Help a newb with a 5f2a journey? King layout

[King Fan]

^^^ what LLC said. I'll mention that, with your good diagram (lo-res here) we can hope to avoid the 50:50 thing. See those phase dots? Red is in phase with black.

These tell us that if the wire colors are right, for single-ended (1-tube) use, "the secondary that goes to the NFB (speaker tip) should be the same phase as the primary that goes to the output tube plate." If *I'm* right, that means green to tip, blue to plate (lug 3).

We do see cases where the wire colors are wrong, so still leave the wires long enough to swap. You can picture how 'out of phase' would turn NFB into PFB -- often results in that loud squeal but other symptoms are possible (eg, muffled or distorted sound).

 

[mabinogeon]

I hope our smart friends have helped with your questions. You have a great idea -- post pics and ask more questions about anything you don't fully understand. As they note, there are a bunch of ways to do a circuit ground bus / wire. In fact, I drew about a dozen each for the 5F2a and the related 5F1, exploring options. This is just one of them...

View attachment 1088218

Note the household safety ground/earth near the PT, and the circuit ground (from the bus) on an input jack. As noted, the bus can be a bare wire -- heavy is nice for rigidity -- or several lengths of standard wire. As the guys note, all you have to do is make sure it doesn't contact any other connections on the board and doesn't run into anything (like the input jacks) in front of the board.

There are several topics that actually need more attention than circuit grounding, even in this simple build. As noted, do you have a single-pole or double-pole switch? Does your PT have a heater center tap? What colors are your PT and OT wires? (Wiring diagrams / schematics can help here.) Then we can think about heater CT elevation, the PT's high-voltage center tap, and several other things.

Let us know what is and isn't clear. And enjoy this part. The research and planning phase is the safest, cheapest, funnest, and most important part of amp building.

Not to derail, but did I miss a discussion on tying the preamp and power amp grounds together? I always separate them.

 

[chas.wahl]

Not to derail, but did I miss a discussion on tying the preamp and power amp grounds together? I always separate them.

Have you been on the International Space Station for awhile, investigating the larger questions of the universe, and just returned? Welcome back to our little world of circuiting obsessions.

 

[King Fan]

Not to derail, but did I miss a discussion on tying the preamp and power amp grounds together? I always separate them.

^^^ what @chas.wahl said.

The best short discussion I know comes from our genius friend Rob explaining how he grounds his optimized 5E3:

"I employed a unified ground bus that is only grounded at the Normal Low input jack.... The typical split-bus ground sends all the preamp return current through the chassis. This ground scheme doesn't flow any current through the chassis which any electrical engineer will tell you is a good thing."

If you want a deep understanding of why a single signal ground (actually just a reference, not a 'ground') is better, most pros will give you a heavy lift about star grounding; see Mr. Keen or Mr. Aiken.

http://www.geofex.com/article_folders/stargnd/stargnd.htm

https://www.aikenamps.com/index.php/grounding

Luckily, before discussing full-on star grounding, Merlin Blencowe explains how a single ground bus can do the same thing as a star, quite simply in our typical home builds, or more optimally with a little more planning. This is still a heavy article, but you can cut straight to Sec. 15.7. Of course, *everyone* should read Sec 15.1 and 15.3.

http://www.valvewizard.co.uk/Grounding.pdf

Again, I don't take sides -- Rob, Keen, and Merlin all tell us in their different ways that we may not need a ground scheme that's theoretically pure. But recognize if there's an EE gold standard, it's the single / unified ground.

 

[mabinogeon]

Have you been on the International Space Station for awhile, investigating the larger questions of the universe, and just returned? Welcome back to our little world of circuiting obsessions.

^^^ what @chas.wahl said.

The best short discussion I know comes from our genius friend Rob explaining how he grounds his optimized 5E3:

"I employed a unified ground bus that is only grounded at the Normal Low input jack.... The typical split-bus ground sends all the preamp return current through the chassis. This ground scheme doesn't flow any current through the chassis which any electrical engineer will tell you is a good thing."

If you want a deep understanding of why a single signal ground (actually just a reference, not a 'ground') is better, most pros will give you a heavy lift about star grounding; see Mr. Keen or Mr. Aiken.

http://www.geofex.com/article_folders/stargnd/stargnd.htm

https://www.aikenamps.com/index.php/grounding

Luckily, before discussing full-on star grounding, Merlin Blencowe explains how a single ground bus can do the same thing as a star, quite simply in our typical home builds, or more optimally with a little more planning. This is still a heavy article, but you can cut straight to Sec. 15.7. Of course, *everyone* should read Sec 15.1 and 15.3.

http://www.valvewizard.co.uk/Grounding.pdf

Again, I don't take sides -- Rob, Keen, and Merlin all tell us in their different ways that we may not need a ground scheme that's theoretically pure. But recognize if there's an EE gold standard, it's the single / unified ground.

Thanks!

Yes, I haven't built an amp since 2018, but we recently moved into a new house and I'm getting my shop unpacked. Itching to get back to building and it sounds like I have some reading up to do

 

[chas.wahl]

As @King Fan's excellent summary notes: no need to "correct" amps built previously (supposed "perfection" being the enemy of the good), unless you are dissatisfied with non-musical noise(s) -- just another wrinkle in evolving theory and practice to consider!

 

[Commodore 64]

I've been out of amp building commission since about 2018 as well. Would like to get back into it, and this thread is fun. You guys are awesome.

And Merlin is God.

 

[prsman1981]

Here comes the fun part. I built a light bulb current limiter as described by Rob Robinette. It worked like a charm on my vox, but I plugged the 5F2A into it and the bulb didn't light. The bulb indicating the amp is powered on lit up.
I read that if the bulb lights to full brightness that indicates a short. But what does it not lighting up indicate?

 

[King Fan]

Here comes the fun part. I built a light bulb current limiter as described by Rob Robinette. It worked like a charm on my vox, but I plugged the 5F2A into it and the bulb didn't light. The bulb indicating the amp is powered on lit up.
I read that if the bulb lights to full brightness that indicates a short. But what does it not lighting up indicate?

I'm going to assume you tested the bulb didn't burn out? Does it still light up on your Vox? What wattage bulb is it? Are you following Rob's startup or some other sequence? IOW, what's hooked up and plugged in beyond the PT?

 

[chas.wahl]

Without knowing the current draw of your Vox, compared to that of your 5F2-A, and the wattage of the bulb you're using, all I can tell you is that whether the lightbulb lights depends on the current being drawn by the amp -- the 5F2-A probably draws considerably less current than the Vox. Higher wattage bulb for higher-current-draw amps, and lower-wattage bulb for lower current amps, for similar lightbulb "experience". A lower-wattage bulb restricts current flow (protecting the amp more if there's a short), but it does so by lowering the voltage provided to the amp more than a higher-wattage bulb. There are several threads hereabouts that touch on this.

 

[King Fan]

Good point, if you have a big-watt bulb in there, I'd try a bulb in the range of 60-150W -- 100W might work well here.

 

[prsman1981]

Without knowing the current draw of your Vox, compared to that of your 5F2-A, and the wattage of the bulb you're using, all I can tell you is that whether the lightbulb lights depends on the current being drawn by the amp -- the 5F2-A probably draws considerably less current than the Vox. Higher wattage bulb for higher-current-draw amps, and lower-wattage bulb for lower current amps, for similar lightbulb "experience". A lower-wattage bulb restricts current flow (protecting the amp more if there's a short), but it does so by lowering the voltage provided to the amp more than a higher-wattage bulb. There are several threads hereabouts that touch on this.

Good call the vox is 15 watts. Not a big one for sure. Used a 60 watt bulb. Saw zero light at all from it so I don't think it burned out. I'll hook the vox back up to verify.
Following Robs start up procedure.
Everything is soldered up at this point. I did measure the filter capacitors and saw zero voltage on them while the amp was on.

 

[Lowerleftcoast]

I did measure the filter capacitors and saw zero voltage on them while the amp was on.

Is the rectifier tube installed? Does it glow? Does it get warm?

 

[prsman1981]

No tubes installed at all.

 

[chas.wahl]

If your limiter is wired correctly and the bulb were burned out, you wouldn't get any voltage to the amp at all -- bulb is wired in series with the power to the amp. Easy enough to screw a bulb into a household socket to check whether the filament is sound. You can also check the voltage supplied to the amp. A correctly functioning lightbulb limiter (especially a smaller-wattage bulb like 60W) should cause some sort of drop in voltage, compared with the supply voltage at the socket. Also, if you short the hot and neutral of the power cord connector from limiter to the amp (with amp disconnected of course), the bulb should glow at "normal brightness" for its wattage, because then it's the only load in the circuit, like a single light bulb operated from a household switch.

There are two apparent schools of thought on the wattage load for a limiter:

1. The Uncle Doug approach, where he hooks up a 300 W bulb, and expects nothing from the bulb, or only a glimmer. This allows the amp to operate at pretty much normal voltage even when hooked up to his limiter, and only lights up if there's a short -- that's the upside. Downside is, if his bulb lights up, he may have already cooked something, because a high-wattage bulb doesn't reduce the voltage or current much.

2. The low-wattage bulb approach, where you accept that the bulb is going to significantly lower voltage (and current) available to the amp. This means that the amp will be well protected, but should not be presumed to be operating "normally". Some people start with a low-wattage load, and progress to a higher one, before accepting that there's no inherent problem in the amp circuit, and forgoing the limiter entirely. With a lower-wattage bulb, the hoped-for experience is that, when first switched on with amp cold, the bulb will glow, maybe even brightly, just briefly until an initial surge is over, and then glow dimly, maybe even not be evident unless ambient light is very low. If the lamp doesn't go dim, then there's a short somewhere, but at least it's at lowered voltage and current.

 

[King Fan]

Great questions and advice from the brain trust. Note with no rectifier, the filter caps see no DC voltage. And 60W is plenty small, should light up here. Can you show us a pic or two of your household and PT wiring? Oh, and an important question: Did you do Rob's pre-start AC wiring check? “Plug a guitar cable into an input jack and check that you have continuity (meter "beep") between the guitar cable's sleeve (the part that isn't the tip) and the amp's power cord ground prong.”

 

[prsman1981]

Continuity is good from input sleeve to ground on the power cord. I also checked continuity of the grounds and that checked out ok.
Makes sense the filter caps wouldn't have anything without the rectifier tube installed now that I think about it.
The bulb no longer lights up, but the filament looks ok and the fuse is fine as well.

Attaching a few pics of the wiring. Going to try to attach a video of the measurement I got from the power to the fuse. It's not steady and starts off low and rises slowly. It is plugged into the limiter at this point.

Edit: For the current limiter. I did break the tabs on the outlet on both sides and checked that there was no continuity between the brass plates where the tab was broken.

 

[prsman1981]

Video of meter reading

 

[King Fan]

The bulb no longer lights up, but the filament looks ok and the fuse is fine as well.

The incandescent light bulb? Or the "pilot lamp"? I'm confused -- did we decide the light bulb works, both in a household socket and in the LBL on the Vox? If it works on the Vox and those tubes warm up, what happens if you then remove the bulb?

IF and when the light bulb lights up and goes dim on this amp, then it's safe (no short) to measure voltages with the LBL removed, and they'll be more accurate.

FWIW, I also like mini-gripper probes for high voltage (ie, amp) testing. Less risk to you and the amp and less risk of wonky readings from poor contact.

 

[prsman1981]

Oh yeah sorry pilot light no longer lights up.
The actual light bulb for the limiter works just fine.