33 Power Pentode Design Pointers Needed

[mario2]

hello friends: I recently bought a bunch of 33 tubes and I want to test them to see how do they sound. With only 1.4 watts output they are not very interesting power wise but the fact that they are directly heated and have only 7% THD make me want to try them.

However I do have several questions on how to bias this tube since I have never used it before and all the examples I could find are in radios where fixed bias is used ("C" battery and one side of the filament grounded) I want to use auto bias but where do I connect the resistor? Negative side of the filament to ground? Or should I use the "pot" scheme commonly seen on 2A3 and other DHT? I will use DC for the heater.

Also these tubes I bought are NU and though they have a date code, I have not been able to find any information on them. The RCA tube manual 1930 says 135V max plate voltage, but the RCA 1937 says 180V max. So not sure how far I can push the plate voltage. Any advise? Sort of suspicious that these tubes are from the 40's but can't be really sure

lastly, this tube seems hard to drive as it has a 18V bias (or 17V "C" battery) so what would you recommend to drive it? I was planning to use a 30 tube to "stay in the family" and since I only need 1V sensitivity I think I should be fine...

Thanks in advance for all the help!

Mario