Your replacements are rated 3A. Far too low for this amp, plus you are using a fast blow fuse which will probably blow due to current inrush at switch on. Just as a comparison, my 4BSST requires a 6.3A slow blow fuse at 240v (I am in the UK). Looks like you are 120v mains, so you will need a higher rating. What does the original fuse say, usually marked on the cap. Or compare it to the working 7b? The 3AG7A marking suggests 7A?
Once you get the correct fuse, and if it still blows, you need to check the amp out as something is not right.