Single Tube Amp (Distortion)

Ron G · #1 01-21-2013, 07:36 AM

I have an Ampeg BA-115 HPT with a single 12AU7 tube. I can't plug and play I have to let it warm up for about 5 minutes if I don't I get a lot of distortion and it will stay the entire time I'm playing. Ampeg replaced my first amp because it had the same problem, but I didn't think to let that one warm up.

Is it normal to let tube amps warm up?

Nashrakh · #2 01-21-2013, 07:46 AM

Power tubes need warmup. Preamp tubes don't. I find the problem you describe to be odd, not playing this amp but as a tube amp user this never happened to me.

Edit: forgot to say, the tube you mention is a preamp tube. As such the amp would not need any warmup because it doesn't have power tubes. In my SS amps there's a bit of a delay between poweron and standby but again, distortion never happened to me there either.

RickenBoogie · #3 01-21-2013, 08:21 AM

The single 12AU7 in your BA-115HPT is only there for "flavor", and should not need more than a few seconds to be up and running. Also, it should not be causing ANY distortion. I suspect what you're hearing is your speaker. I also suspect you may be pushing the volume and the low band of eq, farther than that spkr can handle. What comes next, is a blown spkr. Your amp is capable of pretty decent power output, but the limitations on that will always be the single 15" spkr.

Bassmec · #4 01-21-2013, 08:52 AM

Quote:

Originally Posted by RickenBoogie

The single 12AU7 in your BA-115HPT is only there for "flavor", and should not need more than a few seconds to be up and running. Also, it should not be causing ANY distortion. I suspect what you're hearing is your speaker. I also suspect you may be pushing the volume and the low band of eq, farther than that spkr can handle. What comes next, is a blown spkr. Your amp is capable of pretty decent power output, but the limitations on that will always be the single 15" spkr.

This is perfectly correct.

Ron G · #5 01-21-2013, 09:47 AM

Quote:

Originally Posted by RickenBoogie

The single 12AU7 in your BA-115HPT is only there for "flavor", and should not need more than a few seconds to be up and running. Also, it should not be causing ANY distortion. I suspect what you're hearing is your speaker. I also suspect you may be pushing the volume and the low band of eq, farther than that spkr can handle. What comes next, is a blown spkr. Your amp is capable of pretty decent power output, but the limitations on that will always be the single 15" spkr.

This is my normal amp settings: volume 2 or 3, low;mid;hi 5, master volume 5.

If I let my amp warm up I can play at all volumes.

Bassmec · #6 01-21-2013, 09:58 AM

Quote:

Originally Posted by Ron G

This is my normal amp settings: volume 2 or 3, low;mid;hi 5, master volume 5.

If I let my amp warm up I can play at all volumes.

You see we have no way of telling what the output of your bass guitar might be or how hard you are playing with it.
Unless there is something wrong with the power supply to the tubes heater (normally at 6.3 volts) I can't see what could cause such an extended warm up time.
I am afraid It's tech time unless you can get in there and measure some stuff for us.

Ron G · #7 01-21-2013, 10:04 AM

Thanks, Ampeg replaced my first amp with the same problem two repair shops could not fix it.

B-string · #8 01-21-2013, 10:15 AM

I would suspect the heater voltage to be more like 12-13 volts feed with heaters in series through a dropping resistor from the +15 VDC rail. The dropping resistor could be at fault, the B+ for the plate supply could be soft, the tube could be bad and many other possibilities.
If 5 minutes are required for proper operation that gives a good amount of time for trouble shooting. The shop should be able to find the fault if they are any good at all.

5port · #9 01-21-2013, 10:59 AM

Quote:

Originally Posted by Ron G

Thanks, Ampeg replaced my first amp with the same problem two repair shops could not fix it.

It couldnt hurt to do a line voltage check. Should be between 110 - 120 VAC. If its happening with two amps in a row I suspect something local. Then again the repair shops had the correct voltage.

Mr. Foxen · #10 01-21-2013, 11:03 AM

Odd problem a local tech had in an amp, the heater fuse had gone resistive, so everything measured fine unloaded, but with all the valves going, there was a voltage drop that made it all go wrong.

BogeyBass · #11 01-21-2013, 11:05 AM

or if you had this problem with ampeg before.

then stop buying ampeg.

sounds like a #### thing to say but it really can be that simple

Ron G · #12 01-21-2013, 06:34 PM

Quote:

Originally Posted by BogeyBass

or if you had this problem with ampeg before.

then stop buying ampeg.

sounds like a #### thing to say but it really can be that simple

The first one was under warranty.

JimmyM · #13 01-21-2013, 09:54 PM

Why couldn't it be a tube?

B-string · #14 01-22-2013, 09:54 AM

Quote:

Originally Posted by JimmyM

Why couldn't it be a tube?

Not out of the realm of possibility.

Ron G · #15 01-22-2013, 02:37 PM

Quote:

Originally Posted by JimmyM

Why couldn't it be a tube?

I've replaced the tube in my first amp, the repair shop replaced the tube. It seems the only thing that works in letting it warm up. No more Ampeg's for me!

1958Bassman · #16 01-22-2013, 03:27 PM

Quote:

Originally Posted by Nashrakh

Power tubes need warmup. Preamp tubes don't. I find the problem you describe to be odd, not playing this amp but as a tube amp user this never happened to me.

Edit: forgot to say, the tube you mention is a preamp tube. As such the amp would not need any warmup because it doesn't have power tubes. In my SS amps there's a bit of a delay between poweron and standby but again, distortion never happened to me there either.

Tubes are tubes and it doesn't matter if it's an output or not- the anode still has to be hot to start emitting electrons. Could be a bad tube, weak heater circuit, transformer winding, etc.

B-string · #17 01-22-2013, 03:49 PM

The anode (plate) does not need warming, the cathode does. Just to be clear.

**mjac28** · #18 01-22-2013, 03:52 PM

I agree with that one guy.

gurensan · #19 01-22-2013, 07:29 PM

ALL tubes need to warm up. Not just the big ones.

Ron, allowing time for the tube to heat up is par for the course. Instant power-on is something out of the SS world. Once the tube is ready the distortion should go away. If the heater's power supply is good it can still take 2-3 minutes for it to come up. This is normal.

If the tube never gets warm enough, it'll distort like mad all day.

I haven't taken the dive into that amp's schematic so I don't know for sure if it's got a regulated heater or not. if it doesn't, you could have soft power. If it does, you could have a bad regulator. There are a million things that can cause a tube to distort; a floppy heater supply is only one of them.

Honestly I've never had an issue like this with any tube amp I've ever owned or built. Usually the distortion is on purpose!

Ron G · #20 01-24-2013, 02:27 PM

Quote:

Originally Posted by gurensan

Ron, allowing time for the tube to heat up is par for the course. Instant power-on is something out of the SS world. Once the tube is ready the distortion should go away. If the heater's power supply is good it can still take 2-3 minutes for it to come up. This is normal.

Thanks