finding a bad preamp tube

[sean106alcon]

Hey guys, I know most of it is trial an error; but as i mentioned in a previous thread I am almost positive the hiss or squeal at loud volumes for band situations is due to a bad preamp; again since I first changed them all to begin with. Since the El 34's I put in block access to all the preamp tubes. What is the best or easiest way to find the bad preamp tube with out having the amp on? (some one on another board gave me a good tip to turn the amp up at the hiss volume.....no guitar plugged in but into a cab....and tap each preamp tube with a light stick or pencil...and if a noise is heard from the speakers when I tap it lightly...then that is the culprit.) Also he said V1 is usually the one that causes the horrible feedback at band volumes if it is a bad preamp tube.

How can you do this with the EL34's blocking them all? Meaning is it safe to run the amp with out the El34's in it to do this tap test? Also can you visually just take em all out and see a bad tube like a light bulb that is burnt out? Thanks again

 

[eudaimonia02912]

Given the Mark V's tight layout, you pretty much need to turn the amp off, remove the power tubes, replace one preamp tube, put the power tubes back in, turn the amp on, and see if the problem is solved. If not, repeat the entire process with a different preamp tube.

Otherwise, you could remove the chassis from the headshell and replace the preamp tubes more easily. In that case, you'd just need to put it on standby whenever you replace a preamp tube, but wouldn't need to remove the power tubes or turn the amp off. However, removing the chassis from the shell is a hassle, too.

 

[sean106alcon]

Thanks man. I see four screws on the front grille. Perhaps that is comes off easily and allows easy access to the preamps tubes?

Edit:
Again, is their a visual way to tell if a preamp tube is bad as I mentioned like a light bulb?

-me:Toob Noob :shock:

 

[domct203]

You would hope so but no, just a solid piece of wood back there.

Dom

 

[sean106alcon]

You would hope so but no, just a solid piece of wood back there.

Dom

LOL yup. I just opened it up. Saw the wood. The wood had 4 screws also and was like great....nope. Must be glued in as well **** they make those things tight in there. Well for now I swapped out V1 and tomorrow at jam will see how goes. Thanks.

 

[sean106alcon]

You would hope so but no, just a solid piece of wood back there.

Dom

LOL yup. I just opened it up. Saw the wood. The wood had 4 screws also and was like great....nope. Must be glued in as well **** they make those things tight in there. Well for now I swapped out V1 and tomorrow at jam will see how goes. Thanks.

Well looks like V1 was the culprit for the Hissing. Kept all my stock preamps and just changed one to see and sure enough; no hiss the next day.

Note to boogie. Please make it easier to change preamp tubes? removes the wood glued in after the front panel off or allow it to be screwed off. KThxBy