Common / Ground connection to chassis

[tompagan123]

Hello out there,

My build will have a few  connections to the chassis.   One for the AC safety ground, one for the PT Center Tap and reservoir cap and the third for the floating signal ground buss.  I'm thinking of soldering the safety ground to the chassis near where the AC cord enters.    For the other connections, I'm considering either  installing separate terminal strips near the power transformer.  OR connecting via ring lugs to separate PT bolts.    PT bolts seem to be attractive from a mechanical perspective but I've read in forums that PT bolts are bad connection point for grounding.  I'm wondering if anyone can explain why the PT bolt would be worse than say a terminal strip or soldering directly to the chassis?      Is the concern EMI, resistance, reliability?    What do you folks think about this?

Thanks in advance!

Tom

[sluckey]

I would not solder any connections to chassis. You can't properly solder to an aluminum chassis. Ring lugs and bolts with kep nuts is the way to go. I would use a separate ground point for the AC cord. The PT bolts make a fine ground point.

[terminalgs]

The mains ground connection is all about reliability and durability. A bolt and nut (maybe a locking nut) with a lock washer is minimum.  PT ground bolt is fine.  If you are about to drill 50 holes in a chassis for all the various fasteners, you might consider one more hole for a dedicated bolt & nut that you tighten once, that would never need to be loosened for any amp maintenance.


If you have a blank slate, I'd recommend a ground lift for circuit ground vs. chassis ground.   This means isolated input jacks, and you can't use the pot's case as a convenient circuit ground for the lug's circuit grounding needs.    Valve Wizard has a good chapter on grounding, and covers ground lifts.   Any modern Vox (or probably Marshall or Fender) uses this same basic scheme. 1 cap, 1 resistor, 2-4 diodes.  It isn't required to make an amp quiet, but it is a good measure in that effort. 


this ground lift can be applied to any circuit grounding scheme.  The only draw back is it makes circuit grounding less convenient to a nearby lug, tag, pot shield, etc...

[tompagan123]

Thanks for the tips! My 2009 Merlin book only treats grounding in the chapter on construction tips.  But alas, I found the new Grounding chapter PDF on his website.  It is definitely more in-depth than my hardcover version .  I'll read it over and report back.

[EL34]

Use these ring terminals
#8's for most tranny bolts

They are on this page
http://hoffmanamps.com/MyStore/catalog/parts19.htm

[Fresh_Start]

Any bolt through chassis with Doug's ring terminals can work well. I put a lock washer between chassis and ring terminal and on the opposite side of chassis. Sand around holes in chassis, both sides, and clean with alcohol. Can't have too good a ground connection.

Kevin O'Connor's "galactic ground" and Merlin's approach are very close to Hoffman's when you boil them down. One "Galaxy" for preamp grounded close to input jack, another for power amp at opposite end of chassis, dedicated safety ground. Separate ground for heater center tap.

Cheers,

Chip

[Mike_J]

First off, I always use 4-40 Keps nuts with solder lugs.  I use emery cloth to sand the chassis and each side of the solder lugs before I attach them.  I haven't graduated to Deoxit yet but intend to in the not too distant future.  I buy a lot of Keps nuts from Doug.  I use them for the grounds and to attach tube sockets.  They are efficient to use and make an excellent mechanical connection.

I have also found it very advantageous to run a floating signal ground and to isolate input jacks.  Since I have read the quietest place in the amp is near the input jack it only makes sense to ground these components as close to the input jack as you can reasonably get.

I place a ground for the output transformer ground and the speaker jacks (isolated) close to the speaker jacks.  I have also run them to the main ground containing the PT center tap and HT and screen supply cap grounds.  Either way results in a quiet amp in my experience.  Just seems cleaner to me to run the OT ground with the OT secondary wires.

I have placed the heater tap ground to  the HT  ground and on its own 3 lug strip.  Haven't noticed a difference between the two.  Both are quiet.

I ground the PT CT, and HT and screen supply caps between the transformer bolt and the last power tube socket.  This is about as far as you can get from the inputs.

I also use individual grounds for each power tube cathode to ground using one ohm resistors to simplify biasing.  I have lifted these grounds and run them to the HT ground and have grounded them next to their corresponding power tube and have noticed no difference in noise using either method.

Just my experience.

Thanks
Mike

[tompagan123]

Thanks everyone for the ideas.

TerminalGS,
I checked out Merlin's new grounding chapter. Based on that  I went ahead and replaced my 1K screen dropping resistor with two 470 ohm resistors, one connected to the + side of the reservoir cap (in place of the original 1K), the other from the negative terminal of the reservoir cap to the PT bolt.  It's a clever idea. I don not yet attempt the "ground lift circuit", still pondering it. 


Mike,
Connected directly to the PT bolt mentioned above I've also connected the  PT main and filament center taps.   The speaker   jack is not isolated; it's connected locally to the chassis.   The output tube cathodes connect to a separate grounded terminal strip through 1 ohm resistors.    I then have a floating 18AWG buss running from a separate PT bolt across the PI and preamp ending at the Input jacks.  (The input jacks are isolated).
I see at least two differences between my arrangement and what you described.   1) My filament center tap is not on its own dedicated ground connection and 2) my preamp floating buss-bar connects to the chassis near the PT instead of near the input jacks.    It sounds like I may want to isolate the filament CT to a separate PT bolt, and also drill a hole near the input jacks for a new buss-bar grounding point.   

Thoughts?

[Mike_J]

Thanks everyone for the ideas.

TerminalGS,
I checked out Merlin's new grounding chapter. Based on that  I went ahead and replaced my 1K screen dropping resistor with two 470 ohm resistors, one connected to the + side of the reservoir cap (in place of the original 1K), the other from the negative terminal of the reservoir cap to the PT bolt.  It's a clever idea. I don not yet attempt the "ground lift circuit", still pondering it. 


Mike,
Connected directly to the PT bolt mentioned above I've also connected the  PT main and filament center taps.   The speaker   jack is not isolated; it's connected locally to the chassis.   The output tube cathodes connect to a separate grounded terminal strip through 1 ohm resistors.    I then have a floating 18AWG buss running from a separate PT bolt across the PI and preamp ending at the Input jacks.  (The input jacks are isolated).
I see at least two differences between my arrangement and what you described.   1) My filament center tap is not on its own dedicated ground connection and 2) my preamp floating buss-bar connects to the chassis near the PT instead of near the input jacks.    It sounds like I may want to isolate the filament CT to a separate PT bolt, and also drill a hole near the input jacks for a new buss-bar grounding point.   

Thoughts?

In my opinion you can ground the filament CT at the same point as the HT, PT CT and screen supply ground and it will work as well.  Grounding your buss bar near the input jacks is important in my opinion because it is the quietest place in the amp and should theoretically inject the least amount of noise into the circuit.  Probably not essential to isolate your speaker jacks.  I just do it because that is the way I do it, if that makes any sense and my amps are quiet.

Thanks
Mike

[Mike_J]

Another thing that came to mind is the way you tie your grounds to the negative side of the electrolytic cap for that stage.  Ken's layouts show this very well and are where I got the idea from.  I find it advantageous to tie all grounds that apply to each electrolytic cap together and wire them to the buss bar near the place where the pots for that part of the circuit are grounded.  I read somewhere that it is best to try to place all grounds for each stage as close as possible to the power cap for that stage and it works for me.

As far as noise is concerned a lot can be gained by using metal film resistors on the plates of at least the first preamp tube.  It will make a significant reduction in noise, primarily hiss, versus a carbon comp resistor.  Another source of hum can be old coupling caps that leak DC into the next stage.  Use of new caps should prevent that problem.  If you are going to use vintage caps as coupling caps it is a good idea to test them for DC leakage.  I have found some NOS caps leak too much DC where others, Mustard caps for example, leak much less DC than many modern caps I have tested.

Thanks
Mike