PLease help me troubleshoot which tube/s needs replacing

[Reinhard]

My SFDR has this feint whistling sound, like an old kettle boiling or wind howling around a corner, but obviously much softer.

Any help woould be appreciated.

 

[Diablo1]

If you have one good 12AX7 and one good 12AT7 replacement tube, you just pull preamp tubes one at a time and replace with one of your good replacement tubes. Then turn the amp on and see if the whistling is fixed or not. If not fixed, then you know that your old tube was not the problem, remove the new tube and stick your original tube back in the socket, and move on to the next tube socket and repeat.

In the unlikely event that the problem remains after you've swapped in new preamp tubes in every socket, then you need to try to swap in a pair of 6V6 power tubes.

 

[Reinhard]

Thanks Diablo1! I'll be sure to test the pre-amp tubes first and leave the powertubes as the last resort, as they'll need biasing.

 

[sliberty]

Try gently tapping a pencil / chop stick / other on each preamp tube. if tehre is a microphonic tube, you will hear the tapping brightly coming through the amp. That would be the one to change. My guess is it will be one of the first 2 preamp tubes.

 

[SheltonGuitar]

+1

 

[toxpert]

Check the condition of the tube sockets and cleanliness of tube contact wires.

safety note: before fiddling around with tube contacts or going inside the amp...be sure that all caps are discharged

I recently restored a mid-70s SFDR that came to my possession with RCA preamp tubes from back in the day. Powered up - it had this low volume, cyclic cascading 'surf' sound in the background. Just loud enough that you could hear it when nothing plugged in or when the guitar was plugged in but not playing.

This 'surf' sound was still there after I replaced the electolytic caps for both bypass and filter caps - the original caps did have to go 'cause they were all spewing out the ends.

The tubes felt tight..but I figured I would go ahead and double check tensioning. Pulling the first tube, I pulled out my magnifier lens and there was some kind of tacky residue on the base of each tube socket as well as the bottoms of the corresponding tube. A shot of De-Oxit got rid of that stuff. I then retensioned the sockets. Before putting the tubes back in, I used a soft brass-bristle brush to carefully clean/burnish the contact wires on the base of the tubes.

With the cleaned tubes installed into the retensioned and clean tube sockets.... the amp is now dead quiet. The tone and amp response using those old RCA tubes is great.

I suspect that the previous owner dumped the amp thinking something was dreadfully/terminally wrong with the electronics...

 

[Reinhard]

Thanks for all the info guys! Turns out the 5th tube from the left (looking at the amp from the back) was microphonic. Replaced it and I'll give the amp a workout to see if my problem is sorted.

Thanks again!