Reduce hum in BF Twin Reverb(AB763)

[Alan0354]

OK, that clear up, your reverb circuit is totally innocent. Pull out the 0.02 with V2 still in double proof that the hum is from between the volume pot and the second half of V2. The next step:
Remove the wire from the center of the volume pot and make sure the bright switch is off. Take the wire and ground it onto the ground strip that you use as one point ground and see whether you still have the hum.

BTW, I want to double check that other than the hum, the amp sounds good and have all the power. This is to make sure the circuit itself is build correctly. Then we are approaching the ground trouble shooting!!!

[Twin]

Pulled .02 cap with V2 still in and hum is gone, YES!!!!! OK, will pull center wire from volume pot with bright switch off & ground to terminal... Should I re-solder the .02 cap back in before power up or not?

Yes, the amp sounds beautifully crisp aside from the hum & has no other audible issues that I can hear. The Volume pot on 6 is ear-bleeding loud, just as a Twin should be

[Alan0354]

Yes, you can put everything back. Now, we absolutely rule out the reverb circuit, we rule out the first half of V1 and the tone circuit. We rule out the second half of the V4 with the 10pF//3.3M circuit. So what's left is the volume pot that connect to the second half of V2.
1) Remove the wire from the middle of the volume pot and ground at different points on the chassis and see whether any change of the hum.
2) Remove the ground wire that connect to the 820 ohm and 25uF in parallel that connect to the cathode of V2. Then ground the wire at different point away from the grounding strip and see whether that change the hum. I am trying to separate the ground particularly away from the strip that you have the vibrato and phase splitter tube soldered on.
It is really pointing to grounding issue. One way is to follow the grounding scheme of Fender, but I would still like to make minimal change of your build as it is really beautifully built. Might want to sight yourself up to put another ground strip close to the front end circuit away from the existing strip. But I don't want to get ahead for now. Just one step at a time.

[Twin]

I put everything back together & took the center wire on the Volume pot to the ground terminal & the amp is SILENT! Should I put the center Volume wire back before moving the 22uf ground?

[Alan0354]

I put everything back together & took the center wire on the Volume pot to the ground terminal & the amp is SILENT! Should I put the center Volume wire back before moving the 22uf ground?

Hey hold your horses!!!! I am looking at your ground of the volume pot, It is going the wrong way towards the power amp side!!! You put that on the wrong ground strip!!! That's where the large current from the power amp section goes. Take that wire and put it to the other ground strip you are working on and see what happen.

[Twin]

Shoot! Just saw this after dinner... Oh well, should I still move the 22uf ground? At this point I might put in the ground bus if you think that's the direction we are heading...

[Alan0354]

No, don't move that 22uF ground yet, just move the ground of the volume pot to the ground strip you are working on. Let's solve the problem first.

It is up to you whether you want to change the ground to the Fender Twin. I would if it was my amp. I don't believe in one point ground, it is way over rated. Unless you know exactly what you are doing, it can be dangerous. I'll talk more after we fix the problem.

[Twin]

Alan,

I already did move the ground to another point on the chassis(adjacent cap can) but then moved it back per your instructions...
Anyway, I moved the Volume ground over to the ground terminal next to the Normal channel & the hum is much quieter BUT as you dial up the Volume pot the hum increases, but still not super bad... Heck of a lot better than the hum was before!

[Alan0354]

Alan,

I already did move the ground to another point on the chassis(adjacent cap can) but then moved it back per your instructions...
Anyway, I moved the Volume ground over to the ground terminal next to the Normal channel & the hum is much quieter BUT as you dial up the Volume pot the hum increases, but still not super bad... Heck of a lot better than the hum was before!

I looked closer, your MIDDLE ( tone) pot is grounded on the other strip also, move that over and you should see improvement. But still, it might not be the end of it. Try this and report back.


That's is what I am afraid of and did not express optimism last night because I think you might have more than just that problem. That's the problem with the SINGLE POINT GROUND concept which a lot of people embraced. It is a double edge sword. This is actually electromagnetic wave theory that govern how ground image current travel, people don't know how the ground current travel can really get into trouble by just following the told single point grounding. I just opened a so called boutique amp because it was burned. After I fixed it and test it, at high volume, the volume started to go up and down like amp with vibrato circuit on!!!! It was a single point ground!!! I did not troubleshoot it as I wasn't asked.

Try the middle tone pot wire first and hope that make the amp dead quiet. Then it's up to you whether you still want to fix the ground according to Fender. You don't need to have copper tapes and the conductive glue is not perfect either. Use a big solder iron and you can solder onto the chassis just like Fender did. But if you are happy without doing it, it's fine. Just don't do that again in another build. See how much time it cause you to "do the right thing" from what was told?

[Twin]

Alan,

Yes, and I was hesitant to stack more grounds on the right terminal(already housed the grounds from all other pots, tagboard & 22uf caps!)& felt as though you might be edging this direction, so I yanked the single point terminals & used the copper tape to simulate the vintage brass bus & copied the original ground layout exactly... This works PERFECTLY!!! This is now the quietest amp I own, & with quite a bit better tone than my '73 Twin Reverb!

Thank you SO much for hanging in there & helping me troubleshoot this project, it surely has been an awesome learning experience!
Your knowledge of the circuitry is quite remarkable, are you a vintage Fender aficionado also?

-Thanks again as you have helped me in an absolutely Rocking way!!!!

[Alan0354]

You are welcome. I am happy we solve the problem and it sound that good. Is this a kid, from where? Does it come with all the parts and even the enclosure?

Nop, actually I only worked on two Fender amp in my life. I am an EE for a long time. The things that I worked with you are learned from books and on the job. In fact I only know the vintage Fender amps and a healthy dose of common sense.!!!

Grounding is not limited to amplifiers. Difference is audio amplifies make annoying noise where the other electronics make noise quietly in form of inconsistent data transmission, noisy picture etc. This is quite an involve field. As speed of electronics goes up, it become more and more critical in method of cleaning interference and emission of RF signal to the surrounding that could interfere with other electronics close by. In fact the last 10 years or so, they even have a special kind of engineer called "Signal Integrity engineer" that specialize in just control emission. AND that's where grounding comes into play, mostly in pcb layout. Imperfect grounding cause ground loop that cause EM radiation into the surrounding and cause interference. A lot of the work is done on grounding to reduce ground loop. My experience were high speed analog, high speed digital and RF. Those are the very kind that emit a lot of noise if not done right. Most of the problem are due to bad pcb layout which is no different from bad wiring and wrong grounding point in tube amp case. Because of my experience, I was even put on a contract with KLA Tencor to layout the 3G pixel CCD image sensor. Yes, it is 3X10EE9 pixels compare with less than 20M pixel of the commercial cameras you see in the market!!! It has so many critical they contract output company to do the work and I was the one that did the first design 8 years ago. I was the manager of EE, but when come to critical pcb, I actually lay out the pcb myself as it is more troublesome to instruct a pcb designer to do it right. People never realize how critical is the pcb( components and wiring in this case) layout. You can take the exact circuit and do two different layout and the result is totally different. At over 100MHz, pcb layout is 40% of the battle!!! Concept of grounding is actually electromagnetic theory. Believe it or not, signals are travel in circuit in form of electromagnetic wave, not as current or voltage. This is away from the subject. But it is the knowledge of EM wave propagation that you can predict the ground current.

I never believe in one point ground, if you have to look at things so detail like what we did for the pass three days, it is a bad idea. It require more knowledge to do one point ground than to do the cook book way of treating the chassis as one big ground plane.

When come to guitar amp, I designed one in the 70s and gave up until a few months ago because there is some remote prospect of business. That's when the time I join this forum. No, I am not an expert in guitar amp in any stretch. In fact, I kind of changing my mind as this amplifier thing is more involve than I like to get into. I have a lot of passion in electronics but not necessary in guitar amplifier. I rather get back to my own studies in electromagnetics, RF and antenna. That's where you design capacitors, inductors in some copper trace pattern on the pcb. You look at those pcbs, it look like a maze of copper trace and pattern. That's is really my passion.

[txstrat]

...used the copper tape to simulate the vintage brass bus & copied the original ground layout exactly... This works PERFECTLY!!! This is now the quietest amp I own...

Do you mind to post some pics of your amps grounding as it is now? Or maybe a sketch of the grounding?

[Alan0354]

Do you mind to post some pics of your amps grounding as it is now? Or maybe a sketch of the grounding?

Go to post #20 and download the schematic. The second page is the layout of the amp and it show every single ground points Fender used.

[Greenmachine]

I've built a champ, deluxe, and deluxe reverb using a star ground technique - no hum problems. A single bolt in one place, 10 solder lugs and as many individual direct ground connections as possible. Didn't isolate any jacks, but did use shielded cable from jacks to preamp tubes. I think you're going to find this is one of those issues that people tend to disagree about what's best. I'm just an amateur myself, but this was on the advice of an amp technician who uses the method consistently.

[Alan0354]

I've built a champ, deluxe, and deluxe reverb using a star ground technique - no hum problems. A single bolt in one place, 10 solder lugs and as many individual direct ground connections as possible. Didn't isolate any jacks, but did use shielded cable from jacks to preamp tubes. I think you're going to find this is one of those issues that people tend to disagree about what's best. I'm just an amateur myself, but this was on the advice of an amp technician who uses the method consistently.

You can hit and miss too. I am just sharing my experience as I worked years in grounding designs on circuits that are much more sensitive, I work with uV signals in digital environment with nS rise time. If you want to do star grounding, you better know what you are doing. Whether you want to believe or not, it's the EM wave that travel in the circuit, current and voltage are only the consequence of the boundary condition of the guided EM wave.( even two wires form a guided wave like the flat ribbon antenna cable in TV). I think the guided EM wave is the best way to predict ground current.

With one point ground, one little innocent mistake will give you nightmare to trouble shoot. Look at this post, two ground in the wrong place causing all these. I rest my case.