nF, respectively)
-NFB level switch for stock and very low feedback, adjusted base feedback to match higher gain of EL84
-impedance selector toggle switch (4/8 ohm)
-single input jack
-El84 output pentode, as I have a bunch that I never use
-Premade aluminum chassis
-adding grid stopper, and larger bypass capacitor (100uF) to EL84 cathode
-Gonna get loaded with an old school Sylvania EL84 and a Sovtek 12AX7WA to start. I have lots of Russian surplus 6P14P, and some old US 12AX7's as well.


Here's the initial mockup, chassis is an aluminum BUD Industries AC-403 9-1/2" x 5" x 2"- a real bargain at $16 from Digi-Key. Knobs are temporary until I make some nice hardwood ones or find something else I like that matches the cab once complete.

Here it is stripped and getting prepped for paint, also digging through my tub of resistors to find a few to complete the parts list-

Went with some hammertone paint to hide any scratches, dings, and it just plain looks nice. I really like this stuff. This is over a few coats of automotive primer. Still some dings in the rear, but not too bad.

More to come in another post. I'll show the differences in the tone control with my choice of parts, minor but should be noticeable. Chassis is curing by the fire at home, should be loading it most of the way with parts tonight. It was still a little tacky a day later due to the cold weather when I left it in the uninsulated workshop.

 

[Lingwendil]

Some details on the tone control scheme, I will be substituting the stock 5nF for a 3.3nF, and the 500pF for a 1.5nF. These are already fairly small values, but the changes to the tone control are relatively minor, mainly changing the "knee" of the response curve down a bit, as you can see on this online tone stack calulator- https://www.guitarscience.net/tsc/p...1=22n&C2=4.7n&C3=500p&RT_pot=LogA&RV_pot=LogA

Here's what the stock tone capacitor values frequency curves look like-

And here's the response with my changes-

Looks like relatively minor changes, and should work well. Worst case they can easily be replaced later.

I'll be modifying a schematic with the changes I make later on as the build progresses.

 

[dunner84]

Great build! FWIW I had a love hate relationship with my 5f2a for years... I played with the bias, tubes, bypass caps switchable NFB etc... What worked for me:
- Switchable NFB
- 25uf, 4.7uf, full bypass on the preamp cathode
- replaced stock tone control with Framus control
- EL34 output section
- Diode rectification (needed more volts!)

The amp is an absolute monster now.. The EL34 really made all the difference in the world though. EL84 were quite nice, but lacked a little oomph, the el34 gave me some meat and sparkle.

One other thing. I clipped in one of my pentode preamps one day, and the Vox ef86 preamp into a single ended el34 was absolutely beautiful... So good I ended up adding a "preamp in" jack, so I can play that way whenever I feel like it!

Good Luck with the build! I am curious to know how your tone mods sound.

 

[Lingwendil]

Thanks man. Good to hear that the switchable NFB worked for you. Diodes seem the way to go here too, for sure, want all the volts I can get with a 230VAC transformer! I should be able to get some good operating points without stressing the EL84 here.

How's the Framus tone control compare to the Tweed style? Looks like it wants a low-impedance drive compared to the Tweed style.

Also, pentode preamp for sure sounds fun. I have a couple 6AU6 that I've been thinking of using at some point. Might have to do an early champ style build with 6AU6 and 6AQ5 for fun at some point.Would be a killer clean amp for home use I think.

 

[cometazzi]

Definitely a cool build, and a great writeup. Thanks for sharing! Thanks also for the link to that TSC.

Though your tub o' resistors kinda gives me the heebie-jeebies

meat and sparkle

Dibs on the album name.

 

[Lingwendil]

Not a whole lot of time tonight, but I had an idea at work on a way to label the switches. I grabbed some stainless washers at the hardware store in the way home, and used a set of letter punches to put some quick labels on the switches. Proof of concept is promising, if not a little jerky and serial-killer looking

I also got some of the rest of the parts installed before daddy duty.

I think a little more practice to get the letters aligned nice and clean and they'll look great. Still want to think of a better label for the feedback switch, not sure I'm happy with "LOW" and "HIGH". Maybe "FB ON" and "FB OFF"?

I really like the 4/8 switch though.

I think if I stick the washers on a bolt, and spin them in the chuck of a drill I can polish them with some scotch brite or steel wool.

 

[wangdaning]

Maybe one side NFB and the other side just left blank.

 

[dunner84]

My NFB switch is labelled stock and untamed...

 

[mountainhick]

Maybe "FB ON" and "FB OFF"?

Simpler: "FB" in on position and "OFF"

 

[Lingwendil]

Simpler: "FB" in on position and "OFF"

Brilliant. Why didn't I think of that

I initially thought "TIGHT" and "LOOSE" but not enough room for these punches without it bunching up and looking sloppy

 

[Lingwendil]

Grabbed some walnut with some nice cream color on one edge of the board, and some nice charachter, so i chopped it up and now the simple cab is clamped and glued, gonna dowel the joints and round over the edges. Gonna build something a bit fancier for the next one once I can put together a box joint jig or something for some nicer joinery.

 

[Lingwendil]

So, small update. First run of the amp went swimmingly, and it sounds great.

Some quick notes-
-I'm only getting 240 volts after rectification. Works for me as this will be a home/studio/practice amplifier, so I'm happy. Doing some math I should be getting 3, maybe 3.5 watts or so. As an original Champ/Princeton is about 5 watts and already too loud for some it should be a non-issue. Sure doesn't hurt the sound!
-due to the lower voltage I used a mosfet ripple filter PCB of my own design for the entire power supply. It made it nice and easy to load out and test. Really happy there.
-the wood I clamped for the cabinet shifted in the clamps and will need to be rebuilt. No big deal but a little irritating.
-my tweaks to the caps in the tone control sound great and work well.
-dead silent at idle, only hum comes as soon as the guitar cable is inserted at the guitar. Gotta look at that cheap guitar now!


Some minor issues-

-Two bad switches, had to replace both the NFB and impedance toggle switches. Cheap ten pack from Amazon I've had forever. Easy fixes and verified functionality after. Switchcraft next time.

-During the overall function check shows that the NFB switch does not cause an audible difference in volume or breakup, interestingly. The way I implemented it is a 150k feedback resistor, and switching in a 56k across it to show the higher overall feedback level. I'm thinking that maybe I need to pull the 150k to see if there's more of a difference. I'm using a higher value to account for the higher gm/mu of the EL84. I feel like maybe since there's not much feedback to begin with in this design I can actually go smaller on the feedback resistor overall. Maybe a 22~27k switched in parallel with the 150k would work. Alternatively I can change the switch to add a bypass cap to the cathode instead.



So far so good! Gonna do some more tweaks but I'm super stoked. Now I need to build a speaker cab and get a real guitarist over here to check it out.

 

[2L man]

Perhaps because of lower output power the NFB is not efficient using values which work for higher power? My thinking here is that there come lower NFB signal and assuming the stage which expect the signal its components are the same for higher signal. I hope someone who understand NFB would confirm or repeal it?

When tuning the NFB an oscilloscope is handy. First without NFB output voltage preferably to resistor load is messured and then NFB is connected and how much output drops define the NFB. When voltage drop to half it is 6db. When output drop to 0.707 is 3db. I have a hunch that typical guitar amp NFB effect is closer to 3db than 6db.

If you don't have oscilloscope you could try multimeter AC voltage measuring? Or you can trust what you hear? You could test cellphone db application as well.

 

[Lingwendil]

Yeah, I'm gonna do a test tone into the amps input with a multimeter at the output to check for function. I'll try a stock 22k feedback resistor as well and see. My gut feeling is that the feedback now is too at the low setting to make a big change when flipping the switch.

 

[Tele-Mania]

Nice build.

 

[Lingwendil]

Still haven't had a chance to adjust the feedback resistor, so this is running "wide open" but this is a killer build so far. My father (Tony Flying Squirrel) was in town from Washington state so I had him play with it for a good hour or so.