New Vibroverb AB763 build

[stratomaster]

Yes. I agree it's bad.  I've heard worse from ambient sources though.  Even bad reverb cables can put similar junk in the signal. Putting a baking sheet over the amp takes minutes.  There's no harm in ruling out benchtop noise before taking action.

[Willabe]

But I've never heard anything like that from air born interference. Not that kind of crackling.

From bad plate R's, dirty pots, bad solder joints, yes. But yes, put a metal shield plate over the top of the chassis and rule out air born interference or not. 

Easy to do and well worth it.   

[pdf64]

The only reason I see to use CC resistors there is for aesthetics. 

There is a reason to use carbon R's. They will distort when they have a high dcv on them. But only with a high dcv on them. So for the most past you would only use them for plate and CF K R's.

Ken Fisher used to tell people in Vintage Guitar mag. if you get a hissy/crackling carbon plate R, make sure to put in a carbon R, or the amp will sound different.

There's a short tech paper, only a few paragraphs, that talks about it. Might be on R.G. Keens web site?

http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

There's a case to be made for CCs for anode load resistors in common cathode gain stages, see link.

With cathode load resistor in cathode follower stages, I don't see the same case can be made.

[SleepLess]

http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

There's a case to be made for CCs for anode load resistors in common cathode gain stages, see link.

With cathode load resistor in cathode follower stages, I don't see the same case can be made.

Very interesting article!I placed a baking sheet on top of the chassis and nothing changed. I still get the crackling.

Are these the resistors you recommend?https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film

[pdf64]

...
Are these the resistors you recommend?https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film

Those are for power applications, eg screen grid current limiting in output valves, HT droppers.

Look for a product line that's intended for low noise but can accommodate at least 350V.

Personally I use 1/2W CC for anode loads.

[mresistor]

Are these the resistors you recommend?https://www.tubesandmore.com/products/resistor-pr01-vishay-bc-1-watt-power-metal-film

Actually these are the ones I was talking about, they are precision 1% rated at 500v. This site is Mouser Europe.
https://eu.mouser.com/ProductDetail/Vishay-Dale/CMF60100K00FKEK?qs=LPGv226TKJKhQHe%252BlJF0yQ%3D%3D

[AlNewman]

I think either resistor will work fine in that application.  Dealing with the big suppliers you'll often find resistors on sale, and can buy 50 for the same price as what some of the smaller suppliers charge for 5.

Before going on a shopping spree though, it would be best to pinpoint the actual problem, because as already mentioned it could be any number of things.  Solder connection, leaky capacitor, bad tube, etc.  If you rig up a lead with a .1uf/600v capacitor, and systematically ground the signal to chassis, it won't take long to find exactly what the issue is.

Since it seems to be before the volume pots, I'd start at the input side and work towards the volume.  When the noises disappear, eureka.

[SleepLess]

Hi!I just shot a short video of what I'm getting with the "probe" AlNewman suggested. I already changed the 100K resistor, 22nF cap and 500pF cap on the right side of the board. It didn't change the crackling. I think a video is better:
https://youtube.com/shorts/K7Gf4JYetg8?si=tiSf9fNDhDbnVp-a
What would you do? Thank you!

[AlNewman]

I guess I did a poor job of explaining the principle of what you're trying to do.
When you dump signal to ground anywhere the noise is present, the noise will disappear.  But if you start at the input and work your way through the signal chain, you can find the exact component where the noise disappears.

So, ground out the guitar input.  Noise disappears?  That means the noise must be introduced somewhere in your input jack. 

Noise still there?  That means the noisy component/connection must be after that point.  Move to the next component in the signal chain, which would be after the grid stopper on the 1st gain stage...etc, etc...

[SleepLess]

I guess I did a poor job of explaining the principle of what you're trying to do.
When you dump signal to ground anywhere the noise is present, the noise will disappear.  But if you start at the input and work your way through the signal chain, you can find the exact component where the noise disappears.

So, ground out the guitar input.  Noise disappears?  That means the noise must be introduced somewhere in your input jack. 

Noise still there?  That means the noisy component/connection must be after that point.  Move to the next component in the signal chain, which would be after the grid stopper on the 1st gain stage...etc, etc...

The points where I placed my "probe" in the video are the only ones which suppress the noise. Everywhere else it doesn't change anything. So I actually don't understand what I did wrong in my video. 

[AlNewman]

Here's a schematic of your amp.
Can you put a red dot on all the points you tested in your video which made the noise go away?

https://el34world.com/charts/Schematics/files/Fender/Fender_vibroverb_ab763_schem.pdf

[SleepLess]

There you go.

[stratomaster]

I think he wanted you to keep tracing back to the input jack.

You showed it responds to the volume pot so that means it's further towards the input than you showed by probing. I came to this conclusion because the volume pot at minimum equals grounding the grid of V2b.  So probing the plate connection isn't the latest area in the signal flow that killed the noise. 

With this probe you'll probably want to ensure it's discharged before you probe grids.  But I think you need to at least check the plate connection of V2a.

The 3 areas you probed near V2b plates are basically identical from an AC perspective, so no smoking gun yet.

The two places near PI input were killing both channels and killing the output of just the Vibrato channel.  Didn't tell us much diagnostically other than the Vibrato channel is the biggest contributor to noise.

Keep probing further towards the input.

[SleepLess]

Hi folks!I think I've found it! I touched the grid of V2 with my probe and the crackling ceased. I reflowed the solder joint. I've been playing the amp on 6 for half an hour and it is dead quiet. I'm going to put back the components I had replaced one by one and order the proper pots and a few tubes anyway but I think I've finally solved the issue: a bad solder joint and a low bias. Jeez that was long!I can't thank you all enough! I wouldn't have made it without you. A million thanks!

[Willabe]

Good job, you stuck with it and you found it. 

 

[AlNewman]

Sweet