Ac30 build Hummmmm

[Steelwitch]

What does this mean? How do you ground with a meter?

Taking a resistance reading

[Steelwitch]

Found it. It is in the first stage.. here’s a pic of my scope on the junction of the 100k and 500pf cap on the plate of the input stage. I just changed the resistor to a metal film thinking it might have been a noisy carbon comp I used but no difference. This is def what I’m hearing though!! Any idea what it could be?

https://ibb.co/SRTgLPZ

[Helmholtz]

Taking a resistance reading

In a powered amp?? Might destroy your meter by DC voltage. Anyway it doesn't provide grounding. An Ohmmeter essentially injects a DC current.

[Helmholtz]

Any idea what it could be?

Without knowing the scope settings (vertical sensitivity in V/div and time base in s/div) it is just an unidentified signal.

[Steelwitch]

My mistake on that reading. The guitar I was using had lousy grounding... now using a guitar with better pickups, the first stage looks really quiet on the scope. Maybe it’s possible that it’s the heater supply in V1. I might use a 6v lantern battery to see if this gets rid of the hum

[Chuck H]

My mistake on that reading. The guitar I was using had lousy grounding... now using a guitar with better pickups, the first stage looks really quiet on the scope. Maybe it’s possible that it’s the heater supply in V1. I might use a 6v lantern battery to see if this gets rid of the hum

YOU ALREADY FULLY BYPASSED THE CATHODE CIRCUIT!?! That means any AC there is at ground potential.

Does the hum stop when you pull V1?

[Steelwitch]

Yeah I switched the cathode bypass cap to a 25uf and no difference. I even clipped the cathode straight to ground to hear a difference and there was none, so it must be somewhere else. Maybe it’s the floating triode though, I’m reading 63mv on pins 1 2 and 3 of V1A.

Is there a way for me to make that triode happy? Is it kosher to ground the cathode of that triode with nothing on the plate or grid?

[Chuck H]

Most designers just leave the pins floating without issue. Though I'm not sure I've seen it done with an input tube, so... You could certainly try grounding the unused pins. Since there's no significant voltage or current through the tube I don't see where that should be a problem. You could also wire it in parallel with the input triode. Just wire the plates, grids and cathodes together, half the plate and cathode resistor values in the circuit and double the bypass cap value.

[Steelwitch]

Tried grounding the entire triode and it wasn’t that either. Just put a bus on pins 1,2,3 to ground..

The hum isn’t killed when pulling V1, only when I pull V2

[Steelwitch]

It’s most definitely in the cathode follower. I see the hum on pin 1, 7, and 8 but nowhere before that although it could be a product of something in the previous stage

I do have the first triode in the cathode follower fully bypassed which was not in the original design, but the Hoffman schematic suggested.

[Steelwitch]

I tried unbypassing the first triode and it caused the hum to be unaffected by the volume control.. so at least that’s a different result. The bypassed stage is doing something in terms of AC being sent to ground.

Got 205v on the plate of V2a and 329 on the plate of V2b and 205v on the cathode of V2b

Sorry I keep bumping the thread y’all. Just posting here as I work. You have my gratitude for helping me through this problem!!

[Chuck H]

From post #1

With the volume control turned down completely, the hum is killed,..

Since the volume control follows the first gain stage this statement would indicate that the hum must be entering the circuit prior to the volume control. BUT... Maybe not. Maybe grounding the grid of V2 is what is killing the hum.

But now you say that unbypassing the first triode cathode circuit has rendered the hum unaffected by the volume control.

I'm going to be honest. A lot of your reporting doesn't make any sense. You've been making changes to the amp during the process of isolation and trouble shooting. Since things you have tried haven't worked there is every likelihood you have created another problem in your processes. Symptoms are now inconsistent with where we started and could be indicative of multiple hum sources. We should start over with the following information:

Pulling tubes. NOTHING PLUGGED IN. KEEP THE VOLUME CONTROL AT THE SAME SETTING, all other knobs on five. Staring with the first preamp tube how is the hum affected AS YOU PULL EACH TUBE? Even if the hum is not eliminated. What happens when you pull V1? V2? It would help a lot to know what circuits are where WRT V1, V2, etc. in your amp. Since it not an AC30 or the Hoffman circuit what you have is only implied. Not actually known.

Now you may turn knobs because we need to know how the hum is affected by each of the controls.

DO NOT CHANGE ANYTHING WITH THE SETTINGS OR THE CIRCUIT DURING THE COURSE OF THESE EXAMINATIONS!!! And if you DO change anything in the circuit after the examinations but before we have responded or made suggestions, even if you change it and then change it back, any response we give is rendered absolutely useless.

[Steelwitch]

From post #1



Since the volume control follows the first gain stage this statement would indicate that the hum must be entering the circuit prior to the volume control. BUT... Maybe not. Maybe grounding the grid of V2 is what is killing the hum.

But now you say that unbypassing the first triode cathode circuit has rendered the hum unaffected by the volume control.

I'm going to be honest. A lot of your reporting doesn't make any sense. You've been making changes to the amp during the process of isolation and trouble shooting. Since things you have tried haven't worked there is every likelihood you have created another problem in your processes. Symptoms are now inconsistent with where we started and could be indicative of multiple hum sources. We should start over with the following information:

Pulling tubes. NOTHING PLUGGED IN. KEEP THE VOLUME CONTROL AT THE SAME SETTING, all other knobs on five. Staring with the first preamp tube how is the hum affected AS YOU PULL EACH TUBE? Even if the hum is not eliminated. What happens when you pull V1? V2? It would help a lot to know what circuits are where WRT V1, V2, etc. in your amp. Since it not an AC30 or the Hoffman circuit what you have is only implied. Not actually known.

Now you may turn knobs because we need to know how the hum is affected by each of the controls.

DO NOT CHANGE ANYTHING WITH THE SETTINGS OR THE CIRCUIT DURING THE COURSE OF THESE EXAMINATIONS!!! And if you DO change anything in the circuit after the examinations but before we have responded or made suggestions, even if you change it and then change it back, any response we give is rendered absolutely useless.

I’m sorry if there’s any confusion— everything in the circuit has been brought back to the original Hoffman schematic. The only change I’ve made has to do with output bias. Anything else has just been clipping in to see if there’s any kind of difference.

So— to start over fresh: my volume control is at 10, the rest of the tone controls (treble, bass, high cut) is at 5).

With all tubes installed there’s a combination of hum and hiss.

Pulling V1 gets rid of the hiss completely and I’m left with a hum that sounds like this: https://vimeo.com/367141505

Here it is with my scope on the output jack at .5v/div at 20 microseconds https://ibb.co/fGj8kPd

Pulling V2 stops that hum completely.

With V3 still installed there isn’t any kind of audible hum or hiss but still seeing this on the output— same settings on the scope https://ibb.co/qC2y4PX

Pulling V3 is pure silence

[The Dude]

With volume at 10, you are going to get some hiss. I can't say if you have more hiss than normal, but some gain noise is to be expected.

[Chuck H]

Ok... You can stop looking to the filament circuit as a possible cause. I'll wager two beer PNW that's power supply noise. Even though your scope seems to show about 51kHz!?! Maybe something about the settings is misunderstood?

Alrighty! Now what happens when you ground the input grid of the triode feeding the cathode follower? DO NOT attempt to ground the grid of the cathode follower!!!