I have used my SSE a great deal over the years since I built it, but in the last year it started to become noisy with a loud 'rustling' noise while it sits idle, that appeared to come from both channels. The noise gradually became worse and worse so I recently stopped using it. Now there is rustling with the random pops and crackles. It is not deafening, but it is not the way it used to be and it feels like I am taking a chance by continuing to use it.
Tonight I tried to track down the issue using a scope. It looks like it is actually just on one channel. I can see a very ugly waveform on the speaker posts with no source connected and the sensitivity turned up. I also see a similar picture with the scope on the output tube grid side of the 0.22uF coupling capacitor. I don't see anything with the scope on the connector on the board for the source input, after the potentiometer.
I have eliminated tubes by swapping with other tubes. I had thought it was tired tubes causing the noise.
The evidence appears to be pointing to the CCS on the 12AT7.
The voltages on the 12AT7 are below. There is a bit of an imbalance of the plate voltages, but it does not look too drastic to me.
Interested if anyone has experienced anything similar, or can advise some checks I could do.
Tonight I tried to track down the issue using a scope. It looks like it is actually just on one channel. I can see a very ugly waveform on the speaker posts with no source connected and the sensitivity turned up. I also see a similar picture with the scope on the output tube grid side of the 0.22uF coupling capacitor. I don't see anything with the scope on the connector on the board for the source input, after the potentiometer.
I have eliminated tubes by swapping with other tubes. I had thought it was tired tubes causing the noise.
The evidence appears to be pointing to the CCS on the 12AT7.
The voltages on the 12AT7 are below. There is a bit of an imbalance of the plate voltages, but it does not look too drastic to me.
Interested if anyone has experienced anything similar, or can advise some checks I could do.
| Pin | Role | Vdc |
| 1 | a1 | 252 |
| 2 | g1 | 0.01 |
| 3 | k1 | 2.11 |
| 4 | H | 25 (elevated voltage) |
| 5 | H | 25 |
| 6 | a | 283 |
| 7 | g | 0 |
| 8 | k | 2.03 |
| 9 | H | 25 |
Thanks for the reply. I was just measuring DC voltages, and the AC heater supply is ok, u checked before. The Hester supply is elevated, hence the DC voltage on those pins (4,5,9 Novak, 2,7 octal).
The heater is from the winding on the transformer to the screw connections on the board, but I will try and capture the AC reading next time I check voltages.
The voltage on the output side if the 0.22uF coupling caps is as the g1 reading in the table above. It is not exactly zero, 0.02V, but not an issue I don't think.
Tube idle current is 48mA, (392 - 27) x 0.048 = 17W dissipation.
Comparing with the data on the Tubelab site here ...
SSE - Tubes and Applications
... I am realising I am running this amp quite 'cold' with a 3.2k OPT and 560R cathode bias resistor, but should not be an issue I think.
The first test would be to lift one end of the coupling cap in the offending channel. If the noise goes away the problem is in the driver circuit or the cap itself. If the noise remains it is in the output section.
The usual culprits in this situation are bad parts, usually tubes, connectors, resistors, and semiconductors in that order. Some of the small 1/4 watt resistors can become noisy with age and heat. I have never seen a 10M45S become noisy, but it is a semiconductor so it is possible. The 10K resistor that feeds the 10M45S dissipates a watt or so. It can become noisy after years of use. I have never seen a metalized foil coupling cap become noisy or get leaky if it is not damaged, but some of the boutique film and foil, beeswax, or other exotic caps have gone bad in people's SSEs in the past causing crackling popping and even output tube runaway. The screen grid resistor on the output tubes have been known to get noisy, especially on EL34 builds for some reason.
There are other not so obvious things that can cause this.
First make sure that all of the screw connectors are tight and corrosion free. Try removing each wire tinning it with solder and reinserting it. Re strip as needed if possible.
As stated a decaying solder joint or a dirty or corroded tube socket can too.
If the board was originally soldered with a solder containing water soluble flux (chip quick and others) clean the board with isopropyl alcohol. The residue left behind from a "no clean" or "water clean up" solder flux can absorb moisture and cause all sorts of problems including arc overs.
The usual culprits in this situation are bad parts, usually tubes, connectors, resistors, and semiconductors in that order. Some of the small 1/4 watt resistors can become noisy with age and heat. I have never seen a 10M45S become noisy, but it is a semiconductor so it is possible. The 10K resistor that feeds the 10M45S dissipates a watt or so. It can become noisy after years of use. I have never seen a metalized foil coupling cap become noisy or get leaky if it is not damaged, but some of the boutique film and foil, beeswax, or other exotic caps have gone bad in people's SSEs in the past causing crackling popping and even output tube runaway. The screen grid resistor on the output tubes have been known to get noisy, especially on EL34 builds for some reason.
There are other not so obvious things that can cause this.
First make sure that all of the screw connectors are tight and corrosion free. Try removing each wire tinning it with solder and reinserting it. Re strip as needed if possible.
As stated a decaying solder joint or a dirty or corroded tube socket can too.
If the board was originally soldered with a solder containing water soluble flux (chip quick and others) clean the board with isopropyl alcohol. The residue left behind from a "no clean" or "water clean up" solder flux can absorb moisture and cause all sorts of problems including arc overs.
