trinityamps.com

Although this question is about a non-Trinity amp perhaps someone here can help me. My Fender Twin is emitting a sound (birdlike or radio chatter interference-not loud) sound. The "idiot lights" in the back light green and everything sounds fine when playing the guitar. I turned all lights, electric devices I could to eliminate any possible interference. The sound is intermittent and lasts maybe 5-10 seconds. I spoke to someone at a large music store (repair area) and they said it could be one or more of the 12AX7 preamp tubes going microphonic. The Amp is just a year old played maybe ten hours a week at volume 2-3. I thought the power tubes wear out faster then the preamp tubes. Anybody have ANY ideas...?

bubbleboy

Even though the amp is only a year old, tubes can still die.
the tubes would be the 1st thing to check.
If you have a known good extra 12AX7take your preamp tubes out one at a time and replace with the good tube. You should always suspect the tubes 1st. Plus it's cheaper (well, unless you buy N.O,S. if that's all it was ).
Later

Also, some of the new manufactured tubes have a reputation to go microphonic quickly. I had a new Ei 12ax7 go microphonic in a week.

Try thi tube first.

How do I change the tubes safely. I know, you don't touch the tube glass with your sweaty hands and you probably ease each tube out one at a time etc. But is there something hidden I should not touch while I'm standing in bucket of salt water with wet hands...

Bubbleboy

How do I change the tubes safely. I know, you don't touch the tube glass with your sweaty hands and you probably ease each tube out one at a time etc. But is there something hidden I should not touch while I'm standing in bucket of salt water with wet hands...

Bubbleboy

If the tubes are hot , use either a heat pad or... wait till they cool down. When taking them out just pull slowly and firmly with slight movement from side to side

Let them cool down a bit. Rotate them in a small circle as you pull them out. Not a bad way to put them back in i.e. rotating the top as you push it in.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

The sound that you describe could be cell phone interference. When a digital phone "wakes up" and tries to communicate with the tower it will make a chirping raspy sound for about 5-10 seconds.
Is this a possibility?


LeeMo

I'll say one thing for you Leemo, You have some interesting info stored in your head. I can't say I've ever heard that one. But of course will now store for future reference
By the way that AC-15 site is nice

Thanks for all of your suggestions. Sorry about all these questions but I've never done this before. I'm concerned about putting my hands in that area -I might mistakingly touch a capacitor with 500 V and I'll be "ghost of bubbleboy". I've read that there is a way of bleeding or draining the electricity from these capacitors etc. with a device. Where would I get one of these? When changing or swapping 12AX7 tubes do I have to partly dismantle the amp or can I CAREFULLY reach in and pluck out the tubes. Lastly, the preamp tubes (12AX7) tubes seem to have a metal cover cylindrical cap over them-how do I take off these pieces to get at the tubes?

Once again I apologize for all these questions...

bubbleboy

Bleeder resistor info - refer to viewtopic.php?t=114&highlight=bleeder

and

viewtopic.php?t=245&highlight=bleeder

To remove the metal cover / shield is straighforward. It has a bayonet mount so just push it down, twist it and pull it upwards.

You dont have to dismantle anything. Thats why the sockets are there, so you can easily unplug it.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

i know exactly what you're talking about. my cell phone chirps like that whenever i'm near any speaker, PC speakers, radio, sometimes tube amps etc. sometimes it will super loud at night if i leave my phone near my computer.

Thanks for the info. Now I'm almost as confused as before... How do I know whether my amp (Fender Twin) has a "bleeder"? When I look inside, I see only cube-like forms (Transformers?) some larger then others, and tubes. Where would the "bleeder" be? If this amp doesn't have a "bleeder" where could I get one and wattage should I get for a 100w amp? Last question for the next ten minutes (I must sound like idiot...) where do I put it (I know up my _ss)?

bubbleboy

One of my day jobs is installing GPS tracking systems in "Secondary finance" used cars. The GPS co-ordinates are transmitted over cell phone towers. When I put the antennae too close to the stereo, I'll get that interference. It's a pretty slick system. If the customer doesn't pay, the finance company can beep them to remind them of their obligation. The next step is to disable the starter and locate it for pick-up. They usually have a short grace period before that happens.


LeeMo

As your driving down the road, oops you didn't pay your bill and the next time you stop they shut your car off? Ouch.

They usually set it up to beep 15 times every time the car is started for a period of five days. It won't be there on the sixth day.

LeeMo

oh man, that's tough. I've got a compass Works really well, I think I use that

bubbleboy,
Man , I'm sorry for the high-jack.

Did you find out what the problem was? As long as you don't pull the chassis out of the cabinet, you should be safe. The tubes are easy to change out. You'll probably need to remove the back cover to access them on a Twin. There should be a chart glued to the inside of the cabinet telling you what goes where. In a Twin there should be 4- 6l6's(or 5881's) (The big ones) and 4-12ax7's and 2- 12aT7's. The AT7's are the Phase inverter and the Reverb driver. The Vibrato channel uses 1 & 1/2 -12AX7. The other half is reverb recovery. The normal channel uses both halves of a 12ax7.
If it's a preamp tube that's bad, you should be able to find which one by turning one channels volume down, then the other. You can also remove one tube and test , then the next.
Tube amps are very forgiving when it comes to running without the tubes.

You can look at the layout and schemo on the Fender Field Guide.
http://www.ampwares.com/ffg/
Which model is it?


LeeMo

Bubbleboy! If you're not sure what is what inside the amp chassis (inside the amp where all of the wiring is). do NOT attempt this on your own. You can get hurt bad. Take it to a tube amp tech and have him show you what is what.

The outside of the amp where you can get to the tubes and where the transformers are at is relatively safe (parts get hot). The inside has high voltages that can kill you if you discharge a cap through your body.

The way I do a quick discharge of the caps is to turn the power switch off while playing the guitar and keeping the standby switch on. This gets the caps in my amp down to a few volts. You still have to be careful as some will remind you that caps have a tendancy to partially recharge. But this works for me so far.