Hi, I'm working on a friend's Marshall Valvestate 8040, it is stock, clean, no fiddling, no dust, just scratchy pots. Power supply is steady, no DC on the pots, no leaking coupling caps. Reflowed the solder on pots.

Using quite a bit of contact cleaner. Amp works a lot better now after cleaning the pots, BUT, wondering if these are the "economic" pots that have cardboard backing and will need to be replaced if they go intermitant over a few days?

The amp is over 30yrs old I told my friend it would be a good idea to replace the electrolitycs. Making a PL for him, he said if the cost goes too high he'll sell or scrap  it.

Schematic: https://www.drtube.com/schematics/marshall/8040-iss8.pdf

I get my boxes from Tayda electronics, they can drill them and paint them for you as well if you give them the drill files and paint files.
The last few years I have mainly been making pedals with tubes in them that C2CE makes and sells the PCB's for, they also have drill files and paint files you can use to send to Tayda so they make the boxes up for the pedals.

https://www.taydaelectronics.com/

https://c2celectronics.com/

There will always be some amount of ripple on the supply rails.  Yours are measuring less than 1% of the rail voltage which is quite acceptable.  You did not post measurements for the amount of AC hum across the speaker.  That would be helpful to know the amount of hum we are talking about.  Both with the minimum amount, and with the increased amount when bias is adjusted.
If the hum was still there with the power amp input grounded, that indicates it is coming from within the power amp somewhere.  That is assuming the grounding of the input was a short run to a good ground.  Sometimes our test jumpers to ground the input can pick up hum themselves.

Thank you

I discovered today that my meter is giving me false readings on the power rails. If I reverse the probes I'm getting 0 volts ac on the +42 volt rail and 120 millivolts ac on the -42 volt rail.I assume the meter is being affected by the dc voltage.
So now I'm wondering where the hum is coming from.I have tried it with the preamp connected, disconnected and disconnected with the power amp input grounded.

Right. So you seem determined.

For designing amps, there is several books that I can recommend.

My starter book was from London Power (Kevin O´Connor), TUT2. You have to buy it.

There is also a book by Merlin, Designing Valve Preamps For Guitar And Bass. An early version is available for download. Chapter 12 talks about grounding, since I mentioned that to be of prime importance. I presume it is OK by Merlin, otherwise he would have taken it down.

Although these books are about tubes, the underlying principles are universally valid.

There is also a ton of information available from Rod Elliott.

Those are cook-book guidelines for a tough matter. It does not get simple.

So you don't have any suggestions, just a lecture?

The title of this particular forum is: "The Newcomer's Forum"...am I in the wrong place?

Before I dive into making even a small amp, I estimate that making a pedal is a worthwhile pursuit.
I'm sure there are "premade kits", but what is a resource/company/website you would recommend for the box?
I have a pile of resistors and capacitors in a few bulk buys that I've used for guitars (amplified parts.com), I not only build/service guitars for myself but also for charity:a local music class with lots of kids who come from families who are broke! I build between 5-10 a year, usually. The class itself received in total, 45 last year.
Most of these kids plug into whatever amps are available at class, but I'd like to build some type of pedal to be able to plug into something with a speaker and a power supply.
It might not be an amp, but I'd also just like to build one for myself. The only pedal I ever really use is a 10 band EQ going into my amp. The caps and resistors I have are for guitars, I'm sure I can use some of them for a pedal, but I'll for sure need way more that are amp/pedal specific, I have output/input jacks, and random parts, but a list and a source would be nice. And I'm really not sure what kind of pedal. Distortion, reverb, flange, chorus, just anything to get my feet wet. Suggestions and recommendations welcome. Thanks guys!

Thank you for your help.

Yes, you are right.
The more turned on the outputs are,the more ac is at the speaker.
The dc stays much the same.
There is 120 millivolts ac on the +42 rail. No ac on the -42 rail.

When I first looked at this amp I suspected the smoothing capacitors.
They were reading a little on the low side (4400uf) so thought they should be ok. I checked the bridge rectifier and found no problems.
Then I moved on to looking at the imbalance at the outputs.

Now I have replaced the capacitors and the bridge rectifier but still have 120 millivolts ac on the +42 rail.
I even doubled the capacitance of C320. Although it lowered the hum ever so slightly I still have 120 millivolts ac on the +42 rail.

I have seen it expressed by some fellow (somewhere in the jungle of boards and threads...) that they stay away from discrete circuits, because they grew up with OPAs... That's that excluded anyway.

QuoteI'm interested in learning how amplifiers work and how the components interact with each other.

Well that's a noble approach and a good propellant.

Just saying, that power amplifiers have a side to them, which a normal pedal tinkerer hardly ever crosses into - HF. And also, since comparably  huge current flows are involved, low frequency issues - speak: hum.

Many of the layouts of stompbox PCBs I see on forae are showcase. You can tell that immediately by cosmetically beautiful component placement and by ground planes that flood everything with the totally wrong assumption, that this helps.
With a stomp box, you get away with murder, but a PCB for a power amp is unforgiving. Been there, done that.

This is certainly several levels above stomp boxes.
Except for a few "utilitarian" stomp boxes that I bought kits for, I have always made my PCBs by myself. The were never as posh, but they are quieter, to name one thing. There is lots and lots you can learn from that.

You certainly will choose the right path, and I wish you the best for it.