I've got one I bought for £3.99 from Netto's and it works perfectly well. Obviously I use quality ones in my day to day work but it's handy to have one that can be considered disposable to take if you're eg helping a mate with his car electrics and there is a risk of it getting stood on or having a pipe wrench dropped on it.
The OP is only wanting to measure the bias on his valve amp and this usually means about a volt or so (up to 60V if measuring the actual bias rather than the voltage at the bias test points). I haven't personally ever heard of one blowing up when used to test the mains. I don't doubt that it has been known to happen but this would likely be a particular make and model and not a common issue. I have used my £3.99 one to measure the mains on several occasions with no problems.
The safety issue is worth considering if one intends to use it for high voltages yes but I have trouble in believing such incidents as Marco mentions are more than rare isolated incidents. Caveat emptor though...
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