I recently fixed up a portable Pulser cassette player and quickly had to deal with it’s power supply. It takes C batteries, or I could use it’s built in AC inverter. I decided to wire in a 6V DC power supply and it works great.
Since it takes 4 C batteries I got a 6V power supply. I measured the voltage from the supply and found it was outputting 7 something volts! I found out that when “cheap” power supplies are not under load, they can have a higher than expected voltage. I was a bit nervous the first time I plugged it in, but it worked great. It is much better than the alternatives and I have enjoyed the experience.