Gibson GA-15 RVT Reverb Hum

[Helmholtz]

You mean C14? It actually is missing from this particular amp.

A cathode bypass cap of at at least 25µ may considerably reduce heater hum.

I have a digital scope, but I don't know how to utilize for this application. I'm keen to learn though!

Connect probe to amp's output, choose line triggering and look for hum signal. Then post a picture with scope settings.

[Helmholtz]

It eliminates that hum.

Ok, then try a cathode cap.

[bobloblaws]

Ok, then try a cathode cap.

Unfortunately that didn't help.

[vintagekiki]

Question.
- If R12 (Loudness) is at min (0) and reverb SW is OFF, whether there is hum.
- If R12 (Loudness) is at max (10) and reverb SW is OFF, whether there is hum. ¹⁾
- Is V1 (6EU7) new not "as new". Replacing V1 with V2 does not count as a replace tube.
- Is one end of the heaters grounded.

[Helmholtz]

Unfortunately that didn't help.

Ok, then it's most probably not heater hum (a scope picture would tell if it's heater hum or power supply ripple).

Now please try this: Solder a good (new) 22µ/400V+ cap directly between the node R24/R25 and the ground side of R16 keeping leads as short as possible.

[g1]

Question.
- If R12 (Loudness) is at min (0) and reverb SW is OFF, whether there is hum.

See post #1

[bobloblaws]

Ok, then it's most probably not heater hum (a scope picture would tell if it's heater hum or power supply ripple).

Now please try this: Solder a good (new) 22µ/400V+ cap directly between the node R24/R25 and the ground side of R16 keeping leads as short as possible.

...keeping leads as short as possible. That part could be an issue. This picture shows the way it is currently set up. The red circle is R24/R25, the green circle is where the preamp filters are grounded and the red circle is where R16 is grounded. You can see the long wire connecting the ground points. FWIW these ground points and the other preamp ground points on the eyelet board and chassis grounded at the input jack. When I first looked at this amp the reservoir and screen filters were grounded at the same point as the preamp filters but I changed that to be more like Hoffman's diagram. https://el34world.com/charts/grounds.htm.

Actually, now that I think of it, it doesn't make a lot of sense to keep those preamp filters grounded at the terminal strip if the yellow wire just doubles back and connects to the ground point at R16 all the way over at the eyelet board. I guess I want to change that for starters.

[bobloblaws]

Ok, then it's most probably not heater hum (a scope picture would tell if it's heater hum or power supply ripple).

Now please try this: Solder a good (new) 22µ/400V+ cap directly between the node R24/R25 and the ground side of R16 keeping leads as short as possible.

I tried the 22uF cap, no change.

I'm reading up on how to use trigger function on this scope. I've only ever used the "Run Control/Auto" button to detect a test sine wave. I have an old BK analog scope as well.
This is the user guide for my digital scope.

Rigol 1102E User Guide.pdf

[vintagekiki]

See post #1

Post #1 is talk about V1B. My question relates to V1A

[bobloblaws]

Post #1 is talk about V1B. My question relates to V1A

"One of the footswitch buttons grounds the junction of C15 and R15 when toggled. When that junction is grounded (reverb effect OFF state) the hum in question is not present."

[vintagekiki]

"One of the footswitch buttons grounds the junction of C15 and R15 when toggled. When that junction is grounded (reverb effect OFF state) the hum in question is not present."

See post #19

1)
I'll be free to ask you something.
Did you experiment with the GA-15 RVT in terms of shielded (cable), grounding and wiring amp in general.

2) Underline on 1)

[bobloblaws]

See post #19

Loudness pot is irrelevant, in post #10 I wrote that I bought a brand new tube, yes heater is grounded.

[vintagekiki]

Question
1 Where does the cut red wire (?) go.
2 The blue wire (?) starts from the rectifier diode. The picture does not show where it ends.

[bobloblaws]

Question
1 Where does the cut red wire (?) go.
2 The blue wire (?) starts from the rectifier diode. The picture does not show where it ends.

The red wire that you traced from the rectifier goes to the OT center tap. The blue wire in that vicinity goes from OT to power tube plate.

[bobloblaws]

Ok, then it's most probably not heater hum (a scope picture would tell if it's heater hum or power supply ripple).

Hey, good news! I tried the humdinger and was able to finally eliminate that pesky hum!

So what can I take away from this? It seems the hum was heater related after all. I gather that it leaks into the signal via the cathode of the tube? I was not able to figure out how to gather anything meaningful with my digital scope or my analog scope, in fact I wasn't even sure what I was supposed to be looking for in terms of heater hum vs. ripple. 60Hz vs 120 Hz?

Another question I'm left with. You indicated that the humdinger was something of a band-aid solution and didn't address the root cause. So now that we know this hum can be eliminated by manipulating the heater voltage balance, does it suggest a specific root cause that I could be investigating? I'm curious as well why the bypass cap didn't seem to help as you anticipated if it is in fact heater related.

Anyway, I'm relieved that I at least have the humdinger option. I appreciate the help from you and everyone else, thanks!