Buss Grounding n' Filter Caps..

[paul3i]

Hello everyone!

I just registered, hoping that the forum community can help me with some questions i have..

I'm restoring an old 75' Silverface Super Reverb and i got some questions about bus grounding. I've completely removed the brass under the inputs in order to use a bus wire instead for the grounds.
I've read alot about grounding from Randall Aikens tech info but still got abit confused..

I haven't isolated any input jacks or pots.. That way according to Aiken, the bus should be connected right at the input jack ground on the chassis. (still can bolt the bus on the chassis for extra safety at that point). All the heavy voltage grounds, including the two 100uF/350V filter caps are going to be bolted right at the Power transformer.

Aiken writes for the remaining filter caps (the three 22uF/500V), that their grounds must be connected near the preamp stage that they are decoupling..
So i wont connect the grounds on the filter cap circuit board alltogether. Instead seperate cables should be soldered in each of those three filter cap grounds.

My problem is that i can't figure out at which point on the bus wire should those caps be connected.. Mini "stars" must be formed on the bus but i don't know which components (Main board ground/Tone Control Pots/Filter Cap) must be connected together.
Could someone guide me through ??

P.S
On the layout each of those three caps have letters (B,C,D)

[Tom Phillips]

Welcome to the Forum Paul.
There have been lots of discussions here about grounding schemes. In fact it is one of the more popular topics. I suggest that you do some searches on the forum and read up on the previous discussions before we start adding more here.
Regards,
Tom

[paul3i]

Well it seems that most of my questions were answered as i searched and found a good thread about grounds by R. Aiken
R.A. wrote:

A few things that usually work best for me:

(1) Make sure the center tap of the HT goes directly to the first filter cap ground. There is a lot of high-current noise traveling in this wire. Do not connect this point to the chassis, unless it is the only ground point (see item 4 below).

(2) Locate the remaining preamp filter caps near their respective stages, and connect their ground to the local node grounds of the stage they bypass. Also run a buss wire along the grounds of each the filter caps back to the main filter cap. This is a "modified star/buss" approach where each stage has it's own local star ground and each ties into the global buss.

(3) Make sure you don't route the choke wires near any sensitive preamp nodes - there are also lots of current spikes traveling in these wires, and they will radiate or couple into adjacent wires.

(4) Whether you use a star ground, buss ground, or a modified version of both, connect the entire ground system to only one point in the chassis, preferably right at the input jack, using a PEM stud or secure bolt with lockwasher/loctite. Do not connect the ground system to both the input jack and back at the main power supply ground node or you're asking for ground loop hum. If you are running a buss rail, tie it to the chassis right at the input jack (preferred) or right at the main filter cap, but never both. Do NOT use the input jack nut/solder lug as the ground connection, always use a bolt or PEM fastener that stays securely connected to the chassis.

(5) If you are using isolated input jacks, run a 0.01uF cap from the ground/sleeve connection directly to the chassis with as short a leads as possible. If you are using non-isolated jacks, and you've connected the power supply ground buss/star to ground right near the jack, you're okay, but you may want to add the capacitor anyway in case the connection gets oxidized or corroded.

(6) Never depend on the input jack ground connection through the nut and sleeve lockwasher. Always run a separate ground wire directly from the input jack ground to the bottom of the cathode resistor of the first tube it goes to.

(7) As Mark alluded to earlier in this thread, never ground the pots/switches/jacks to the power supply ground rail or buss. Treat every amplification stage as if it were a differential amplifier that amplifies the difference between it's grid and it's cathode (or the bottom of the cathode resistor). If you have a volume pot that goes to the grid of a tube, run a wire from the ground of that pot to the bottom of the cathode resistor for that tube. Then run a wire from that point down to the supply buss ground or star ground.

(8) Always reference circuit grounds to the later stage that is receiving the signal, not the driving stage's ground. For example, in the volume pot example above, reference the pot ground to the stage that the wiper is connected to, not the stage that drives the top side of the pot. If you have a tone stack circuit, reference the ground of the tone stack to the stage following the tone stack, not the stage driving it.

(9) Always connect the AC mains safety ground (green wire) directly to the chassis, close to the point where it enters, with enough slack in the wire to maintain the ground wire integrity if the other wires get pulled out by accident.

Randall Aiken

I'm posting this cause it may be usefull to people that have similar questions..
Just one last thing that isn't mentioned..

Does anyone know where should the bias supply ground be connected? At the bus or at the Power transformer main star?

[Tom Phillips]

...Does anyone know where should the bias supply ground be connected? At the bus or at the Power transformer main star?

A common approach is to connect it to the same ground node where the power tube cathodes are grounded.
Cheers,
Tom

[paul3i]

Thank you very much Tom!

[Tom Phillips]

Pavlos,
I'm glad you are finding the information you wanted. The Randall Aiken information you posted is good and all correct but it does not include the specifics of hooking up the various the ground connections in an amp. I suggest that you keep searching out the other discussions on the forum. There are many that show schematic diagrams and layouts that emphasize the various approaches to a grounding scheme.
Regards,
Tom

[paul3i]

Another question came up..
Where should the output transformer secondary "common" wire be connected?
From what i 've read so far i should ground the OT to the first Filter Cap (B+) right at the main ground. Then another cable should go from the output ground to the point where the Phase Inverter is grounded on the bus wire..
Am i right?

P.S.
1)Im going to isolate the output speaker jacks.
2)The amp has already negative feedback and a cable is going from the output jacks back to the PI throufh a 820 Ohm resistor.

[Tom Phillips]

Given that your amp uses negative feedback AND that you are going to isolate the speaker output jack from the chassis I would recommend that you connect the output jack ground lug to the point where the Phase Inverter is grounded on the bus wire. There is no need to run an additional ground to the first filter cap ground. That would just form a ground loop.

Note that both secondary wires (hot and ground) from the OT should connect directly to the speaker jack. Then you run an additional ground lead from the speaker jack ground to the PI circuit ground. That provides a ground reference for the feedback signal.
Cheers,
Tom

[Bruce / Mission Amps]

I'm curious why you are going through all this brain damage for what is already a fairly quiet and hum free amp design?
And why remove the main grounding bus brass plate... that makes no sense. Isolated input jacks... what the heck have you been reading and convinced of?

[Tom Phillips]

Bruce makes a good point.
When you read about grounding design methods you would think that the Fender scheme would give really bad results. However, it works amazingly well for their low gain amps. A pitfall to watch out for is that an "improvement" in one area of the grounding can actually cause more hum. This is because the amp circuits work together to form a whole system. Therefore, once you get started, you sometimes need to change several things to get an overall performance improvement. There are lots of interactions in play.

If you are doing this as a learning experience then you will certainly learn a lot before you are done. However, if you just want a nice clone of an old Fender amp design then consider carefully what Bruce said.

Cheers,
Tom

[paul3i]

First of all thank you for all the usefull advices.

All this started cause i heard an old blackface twin reverb.. The amp had amazing sound but also had an annoying hum even that it had its electrolytics replaced and its groundings/components checked over and over again.
Maybe you are right about it and if i've just followed the Fender schematic/layout i would have a very good result, but it seems that at this point its something i'll never find out..
In the end i hope that all this "improvements" will give a satisfying result and that my build will be quiet.

Could you please explain why using a bus ground wire instead of the fender brass plate is wrong?
Also what is wrong with isolated input/output jacks?

All the information about grounding i've read, are mostly tech advices by R. Aiken's website and i don't think that they cause brain damage

P.S.
Not being aggressive, just curious

[Steve Conner]

Old Fender amps have low enough gain that almost any ground scheme will give acceptable results. The old brass plate was fine. If the amp hums, it's more probably because of mismatched power tubes, or a preamp tube with heater-to-cathode leakage.

This agonizing about grounding is mostly for high-gain metal amps, hi-fi amps where you are aiming for 90dB SNR, and scientific instruments. Having said that, Randall Aiken's point #1 is always valid, it can inject enough hum to wreck any amp.

[Tom Phillips]

Could you please explain why using a bus ground wire instead of the fender brass plate is wrong? Also what is wrong with isolated input/output jacks?

Neither of those things are “wrong.” The Fender brass plate and associated random grounding technique was adequate for their purpose at the time. The more advanced grounding methods that you have been pursuing are in fact better and they are the methods I use when building from scratch.
Grounding is a complex issue. It only takes one little mistake to mess up a grounding scheme. The different approaches are not 100% compatible so one needs to understand the theory behind why you connect / isolate the various parts of the circuit in a certain manner. There are many reasons why the particular Twin Reverb amp that set you on this path may have been hummy and we won’t ever find out what was up with that amp. It is possible to improve on the original Fender design. There was even some slight evolution toward a more sophisticated grounding layout in the Silverface years.
Have fun and enjoy the quest for knowledge.
Tom

[Randall]

I always ask myself if Leo Fender was standing looking over my shoulder would I still really be asking if the internet and I are going to improve on his designs? That usually settles me right down in a hurry.

[Enzo]

It is tempting to look at things as right and wrong, and as best and worst. But we must always look at things as a system, not as a list of parts. You cannot look at grounding the input jack for example by itself. You can only look at it productively in the context of the whole system. You can look at a number of grounding schemes, but you still need to consider them in the context of the whole amp. Just as looking at the NFB as a resistor. it isn't a resistor, it is a voltage divider in an active feedback system. So to ask if 820 ohms is better than 10k is meaningless without knowing the rest of the circuit.

Fender used heavy steel chassis. Ever try to solder to the chassis? Ain't easy. Thin brass plate? Easy to solder to. SO that is what the plate is about, it is clamped to the chassis by the controls, making fine contact with the chassis, but it allows the production people to solder to the thin brass plate instead of the thick chassis. Nothing cosmic about it, nothing chemical, it is just a way to solder to chassis without soldering to the actual chassis.

I can't tell you if your bus wire is going to work better or worse in your amp. I can see differences. The brass plate is connected to chassis all over. your bus wire is connected to ground at one or two places. I see that as greater potential for ground related hum... maybe.

As I mentioned, grounding jacks is not a question to ask in isolation. It is all about ground currents. If you ground the input jack somewhere, then what other things share that ground path with their currents? The answer to that is what determines if it was a good idea or not. By itself, grounding a jack is neither good nor bad.