SOLVED Seeking 5F6A Build Help: Tests and sounds great, but bypasses tone stack?

[Willabe]

I can still move that B+1 cap set. I’m trying to keep a somewhat traditional layout, but I’ll think about this.

If you do move the 1st filter cap B+ node A, I'd try and place the caps left/right, parallel with the chassis. With the ground towards the PT and the + towards the eyelet board. Shorter wiring that way.

If they wont fit that way, I'd place them so the ground end is towards the power amp chassis ground, again, for shorter wiring.

You don't have to move them. If it were my amp, I think I probably would. But twisting those 3 pairs of B+ wires will probably take care of it. I don't know if robrob wired up his amp the way Fender did.  It's just a couple things he did with the grounds are not what several authors/builders say not to do.

I normally use a blank chassis when I build an amp, so I can do some things like laying out the circuits/parts/PT/OT/B+ filter caps so I can use twisted pairs and wire up the grounds like I want to. But most guys don't bother with the time it takes to do this.

To be fair, most of them end up with very quiet amps.   

[gfarina55]

Quick troubleshooting question: Would using 18ga stranded speaker wire (https://www.angela.com/clothcoveredwire18gaugeblackperfoot.aspx) cause a pretty significant lack of high end and volume on this 5f6A?

[PRR]

How long is it?

I'd run 18ga all over a livingroom no concern. 50 feet may be just audible. Lack of highs is unlikely.

If you don't believe me, cut the cord off a power tool and try it. But my 2 cents is on something else.

[gfarina55]

Not long. It's in a 4 x10 combo cab.

I finished rebuilding the amp (and did end up moving one of the filter cap nodes...more on that soon). All voltages are correct and everything seems in order, except the amp produces about 2/3 of the volume that it should produce and sounds muffled with little high end. The speaker wire gauge was my last best theory....

I went through the obvious troubleshooting (double checked speaker wiring, jacks, tubes, chopstick, etc.) but I can't figure it out. I thought it might have had something to do with the negative feedback b/c the Presence pot doesn't seem to work (or I can't hear it working), but I disconnected the neg. feedback resistor and got the same results. Here's a pic of the "vintage style" speaker wiring. Am I making some obvious mistake?

[SILVERGUN]

What is the value of the cap in this snip?

[gfarina55]

Aha, it's .047 mfd....I had thought this was the same as 47pf but now I see that I was WAYYY off! Good eye Silvergun!

[Willabe]

Good eye SG indeed! 

If you don't have the correct value, for now, just take that cap out and see if that fixes the volume. That cap is just for keeping the amp stable, it should work fine without it for now.

The value you have in there now is causing to much signal to get canceled across the PI's output. The small 47pF cap only cancels a small amount of very high end frequency.   

[SILVERGUN]

Aha, it's .047 mfd....I had thought this was the same as 47pf but now I see that I was WAYYY off! Good eye Silvergun!

Yeah, that's where all your highs went.
It should be .000047uf, or just 47pf, but it really doesn't even have to be there. You can snip it out clean and fire away.


I'm glad you posted that pic!

[Willabe]

Has that .047 cap always been in there?

[66Strat]

Aha, it's .047 mfd....I had thought this was the same as 47pf but now I see that I was WAYYY off! Good eye Silvergun!

Yeah, that's where all your highs went.
It should be .000047uf, or just 47pf, but it really doesn't even have to be there. You can snip it out clean and fire away.


I'm glad you posted that pic!

Good eye.

[gfarina55]

No, I replaced all the caps and resistors in the rebuild. That cap wasn't in the other working version of the amp.

I do have some 100pf caps. I'll throw one in there and report back!

[gfarina55]

The cap swap did the trick! The amp sounds perfect (at least at "apartment volume"), all voltages are correct, and aside from a bit of hiss (CC resistors throughout), there is virtually no hum or any other noise when the amp is cranked. I’ll take the amp to the studio on Friday and give it a good workout at full volume.

Willabe, I did end up moving the B+1 node into the chassis with the neg. side right on the power amp ground terminal as you suggested. Photos below. You can see the PT CT & Filament CT go right to the power amp ground, as do the bias ground (yellow wire) and the neg side of the other two B+ nodes from the doghouse (white wire). At the top of the amp, you can see 2 pairs of red/white wires from B+2 and B+3 on the board, and each pair goes to a corresponding filter cap in the doghouse. I twisted these really tight and into the doghouse. In these red/white pairs, one of the white wires returns from the doghouse to the power amp ground (as stated above), and the other goes from the doghouse to the Presence ground. In other words, I've wired the grounding exactly like the chart I drew in my earlier post based on the info you provided.

The top/left side of the amp isn’t as neat is it could be with the additional wiring, but the hum is virtually gone so I think it’s a good tradeoff. I did spend some time moving wires around so that "different" wires only cross at right angles. I’m also going to put a bit of silicone under the B+1 caps so that they’re supported by more than their leads.

I’m really glad I rebuilt the amp from scratch. The CC resistors are quieter than I expected, and I doubt I’ll ever work with PVC stranded wire again. The cloth pushback wire is so much easier to use, looks better, and allows for neater layout IMO.

So, thank you all, and if I don't find any issues when I crank the amp this Friday, I'll start the rebuild of my 5E3!

[Willabe]

.... there is virtually no hum or any other noise when the amp is cranked.

Just to be clear, redoing the grounds like I showed you got rid of the last bit of hum? Or it made no difference at all?

Looks yery nice.

Only 1 thing, get rid of that wire nut, that's not safe.

[Willabe]

Oh, and the presence control works again? 

[gfarina55]

.... there is virtually no hum or any other noise when the amp is cranked.

Just to be clear, redoing the grounds like I showed you got rid of the last bit of hum? Or it made no difference at all?

Looks yery nice.

Only 1 thing, get rid of that wire nut, that's not safe.

Yes, it made a difference. The 60-cycle hum wasn't so bad in the old version, but yes, your advice completely eliminated the hum AND you taught me a ton about grounding, so thank you!! I'll get rid of the wire nut. The Presence control works, and I'm wondering if it was ever broken, or if I just couldn't hear it working when there was no high-end.

I managed to sneak into ESS this afternoon and crank the amp for a full hour and it's perfect. FWIW, the new version with all CC resistors and fancy caps sounds/feels significantly better than the old version with modern resistors, and keep in mind the transformers, speakers, and cab are the same. The voltages are nearly identical in both versions. (I measured every resistor before installing them both times, so this is not unexpected.) The old version sounded great but had a harsher distortion ("crossover distortion" I believe) when cranked, and and comparatively it felt a bit more sterile and unforgiving. The new CC version just sings, and the overdrive is noticeably smoother and more musical. I'm a player and not a builder (clearly), but my assessment is that the CC resistors and fancy caps made the difference between a good amp and an inspiring amp. My Centaur just sat in my case today as the amp's natural overdrive sounded so sweet.