Reverb Circuit Hum

[mwaller]

So, I was able to filter out the hum by replacing the .01uf that connects to the reverb pot with 500pf.
However, I still have a residual buzz that is quite annoying.
Could this be resolved by bypassing the filter caps with a film cap? What value should I try? Other ideas?

[Stevem]

The grounds for any of the preamp stage and reverb power supply filters should be landed where that tubes cathode resistor gets its ground from.
A buzz issue can be tamed by trying this, tape a sheet of Aluminum foil to a section of card board that's big enough to cover the bottom of the chassis and ground it with a clip lead, this should kill the buzz issue, or minimize it a bunch!
If so then just get some thin sheet metal to bolt the amp down to when it's in its cabinet.

Fender did not even use a sheet of metal , they stapled metal window screening to the roof of the cabinet.

There later amps use foil to line the left and right sides of the cabinet and the solid back panel.
Can you post a bunch pictures of the build?

[mwaller]

Here's the most current picture of the chassis:

IMG_20190127_210917703.jpg

I finally installed the chassis into a cabinet, which has aluminum tape beneath the top panel. Unfortunately, this did nothing to reduce the buzz from the reverb circuit.
What's worse, the amp now has an audible hum that was probably hidden by the low efficiency speaker I did my testing with... Pretty frustrating!

[Stevem]

We need to nail down what your hearing!
Low E on a guitar is 82 HZ, so is the hum your hearing below that which would be 60hz like coming from the heaters , or is it above 82 HZ which would a 120 HZ ( a octave above 60 HZ) and coming from the D.C. Side of the power supply.

One thing that gets folks sometimes is the heater wiring where for example they do not have the same wire feeding pins 4 and 5 or pin 9 on every tube.

Also how much AC ripple voltage are you reading on the plates of all the preamp tubes?

What is the pot on the rear of the amp?

[mwaller]

We need to nail down what your hearing!
Low E on a guitar is 82 HZ, so is the hum your hearing below that which would be 60hz like coming from the heaters , or is it above 82 HZ which would a 120 HZ ( a octave above 60 HZ) and coming from the D.C. Side of the power supply.

One thing that gets folks sometimes is the heater wiring where for example they do not have the same wire feeding pins 4 and 5 or pin 9 on every tube.

Also how much AC ripple voltage are you reading on the plates of all the preamp tubes?

What is the pot on the rear of the amp?

Thanks for your comments! I may try to make video so you can hear what I'm experiencing. I did a bit more investigation last night and determined that I also have a mechanical hum issue (AARGH!). With the rectifier and power tubes removed, I can still hear a slight hum that is not coming from the speaker. I never noticed this when the chassis was simply supported between two small chairs. The mechanical hum sounds like 120Hz, though I have to assume it is harmonic of 60Hz hum from the heater wires. What is the best way to tackle this mechanical issue?! I currently have the heater wires pressed against the chassis where possible, but perhaps this is part of the problem...

I'm not sure about the ripple on plates... I don't have an o-scope. Will a Fluke DMM give me give me any useful information in this regard?

The pot on the back is the 1M Reverb Dwell pot. It adjusts the amount of dry signal that gets fed to the reverb circuit.

[Stevem]

You can just measure the ripple voltage with your voltmeter set for AC volts.

You need to narrow down which gain stage the issue is starting with.
Starting with the first preamp gain stage lift the plate load resistor to unpower the tube and keep going down stream unpowering gain stages untill the issue stops and then you will know which area of the amp to focus on.

You might also try standing the heater wires up above the tubes like Fender does.

[mwaller]

You need to narrow down which gain stage the issue is starting with.
Starting with the first preamp gain stage lift the plate load resistor to unpower the tube and keep going down stream unpowering gain stages untill the issue stops and then you will know which area of the amp to focus on.

Would pulling the tube have the same effect?

[Stevem]

Yes, but then your maybe killing two gain stages at the same time since each preamp tube may be doing 2 gain functions at the same time and we want to nail it down to one.

[Roe]

I find that fender type reverb circtuits ala ab763 are always somewhat noisy. I rebuilt a reverb circuit to roughly the two rock mayer specs and it is less more quiet than it used to be. Shared cathode on v4 of an ab763 is noisier than a split cathode. and the 3m3 resistor adds hiss

[Stevem]

The sharded cathode and plate in the fender circuit is on the 12AT7 which is V3" not 4.
Unless you want to use a real power tube to drive the input of a 8 ohm reverb pan then strapping the triode is the only way to go!

[Roe]

The sharded cathode and plate in the fender circuit is on the 12AT7 which is V3" not 4.
Unless you want to use a real power tube to drive the input of a 8 ohm reverb pan then strapping the triode is the only way to go!

both v3 and v4 have shared cathodes in ab763. the shared cathode on v3 is fine, but its not fine on v4

[mwaller]

The sharded cathode and plate in the fender circuit is on the 12AT7 which is V3" not 4.
Unless you want to use a real power tube to drive the input of a 8 ohm reverb pan then strapping the triode is the only way to go!

The reverb design I uses a single triode to drive the pan via a transformer. The recovery stage is the other triode in the same tube.

If anything, the reverb is too strong. With a 12AU7 installed, the reverb is overwhelming in settings above ~3.

[mwaller]

I'm feeling a bit more optimistic about my project tonight. I've always been a bit suspicious of the original 1960's wiring in the downstairs of my home, so I decided to move the project upstairs to a clean power source. The mechanical hum is still present, but at a much lower intensity.
As I began adding tubes back, I didn't experience much hum until I plugged in V3. When I turned the reverb up, I started to get loud squeals! I did some gentle taps and discovered that the two vintage 12au7 were both quite microphonic. This could certainly account for the most recent bout of maladies. I hope to try with new tubes later tonight.

[mwaller]

Here's a sample of what I'm hearing. My cellphone mic seemed to pick up some high frequency harmonics that aren't really noticeable in person. But the fundamentals are accurate. There seems to be a rushing water sound now that I never noticed with the other speaker!?
https://youtu.be/F--kBveuOpU

[Roe]

have you tried a 100k reverb pot ala fender?