Power heads separated from speakers.

Keith Hilton · Post by **Keith Hilton** » 19 Mar 2004 8:19 pm

I am working on a friends bass amp tomarrow, and something important came to mind. The amp is one of the old Ampeg tube bass amps with a separate power head, and a separate speaker cabinet. If I remember correctly the head hooks to the cabinet with a 1/4 inch plug. The wire is not shielded of course. The same connection system was on many of the old amps, including the old Fenders. There is "no way" a common 1/4 inch plug can be plugged into a 1/4 inch jack, without the ground touching the signal connection. When the ground touches the signal, you don't have 16 ohms, you don't have 8 ohms, you don't have 4 ohms, you have ZERO ohms. "0" ---Ohms means a dead short.
A speaker amplifies an AC signal. What tears up a speaker is DC current. A dead short is DC current. So, plugging in one of these separate power heads to a speaker cabinet could destroy a speaker if the power head was on. It is obvious the connection should be made when power to the head is off. What I am wondering is how companies like Ampeg and Fender were able to guarantee anything with guys connecting up with the power on. I am certain it happened all the time and is still happening. I just wonder how much equipment has been cooked this way?

John Floyd · Post by **John Floyd** » 19 Mar 2004 9:18 pm

It never was a problem with Tube Equipment, But there has been many a solid state amp blown by doing this. Its only Shorted for a moment and only on the Speaker cabinet end, It never shorts when you plug in on the Amplifier end.[This message was edited by John Floyd on 19 March 2004 at 09:26 PM.]

Bill Crook · Post by **Bill Crook** » 20 Mar 2004 8:01 am

As long as there is NO signal being applied to the input of the amp,the AC signal at the output (speaker)jack is 0 (Zero). In theory,this is true but,because like a tube set, the output q's are not balanced. Therefore,one may see maby 0.3volts +/- at the speaker connection. So,as long as no signal is being applied to the speaker jack, it is perfectly O.K. to plug and un-plug the speaker cables (either end).

Remember, transistor amps do NOT have or need a output transformer 100% of the time.

Now with that said.............

I don't understand why anyone would want to change a speaker setup while a signal is being applied to the device.

John Floyd · Post by **John Floyd** » 20 Mar 2004 10:38 am

So,as long as no signal is being applied to the speaker jack, it is perfectly O.K. to plug and un-plug the speaker cables (either end).

Theory is a wonderful thing, but just when you think you have all the answers it will reach out and bite you in the hind parts.

Some of the Older Solid State amps like Shobud, Standel and others with not a lot of protective circuitry in them are looking for an excuse to become unstable and blow sky high.

On the Speaker cabinet end with a standard 1/4 plug and jack when you insert that end into the jack you are shorting the tip to ground until it is seated completely in the jack. I've repaired a lot of amps that have blown from doing this.

I would never reccomend shorting the output of a solid state amp to ground under any circumstances, It will no doubt make some shop owner very happy. The repairman could care less, because he probably isn't making any more money either way.

Keith Hilton · Post by **Keith Hilton** » 20 Mar 2004 11:33 am

If there is one thing I've learned from manufacturing electronics, it is putting all the protective devices on the electronics I can dream up. I'm sure Ampeg, Fender and the other companies had ways of knowing when a user connected the head with cabinet with the power on. Hartley Peavey once told me this, "You would not believe how many people plug the two wires of a speaker into a 120 volt wall receptical." I guess they do this just to see what will happen.

john widgren · Post by **john widgren** » 20 Mar 2004 12:50 pm

I am a big fan of the Marrs ultralight cabs. To reiterate Segal's comments, I agree that the closed backs sound more consistant under a wider variety of stage settings. I belive that the closed back does give a few db increase in Bass "punch" which can be easily EQ'd if you don't like. The reduced weight and size of the cabs is a huge plus in my book. I have played them in both open and closed back configurations, and have come to prefer the closed back setup.

Paul Osbty · Post by **Paul Osbty** » 20 Mar 2004 1:52 pm

This really isn't a problem.

Say the cable is connected to the head. The signal is on the tip of the other end of the cable. Now touch that tip to the "ground" of the speaker cabinet. That "ground" is floating. It is not connected to earth ground.

Do the opposite. Plug the cabinet in first. Now touch the tip of the other end of the cable to the "ground" of the speaker jack on the head. There is no signal coming from the cabinet. It is floating.

Perhaps the inrush of current to the speakers (when it is connected with a signal applied) may pop a fuse. I've never tried, but I don't see any huge problem. To be safe, I wouldn't suggest doing this.

The one thing to avoid is having the tip and sleeve shorting to each other.[This message was edited by Paul Osbty on 20 March 2004 at 01:56 PM.]

John Floyd · Post by **John Floyd** » 20 Mar 2004 6:32 pm

Paul
You miss the point entirely, when you insert the tip of the plug in the jack, The tip and sleeve are shorted for a second or so. I wasn't talking about the Jack body being grounded.

Paul Osbty · Post by **Paul Osbty** » 21 Mar 2004 11:45 am

Okay, John. I was trying to partake in the discussion. In the future I won't help when you've posted.

Bill Crook · Post by **Bill Crook** » 21 Mar 2004 4:12 pm

Stay with us, Paul.............

Seldom have I ever seen a self-shorting jack in a speaker cab so your thoughts are correct !! NO shorting of signal line as the jack dosen't short tip to sleeve as the instrument input jack does.

I just ran several test useing my "Peavey" 112e cabs. I couldn't dupicate the error.

Again, I say....
As long as there is NO signal being applies to the speaker connection, it is O.K. to connect/disconnect a speaker jack.

John Floyd · Post by **John Floyd** » 22 Mar 2004 12:22 am

Paul
I apologize if I sounded short with you.

If you have an old switchcraft jack laying around, Put a 1/4 plug in to that jack, observing the tip and sleeve of the plug while you are doing it. What happens as you are inserting it? You momentarily short the tip to sleeve on the plug, Right? If that other end of the cord is plugged into an amp output, you have shorted the amp output to ground, just for a second, but sometimes that is enough.

Bill
I still stand by my statement, that it is dangerous to a solid state amp to short the output to ground under any circumstances. Signal or no signal. There is a way to do this semi safe and that is to have the cord plugged into the speaker cabinet first and plug into the amp head last. That way no dead short has a chance of happening. When Disconnecting, Do the reverse, unplug at the amp head first. There is always some signal present at an amp when it is powered up, noise, hum, etc. I'm just not willing to take that chance with my equipment, when Its easier just to power down for a minute to make the switch to another cabinet or whatever needs to be done.[This message was edited by John Floyd on 22 March 2004 at 12:33 AM.]

David Doggett · Post by **David Doggett** » 22 Mar 2004 10:25 am

Is it okay to plug and unplug speakers with a tube amphead on standby?

John Floyd · Post by **John Floyd** » 22 Mar 2004 10:32 am

Nicholas Dedring · Post by **Nicholas Dedring** » 22 Mar 2004 10:52 am

Doesn't turning a tube amp on without a speaker load really, really bad?

I know THD mentions for the Bi-Valve and Univalve that they have a built in dummy load, so they can be played without a speaker attached... which would mean that turning on amps without the dummy load, without the speakers plugged in would put some kind of a hurt on the amplifier... standby is one thing, but the amp powered on will be damaged in the manner we're discussing here. As far as I understand, at least.

Scott Appleton · Post by **Scott Appleton** » 22 Mar 2004 1:26 pm

Again the way to do this is simple plug in the speaker cab then turn on the amp.
Ive been doing that for 30 years and never blown an
amp or speaker. If its an amp with a standby switch
I will leave it in stand by if the main power switch is on while changeing speaker cabs.

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