OTL amps, my pick for the tube used.

The 633C initially became popular for its unusual shape and very low cost. Even in small quantities they could be had for $13 including socket. Very unusual to sell a tube with a socket. They actually recommenced replacing the socket with each tube renewal!

Now I see M Mathews is offering them for $142 so this is either new production or NOS. I was told that there were warehouses everywhere with this tube stacked to the ceiling. Don't worry, never will run out, I was told.

This tube is very close to our 6336 made only by Tungsol and equals 3 x 6AS7 in parallel. It has other features and I recommend take the time to read the 3 paragraph description. The number is strangely similar.

https://frank.pocnet.net/sheets/127/6/6336A.pdf

I recently worked on a VK-75 amp which uses 4 of the 6C33. When turning the amp over to work in it the tubes keept falling out of their sockets. Replacing the sockets in most 6C33 amps I have seen is not always easy... but it should be. 

On to the tube I use, the 26DQ5 has all the attributes one can desire in a OTL output tube. Those are

• High Current per tube
• High overload current. RCA advertised 400% for a few seconds.
• Low screen voltage, ability to run in Triode
• Octal socket holds the tube firmly. Much better than wirepins.

The thing that has always concerned me is the cathode flaking I see with Pass Tubes.That tells me they are not up for high peak currents like Sweep tubes are. I have to be very careful while curve tracing these Pass Tubes so as not to flake the cathode. It lowers the peak current and can cause quite a bias shift.

When you get right down to it Sweep Tubes are the only choice for making OTL amplifiers.