Bob,
The 6ag5 was probably chosen as a lower noise tube. (or down there that was what the guy could get) (or possibly to get better sensitivity on the higher bands)If the original octal socket is still intact, you might want to try swapping the 6SK7 for a 6AB7 or AC7. Sometimes this does wonders for the noise figure on some of the "older" radios.
My SX-28 has a 6AB7 in the first (or maybe second) RF amp stage and has surprisingly good sensitvity all the way up to 10M. Most of those radios from that era were pretty deaf on the higher bands.
I also had an SX-62 that someone over the years changed the IF amp tubes to miniatures (6AG5s) and it worked very well so I left it that way. I believe this was a popular practice some years back.
The Slab Bacon
The original socket has been replaced with a miniature type and due to the difficulty of getting at it under the tuning section I think I will leave it alone at this stage. The radio appears to be working well on the lower bands and WWV coming in well on 10Mhz. The S meter appears to function correctly.
There is one minor problem I have yet to come to grips with. The BFO works as it should on an SSB signal but as soon as it is switched on the S meter rises to S9 +10 and when the BFO control is advanced fully clockwise the meter rises to +40. It is the same result as turning the RF gain down. Any suggestions?
I came across an earlier post by Carl regarding the value of the filter caps. They had already been replaced but with a dual 50mfd can type so I disconnected the input side and replaced that with an 8mfd. The B+ voltage is now down to around 260V about 20V lower than it should be but the rectifier tube should be a lot happier with less C hanging on it. The plate resistor on the 6SQ7 had gone from 250K to 340K so that was replaced at the same time.
On a different subject, I was recently given a Spot Frequency Alignment Oscillator (see pictures) They were manufactured in Australia by R W Steane & Co probably around the 1940's. Did any of them make it to the USA?