About the only drop in I know of is a M-2057, and it is just as hard to find as the 8950 is. The 8950 is a 12 volt version of a 6LF6, the heater runs on 12 volts on the 8950/m-2057 and the heater on the 6lf6 is 6 volt.
I know the Swan 1200X Amps used the 8950`s ( 4 of them) and many were converted to run 6lf6`s, 6kd6`s and so on.
So If you can dig up a 6 volt filament transformer
, change a few things and re-wire the socket for 6 volt you can get away with a 6lf6,6kd6 or a 6lb6. Another way to go for the 3 tubes is if you can find 3, 40lf6 tubes, you could run the filaments in series, and heat them from a 120 volt line (40+40+40 =120), but you are still stuck with a 8950 driving them.
Sweep tubes are getting harder and harder to find, and the price just goes up...I remember when you could buy 8950`s from Thrifty Drug stores for around $18 each, they had a box of them in the big tube tester by the front door...If I knew then...... I would have bought a truck load of them.
I now see 8950`s go for $70 to $80 NOS
At times you can find them on E-Bay used in the 25 to 35 dollar range, but that is a toss of the dice.
There are 5 on E-Bay right now, starting bid is $100 http://cgi.ebay.com/5-electron-tubes...QQcmdZViewItem
Or here is a Guy that says he has lots of new ones, $75 Bucks http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180170881989&ssPageNam e=MERCOSI_VI_ROSI_PR4_PCN_BIX&refitem=270208062593 &itemcount=4&refwidgetloc=closed_view_item&refwidg ettype=osi_widget&_trksid=p284.m185&_trkparms=algo %3DSI%26its%3DI%252BIA%26itu%3DCR%252BIA%252BUCI%2 6otn%3D4%26ps%3D42
I got rid of my last sweep tube amp long ago, and got a Al-811H, tubes are cheap, and easy to find.