I’ll have to go on Ebay and get some spare QB5/1750 tubes for it.I see that some sellers on eBay are offering GU-46 tubes for relatively low prices, and are indicating that they are equivalent to the QB5/1750. However the GU-46 is specified as being usable up to a maximum anode voltage of 3000 volts, while the corresponding spec for the QB5/1750 is 5000 volts. There might also be other significant spec differences between the two tube types; I haven’t checked further. So I would suggest that you avoid the GU-46 unless you are able to obtain confirmation from NAT that it would be suitable for use in this amp.
Regards,
-- Al
Edit: For that matter, I see that the GU-46 and the QB5/1750 don’t even have the same number of pins in their bases. So the statements in some of the eBay listings for the GU-46 that it is similar to or equivalent to the QB5/1750 are definitely to be ignored.