Sleeping 6L6 power tube?

[Skrazz]

I have myself a Fender 75 Combo amp (1x15) and very simply... im in love with it. The thing is loud and the tone is wonderful. Well, the other day i just so happened to notice one of the two 6L6's was not glowing, nor was it hot. I switched the 2 power amp tubes to see if it was the tube itself but this was not the case. The only logical thing i can come up with is maybe this tube is some sort of reserve for when an 8 Ohm cabinet is attached. yes this probably sounds silly but since there doesnt seem to be any problem with my amp other than the visual. Is this at all a possible scenario? i dont want to run this thing if there's some fault with it but i really dont hear any problems with the sound coming out of this thing. any tube related help would be great even if it's not specific to my model of amp!

-josh

[Skrazz]

http://a549.ac-images.myspacecdn.com...9eefb946e4.jpg

[corworld]

Nice theory about a sleeping tube, if only it were true. The amps needs both tubes working to sound right. At a really low volume, one tube might work fine but at a loud volume it would sound bad with only one tube. I'm surprised you don't hear anything wrong. Are you sure the tube is not working? Pull one and see if it sounds worse. The only thing that should light up is the filaments. If the plates are glowing that is bad. The sound may be fine but the tube will fail at some point. Do a search on tubes if you don't know where a plate or a filament is on a 6L6

[hangman]

something is wrong with your amp. the tube should at the very least be quite warm.

my guess is that something is disconnected inside, or the tube socket needs to be tensioned because it is not making good contact with the tube.

have someone who knows what they're doing look at it.

[Skrazz]

Alright, so it IS in fact a sick amp. Last night i flipped on the power switch and both tubes came on fine. the amp was MUCH louder than before. i'm going to get it checked out before this problem happens again. thank you all for your time and help!

[Wavebourn]

Check wires going to 2 and 7 legs of that tube and resolder where the contact was lost.

...it is amazing how tubes can fool human perception!
During the last concert I've discovered one 6P3S in my amp has glowing anode. It turned to be I've overbiased them, so evaporated anode metal shorted the path between grid and cathode so only -1V of bias voltage left in one shoulder (2 tubes in parallel)... Actually, it was a single ended kind of an amp as the result with much less output power and higher distortions if to measure them, but people kept saying it sounds brilliant... Go figure...

[ulysses]

I had a '68 Bassman once and the output transformer "burned out". One half of the primary turned open-circuit because the wire melted (just in one little spot, where the internal wire joined with the external wire). This caused one of the tubes to not get any plate current, which turned it into a "single-ended" amplifier. Except of course the transformer wasn't gapped for single-ended operation. The result was a saturated transformer and an increase 2nd-order distortion (push-pull amps naturally cancel even-order distortion). These could be considered positive attributes to many guitar players. Of course there's also a loss of output power and a loss of headroom, but those are a matter of degree. So don't feel bad about not thinking your amp "sounded broken".

In your case, since the tube didn't heat up at all, I would suspect the heater supply. With a cold cathode, the tube won't conduct any plate or screen current. The fact that it's intermittent has to make you wonder though - you'd expect a high-current heater supply to be either connected or not. Maybe the tube filament itself is broken. I'm sure you've seen light bulbs like that, where one end of the filament is broken but if you hold it just right it swings into place and lights up.

Anyway, good luck! I hope you bring it to (and are able to find) a good, reliable amp tech that doesn't charge too much money, take too long, or totally change the sound of your amp! If you find such a tech, ask him how Big Foot and the Loch Ness Monster are doing.

[Wavebourn]

Ulysses,
Bassman does not have class A biased output stage, so you had a full spectrum of harmonics, plus DC following an envelope. Mine is a class A amp, and one shoulder biased the output tranny, so it turned into a fair single ended amp, however too much power was going to heat a room... I am going to repeat that now intentionally, using a FET to bias a push-pull OPT for SE. However, costs of the transformer saved will go to Pacific Gas and Electricity...

Skrazz, if you don't know how to check 6.3V between 2 and 7 pins of the tube and resolder wires it is indeed better to bring it to some experienced tech.

[Skrazz]

i'll give it a look in the morning. fortunately i know of a great tech who takes good care of his clients. it'll definitely help knowing what's wrong with it before i take it in though. all your tube talkin' has inspired me to read up on these gems