Main Reservoir Cap

[jordan86]

I continue to enjoy experimenting with my Single Ended Plexi build that I have posted about here many times. I've learned so much. Curious about filter caps though. I still have a faint bit of hum in my build. Even in "standby" and without tubes. I recently added a filter choke between the main res cap and the first filter node to the output plate/OT. 150ma/5H. No real improvement in hum. I presume it's probably grounding or transformer placement more than anything. Attached the Frankenstein'd schematic. And a picture of the current layout. Grounding bar is certainly a bit janky. This started out as an AX84 SEL that a friend built for me 13 years ago.

It's got 47uf at every filter stage. Wondering if doing 80-100uf on the first two stage and 33uf on the two preamp stages would be any better or would it be a "wash".

1) Is there a LIMIT on res cap capacitance in SS amp?  For instance, I know Dr. Z's Z Plus is single ended and he uses 100uf on all the stages. Albeit, a much different circuit. 
2) The screen has its own extra filter stage, which is pretty interesting. Never seen that before. Could that be improved?
3) My B+2 voltage are actually lower than listed here. Probably around 300v? I replaced R3's stock 1K value with a 10K (more so for the preamp/CF), but because of the wonky layout design, my screen voltage is much lower as well. Considering replacing the R6 (5K) with a 1K to get that back up closer to the plate voltage. That maximizes output power when the screen and plates are closer to each other, correct?

Grounding is also similar Merlin's ground bus. Kind of a mix of his ground bus and improved ground bus. Everything is on one single bus bar that goes through the input jack to ground.

Sorry I know that's like a half dozen different questions. Just curious if you guys think experimenting is worthwhile or if I'm just spinning my wheels for no reason. It's a good sounding amp and I enjoy it. Just wondering if my effort will be worth the return.

[sluckey]

I still have a faint bit of hum in my build. Even in "standby" and without tubes.

Messing with filter caps can't fix that.

[jordan86]

Thanks. I figured. Any tone benefits to be had though for these?

1) Higher (100uf) at first stage
2) Getting screen voltage closer to plate
3) Lower fitlering/capacitance in the preamp?

[sluckey]

Probably not. But it's your amp, your ears, and your time. Fire at will.   

[shooter]

Even in "standby" and without tubes.

guessing "mechanical" hum, got a stethoscope?
if not a metal rod, like we use for listen'n to knocking engines
check both trannies, I had a PT that became a bench PT because it hummed just enough to bug me

[sluckey]

Post a directly overhead pic of the top side of the chassis so we can get a sense of the layout and transformer orientation.

[Joel]

Or you could try a capacitor multiplier.  All the filtering benefits of a MASSIVE filter cap without the expense.

[pdf64]

As the hum is reported to occur in the absence of HT current flow, tweaking the HT filtering would seem to be wasted effort, as it must be unrelated to HT ripple.

[jordan86]

Post a directly overhead pic of the top side of the chassis so we can get a sense of the layout and transformer orientation.

Here's the top side. I did not build it originally or place the OT/PT. Hum was slightly improved with the recently added choke. The 45 degree orientation on the choke was the quietest to my ear when testing. Didn't do the headphone trick thing, just tack soldered in and tested with placement.

[Mungo Park]

 Sorry for the off topic question but if find this an Interesting looking amp
How do you find the amps sound/tone with so many preamp tubes.
With the MV do you get noticeable different preamp gain.

[jordan86]

Will shoot you a PM, Mungo.

[pdf64]

…
How do you find the amps sound/tone with so many preamp tubes.
With the MV do you get noticeable different preamp gain.

What makes you think there’s anything unusual about it?
Almost any medium gain preamp channel, eg Marshall 2204, has a similar arrangement https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_2204_lead_series.pdf

[Ed_Chambley]

There is a dude inside your tube that is taking photos of us.


I can see one place, but probably just the photo.  There appears to be a bad solder joint on that first black wire.  Anyway, if it is mechanical get you some little football players and see if they will run around on the chassis.


I could give you all the things you "could" try or just give you the link


http://www.geofex.com/ampdbug/ampdebug.htm


Check it out if you find it to not be =mechanical.

[RalphL]

Here's a possibility: The proximity and  symmetry of your transformers. They radiate emf, right? So being close, even at 90 degree angle to each other,  can cause hum in the OT. And even at 90 degrees if they are out of symmetry, the centers of the trannys not lining up, then you'll get  interference which no amount of ground schemes or filtering will change.  Note the center line of the OT doesn't line up with the center line of the PT and they're about as close as they can be. I've built several bigger amps, bigger chassis, and a bunch of little ones and unless I pay attention to this aspect my smaller ones are universally noisier.

[Fresh_Start]

Is it 60 Hz hum?  It looks like the first stage grid stoppers are parallel with and close to heaters. I only mention this because it would be easy to try a fix.

Cheers,
Chip