Unplugging speaker cabs on a tube amp

[rickaddict]

Hi guys!

I know I'm not supposed to power up my tube amps unless they are plugged into a speaker cab, but what about "Standby" mode?

Can I unplug speakers and/or switch cabs while my amp is on Standby?

Thanks in advance!

[cjj]

Should be OK since standby is generally just running the tube heaters to keep 'em warm. But it will depend on how the actual amp circuit is designed, etc., etc. (general disclaimer denying any responsibility and whatnot)...

[rickaddict]

Should be OK since standby is generally just running the tube heaters to keep 'em warm. But it will depend on how the actual amp circuit is designed, etc., etc. (general disclaimer denying any responsibility and whatnot)...

I guess I should have included the amp model. One is an SVT-CL from 2005 or so, the other is a B-15R.

[cjj]

Well, I couldn't find a schematic for a B-15R, just a B-15N. It looks like both the B-15N & the SVT just leave the tube heaters on in standby and apply power to the rest when not in standby. So, you should be OK, just don't forget the put it in standby or reconnect speakers. Same disclaimer though...

[rickaddict]

Thanks CJ!

[soundmasterg]

With just about any tube amp, if there is no signal running through it, you are safe to run it without a load. I've done this many times for testing reasons. I've even tested amps with no load and a signal going through them for specific reasons with no problems. That said, for someone who doesn't have test equipment and doesn't know exactly what they are doing, it is very easy to forget and run a signal through it and forget to hook up the speakers, so better safe than sorry. Run a load at all times and you won't have a problem.

The thing is, some amps don't use shorting input jacks (vintage Silvertones come to mind), so even without something plugged in, the jack is not shorting to ground, which means the grid of the first stage(s) in the amp are not shorted to ground, which means the tube is amplifying noise. In an amp where the standby shuts off the B+ voltage, then it doesn't matter because the tube is essentially off. In other amps where the standby is achieved some other way than interrupting the B+, (agains, Silvertones come to mind) then these amps could theoretically have a possible problem with no load attached to the amp. In reality the risk is low without a signal, but depending on the amp, it could be a concern.....something very high voltage with a low primary impedance on the output transformer like an SVT would make me think twice.

Greg

[gibsonlp]

Greg - What were you testing in your amp without putting a load on it?
I bought a pair of 200watt @ 8ohm power resistors and plugged them in parallel to my SVT in order to measure harmonic distortion with a scope, when the load was wrong (IE: only 8 ohms) the signal looked much different.

[soundmasterg]

Sorry for the delay in getting back to you on this. I've gone back to school for an EE and am quite busy with homework. In the specific instance I mentioned, I was testing to see how well the NFB was working. There are quite a few other reasons why you might want to test without a load, but this type of thing should only be done under lab or workbench conditions when you know what to look for if there is going to be a problem. You wouldn't want to do the typical thing where a musician hooks their amp up and blares a guitar with pedals into it with no load hooked up because they forgot. That could be problematic depending on the amp and the power levels involved.

Greg