My lil 5F2-A build

[tubedood]

Hi all...
First a quick thank you for being a great source of valuable info to me for the last few years!
I have thrown out a few questions here and there about old builds that had been failures and about ideas I have wanted to try. It is always nice to be a member of a community that is like minded and supportive.

I decided last year to make what was to be at the time an accurate era correct old 5F2-A Princeton. I had the mindset to use as much old school parts as I could so of course I obtained a bunch of old 1 watt carbon comp resistors ( I read that upping the value from 1/2 watt to 1 watt helps reduce the carbon comp hiss and pop syndrome). I did use a few old Piher 68k input resistors I had in the drawer as that's not where I wanted the carbon comp to raise its 50+ year old fist and make noise (they might not be from that late 50's era but who can argue with a little plexi vibe going on)!

The power supply I did use new electrolytics. I upped the values a bit both for a little more filtering (and man they were cheaper) from the original 16-16-8-8 to a modern 20-20-10-10uf. For tubes I happened to have some old great testing glass, a GE 5y3GT, a Sylvania 6V6GT branded as Webcor and an old favorite of mine a good ole Sylvania branded as Baldwin grey plate 12AX7 all from that wonderful 1959 era. My quest was off to a good start!

I ordered my turret board from Doug at Hoffman Amplifiers and that was a neat experience as I made it from scratch to facilitate the few modifications I was going to go with on this build. I wanted a switchable "Voicing" on the 1st gain stage to let me use that 25uf fender bypass capacitor on the cathode or a smaller 1uf which should help if one has humbucker pickups or wants a bit more brightness in the tone.

Here is the board and a few pics of the parts to start the thread. I had a great time with the build and hope to try a harder one soon!

HoffmanTurret.JPG

Fitting Filter Capacitors.JPG

loaded board.jpg

_DSC0109.JPG

[tubedood]

The chassis I went with was also from Hoffman Amplifiers, the traditional 5F2-A chrome chassis. Shes a real looker! Here she is with the transformers I used.

The power transformer is a Weber W022772 which was a great price and offer us a 300-0-300 or 330-0-330 secondary voltage and also a 120 or 125 volt primary to adjust for your wall voltage. All this for like 45 dollars. I couldn't say no.
The Output transformer is from Allen Amplifiers (Dave is a pleasure to deal with I recommend highly) and it is the TO11S which is great for 8k (a 6V6 tube) to 8 or 16 ohm speakers and can be also used for 4k to 8 or 4 ohm speakers if you have a 6L6 or EL34 output tube. It can be driven with 95ma so it has some weight and size.
Here is the chassis and transformers.

TO11S_1.JPG

[tubedood]

For a speaker I soooo was going to grab a Rock n Roll Celestion as that name screams to me as quality but this amplifier is refined and old school remember hahaaa.

I searched the 'Bay and found a set of 4 old Fender blue frame Alnico speakers from an old Super amp from early 90's. I researched them and they are made by Eminence and have a paper former for the voice coil. Folks say they are the shiznit but they cant be for a higher watt amplifier. I don't see this as an issue in this build so I grab one.

Did I say one? Man I wish I had grabbed all four. These are great old speakers that I can already recommend for any build where your after a clean glassy sound that breaks up lightly under pressure.

Alnico Speaker 5F2-A.jpg

[tubedood]

I am not a woodworker so this part of the build was very intimidating. THE CABINET.

I was going to just order one all done but first off they were a lil pricey at $275 or so... but secondly they all had like a 6 week lead time. Time for Cheapo Depot.

I think I grabbed enough pine planks and Baltic Birch plywood to do this project (plus a few ooopsies) for like 50 dollars. It did cost a little more for the stains I used on the inside and the additional materials such as glue etc. I had practiced some nice looking box joints (practiced!) but the finished product is butt jointed and glued and screwed.

Next cabinet I will work on my woodmanship skills.

Cabinet back.jpg

_DSC0396.JPG

[tubedood]

The colors I decided on are nice I think and kind of vintage. The Burgundy I liked was called either "Buggy Whip" or also "Wine Taurus". The cream color I ordered is "Ivory Bronco". A great looking combo I must say!

Here is the Wine and the speaker grill I chose... "Fender Wheat"

Wheat and Wine.jpg

[tubedood]

The second tone mod I wanted was a feature that came from an old amp build I did in the 90's. It is a "Brilliance" function that adds a treble boost to the sound but it isn't a traditional bright capacitor that loses effectiveness as the volume is increased. It is a bypass capacitor on the cathode so the spank it adds is always present not only at lower volumes. The only problem incorporating it in this lil build is I only had two gain stages to use and the one I had available was using the negative feedback circuit which I didn't want to remove.

Here was the plan...
First off I reduced the amount of negative feedback on this lil amp by increasing the resistor value to what I had in the drawer for my carbon comps... 47k it is. That is less negative feedback right outta the gate so we have a little more drive and crunch already.

To not cancel all that effort out once the "Brightness" capacitor is deployed we have to limit its authority so it doesn't cancel it all out. You really cant employ traditional "Negative Feedback" on a gain stage that has a fully bypassed cathode.

After researching a bit and asking around I added a lil cheat, a 1k resistor limits the "Brightness" capacitors effect and keeps the negative feedback in check. That is a lil win/win.

Brilliance.JPG

[tubedood]

It took a little time to make the cabinet but I can say I am pleased with how it turned out.

5F2-A Front.jpg

5F2_A Side.jpg

_DSC0372.JPG

[tubedood]

Soooo after lighting this thing up I end up with these numbers for my voltages... I used the 330-0-330 secondary taps and my plate voltage is 334 volts. I have an 18.5 volt drop across the 488 ohm cathode resistor.
The numbers point to about 11.97 watts out of the 12 watt design of the 6V6GT tube.

I think it is spot on and the ol' Alnico speaker makes it chime and sing. On lower notes I don't have any farting out it seems ok with whatever I throw at it so far.
Once the "Brilliance" is turned on the gain steps up a hair and some sparkle coloring is added. It breaks up nicely starting about 5 on the dial (or lower if you really dig in on your playing). At 10 it is definitely loud and full of crunch but not in an 80's way but I don't think I can get that flavor out of this lil guy. I might try on overdrive pedal in front of it and see how it sounds.

Speaking of pedals do you folks find a little circuit like a Champ or Princeton a nice pedal platform to color the sound and add some versatility? I think it starts to break up a little low on the dial so maybe it isn't?
Thoughts?

Dale

PS here is the schematic for this fun build.

5F2-A '59_A.jpg

[Colossal]

Really nice work, all around!

[tubedood]

Thank you Colossal... this was a fun build and I learned a lot.

I hope to improve my cabinet work and try some box or dovetail joints on my next amplifier but I am not very savvy when it comes to wood working. I grabbed a router and I am going to practice some cabinet skills!

[Faze]

Why the 1K resistor before feeding the B+ to the output transformer. Seems like that would reduce the current to the output tube. usually its a direct feed from the first filter cap off the rectifier.

[statorvane]

Really nice cabinet. Well done!

Why the 1K resistor before feeding the B+ to the output transformer.

That is typical practice for single ended amplifiers.

[martin manning]

I agree, the voltages look great, well matched to the 8k primary, and the mods seem like they will be very useful.

Why the 1K resistor before feeding the B+ to the output transformer. Seems like that would reduce the current to the output tube. usually its a direct feed from the first filter cap off the rectifier.

It's not just the resistor, there is an extra RC filter (1k and 20uF to ground) to reduce ripple voltage and resulting 120Hz hum. Unlike PP, SE amps do not have the benefit of common mode cancellation.

To not cancel all that effort out once the "Brightness" capacitor is deployed we have to limit its authority so it doesn't cancel it all out. You really cant employ traditional "Negative Feedback" on a gain stage that has a fully bypassed cathode.

After researching a bit and asking around I added a lil cheat, a 1k resistor limits the "Brightness" capacitors effect and keeps the negative feedback in check.

There are two things going on there. One is the partial cathode bypass with the 150n and 1k, which will give that stage some treble boost all by itself. Then, the NFB is reduced at HF, the same as in a presence (or in the case of a fixed value for the FB, an "accent" switch). That will preferentially reduce low end for more apparent treble boost. Another way to go about it is to bypass the cathode resistor, connect the FB at the bottom of the parallel RC, and add a small resistance between there and ground.

[Colossal]

I found it interesting that the builder increased the output coupling cap to 0.1uF, up from the stock 0.022uF.

[M Fowler]

Very nice build and one of my favorite small amp circuits.