Can I change the Tube in an Ampeg BA115HPT?

Do not mess with this yourself. Take it to a repair shop. The switch needs to be replaced, at a minimum and possibly other parts. There is lethal voltage involved in a tube amp.

Sounds like a bad power supply filter cap...the buzz you're hearing is the 60 cycles hum that filter caps reduce or (hopefully) eliminate altogether. It may also be related to the tubes but finding the bad one will require some work. Any friends have this same amp? If so, try swapping one tube at a time and see if you get lucky. If this amp is new take it back ASAP as it is a manufacturing defect.

You have probably blown a tube. This amp has one 12ax7 and one 12au7 in the vintage preamp. You can try replacing the preamp tubes, but it can be quite dangerous if you don't know what you're doing!

I assume that the hum exists without any cables plugged into the input. If that is not the case, then make sure all input cables and source are using BALANCED lines. If the hum is present with no input cables, since this is a tube type preamp, the input tube for the bad channel is first suspect. It is likely a 12AX7 or 12AT7 type tube. Since there are two channels, the first thing is to SWAP the two channels and see if the hum moves with the tube... if it does replace the tube. I have seen another problem with preamps in general where bypass capacitors from either side of the XLR connectors to ground get zapped with static and one side shorts. This reduces the gain by 6 DB and leads to hum.

Here is the schematic:

http://elektrotanya.com/fender_vibrolux_6g11_schem.pdf/download.html

Check the quality of the DC bias as being free of ripple.across teh 25 mfd/50v cap filtering the bias.
If much ripple there, replace the cap. Also the .05 bypass on the swinger of the intensity control.

This should be an easy fix, but you might need an oscilloscope to diagnose it quickly... or just parallel caps across these to see if it kills the hum.

The main high voltage filter caps are also suspect as well as the bypass of power to the ealier inverter stage a 16mfd/450v

There are lethal voltages in this so use great caution when troubleshooting... the caps hold a nasty charge.

If none of the inputs work, then you may have lost part of the power supply. DO NOT go into this amp under any circumstances !!!!...there is enough voltage in here to kill you crispy !! I would strongly recommend that you take it to a respected repair shop that specializes in guitar amps. Possibly visit the ampeg website for an authorized repair center. It might cost a few bucks, but the Ampeg is highly sought after by musicians everywhere !!!....thanks... Rob

okay so buzzing noise. i have seen this once before. the extension cord feeding a power strip with two amps plugged in and powered on off of a low amp electrical outlet with fluorescent lights overhead.......try running only one power unit off of the electrical outlet you are using, just one straight into the wall with the lights out. if you still hear it i wonder if the output transformer is starting to go south. if you listen close sometimes you can hear the buzz of a transformer powered on, example: you can hear the buzz of a power transformer on a power pole in peak hours. tubes usually make hisssing, crackling type noises. next would be to tap on the tubes while in play with an orange stick or pencil eraser to listen for microphics or the tapping itself thru the speakers. if you hear with out a doubt change them.

NEVER use switch cleaner such as DeOxit on pots !!!!!

I have used CRC226 available at Home Depot electrical dept.

I SUSPECT that the resistance element in that pot is cracked. This is common if the knob sustained a hit. Replacing the pot is the only cure if the element is cracked.

Reviewing the schematic, ALL audio passes through the wiper on the HIGH tone pots. If the wiper is intermittent, so will be the sound.

If your unit is solid state amp, the pot looks like a 10K ohm LINEAR taper.
If you have a tube type, then the pot is a 250K ohm LINEAR.

Note that the other tone pots in the units are AUDIO tapers, but the high is a linear taper.

Get repair parts from either DigiKey.com or Mouser.com

Well whatever you do do not fire it up. If you have a VARIAC or know someone wh has a variac use that to bring the supply voltage up gradually in case a output cap is dried out. What kind of amp is it? The tubes are stuck due to oxidation of the tube socket and pin on the tube. If you have already powered it up and it didn't smoke your input transformer and you didn't hear any humming or buzzing I would say the next step would be to check the tubes out. If you have access to a tube tester obviously use that however you can check the bias after they warm up. You will also want to do the usual maintenance stuff like cleaning the potentiometers with contact cleaner and also check your 1/4 input and output jacks.

Ah yes: complete catastrophe. Usually when everything is not working, it is a single reason and when that single reason is fixed, so is the amp.

Based on what you tell me, I am going to say you have a bad power tube. And I will even go as far to say that you buy a set of tubes and play on them as long as you can. If so, then you not only burned up the tube, but weakened the power section and may have a slooow tube frier. I always tell customers: modern tubes have an effective life of less that 1200 or so hours and that varies. Replace the tubes once per year if you gig with the amp: period. Otherwise, guys like me (doing repairs) stay busy and the tube companies get rich selling tubes to amps with weakened power sections.
It sounds like you burned a power tube and blew a fuse. First, you need a new set of power tubes and some extra fuses (go to Radio Shack and set the value you need in SLOW BLOW fuses).
Then, replace all the power tubes. Don't worry about biasing yet, we are just seeing if there is a problem. Next, replace the fuse.
Turn on the amp and play on it at various volumes and settings. If all is well, take the amp to a tech and get it rebiased. If you can afford it, pay to have the grid/plate and other resistors changed so the power section will be like new (clean slate with new tubes). Your amp will love you for it.
Almost all of the amps I have worked on for performance problems (cant keep tubes to stay alive for very long) are directly related to end user use. When you use a tube until it blows it ALWAYS TAKES SOMETHING WITH THE TUBE WHEN IT GOES (like the power section components). The compents will be weakened and the tubes will 'wear' at different rates that can even move them out of the 20% tolerence they must be within to sound good. 99% of the time a board repair with a retube after a catastrophe fixes the amp until the next time it is 'run into the ground'. Tube amps are NOT invinceble: they are weak compared to solid state and expensive to own. But we love tubes because they sound great. I have solid state to knock around on, when when it counts, I play only with tubes. I have spent hundreds on good tubes because you do get what you pay for.

Hope this helps!
-mike