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Old 10th August 2020
  #9
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You have two choices. Either you buy whatever contactors graybar has on sale this week which have wiping contacts, or you buy something with silver or gold contacts. With big electricity, the contacts arc when they open or close, but with small electricity if schmutz builds up on the contacts it will form a semiconducting layer that is a problem. The two solutions for this are to use contactors that scrape the contacts when they are cycled, or contacts and switches with precious metal contacts (which are not that expensive). If you use those ubiquitous ice cube relays, order the precious metal option.

Something like this is fine: https://www.amazon.com/Idec-RH2B-UAC.../dp/B005T5J5NO

Ampex tape machines use plug-in relays for the transport and for routing audio and they all look the same but they have different part numbers because they have different contact materials. People swap them and then stuff stops working because the audio relays can't handle the current of the motor and the motor relays get dirty and stop passing audio.

As mentioned before, don't leave a tube amplifier without a load on it. (That's the case for a few very old solid state designs too, which use output transformers). Doesn't need to be a super load, a 10W 50 ohm resistor is enough. Just make sure the amplifier always sees a speaker or a resistor.
--scott