crackling sound on my SF super reverb

[qwixzh]

hi all,

I've recently experienced a crackling sound on my 74 super reverb. It is happening on both channels even with the volume control at zero (1 for this based on the knobs ). Its happening almost instantly at the time i turned it ON. The crackling sound increases volume as i increase the volume control.

What should i look for initially?

thanks!
qwixzh

[mikeboone]

Check the preamp tubes... You can quickly isolate things by removing V6 (Phase Inverter) closest to your power tubes. If the problem is there, its in your power section, if not, its in the preamp.

Replace V6, then starting with V1 (on your right as you look into the back of the amp), remove this tube and power it back up and see if the issue remains. Repeat this for V2, 3, etc. and see where (if at all) the problem stops.

Be very careful when replacing these tubes as they dont have index key like the power tubes do. Just pay attention to the gap between pins 9 and 1.

Another thing it could be is your Master Volume as it affects both channels. You will need to spray it with a cleaner. Make sure it is made for cleaning electronics and is safe on plastics. Radio Shack carries a a product called Precision Electronics Cleaner that works quite well.

[jmaf]

What should i look for initially?

This may sound obvious, but how long has it been since you last changed tubes?

If you have a set of spare tubes known to be good on another amp you might want to substitute them and see. If the crackling continues then you've eliminated that possibility and you can go on to checking pots and solder joints with a chopstick.

[qwixzh]

Check the preamp tubes... You can quickly isolate things by removing V6 (Phase Inverter) closest to your power tubes. If the problem is there, its in your power section, if not, its in the preamp..

will do this later

Replace V6, then starting with V1 (on your right as you look into the back of the amp), remove this tube and power it back up and see if the issue remains. Repeat this for V2, 3, etc. and see where (if at all) the problem stops.

already replaced V6 with a new matched triode but problem is still there

Another thing it could be is your Master Volume as it affects both channels. You will need to spray it with a cleaner. Make sure it is made for cleaning electronics and is safe on plastics. Radio Shack carries a a product called Precision Electronics Cleaner that works quite well.

no master volume on this

This may sound obvious, but how long has it been since you last changed tubes?

If you have a set of spare tubes known to be good on another amp you might want to substitute them and see. If the crackling continues then you've eliminated that possibility and you can go on to checking pots and solder joints with a chopstick. .

probably 3-4months ago since i last change.

[mikeboone]

I didnt want to change V6, i was merely suggesting to place it back in the amplifier since the sentence prior would have you removing it.

The idea is to see if the noise stops when you pull the PI, simply dividing the circuit.

[JoeM]

I'm not sure what exactly you mean by a "crackling sound", but my SFSR (about the same year as yours more or less), had what I'd describe more like a 'frying egg' sound that turned out to be the carbon composition plate resistors on the 12AX7s preamps. I put in carbon film resistors (100k 1W) and that took care of it. On other amps I've had noises that were solved by cleaning up the tube pins with Caig Pro-gold, also sort of a sizzling type of sound.

[qwixzh]

I didnt want to change V6, i was merely suggesting to place it back in the amplifier since the sentence prior would have you removing it.

The idea is to see if the noise stops when you pull the PI, simply dividing the circuit.

I see, sorry i misqouted you. haha I havent touch the amp yet. probably tonight after work. thanks!

I'm not sure what exactly you mean by a "crackling sound", but my SFSR (about the same year as yours more or less), had what I'd describe more like a 'frying egg' sound that turned out to be the carbon composition plate resistors on the 12AX7s preamps. I put in carbon film resistors (100k 1W) and that took care of it. On other amps I've had noises that were solved by cleaning up the tube pins with Caig Pro-gold, also sort of a sizzling type of sound.

'frying egg', yes, similar to that sound. I already did changed the 100k plate resistors about a month ago, will check the sockets later. thanks!

[jmaf]

probably 3-4months ago since i last change.

They're brand new. If I were you I'd follow JoeM's advice, could be a bad resistor.

[Gtr_tech]

Did you change the plate loads in the PI section? I see those fail alot in the newer Devilles and the like.

[qwixzh]

by plate load, you mean the 82k and 100k resistors for a BF and 2pcs 47k for an SF?

havent touched that one either. will look at it also. been very busy lately...

thanks!