Loud Pop when turning off Standby Switch, Snubber Cap?

  • Thread starter Thread starter BesaMoogie
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BesaMoogie

BesaMoogie

Well-known member
Hi, maybe someone can help me out here.
I built a clone of an 1959 SLP where the Standy Switch sometimes makes a loud pop noise. I heard from two techs, that this is quite common in those old circuits. And depending on the phase of the AC you catch when turning off, you get the pop or not.

So after some research I found out, that possibly a cap across the Standby Switch can prevent the popping that seems to come from a voltage spike when turning the switch to off. However, I find lot of different information on this. Sometimes an 220k 2W Resistor is recommended, sometimes a 600 - 1000V cap with varying values and sometimes even a Resistor in parallel with a cap.

So can someone more experienced shed some light in this what would be a good solution?
 
The standby switch is bad and needs replaced. They don’t pop normally. A cap across the switch in an SLP is a bandaid fix.

The pop you hear in your speakers is the contacts themselves arcing across when a gap is presented. The switch cannot disconnect fast enough causing an electrical spark gap that is heard. Large filter caps can cause the problem to be worse since there’s more stored Joules of energy.

Replace the switches. I like to use Marshall branded switches as they’re rated well beyond what is needed and well built.
 
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glpg80 said:
The standby switch is bad and needs replaced. They don’t pop normally. A cap across the switch in an SLP is a bandaid fix.

The pop you hear in your speakers is the contacts themselves arcing across when a gap is presented. The switch cannot disconnect fast enough causing an electrical spark gap that is heard. Large filter caps can cause the problem to be worse since there’s more stored Joules of energy.

Replace the switches. I like to use Marshall branded switches as they’re rated well beyond what is needed and well built.
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Thanks for the recommendation.
Do you have any suggestion, which switch to use? As it is a clone build, the switch I used was new. But maybe I got a bad one or it is low quality in general.
 
If you are shutting off the amp just turn off the power switch first. I only use it when turning the amp on. That cuts the times that switch has to switch high voltage in half.
 
SpiderWars said:
If you are shutting off the amp just turn off the power switch first. I only use it when turning the amp on. That cuts the times that switch has to switch high voltage in half.
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I don’t like to turn it off that way.

The negative grid voltage will discharge at a faster rate than the filter caps if the filter caps aren’t close to stock 32uF ranges, which means you’ll be discharging current and the filter caps through the power tube cathodes since the cathodes themselves are indirectly warm enough without the heater filaments having voltage across them to still allow the cathode to indirectly draw electrons. The choke will limit the inrush to the tubes to a limit but you will shorten the life of the tubes over time. The turning off of an amp is a transient effect of variables and may be fine for some amps but not others especially those with a high b+ and large filtering of the screens and plate caps.

I use the standby switch to remove B+ from the power tubes before shutting down as to not allow the filter caps to drain through them in parallel with the shunt resistors across the filter caps.
 
I’ll need gut shots. Something isn’t right or there’s components missing in the schematic. I’m not saying you’ve done anything wrong, but it’s a kit and I always get suspicious.

If you have the schematic you used, that would be helpful.

You’re 220v?
 
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glpg80 said:
I’ll need gut shots. Something isn’t right or there’s components missing in the schematic. I’m not saying you’ve done anything wrong, but it’s a kit and I always get suspicious.

If you have the schematic you used, that would be helpful.

You’re 220v?
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No reason for me to be offended here. It was my first build and I consider myself still in the learning process.
I can PM you pictures and schematics, so maybe there`s something wrong.
It`s a 230v kit.
 
I’ll look it over and let you know if I find anything. Could be related to your grounding scheme or something else. I make no promises but I will do what I can to spot something :)

Cheers
 
glpg80 said:
I’ll look it over and let you know if I find anything. Could be related to your grounding scheme or something else. I make no promises but I will do what I can to spot something :)

Cheers
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Thanks man, really appreciate this!
 
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