Question about hum and ground loops in original Fender AB763

[mikepukmel]

Hi All,
If I look at the original schematic/layout diagram, it has a whole bunch of circuit points tied to the chassis, something like 17. I went back and re-read Merlin's chapter on grounding. and ground loops, the suggestions there are to tie circuit reference to the chassis at one point, not multiple, points and he gives a diagram that shows ground loop and getting rid of it by removing one place in the circuit reference is tied to chassis (ground).

So, didn't the original Fender design have lots of ground loops? How did it work at all? I had a few of the old amps back in the day, and its been a very long time, but I remember mostly pickup hum, not a whole lot of amp hum. The Princeton reverb I had, i remember cranking the thing and it was extremely quiet, with no signal.

To "improve" I went back and looked at some older posts on fixing, improving hum. One build had a brass or steel rod hanging between the eyelet board, and pots. The grounds from the pots and that side of the circuit board led to that bar. But I did not see where that bar was tied to the chassis. It looked like there was a second place tied to the chassis, over by the power transformer, that tied the negative side of the power supply reservior caps and the PT center tap and also the + side on the bias ckt.

If you have two points tied to the chassis, one for the power supply, and one for the "reference" bar, won't that also create a ground looop?

Thanks,
MP

[Mike K]

My method of grounding requires looking at the circuit, not the layout. I use what I call a modified star or bus. Yes, you ground at only one point (other than the the earth bond for the mains) and that is right next to the input jack. You hook everything into the "bus" exactly as current would flow from "dirtiest" or latest in the amp, to "cleanest" or near the input. The capacitors that supply each of these should be tied into the "bus" directly before the part of the circuit they supply. This gives you essentially what is a star ground, but instead of using separate leads for each supply cap to the ground point, you move along a bus in cascading order. If done correctly, I've found it give identical performance to the star.

So if you follow what Merlin shows in his books, start at the circuit and isolate supplies from each cap and how current flows there, then either tie these together as star points or align them in order as they run in the amp from dirty to clean with the cap preceding them and orient them along the bus from dirty to clean, ending at the input. Either that or use multiple leads and star ground them at the input. Either way, it all starts with understanding the circuit flow.

[mikepukmel]

Thanks. So, here, when you say "ground at only one point" you mean, tie to the chassis? I found another example, not build photos, but a layout diagram (graphic). I think it does what you are suggesting. There is one point where the curcuit is tied to teh chassis right at the first input jack. From there, a wire leads to the circuit board, and has connections to each point where in the original Fender, there would have been a wire down to that brass plate. Finally, it ends where the dropping cap negative are tied in. But there is still one additional point where the reserviour caps and transformer center taps are tied together. This is the "dirty" section? So if there are two points, one is far from the other, and all the 'clean' stuff is tied to chassis at one point, and all the "dirty" stuff tied to the chassis at the other, that is a good compromise?

[mikepukmel]

I don't have the engineering chops to converse about this stuff intelligently, but if I look at photos of the original chassis wiring, the 64 and earlier anyway, they are extremely 'neat' looking. It looks like, without having engineering sense about this, that Fender tried to reduce the length of each physical lead as much as possible, i.e. make all wires added to the circuit as short as possible. The wires are all very short! Is that what they did?

[Mike K]

I don't know. I never do layouts like Fender or any of those vintage designs. To me they are just asking for trouble and it's not hard to modify them to the modern method of grounding.

[Mike K]

Also, not to be rude, but I've read some of your other posts on the forum and you seem to be having some issues with your builds.

My advice would be this: Dial it back and build some simpler amps. It's not that a JTM45 is all that difficult, but it does have a fair deal of parts. Reverb amps can be very fussy, and I wouldn't recommend them until you've really mastered everything else. If you had issues with the 5E3, that's about as simple as you can get with a push-pull amp, I'd really suggest you put this other stuff aside and build some single-ended amps. AC4, 5C1, 5F2A, BF Champ, AX84 P1. Any of those. And although you read my posts it would probably sound like I would recommend good components. No. I'd buy the cheapest crap you can get and make those sound the best you can with that. Then start modifying, swapping, testing, testing, testing. Then, and only then invest in the big money components. Those will never sound good if you can't master the basics.

All my first amps were SE. None had reverb. All single channel. All used Hammond transformers, usually the universal SE type and whatever PT spec I could figure from the universal tube supplies. All used cheap caps, resistors and tubes. Once I had mastered them, then I started tweaking. And I'll say, you hit a limit pretty fast with a 2-tube SE amp. But see what you can do with. You should be able to make it quiet and sound reasonably good. Then go back to your 5E3 and perfect that. Next I'd say would be the JTM45. And lastly, this Deluxe Reverb.

[Mike K]

AX84 High Octane






AX84 P1










AX84 P1x









AC4:






Tweed Princeton 5F2A








Some of these amps were later on, so I had more experience, or they were for a customer, so I used higher end stuff. Some were my second build of something, so I'd figured some stuff out. Some I don't have any good pics of the first build. As you can see, I tried different ways of building stuff. Some amps I built on boards, other PTP on terminal strips.

[mikepukmel]

Also, not to be rude, but I've read some of your other posts on the forum and you seem to be having some issues with your builds.

My advice would be this: Dial it back and build some simpler amps. It's not that a JTM45 is all that difficult, but it does have a fair deal of parts. Reverb amps can be very fussy, and I wouldn't recommend them until you've really mastered everything else. If you had issues with the 5E3, that's about as simple as you can get with a push-pull amp, I'd really suggest you put this other stuff aside and build some single-ended amps. AC4, 5C1, 5F2A, BF Champ, AX84 P1. Any of those. And although you read my posts it would probably sound like I would recommend good components. No. I'd buy the cheapest crap you can get and make those sound the best you can with that. Then start modifying, swapping, testing, testing, testing. Then, and only then invest in the big money components. Those will never sound good if you can't master the basics.

All my first amps were SE. None had reverb. All single channel. All used Hammond transformers, usually the universal SE type and whatever PT spec I could figure from the universal tube supplies. All used cheap caps, resistors and tubes. Once I had mastered them, then I started tweaking. And I'll say, you hit a limit pretty fast with a 2-tube SE amp. But see what you can do with. You should be able to make it quiet and sound reasonably good. Then go back to your 5E3 and perfect that. Next I'd say would be the JTM45. And lastly, this Deluxe Reverb.

Not rude *at* all! Thanks for any suggestions, criticism, etc. Its a fun project kind of thing, I have a physics background, but very little electronics, and practical amp building/debugging knowledge. Thanks for the suggestions. Im saving your suggestions to pdf, so I won't lost them (mea culpa, sometimes I ask a question, got very good answers/suggestions, got delayed due to work etc, and could not find later).

[mikepukmel]

Re your photos, thanks! Very clean builds!!! Super clean/neat wiring.

[loudthud]

I don't know if I ever built an amp where the parts didn't touch somewhere.

[mikepukmel]

if you use a thick fiberglass board with turrets, like the one in the photo, i.e. its not the old fender style black floppy board with another blank underneath for insulation, then how far do you mount the turret board up off the chassis, to prevent arc from the board to the chassis? I think mine looks too far from the chassis, compared to the really neat builds like these.

[g1]

Here is an article that addresses some of your grounding questions: http://www.geofex.com/article_folder...nd/stargnd.htm

[loudthud]

There are a couple of subtle things that Fender did that made hum a non-issue. First is the relatively low gain. Next was the general layout with the power supply and output stage on one end of the chassis and the preamp on the other end. You will usually notice a black ground wire comes out of the dog house and grounds in the preamp and another grounds near the power amp. This is important to keep hum out of the preamp.

Many people have tried to "improve" the Fender grounding scheme. Most failed miserably because they didn't understand how ground works or they tried to implement a star ground.

[Mike K]

Not rude *at* all! Thanks for any suggestions, criticism, etc. Its a fun project kind of thing, I have a physics background, but very little electronics, and practical amp building/debugging knowledge. Thanks for the suggestions. Im saving your suggestions to pdf, so I won't lost them (mea culpa, sometimes I ask a question, got very good answers/suggestions, got delayed due to work etc, and could not find later).

Excellent. You definitely have the right attitude!

My photos weren't to brag - just to show a few different SE amps and the complexity of the wiring as well a progression. A keen eye might notice that I didn't always use star or modified bus grounding, in some of my middle amps I did more what Fender did on their vintage amps and that was a separate preamp and power amp ground. I also tended to ground the secondary of the OT to the chassis. This works fine on most low gain amps, but it's not as good as the former options. I've built many Fender type reverb amps using modified bus and they were perfectly quiet, so no "miserable" failing if done correctly. There are many ways to successfully ground, but some always work well no matter what the amp, and I've found modified bus is one of those methods. Star works well too, but is messier from a wiring perspective.


Sorry to derail you from your original post, but it seems you have a couple going, and again, I'm trying to derail you to building something simpler. I'd equate this to learning how to play Ramones before you learn how to play Hendrix. Nail those power chords first, then come back and work on the other. You'll have much better and cleaner amps in the end.

I think a lot of people on this forum are techs. I'm going to come out and say 1) you don't need a ton of technical knowledge to understand amp building, design and debug and 2) you don't need a ton of tools. My opinion is you need a lot of time, and a lot of curiosity. That's why when I say test, test, test I mean play with the amp - change values and figure out what each of them do. Play with the layout, figure it out PTP and on a turret board. Look at the Fender layout and see what you think might be improved. They aren't very good, I'll be quite honest. They were mostly sloppily slapped together by low paid women first joining the workforce. Fender was looking to make money, not make pretty amps. There's a lot of variation too. We only tend to see the best examples alive today but Fender made millions of amps over the years and the vast majority went into landfills.

You have Merlin's book, and that's a great reference. Go back and start simple and make sure you really get it all on a two tube amp. And I don't mean all the nitty gritty little details, you don't need that to make a good sounding amp. You need a good understanding of the fundamentals. Many things simply don't make a lick of difference and IME most things actually listening and playing will tell you what is "good" and what isn't. A scope and audio signal generator are super useful for debugging, but I didn't have those for my first few amps, and I got through just fine.

[Mike K]

if you use a thick fiberglass board with turrets, like the one in the photo, i.e. its not the old fender style black floppy board with another blank underneath for insulation, then how far do you mount the turret board up off the chassis, to prevent arc from the board to the chassis? I think mine looks too far from the chassis, compared to the really neat builds like these.

I think usually 1/2-5/8" is what I use for standoffs, but I might be wrong. You don't need much and as long as your wires running under the board actually have insulation on them except where they go into the eyelets or bottom of the turret, you won't have any issue. Air has a high dielectric strength. You have to be really close to arc.


I typically figure this out based on my chassis though. If I'm using a 2" chassis I check to make sure my larges caps (or any other component that is large) won't interfere when I actually mount it. Just do a simple vertical stack, make sure you have clearance. If it's tight, I'll mount directly on the chassis and use a terminal strip to secure. I did that in a pic above but it wasn't for clearance. If you ever try to use film caps for your power supply (which have advantages, although they are HUGE) you may need to mount things differently. There's one of those in the preamp of the AX84 P1 above and you can see the size difference compared to an electrolytic.