I'm planning on converting the amp to 9 pin mini sockets, and am considering going the tried-and-true route (El84s- which I'll have to buy) or using a pair of NOS RCA 6973s I've got. Any thoughts?
6973's seem to run about $60 each, when you can find them. Would it not make sense to sell the pair to someone who needs them for their oddball Gibson, and buy a stash of EL84's? Unless you've got a bunch of spare 6973's laying around as replacements (of course, that could mean many, many, many spare EL84's in trade...).
... the 6973s seem potentially more robust than El84s in terms of their dissipation ratings and such... Also, are the 6973s pentodes or beam power tubes? I've seen conflicting info on data sheets.
The only data sheets I saw for the 6973 were from RCA, and they call the tube a beam power tube. G3 is connected internally to the cathode. You could look at the tube in-hand; if you can see at all within the plate structure, you'd be able to tell if the tube is a pentode (with an actual suppressor grid spiral) or a beam power tube (with beam-forming plates having openings facing the plate, usually at the plate seam).
These sheets also say the 6973 is a 12w tube (otherwise like an EL84, 6V6, 6AQ5, etc). The Design Max plate voltage of 440v seems like an improvement, until you see the Design Max screen voltage of 330v. Since most guitar amps run the screens at about the same voltage as the plates (for economical power supply design), then high plate voltage rating is little-use outside a specialized circuit. For what it's worth, the Mullard EL84 sheet says max plate voltage is 500v, but max screen voltage is only 300v. On this standpoint, it seems like a wash.
The 6973's transconductance on the sheet is only ~4800 micromhos (or 4.8mA/V). An EL84's Gm under similar conditions is 11,800 micromhos (or 11.8mA/V), meaning the EL84 is very much easier to drive. Said another way, a smaller input signal will push the EL84 to max power output and distortion, while the 6973 is outputting less power and cleanly for the same drive. With a direct swap, the 6973 will seem to lack power (although it's only a matter of turning up the volume).
Looking at a 6AQ5 data sheet, all ratings are broadly the same as for the 6973, except for the latter's higher plate and screen voltage maximums. I know squat about the 6973, but it looks to me like an "uprated 6AQ5" which is also just a 6V6 in a 9-pin bottle.
Given all this, and if the 6973 is really just a special-6AQ5 and therefore a 6V6-style tube, you may be best served with an actual EL84 for "chime." True pentodes have a different sound than beam power tubes, which some folks put down to additional odd-harmonic distortion which gives a bright edge.
Topic: AC 15 with 6973 tubes (Read 7053 times)