Bad noise (static?) - asking help for troubleshooting

[slidincharlie (Carlo P)]

Hello guys.
Now I need some help for troubleshooting a very annoying buzz/fart noise in my late-60s Silvertone 1482 amp.
The chassis is housed in a nice, sturdy custom-made cabinet made of birch plywood.
The speaker is a nice Weber ceramic 12" unit.
The noise has developed since a few months, but I can't trace a clear origin to it.
It is a sort of farting buzz (sorry for the unpolite term) on top of some low-frequency notes. The noise is stronger with the neck or neck+bridge pickups engaged, while it is absent if I use the bridge pickup alone. So it IS linked to the production of low-end notes.
Please listen to the attached audio sample. I play the three lower strings of a standard E chord in sequence: the noise is on top of the second and third notes.
I have swapped speaker and cabinet and the buzz is still there, so I believe that it originates from the chassis/circuit. Yet it comes out of the speaker, not from rattling parts in the chassis.
I wonder if a component can be the culprit.
Thanks in advance.
--Carlo
Farting low notes.mp3

[nickb]

Schematic below. Basic stuff first:- Have you tried swapping the tubes?

[The Dude]

Much of the time, when I hear that symptom described, it's failing filter caps. Check your B+ nodes for low DC and/or too much AC. Or, clip a known good cap across them and see if the symptom goes away.

[pdf64]

...I have swapped speaker and cabinet and the buzz is still there, so I believe that it originates from the chassis/circuit...

Is the problem there if the amp chassis and speaker cab are physically separated, ie such that vibration from the speaker can't shake the amp?

[slidincharlie (Carlo P)]

Is the problem there if the amp chassis and speaker cab are physically separated, ie such that vibration from the speaker can't shake the amp?

Yes, it is.

P.S.: Replies to the other posts above will follow very soon.

[slidincharlie (Carlo P)]

Schematic below. Basic stuff first:- Have you tried swapping the tubes?

I have now swapped all preamp and power tubes, including three different 6V6 pairs. The noise is still there. I haven't swapped the rectifier and tremolo tubes though.

Much of the time, when I hear that symptom described, it's failing filter caps. Check your B+ nodes for low DC and/or too much AC. Or, clip a known good cap across them and see if the symptom goes away.

I don't have any filter cap of the right value at hand, unfortunately.
I replaced all filter caps a few years ago with Sprague Atom units (instead of a multicap can like in the stock amp).
The B+ at the filter cap nodes is 30-40 VDC lower than specs.
As regards the AC: should I simply read it at the same nodes with the DMM set to VAC readings? If so, I got the following readings: 4.9, 1.5 and .2 at the 20uF, 10uF and 5uF nodes, respectively.
Is there anything wrong with such values?
--Carlo

[slidincharlie (Carlo P)]

I was wondering if the AC readings give you guys any clue about the origin of the noise in my amp...

[nickb]

I was wondering if the AC readings give you guys any clue about the origin of the noise in my amp...

What effect do the controls have on the issue?

DC readings might be more helpful as I'm thinking that something has drifted off leading to a lack of headroom. Tabulate plate, cathode and grid for each tube.

This the kind a problem where a scope is invaluable.

[pdf64]

The Vac readings from the HT nodes indicate that the filter caps have some degree of functionality.
It would still be a good idea to tack a small known good cap across each in turn.

The amp doesn't seem to have grid stoppers / screen grid resistors on the power tubes; they act to mitigate instability, which may be the issue with your amp..
Suggest a 1K - 10k low power series resistor mounted at each power tube socket terminal 5, resistor body close to the terminal, and 100 - 470 ohm MO / MF flameproof 1 watt series resistor mounted at each power tube socket terminal 4, resistor body close to the terminal.

Whilst you're doing that, a grid stopper on the microphone input triode would be a good idea too, eg 10k - 100k low power series resistor mounted at the tube grid socket terminal, resistor body close to the terminal.

[g1]

I don't think you have ruled out the rectifier tube yet either (or it's connections).

[slidincharlie (Carlo P)]

What effect do the controls have on the issue?
DC readings might be more helpful as I'm thinking that something has drifted off leading to a lack of headroom. Tabulate plate, cathode and grid for each tube.
This the kind a problem where a scope is invaluable.

1) Tone controls do not affect the crackling noise in any way. As for the volume, the crackling noise increases with increasing volume level. There is no noise if the volume is very low.
2) hmmm, I'm not sure that this kind of crackling noise sounds like a lack of headroom.
Vdc readings on the power tubes are the following:
306 V on the plate
300 V on the screen grid (pin 4)
18 V on the cathode.
I don't have a scope unfortunately (and I don't know how to use it anyway) .

The Vac readings from the HT nodes indicate that the filter caps have some degree of functionality.
It would still be a good idea to tack a small known good cap across each in turn.
The amp doesn't seem to have grid stoppers / screen grid resistors on the power tubes; they act to mitigate instability, which may be the issue with your amp..
Suggest a 1K - 10k low power series resistor mounted at each power tube socket terminal 5, resistor body close to the terminal, and 100 - 470 ohm MO / MF flameproof 1 watt series resistor mounted at each power tube socket terminal 4, resistor body close to the terminal.
Whilst you're doing that, a grid stopper on the microphone input triode would be a good idea too, eg 10k - 100k low power series resistor mounted at the tube grid socket terminal, resistor body close to the terminal.

A few questions:
1) What sort of "small" cap should I put across each current cap? I only have a few small e-lytic caps in the range of 5-50 uF @ 10-50V.
2) Does "across" mean in parallel to each current cap?
3) Re: grid resistors: do you mean like in a classic blackface Fender design?

I don't think you have ruled out the rectifier tube yet either (or it's connections).

I have swapped in another 6X4 rectifier to no avail.
Actually the rectifier section and the preamp section were two parts that I have modified just before the crackling noise appeared. Previously I had installed two diodes in place of the 6X4 and I had no trouble. After reverting the circuit back to original specs I started to hear the crackling noise...

[g1]

After reverting the circuit back to original specs I started to hear the crackling noise...

Then you need to triple (or quadruple) check your work, maybe resolder your connections anyway.

[nickb]

1) Actually the rectifier section and the preamp section were two parts that I have modified just before the crackling noise appeared. Previously I had installed two diodes in place of the 6X4 and I had no trouble. After reverting the circuit back to original specs I started to hear the crackling noise...

Oh really? Do tell us more about what you did. It might have been nice to know that up front

[pdf64]

1) What sort of "small" cap should I put across each current cap? I only have a few small e-lytic caps in the range of 5-50 uF @ 10-50V.
2) Does "across" mean in parallel to each current cap?
3) Re: grid resistors: do you mean like in a classic blackface Fender design?

By small I mean that value of the additional cap should be less that the installed cap that it is in parallel with, thereby avoiding changing circuit operation / conditions / stressing the rectifier, much. Given the values in the amp, anything in the range 0.5uF to 5 uF would be fine.
The voltage rating must be at least as much as the max voltage it will be subjected to in the amp, eg at start up. I hope you can see that a 50V cap would be totally unsuitable, may explode in short order.
It doesn't have to be an e cap, film caps tend to have much better performance, eg a 0.47uF 400V film cap should be fine.
Yes, the additional cap should be fitted in parallel with each HT cap in turn.
The installed caps may have excessive ESR; the additional cap will bypass it.
So if the bypass cures the noise, the installed cap it's across is suspect.

Power tube grid resistors like (most) BF Fenders, eg 1k5 and 470 ohm 1 W.

Regarding the mic input stage grid stopper, no, not remotely mounted as Fender did.
Grid stoppers act effectively when the conductor length between the tube's grid pin and resistor body is as short as possible.
Hence mounted at the tube socket terminal, rather than on a circuit board.

[slidincharlie (Carlo P)]

A little update.
1) The mods that I did a while ago to the amp were:
- a re-arrangement of the preamp section following a Fender tweed Deluxe design, that quite a few people have tried with good results in this particular model;
- a replacement of the rectifier tube with diodes.
Then, a few months ago I returned to the stock configuration.
2) I tried the cap-across-cap experiment using a .05@500V cap. Unfortunately it did not bring any good result. The crackling noise is still there.
3) I resoldered one suspect multi-connection and it *seems* it helped. I can't play the amp at adequate volume until Monday night at the rehearsals. I'll post the results.
For the rest I will have to try in the next days, hopefully tomorrow.