mixing tubes and transistors

Maybe or maybe not a hazard; damage potential may indeed exist. It depends upon what the preamp does at its outputs when power is applied &/or removed. Some are safe & some are not; if it outputs any transient voltages then that would be coupled to the power amp & then amplified, possibly causing a blown amp fuse or even speaker damage. The safest way to find out is to use an oscilloscope to monitor the preamp's outputs, then turn power on & off while looking for DC component or significant (>100mV) transient activity. If you don't have access to a scope then a VOM might work, or else ask a dealer's service dept. to check this for you; it doesn't take very long & so shouldn't be very costly. Another way to test would be to wire up a cheap pair of speakers & live dangerously. Use some old 4X5 oval speakers etc, from junk.

Many tube pre-amps have no problem being left on all the time. In fact many recommend it. Tubes in many will last years depending on how much voltage the manufacturer runs them at. Some also have a standby mode. We never turn any of our tube pre's off. This is not the case with all of them and you should check with the selling dealer as to his experience.

A number of preamp tubes do well if left on (last and can perform well depending on time constants in design & warm up time). Without knowing yours I can't comment further, but your dealer and manufacturer can give some advice.

Even with mute activated, I have found (through a well-meaning visitor who decided to "help" me) that some preamps do give a pretty significant pop when turned off. As others have commented, I would worry about damage to the speakers. Bob_bundus' suggestion seems excellent to me...

Good luck,
Jacob

If I am in a situation where I want to leave the SS amp on and turn off the Tube preamp, I shut the amp off first; then turn the amp back on after I turn off the preamp.

Then reverse the process turning the preamp back on; ie, shut the amp off first before turning on the preamp.

Leaving low level tubes turned on in standby mode is typically okay and recommended by several different manufacturers. My Father has been doing this for appr 15 years and only had to change tubes one time. Then again, if one uses "low grade" tubes or the component is not properly designed, you can expect significantly shorter lifespan than what he was been able to obtain with his. Sean
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