Did I blow my tube amp?

It is not that likely that having no speakers connected accounts for your problem. That could overload the primary side of the output transformer, but, that usually happens when there is an input and the amp is supposedly putting out a lot of power, except, there is no load attached. If you had no signal going to the input, I don't think you caused the problem.

Perhaps an output tube was bad (shorted) and that caused the main fuse to blow. Check all fuses first; if you are lucky it is just the fuse.

I can't say from the facts stated whether or not the output transformers are blown, or if something else could be causing the problem. However, I would make two points:

1)Whether or not damage will result from powering up a tube amp (that has output transformers) and subsequently powering it down without having anything connected to the inputs and outputs depends on many aspects of the design of the amp. Experience with a different model amp has no relevance.

2)Regardless of whether or not damage might result with a particular amp design, IMO doing that is ALWAYS bad practice, especially if shorting plugs are not used on the inputs. In addition to the possibilities that have been stated, turn-on or turn-off transients could in effect put a signal through the amp. That is particularly likely to occur when the amp is being turned off, if shorting plugs are not connected to the inputs. Transients generated at the instant the power switch opens could couple into the inputs of the amp (to a greater extent than they normally would, because of the high impedance that is present at that point when the inputs are unconnected), then be amplified through the amp's signal path, resulting in huge voltage transients being caused by inductive kickback in the unloaded output transformers (as described in the paper Jim provided).

How likely is that to occur and cause damage with any given amp design? Who knows? But IMO it is a risk that there no point in taking.

Regards,
-- Al

Though I have never powered up a tube amp without the speakers connected this thread has made me think, WHAT IF?.... What if a speaker cable spade come lose because I was changing ICs or what ever? How many here always check the speaker cable terminations before turning on the amp?

I read the owners manual for my ARC tube amp cover to cover twice.... No mention whats so ever.... No WARNING.... Nothing about not running the amp without the speakers connected. I know better.... But what if?

I am going to give ARC service a call and ask, WHAT IF!

Cary Audio.

How do you correctly bias a Cary Audio vacuum tube amplifier?

Disconnect the speaker cables and the input RCA cable or XLR cable from the input jack.
Start with the amplifier warmed up for 5-10 minutes and then turn it off to connect the bias cable.
Connect the provided Bias Cord into the bias connection jack. It has a 1/4 inch mono 'phone plug' on one end and a pair of alligator clips on the other end. Plug the phone plug into the jack on the chassis. The red alligator clip is for positive electrical connections, the black one is for negative electrical connections on the milli ampere (mA) meter.
Set your meter to the milli amperes (mA) reading range. NOT to the Milli Volts (mV) range!
Turn the bias adjustment screw on the chassis all the way counterclockwise so that the meter will read zero when you turn the power on. Now, turn the AC power on. Wait for 60 seconds to warm up the tubes.
SLOWLY turn the bias adjustment screw clockwise while you watch the meter for a mA reading.
Any version Rocket 88, for example, should be set to 175-200 milliamperes with 160-240 range mA setting range OK. Any version V12 should be 250-275 mA with 220-330 overall as the possible mA setting range. Let the amplifier run without an input signal for another 5-10 minutes and check it again. Readjust the bias setting again, if necessary. New output tubes should be checked a couple of times in the first two weeks as they 'burn in' to their normal operating mode. After that, check them every 3 months just to be sure all is well.

Turn off the amplifier, reconnect the speaker cables and the RCA or XLR input cable, turn on the amplifier and enjoy the music!

NOTE: Some other tube amplifier designs will oscillate or go into overload if they are used without a speaker or resistive load attached to the speaker output terminals .Cary Audio Design tube amplifiers are inherently STABLE DESIGNS and may be operated without a load on the output terminals for adjusting bias or while in burn in.

I have, on several occasions, turned on my amps and noticed that one channel was not working because a spade has worked itself loose. Nothing has ever happened.

I never start a listening session with the preamp turned up very much, so I do not subject the amp to high level output into such a no load situation. I know it is not a good thing to do, but, I have never, nor has anyone I know, experienced damage from operating without a load attached.