Planning a 5F2a

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,166
Location
Salt Lake City
Those are cool observations and info points. Just wondering though about the 2019 thing. This current thread? Or some other thread in a link? I may be missing something….
So, I found it interesting that the TDPRI site would list this thread in its weekly email menu of trending topics, particularly since it appears the last entry was back in 2019.
322D839B-7596-486B-AEB1-2DCD2A01B967.jpeg
 

tubeswell

Friend of Leo's
Joined
Jul 1, 2008
Posts
2,452
Location
NZ
Going by the size of that choke, and that it looks like an HT choke, and that you know it came out of a Hammond, (and assuming there’s nothing wrong with it*) then yes, you (probably) can happily use it for a whole-of-supply choke in a CLC filter in a single-ended amp running a 6V6 and a preamp tube - because that sort of amp will only draw about 40-50mA tops, and what you’re holding looks like a 40-50mA choke. (No guarantees implied or given.)

* If the amp blows mains fuses when you first turn it on, it’s probably an indication that it won’t work.
 
Last edited:

Phrygian77

Poster Extraordinaire
Gold Supporter
Joined
Apr 30, 2016
Posts
5,471
Location
Crawfordville, FL
You amp tech dudes are cool. I wish I understood 1/10 of what ya'll are talkin' about, but I like to read threads like this in the hopes of learning something. Don't want to derail the thread, but I'm curious - How do things like chokes and PT's go bad over the years? With normal use, I'd think they'd last forever. Obviously they don't, but I just don't know why they don't I suppose. As one person mentioned "looks pretty rotted" how does something like this "rot" or "fall apart"?

Humidity and corrosion can lead to the early death of transformers.

I highly recommend Blencowe's books. The guitar oriented book is intermediate level, but his hi-fi book starts with some relatively basic electronics info. You really need to get your head around alternating current, amplitude/period/frequency/phase (periodic functions), and impedance.
 
Last edited:

chas.wahl

Tele-Meister
Joined
Dec 8, 2012
Posts
419
Location
NYC
FWIW, here's the poor man's 5F2a schematic with voltages. (Or rich man's? Only a very few 6G10s were ever built.) Same circuit….
Same circuit, but note that the power transformers listed on 5F2-A and 6G10 are different (66709 vs 125P1A). I believe there's a change in vendor in there (Triad to Schumacher? or vice-versa, or is that OTs?) but my suspicion is that a 6G10s B+ voltages were significantly higher (though within Fender's "20%" fudge factor).

To confuse matters, I have a photo of a 5F2 that's clearly got a Triad PT numbered 7787 (model or serial #?).
5F2-transformers.jpg


And here's a 1960-61 5F2-A that was for sale (for 2000 GBP! [the transformer in the cab corner is obviously a 125>220 converter]) with a slew of pictures, showing a PT with apparent model # OP-250B bearing also number 606036 (there is a better closeup photo of just the PT), which I thought might be a serial, except that I found post in a thread (search for either number if my link doesn't go directly to it) discussing a 1954-55 Champ with exactly the same numbers on the PT. According to the poster, that's a Schumacher. I actually think that the 6-digit number might be a date or date/serial code, since I've found a few references to "606" numbered 5F{1|2-A}s, with different last 3 digits. Note, interestingly, that the OT in the picture is a 125A2A, which is exactly what's listed on the 6G10 schematic @King Fan has posted above.
5F2-A 1960-61 transformers.jpg


Nevertheless, I suspect that the PT used for Tweed Champ and Princeton was the same item (why change it for a tone stack?), and the 5F1 layout clearly has 340 V as the B+ at filter cap 1. That's what I'll be shooting for. BTW, the thread linked-to above has a reply, just below the one mentioning 606036, where the poster reverse-engineers the PT voltage:
"5F1 used a 275-0-275 I think. Here's how I came up with that...
According to the 5F1 layout Fender shows 340V at the first filter cap. Add 50V to that (the typical voltage drop from a 5Y3 rectifier at those low voltages and current draws)... 340V + 50V = 390V

"With a full-wave rectifier on a center-tapped transformer, divide that by 0.71 to get the un-rectified AC input voltage... 390V / 0.71 = 549.3V

"Since it's a center-tapped transformer, divide that by 2: 549V / 2 = 274.5V

"So, a 275-0-275 transformer for a 5F1 is what you are after to stay true to the original.
"
 

Mark1406

TDPRI Member
Joined
Nov 5, 2013
Posts
86
Location
Sydney
There is some good info here like the size of the filter caps but the info wasn’t correct. As shown the maximum capacitance is 32uF.

Also you don’t want your rail voltage to exceed 360VDC or you will need a different cathode resistor on the 6V6.

The big question is how are you going to use the amp?
If you are going to drive the amp hard and loud I wouldn’t use a choke from the first B+ node to the screen grid B+ node, the voltage will be too high and you will destroy 6V6’s. In this case the 10K resistor is the better option.

Personally, I would prefer to use a sturdy choke before the B+ to the output transformer to filter out mains hum. There has been a post suggesting to use Pi filters with resistors instead of chokes. The problem here is sag, the amp for lack of a better term browns out and sounds muddy.

Then there is the guitar you will use with the amp, if you are going to use a Fender style guitar stick with Fender plate and cathode values, if you are going to use Gibson style pickups then consider Skylark plate and cathode resistor values. Both are good amps.

Good luck with your build.

Regards

Mark
 

Attachments

  • 0C291038-AEC9-4B06-ABA6-29D640D31DBD.png
    0C291038-AEC9-4B06-ABA6-29D640D31DBD.png
    366.8 KB · Views: 9

chas.wahl

Tele-Meister
Joined
Dec 8, 2012
Posts
419
Location
NYC
Brimar says potayto:
Brimar 5Y3GT 19??.jpg


Others say potahto:
RCA Receiving Tube Manual 19??
RCA 5Y3GT RTM 19??.jpg


GE 1955
GE 5Y3GT 1955.jpg


Tung-sol 1959
Tung-Sol 5Y3GT 1959.jpg


RCA 1961
RCA 5Y3GT 1961.jpg


And finally, we have the real-life examples of people who load the 5Y3GT with twice the "typical operation" 20 uF and more, and there doesn't seem to be a literature supporting failures as a result of that.
 

ArcticWhite

Tele-Afflicted
Joined
Apr 14, 2009
Posts
1,544
Location
Portland Oregon
d like to know how the OP made out with his build! If he still get's this thread, maybe he can post some images of his final product? I'll just close out my 'contribution' here by posting a few shots of the 5F1 'clone' that I build. From 'empty' chassis with just iron, tube sockets, and control panel stuff mounted, to final product.

I just started on Tuesday! (Of 2022)
Anyway, here's a couple progress shots.
 

Attachments

  • 20220510_071513.jpg
    20220510_071513.jpg
    167.9 KB · Views: 24
  • 20220505_132922.jpg
    20220505_132922.jpg
    173.4 KB · Views: 23
  • 20220505_132910.jpg
    20220505_132910.jpg
    144.6 KB · Views: 23

King Fan

Poster Extraordinaire
Ad Free Member
Joined
Jan 1, 2013
Posts
8,166
Location
Salt Lake City
And finally, we have the real-life examples of people who load the 5Y3GT with twice the "typical operation" 20 uF and more, and there doesn't seem to be a literature supporting failures as a result of that.

Right; lots of good info in your post. Among those who've loaded the 5Y3GT with double the 20uF 'typical' capacitance is Fender; although the early schematic didn't show it, most VibroChamps and all Broncos were shipped with the first 2x20 in the cap can jumper'd together to make 40uF. These amps have never been noted to eat 5Y3s. The 10-20 uF "typical" and 20-32uF "max" values we hear (and see in data sheets) are said to be a throwback to the early years; when RCA introduced the 5Y3 in 1935, caps bigger than 10uF were rare -- and expensive. For that matter, note Leo ran 2x16 to get 32 in the 5F2a even as late as the '50s.

Here's my '67 VibroChamp...

1652501089352.png


Here's the Bronco...

1652500080906.png
 
Last edited:

2L man

Tele-Afflicted
Joined
Nov 23, 2020
Posts
1,400
Age
62
Location
Finland
I will try to follow this up in the literature (to understand what's the difference between "choke input to filter" and "capacitor [or choke] input to filter"), and re-read the thread that I linked-to above in post #6; but I'll just say this: take a look at a Tweed Tremolux (5G9) or Bassman (5F6-A) schematic with the voltages shown, and you'll see that the drop with a choke only is about 2-3 V. To me, that's going to be a lot less than having a 10k resistor there, as in the 5F2-A circuit. OTOH, the drop shown on the 5F2 due to choke is 320-305 = 15 V, so go figure (unfortunately, despite its enduring popularity, the 5F2-A schematic and layout don't have any voltages listed).

Apart from the 5F2, which as I've pointed out, has OT leg and screens on the same B+ feed before the choke (seems a bit dangerous to me), the other Fender schematics I've referenced that have a choke also have 470 ohm resistors between the choke output and the power tube screen grids, with the effect/object of lowering the voltage to them more than the drop incurred by the choke.

I know that I'm mixing and matching simpler SE circuits with more complex PP ones, but still . . .
There Choke is used after the first filter capacitor and the voltage drop comes mostly of the coil resistance and less of a Choke charging the current and releasing the current. That change the filter operation to CLC. I think choke gets mostly "wasted" as voltage regulating component but still does filter the Mains Hum. Obviously this CL order is used because sag-effect is considered good and it comes when regulating is not good.

Sometimes of even more often on push pull instrument amps the Choke is used after B+1/OT input but before B+2/Screens supply (together with Screen dropper resistors) and purpose there seems to be prevent stages interact with each other.
 

ArcticWhite

Tele-Afflicted
Joined
Apr 14, 2009
Posts
1,544
Location
Portland Oregon
A couple more progress shots. All the hardware is in, but I need more room for the big caps, so I'm going to move the fuse to the opposite face, next to the power switch.

Planning to use metal film resistors in the signal path. Can anyone tell me where I should use carbon comp instead?

BTW, the choke is in the middle. I'm going to start with the little OT that came with the amp, and probably sub in a larger one later for comparison.


20220515_190257.jpg
20220515_190314.jpg
20220515_190308.jpg
 

ArcticWhite

Tele-Afflicted
Joined
Apr 14, 2009
Posts
1,544
Location
Portland Oregon
OP here.
I built the amp as a P2P, but attempted to follow current best practices grounding schemes, so there's a lot of ground wire running all over the place. It's kind of a mess to look at.

It worked pretty great for about thirty minutes, and then started making loud click/pop noises when I hit A string notes, hard at 70% volume.

Chopsticked it and reflowed everything, tried different 8ohm speakers, and swapped a bunch of tubes with no change.

Then the OT gave it up with a puff of smoke. It still works, but now there's a lot of noise, so I'm assuming the OT is partially fried.

I used the smaller of the two OTs from the original stereo SE amp because it didn't have the center tap that the other one has. I assumed it was compatible with an 8ohm speaker but looking at this schematic it's very hard to tell.

Anyone want to take a guess at what impedance the center tapped OT would like to see? I've included a closer view, but the text is pretty illegible.


Screenshot_20220526-084923_Gallery.jpg

IMG_20210720_001114 (1).jpg
Screenshot_20220526-084923_Gallery.jpg
IMG_20210720_001114 (1).jpg
20220524_203502.jpg
 




Top