Night Train: Did I cause any permanent damage?

[Henry909]

I have a Vox Night Train which I have been using with the speaker from a Marshall JCM 600 1x12 combo amp as a cab for the NT. I mounted a jack connected to the speaker on the back of the Marshall and plug the NT head into it. When I first started doing this I disconnected the cable that originally went from the speaker to the internal speaker jack on the back of the Marshall. I always used the NT with the Marshall JCM 600/Cab without the original speaker cable connected to the Internal jack until last week when I plugged the original speaker cable into the internal jack (I did this without thinking about it) at the same time that I was using the speaker as a cab for the NT. I was wondering why the NT was all of a sudden noisier and running hotter. I installed new tubes today and fired up the amp for about five minutes to hear what the new tubes sounded like then turned it off. It was still noisy. I tested the impedance and found that it was very low. Then I disconnected the original internal speaker cable and found that the impedance now measured normal. Then I tried the NT with the Marshall cab again and it sounded fine with no noise.

So: I learned the hard way that having the original speaker cable connected to the internal speaker jack at the same time that you have the NT head connected to the speaker REDUCES IMPEDANCE from 16 ohms to I am not sure what ... Why was the impedance reduced? Was this the same as running the head without a load? Have I caused any damage to the NT? How long does it take to ruin the tubes this way?

Thanks in advance for your replies to this.

[52 Bill]

Welcome to the place.

Both of your amps are tube driven, and both amps have output transformers that connect to the speaker. When you left the JCM600 connected to the speaker, its' output transformer was still connected to the circuit and caused your impedance problem. You probably did no permanent damage to either amp, but you should not repeat this mistake.