alternatives to the Raytheon 5840 in a submini tube booster

[mordechai]

I'm going to build up Rick Holt's very cool looking 5840 tube booster...and I'll probably stick a simple JFET booster in front of it to drive it a little harder.  I'm wondering if anyone has any thoughts about using alternatives to this tube, though.  I've seen some posts elsewhere about 5899 tubes as replacements, or CV477's.  Would they perform as well?  better?  worse? 

[patricks]

Check out the Tube Datasheet Locator at Duncan Amps
It says that the CV3929, CV8039, and CV9531 are close or identical. No mention of the 5899 or CV477, but check the datasheets, they may well work fine as a direct swap, or you might have to change the pinout, HT voltage, etc.

If you're thinking of going down the path of building or modding tube amps, Duncan's TDSL page and Merlin's Valve Wizard site are indispensable

[jonnyeye]

The 5899 is semi-remote cutoff; it is close enough in spec to the 5840 to work in most situations but gets progressively less gain the more negative the input signal gets - this was intended to be used as a way of controlling the gain of the tube, but in a booster will have additional (mostly 2nd harmonic) distortion with large (>2V pk) input swings... which could sound nice. Up to your ears to decide.

Other tubes to check out are the 5702 and 6205 (both sharp cutoff) and their semi-remote cutoff variants, the 6872 and 6206.  They'll all be similar enough to substitute for each other with perhaps minor tweaks to the cathode resistor value.