Two dumb but related questions: 5F2a impedance and NFB

[King Fan]

Question 1: What was the (most common) 'native' output impedance of the 5F2a? Complex but informative answers in this 2015 thread suggest to me it was (most often) 8 ohms, though, Fender being Fender, 4 was also possible, especially in late production.

Question 2: If it *was* 8 ohms, and yet the 5F2a kept the 5F1's 22K NFB resistor, then of course that's an obvious reason the 5F2a is somewhat less wild and wooly than its little brother. And also the obvious reason folks like to bump up the Princeton's NFB resistor, often on a switch, to get a little hair back, noting a full cut is pretty extreme.

Which also suggests a possible answer to the common question of how much to bump up that resistor. Rob's "Whole Earth Catalog" of amp knowledge disguised as a 5E3 mods page tells us that "When changing the feedback source from one output transformer secondary to another you change the feedback resistance by a factor of 1.41 for one step change (for example going from a 2 ohm speaker tap to 4 ohm)"... where 1.41 of course is SQRT(2). If so, and if I'm right, to get Champ feedback in an 8-ohm Princeton you'd want about 30-33K? I get the feeling that values up to 50K are often used, especially if on a pot.

 

[King Fan]

And an informal poll: If you own an original or clone 5F2a (and please tell us which) what's the OT/speaker impedance? I'll start; my clone is 8Ω.

 

[echuta13]

I've built a few 5f2a's at this point? I can't comment too much on stock values other than I think the Vibro-Champs I have are loaded with 4 ohm speakers as well? I've never liked the NFB as it stands with a 22K value. The amp sounded like it had a wool blanket over it. Messing with values, I found that I liked a 100K in that spot or completely removed. Without it, the amp definitely pushes a more prominent mid range and is overly gainy. I usually change the volume pot to a 500K to compensate for this plus I add a grid stopper to the second triode and use a 10uF bypass cap on the first triode as part of the tuning process (full but firm bottom). I recall just putting a pot in it's place with the 22k inline to figure out where the sweat spot was for me. This is just my experience with the 5f2a and tone as always.. is a subjective thing.

 

[Bitsleftover]

King Fan, You have an uncanny knack of starting discussing the issues I was just about to ask about.
Ive read dozens of threads relating to 5F2a NFB over the past few weeks and am not really any closer to finding a definitive answer. Probably because there isn’t one.
I’d come to the same conclusion as you really.
Generally speaking though, I only really like NFB if its feeding a presence circuit. Every time I play an amp with a switch, I find myself turning the NFB off.
But then my tastes are about as subtle as a house brick!
I’m going to include a switch on my current 8ohm build so Ill be watching this thread with interest.

 

[King Fan]

Messing with values, I found that I liked a 100K in that spot

All excellent points. Your experience agrees with mine. In fact, I ended up adding 51K on a switch, so my 'more hair' option is 22K + 51K = 73K -- and it doesn't get so much super-hairy as louder and more present, moving edge-of-dirt more clearly onto the outfield track.

Another data point is the GA-5, which is almost identical to the 5F2a except for a few resistors -- including it's gainier 2.2K preamp cathode resistors and 47K NFB resistor. Looking at crude NFB ratios, the 5F2a is at 14.7:1, while the Gibby is at 21:1. Of course, 21:1 would put the 5F2a exactly into that 30-33K zone we got from the SQRT(2) thing. OTOH, the GA-5, being gainier, doesn't need an NFB cut as much....

 

[King Fan]

Generally speaking though, I only really like NFB if its feeding a presence circuit. Every time I play an amp with a switch, I find myself turning the NFB off.
But then my tastes are about as subtle as a house brick!
I’m going to include a switch on my current 8ohm build so Ill be watching this thread with interest.

We're both just surfing the zeitgeist, my friend, and sharing the same wave once in a while.

in my limited experience, the effect of a full NFB cut varies a lot from amp to amp, both with the amp's native personality and its original NFB ratio. My 6G2 loves a full cut, my PR mostly hates it; the most common reaction I've heard on the 5F2a is that the full cut's a bit too much. You may be the perfect candidate for that pot (50K?, or better maybe 100K) in line with the 22K. Then, as smart folks always say, once you find a pot setting you really like, you measure that resistance and put a fixed resistor of that size on a switch, so you don't have to fiddle around with a knob to find your tone. But I'm sure you know that already...

 

[Ten Over]

Do both OT primaries have the same impedance?

 

[muscmp]

i have a 59 princeton 5f2a and it is 4 ohm, actually 3.4 ohm. it has the 125a2a OT which apparently was used more in the harvard. 606943 schumacher code. when i got it there was a 65 utah in it that i've replaced with a great weber.

PT is triad 66079.

i keep thinking about adding nfb but it sounds so good as is. i mainly use it for harp.

play music!

 

[King Fan]

Do both OT primaries have the same impedance?

That's a good question, whether about the two (or more) OTs Fender put in the Princeton and 6G10 'Harvard' over the years, or about the 5F2a vs. the 5F1 (again, did the Champ get more than one?), or about the 5F2a vs. the GA-5. I dunno, in any of these cases. The Fender OTs may have data available; their model numbers are as per @muscmp, altho I'm not sure there wasn't a third one in play. The GA-5 PT and OT both had really helpful model numbers (not): GA5-P and GA5-O.

In fact, 'what primary winding?' would be another great 5F2a question. The advice I got when I built mine was that 8K:8Ω was a 'good' setup for the 5F2a. At any rate, it's what I have, and I'm sticking with it. But it'd make a difference in this whole question.

 

[jsnwhite619]

One of my threads a long time ago discussed the NFB value for the 8R speaker. It was mid-30k range I think. I tend to use a 56k if I'm not adding a switch, handles humbuckers ok still at that point. That and a .033uf cap up front can really make it sound bigger. I prefer an 8k primary with 6v6, and stick with 8R OT/speaker just for available options.

 

[echuta13]

I tend to use a 56k if I'm not adding a switch, handles humbuckers ok still at that point.

That was the reason I had to cut the gain a bit in the 5f2a's & add a screen resistor (with no NFB or the 100k in there). It became a fart-beast with humbuckers!

 

[Bitsleftover]

I've built a few 5f2a's at this point? I can't comment too much on stock values other than I think the Vibro-Champs I have are loaded with 4 ohm speakers as well? I've never liked the NFB as it stands with a 22K value. The amp sounded like it had a wool blanket over it. Messing with values, I found that I liked a 100K in that spot or completely removed. Without it, the amp definitely pushes a more prominent mid range and is overly gainy. I usually change the volume pot to a 500K to compensate for this plus I add a grid stopper to the second triode and use a 10uF bypass cap on the first triode as part of the tuning process (full but firm bottom). I recall just putting a pot in it's place with the 22k inline to figure out where the sweat spot was for me. This is just my experience with the 5f2a and tone as always.. is a subjective thing.

If you don’t mind me asking, what size grid stopper are you using on the second triode? Is that the 100k you mention?

 

[Mongo Park]

Yes a good point to iron out.
I build the 5f2a with a bigger output transformer with a 8 ohm 12” speaker and have gone with the standard 22k NFB resistor. I put the NFB on a switch. I find no NFB is a ok option, must be the bigger op transformer and speaker that is making this a useable option. It provides distortion but not so much it is poop. I certainly will try different resistors to see if this will expand the range of clean. With the switch and a higher value NFB resistor I should be able to get some more clean on up the volume dial, which would be a good thing.
I use a op transformer which is rated for 10 watts or maybe more. The concept the the under rated op transformer adds to the charm of the amp never appealed to me. I am guessing the bigger op transformer is giving me more clean as a baseline as well as keeping the bass from farting out. Also allowing the no NFB to be within a useable range. This also seems to keep from having to add grid stoppers that some people use
I guess a little less germaine to the thread but maybe more FYI.

 

[King Fan]

I build the 5f2a with a bigger output transformer with

Right, the *size* of the OT is yet another 5F2a variable. I have the simplistic idea the original OT was upsized from the Champ, even when the Princeton ran an 8" speaker. Was/were the vintage OTs ~15W? I thought the 'official' real-world differences from the 5F1 were the tone knob, the extra filtering, and the bigger OT. I had the impression the filtering and especially the OT size were bigger factors than the NFB thing on sonic differences from the Champ... EDIT: oh, and the cab size, duh.

Of course speakers are always a dominant variable, especially size here (the 6G10 10" is super-popular in modern Princeton clones), but also, in the 5F2a, even more than overall performance, bass handling?

Since speakers are always a huge wild card, I'll just add OT size to my informal / occasional poll. Mine happens to be nominal 15W.

But hey, I know some folks here (@jsnwhite619 ?) also increase filter sizes in small Fenders. So many variables! It's almost like amplifiers were more art than science. But nah, that could never be...

 

[echuta13]

If you don’t mind me asking, what size grid stopper are you using on the second triode? Is that the 100k you mention?

The 100K was the pot for the variable NFB, but up to 100K is okay for the grid stopper. 33K-40K would be in the realm of what I would use. You're adding reverb into yours and I've seen quite a few schematics use a 100K with the reverb insertion points before and after it.

I'm running slightly beefier O/T's like Mongo Park as well (and 12" speakers). I can't really say there's that much 5f2a left in most of the builds (but it is in the DNA so to speak).

I think my first 5f2a build had 5 pots, and 2-3 switches?!? Once I dialed in settings, most of them were set & forget.

 

[Mongo Park]

Yes l did think of mentioning the filter caps but that is a whole nuther topic I think.
So yes op transformer and NFB resistor value are a choice that should be made together as one effects the other, more so to me anyways. As for the 5f2a being changed so much I like to think what would Leo likely done without money/limited technology restrictions, what parts would he changed. So I think of this thread through that lens. I think it is a given such things would have occurred, but sorting out what the original proper value would be for an 4/8 ohm speaker is need befor it can be built on, as this thread is titled.
So now I have to go back and try a pot or various resistors in the NFB position, to extrapolate for the bigger op transformer and build form there.
I am not really in tune with the exact original op transformers values used.
Filter caps for another day.
As far as speaker goes to me it is a mater of taste and less technical elements there. If you like the sound of that speaker it is all good is about all I get from those long thread with descriptions like it has a hint of cork on the edges and overtones of fresh loam on a warm day.

 

[jsnwhite619]

I've had two that I built side by side, one with a 5W Champ OT, the other with a 15W "upgrade". Slightly warmer/more bass overtone with the bigger one, but not enough difference for me to warrant the price or weight increase if it's only for a 6v6. The 5k & 8k options make a bigger difference to me than the OT size/rating, and the 8k is what has the "right" sound to me.

I am definitely a fan of increasing the filter cap value on the 6v6 node. If I were building another right now and wanted to get the most out of it with the least amount of mods and work, these three things are where I would start.





This Champ from several years ago had the ClassicTone 5W OT, about the size of a golf ball, plugged into this 4x12 and P90's running through it.

 

[King Fan]

Good notes all around. I've been trying to help out on various 5F1 threads for a few months, and I've come to realize that even the simplest amps present us with tons of choices. Some choices we discuss to death probably make no difference in small amps, but others we take as 'given' -- Champ OT specs for example -- are actually pretty complicated. Speaking of golf-ball OTs, ClassicTone used to make a "tiny" reverse engineered Champ OT with these unusual specs. Yeah, 17KΩ into 4Ω.

 

[jsnwhite619]

@King Fan I haven't been down the 17k primary rabbit hole in a while. Is my math correct that if you wired an 8k primary for a 4R speaker, but ran it through an 8R speaker instead, you'd have 16k primary reflected and be able to compare the two?

When I built that one of those 5881 Princeton hot rods, I tried the 8k & 5k options with it. The 8k with a 5881 cut the headroom to zero and kinda had a Black Keys vibe to it, so I went with the 5k with the big tube. But I definitely prefer 8k primary with the 6v6.

 

[bikescene]

I built a 52FA-ish amp that could switch between the stock 22k value and a 50k pot off an 8 ohm tap. I never really experimented much though. I always stuck with the 22k value.

This thread has me interested in trying out the 30k-ish values.