Help a newb with a 5f2a journey? King layout

[Lowerleftcoast]

They are not polarized. One leg will connect to the outer film which can act as a shield so some builders will try to identify the outer foil leg. There are several threads discussing *how to*. I doubt the orientation of the coupling caps in a 5F2A will result in any improvement in noise level. Their position on the board has a good distance from other components so the shielding property will likely not be an advantage.

 

[prsman1981]

Is this looking ok? Did a dry fit whilst waiting on the resistor to drain the cap as shown in King's layout.

 

[Lowerleftcoast]

Looking good!

The 500 Ohm cathode resistor will get hot so make sure the cathode cap has a little space to keep cool. (You may have enough as shown but it is something to be mindful of.)

Some of the Shock Brothers and Sisters give points for style. They prefer to see the resistor color bands all the same direction. I guess they can't read backwards.

 

[prsman1981]

Some of the Shock Brothers and Sisters give points for style. They prefer to see the resistor color bands all the same direction. I guess they can't read backwards.

Good to know. Lol. I'm colorblind so that never crossed my mind. I'll see what I can do for the coveted internet cool points.

 

[prsman1981]

Would this be enough space for the heat concerns?

 

[prsman1981]

I made this template from cardboard. Marked all the component locations and holes for wiring to help me visualize all of this for board placement. Would this be ok? My main thought was input jacks. But looking at it now wondering if there will be enough clearance for wiring the tube socket to the left of the speaker jack. (Sick brain can't think of the tube name right now)

 

[King Fan]

The real board makes the perfect 'mockup' for fitting in the chassis, and for sure you don't want it longer or wider than the Fender version; I assume that's just a cardboard version of your real board? I just use the real thing, though I have done a cardboard template *of the front panel* to lay out wiring to the pots before I move it all into the amp.

BTW, did your board come with a backing board? It's a perfectly adequate template too -- component location is fixed, and moving the board far right won't lengthen your wire runs to any appreciable amount.

There's very little space in that chassis; IME you want the board far right. In fact, that's almost always the way folks do tweeds. FWIW here's mine during early wire-up... ignore my 'switched tone' hack.

 

[theprofessor]

View attachment 1092047
Would this be enough space for the heat concerns?

Yes. And I agree with @King Fan ’s post above: place the board farther to the right.

 

[prsman1981]

Appreciate all the help so far. Going to work on mounting holes for the board and ground wire today. Went to ace yesterday and got star washers and kep nuts.

 

[pinchegil]

Good to know. Lol. I'm colorblind so that never crossed my mind. I'll see what I can do for the coveted internet cool points.

If you are really colorblind, on your next build, and there will be more, look at vishay dale CMF 60 resistors. The values are numeric. Just make sure when u install the the values are on top and they are all in the same direction

 

[King Fan]

If you are really colorblind, on your next build, and there will be more, look at vishay dale CMF 60 resistors. The values are numeric. Just make sure when u install the the values are on top and they are all in the same direction

Heh. Most of us might as well be colorblind when it comes to resistor codes. Smart guys learn to read at least the multiplier band so they can tell 470Ω from 470K, but even that is tricky. Your multimeter is your best friend whether or not you're colorblind. We see resistor misplacement/misreading (often by multiplier orders of magnitude) in an amazingly large number of "my amp doesn't work" threads.

Here's one of the Dale milspec resistors @pinchegil mentions. He's right. The K is easier to see here than it is on my multimeter. The 1% tolerance is nice too, and being metal film, they're about as quiet as small resistors can be. Though you can source these with the big shops like Mouser or Digi-Key, I get mine from TubeDepot (no affiliation), where they only sell the 1/2W amp-specific kind, in amp-likely sizes, which avoids sorting through zillions of options.

And I know you're kidding about internet cool points. Orienting color bands all the same way is less about style and more about increasing the ability to decode them. If the tolerance band is always to the right or down (or left and up) those darn brown bands get easier to figure out. And yeah, resistors are less important than orienting caps with their label facing up. *That's* like orienting the rearview mirror to face the driver, not the passenger....

 

[prsman1981]

I've got a few more questions for you kind folks.

First would be about reading the schematic for the power transformer. It has a center tap, but I'm unclear which wire that would be for each winding? The center? (4 and 9)?

Second question is about the ground tabs that came with my kit. Since I'm using the ground bus from Kings layout will I still use any of them?

 

[theprofessor]

Second question first: You don't have to use any of the terminal strips unless you need to/want to. You won't need to make an "artificial heater/filament center tap" using two 100 ohm resistors to ground, since your power transformer has a 6.3V heater/filament center tap (green and yellow). You'll simply ground that green/yellow wire in an appropriate location. Exactly where that location is, is a matter of opinion. Since you have one power tube on this 5f2-a that is cathode-biased, I'd suggest you run it to the cathode of that power tube and use that cathode pin (in this case, that's pin 8) as your grounding point. Now to the first question: Your high voltage secondary center tap is the red/yellow wire on your diagram (with a "4" to the right of it). The all-red wires (3 and 5) will go to your rectifier.

As for reading that diagram, you've got primary connections on the left, and secondary connections on the right

PRIMARY || SECONDARY

Over on the secondary side, you've got three batches:

Red | Red-Yellow | Red are your high voltage connections. The 70 mA is the current the PT is capable of supplying. The 650V may also be written 325-0-325, where "0" is your center tap. That's what "CT" stands for.

The two Yellow wires that are labeled 5V @ 3A are filament/heater supplies for your rectifier tube.

The two Green wires that are labeled 6.3V @ 2.5A are filament/heater supplies for your other tubes (6V6 and 12ax7). The green-yellow one is the aforementioned heater/filament center tap.

The job of the rectifier is to convert wall voltage (AC; alternating current) into direct current (DC). The green wires actually function on AC in this amp. Since the other wires (red batch and yellow batch) both run to the rectifier first, there is no particular directionality in which those wires need to be hooked up to the rectifier. Obviously, they need to be connected to the correct pairs of pins; but what I mean is, 3 and 5 are interchangeable in this amp design, as are 6 and 7. The current alternates (i.e., flows in both directions) before the rectifier rectifies it, so they are interchangeable.

 

[King Fan]

^^^ you can't go wrong learning from @theprofessor; what a smart, clear, complete post. He explains both reasoning and options in perfect detail.

It has a center tap, but I'm unclear which wire that would be for each winding? The center? (4 and 9)?

I like maps and pictures, and happened to notice your PT happens to match its wire colors with the layout I happened to post. Happy happenstance.

Two details may deserve a mention; they also happen to be in my layout. It's optional, but the amp may be a little quieter if you run the heater 6.3V CT all the way to the 6V6 cathode, pin 8. The roughly 20V DC current on that pin will 'elevate' the 6.3V AC (like waves on a reservoir) which makes 'heater hum' less likely to couple or leak into your signal.

An even simpler noise preventer is to run the red/yellow HV (high voltage) CT directly to the first filter cap negative pole. This lets the huge-ish ripple in the return current go straight home to its source in the PT, keeping it off the grounds and therefore off the rest of the circuit.

 

[prsman1981]

Well things have been going smoothly. Been taking my time and having quite a bit of fun with it. One of the last things I need to hookup is my output transformer. So I thought I'd ask a question about that.

1. Being colorblind do I just want to ask if my colors match those on the layout.

I have an 8 ohm 10" speaker I'll be using on it.

So on the output black goes to the ground of the speaker jack and green to tip?

And input side red to pin 3 of the 6V6
And blue to the positive side of the far left capacitor?

 

[King Fan]

Well things have been going smoothly. Been taking my time and having quite a bit of fun with it. One of the last things I need to hookup is my output transformer. So I thought I'd ask a question about that.

1. Being colorblind do I just want to ask if my colors match those on the layout.

I have an 8 ohm 10" speaker I'll be using on it.

So on the output black goes to the ground of the speaker jack and green to tip?

And input side red to pin 3 of the 6V6
And blue to the positive side of the far left capacitor?

Do you have a schematic / wiring diagram for your OT? Forgive me if you shared it already, but they vary. And even when you have the 1° and 2° colors sorted, there are often 50:50 odds on which color goes to which lug (on the 6V6 or the output jack) so you want to leave the wires long enough to swap. We can discuss more -- but a schematic would help.

You're really smart to take it slow and enjoy it. As always, let us know if anything doesn't make sense...

 

[prsman1981]

Do you have a schematic / wiring diagram for your OT? Forgive me if you shared it already, but they vary. And even when you have the 1° and 2° colors sorted, there are often 50:50 odds on which color goes to which lug (on the 6V6 or the output jack) so you want to leave the wires long enough to swap. We can discuss more -- but a schematic would help.

You're really smart to take it slow and enjoy it. As always, let us know if anything doesn't make sense...

Yeah I had it on the post but apparently I can only upload one file at a time.

 

[Lowerleftcoast]

So on the output black goes to the ground of the speaker jack and green to tip?

Yes, use the black to ground and green to tip.

And input side red to pin 3 of the 6V6
And blue to the positive side of the far left capacitor?

(The positive side of the far left capacitor is the B+.) The layout shows the color connections just as you have described. Leave the blue and red wires long enough to swap their positions.

There is what is known as the 50/50/90 rule. You have a 50/50 chance the wires will be hooked up so the negative feedback circuit actually has NFB. If the wires are reversed the circuit will have positive feedback. Positive feedback will make some odd oscillations and many times the amp will squeal loudly. So you have a 50/50 chance of getting it right. The thing is, 90% of the time we get it wrong.

Be prepared to quickly shut the amp down at first start up. That squeal can be loud.
If the amp squeals, swap the red and blue wires and try the start up again.

 

[Lowerleftcoast]

I just noticed these two resistors need to swap places.

 

[prsman1981]

I just noticed these two resistors need to swap places.

View attachment 1098905

Oh snap good catch!