5150 - Powered up with no speaker cable

[guitman32]

Hi all

Back story: A few friends of mine are recording an album. They wanted some rich tones so they asked to borrow my early model 5150 and my Lee Jackson to track. I said OK, and didn’t think much of it because the producer isn’t an idiot (or so I thought), and I usually help them in the studio so I wouldn’t be too far from my babies anyway...

Issue: I arrive a bit late today, to find them recording some tunes with the Lee Jackson. To my horror, I see the 5150 still fully powered up with NO CABINET CONNECTED!! I immediately shut it off and proceed to ask what steps they took to switch speaker cables from the 5150 to the Lee Jackson. According to the guitarist, they removed the speaker cable from the 5150 while it was still powered on and connected it to the Lee Jackson. They then fired up the Lee, while leaving the 5150 on. They couldn’t tell me how long it was on for with no speaker connected, and I can only hope they didn’t leave the guitar plugged in to the 5150 before switching over to the Lee.

At this point I proceed to plug in the 5150 to asses the damage. I plugged into the High gain input (the one they were using), and begin a low volume. I was a bit distracted at this point by someone, so I missed looking/hearing anything out of the ordinary on power up. Everything actually sounded OK initially, until I turned the Pre-Amp gain to ~5.5-6. At this point I began to hear nasty feedback (white noise + squeal) when I turned the guitar volume up. It would feedback when not playing, and when I played, the feedback would stop until I stopped playing. Prior to this incident, this or anything like it had never occurred. Anything below 5.5 on the pre-amp gain would minimize and eventually eliminate the feedback. When playing, however, everything sounded pretty good (ie the tone seemed to sound fine. Unfortunately I was rushing things so I didn’t get a really good listen to the amp in fear of frying her even more) At this point I proceeded to examine the power tubes and noticed a blue glow at the bottom of each tube. This blue glow was never present before, and would get brighter and fluctuate if I hit a note/chord. I figured this was bad, so I shut everything down and came here for help (I have been lurking here for a while).

I am wondering how I can asses the extent of the damage. Are the power tubes damaged, pre-amp tubes damaged, or both? Is there a high likelihood of damage to the internals of the amp? Obviously best case scenario would be a simple tube replacement. I made it quite clear that whatever repairs would be on their dime, so I want to make sure I get everything checked out.

[HTH]

preamp tubes will be fine.

output tubes - that blue glow is fine, many tubes have them. have a read of this to understand a little more... http://members.aol.com/larrysb/blue_glow.html

as for the output transformer, you should probably be ok as long as there was no guitar plugged in. of course you never know and leaving a tube amp with no load is never a good idea.

[Enzo]

It doesn't hurt a tube amp to be powered up without a speaker as long as you are not playing through it. The occasional noise might offer some tiny risk, but until you bang out someting in the guitar into it, there is not much risk.

When you play with no speaker load, the large signal can reflect back to the primary side and arc over.

ANy damage to the amp would be to the output transformer, and it would either make the amp silent of have it blowing fuses more than anything else. It should have zero effect on the preamp circuits.

[Amp Kat]

At this point I began to hear nasty feedback (white noise + squeal) when I turned the guitar volume up. It would feedback when not playing, and when I played, the feedback would stop until I stopped playing.

Yup, now that sounds like the 5150 I know. Another thing that determines when the OT blows is how high the post and pregain are set to. If it's cranked the chances of blowing it are even greater with signal plugged in.

[guitman32]

Thanks for the responses!

I just spoke with the guitarist a bit more, and asked for details. Basically, the 5150 was on for a long time (at least 2 hours, possibly more) at idle while they were tracking with the Lee. I then asked specifically if a guitar was plugged in while they made the switch, and he response was that it is possible, but he doesn't think it was actually played with no speaker. What I think happened is that he turned the volume down on his guitar, removed the speaker cable, then moved the guitar cable over the the Lee.

Yup, now that sounds like the 5150 I know. Another thing that determines when the OT blows is how high the post and pregain are set to. If it's cranked the chances of blowing it are even greater with signal plugged in.

The pre gain was set to 6, and the post was at about the same at the time. This is pretty cranked in by book, but what do you think? Also, this feeback was not normal. The above pre and post gain settings prior to the incident resulted in no feedback or unusual white noise with the guitar volume up. Obviously there will be a normal about of hum at volume with any tube amp, but this was definitely out of the ordinary. It basically made recording anything but constant playing impossible.

ANy damage to the amp would be to the output transformer, and it would either make the amp silent of have it blowing fuses more than anything else. It should have zero effect on the preamp circuits.

Do you mean that even if the output transformer is even slightly damaged it would result in either blowing fuses or no sound, but nothing else short of this? Could the tubes be shot and causing the unusual feedback? Maybe the tube sockets? Could the long period of idling with no speaker have an effect on anything other than the tubes, tube sockets and output transformer?

[Enzo]

I won't go so far as to say it is all or nothing, but it is a large signal that does the damage. The only danger from idling is the case where some glitch on the power line or something makes a loud noise through the amp.

Idling won't hurt the amp.

The nature of the damage arcing causes is such that typically it will short windings together or to frame. If short to frame, then the fuses vlow right now. POOF. if turns short together, it will diminish the output at the very least, not make it more gainy.

If you think the tubes are causing feedback, rap on them with your knuckle. Uf they respond out the speaker to that, then they are microphonic. That is not likely caused by arcing in the transformer.

If you think the noise is excessive, turn the amp on with NOTHING plugged into the inputs. Turn the amp up to wherever. Now the noise you hear is what the amp contributes. Now plug the guitar in and turn the guitar volume control to zero. SHould sound about the same. Any additional noise now is from the guitar cord and the guitar. Now turn the guitar up. ANy increase in noise now is from the pickups, not the amp.

[MitchK]

Also, this feeback was not normal. The above pre and post gain settings prior to the incident resulted in no feedback or unusual white noise with the guitar volume up. Obviously there will be a normal about of hum at volume with any tube amp, but this was definitely out of the ordinary. It basically made recording anything but constant playing impossible.

Were you by any chance testing this with a different guitar than normal? some picks are shielded better than others. I have a early model 5150 and in general it squeels like a pig at high volumes, but if I play my fender strat that has an EMG 81 in it it certainly feeds back less than if I plug in my 1970 les paul with its old not so well shielded pickups and wiring. Also your EQ settings will have an effect on this.