New Grounding Scheme - wisdom to find hum and hiss

[bxershrts]

Hello,

I built a deluxe reverb style amp and thought I would try a new grounding scheme. Yet at idle, with all pots counterclockwise there is 60cycle hum and hiss, albeit minimal - yet there.

Having played a 65 Deluxe reverb, with the amp on, even all the original paper filter caps, all pots counterclockwise, it was so quiet that you would second guess if it was even on.

In my build - All Pots are bussed to ground at the normal input jack lug, pre amp section, and filter caps as well - (preamp , phase inverter and screens).

Power tube plates are grounded by the bias pot and the center taps.

AC Ground is near the strain relief at the cord.

The bias section and virtual center tap are grounded by the light.

Rca jacks are decoupled from the chassis via plastic washers and sheath on the threads and grounded to the lug on the reverb input, and the reverb transformer is grounded there as well.

Have I missed something ?

Filter caps are f&t 22uf 450v.
Bias cap is upped 20 100uf/100v

Any suggestions on what could be the source of the hum?

Please find the build pictures and a picture representation of the grounding scheme

The circuit sounds great and functions flawlessly - idle hum is more than I would prefer.

Thanks for all your help and suggestions!

Bxer

[shooter]

you show the bias pot grounded to the intensity pot, ground it where Fender did or at the ground end of the bias filter cap.


finding hum, quick n dirty
Power down-wait
short out the input jacks
pull all tube except PI n PA
power on
HUM?


if yes head to pi/PA
if no add rev tube


??
keep working back til it shows up

[tubeswell]

You've got multiple ground return attachment points around the chassis, which creates turmoil in the ground return current.

(Edit: and you’ve split your ground buss up into several components- so the ground return path from each part of the preamp is no longer distinct - rather it’s mixed up with the ground return paths from other parts of the preamp)

For better grounding, check out the chapter on this link> https://valvewizard.co.uk/Grounding.pdf

[stratomaster]

One often overlooked issue is the choice of speaker. Modern speakers tend to be more efficient and have better response at 50/60hz and an extended high end compared to vintage (potentially tired) speakers, so the noise floor seems higher by comparison.

Also h-K leakage is a prevalent issue--even in seemingly fully bypassed stages.  A little heater elevation goes a long way.

I'm not saying your ground scheme can't be improved--but that there are also other issues at play.

[mresistor]

Post a guts photo that shows the entire Power Supply end of the amp. Thx.

[elams1894]

As Tubeswell has outlined, multiple ground points make for a hum playground. In my experience with push pull style amps, the preamp (including tonestack) with associated electrolytic filter caps, are best ground at a single point at, or near, the the input.

The Poweramp (including phase inverter) with associated electrolytic filter caps, are best attached to the chassis at a single point that is either at the input of the poweramp section (usually from the phase inverter electrolytic cap negative), or brought to a chassis point that is of greatest convenience (in my builds, that is normally next to, but separate from, the mains chassis ground point).

The mains earth, for safety reasons, should ideally have its own chassis attachment, and the only other chassis attachment ground points are one each for the preamp and poweramp.

The centre taps of the Power Transformer secondary windings should be attached to the negative side of the first filter cap (or filter cap arrangement).

In instances of excessive hum, splitting the poweramp from the preamp in a grounding sense would be the first thing I would consider.

I have also found hum can manifest in a poorly grounded preamp input. Tightening, or checking, preamp earth connection has cured issues in a few of my builds.

A cold solder joint on an electrolytic earth has been the cause of hum for me in the past. I stumbled upon the cold joint by accident when the amp was knocked whilst being turned on and a loud 'thump' was noticed. This has been a good trouble shooting trick for me ever since. Any major cold solder issues will dislodge momentarily and produce a heavy 'thump' when gently knocked. Even lifting the whole chassis slightly from the bench and letting it drop can be telling. You dont want any thumps at all. Dont worry, your amp will be fine.

Positioning of the Output Transformer with regard to Power Transformer can be critical as well in terms of least hum. Tighten all bolts of the Power Transformer if it is a laydown type, as only a slight warping, or misalignment can produce hum. Normally, the Output Transformer is best positioned 90 degrees orientation from the Power Transformer (PT laminations pointing to OT bell ends in upright style) situated along the same axis. However this is only a rough guide as in some situations I have found that a slight offset from the longitudinal axis can kill hum completely.

Best of luck.

[bxershrts]

Thank you for all your replies!

I agree that speakers are much more efficient and do reveal much more of the amp than the worn out vintage ones.

Correct me if I am misunderstanding this wrong. In a stock "layout" - there are MANY grounding points - that as mentioned , can lead to playground loop havoc. Yet in stock form there is a playground of many grounds- and this is relatively quiet. Please find provided pictures to show the grounding points of a stock amp.

Using the concept of splitting up the pre and power grounds , this was the goal I looked to implement -  having all the pre amp section terminate at the input jack lug on the right and the power amp on the left and having very few ground points. * the bias pot ground was moved to the center tap ground location from the pre amp with zero discernable difference.

Please view the only grounding points on the second picture.

With only the 6v6 tubes and Pi and rectifier installed -there is hum - which does feel like magnetic field interaction from the transformers.
I will note that increasing the value of the the stock 25uf bias capacitor to 100uf did decrease the idle hum substantially. Yes there is 60cycle still present as noted with only the PA and Pi.

I will try moving the output transformer to see if any interatction happens.

Thank you for all your ideas

[stratomaster]

With only the 6v6 tubes and Pi and rectifier installed -there is hum - which does feel like magnetic field interaction from the transformers.

Try grounding the input to the PI then reassess.  Search the forum for "headphone trick" to definitely locate and remedy transformer interaction.  No feelings required.

I will note that increasing the value of the the stock 25uf bias capacitor to 100uf did decrease the idle hum substantially. Yes there is 60cycle still present as noted with only the PA and Pi.

Put the old 25μF cap on the wiper of the bias pot to ground (same ground as the limiting resistor off the outer lug).  Positive to ground.  This will create a 2nd filtering pole and cut the ripple by a factor of 10+.  It will be a non issue.

[Wonder2121]

Also to add, the chassis painted, make sure ground lugs make metal on metal contact..
You can slide a piece of steel inbetween transformers to check flux hum..

[bxershrts]

Hello everyone,

I just wanted to update the 60cycle hum and the fix for it.

With this new grounding scheme, terminating the whole preamp section to the ground lug at the normal channel, the remaining power amp , bias and heater center tap grounded at the right side of the chassis....i disconnected the HV CT grounded to the chassis in the middle and ran it to the dog house -grounded it at the power tube plates , now the amp is silent as can be.