12BZ7 ///12AX7

The 12bz7 is basically a double 12ax7, ie. Each 12bz7 triode is similar to two 12ax7 triodes in parallel. It draws double the heater current of a 12ax7 (600mA vs 300mA) and has twice the transconductance. It has a gain of 100, like the 12ax7. Not sure if your Defy 7 can take it, you might need to ask Jadis for help with that. If the circuit was designed for 12ax7s, dropping in a 12bz7 might cause the circuit not to work properly.
In the same way a 12bh7 is similar to a double 12au7 and also draws twice the heater current, and a gain of 20.
I used to use 12bz7s in my tube phono pre of the 90s.

+1 ndevamp.
Check with the manufacturer.

Jadis always *Please relate all concerns to our USA representative dealer*.
Bluebird, i believe, which is a dead end as for *~~cap I swap tubes,,,~~~
~~what we suggest,,,,,,~~ no doubt will be the answer..
As Blue and Jadis do not want to give the green light and if a curcuit blows, , can’t take the risk for blame..
Seems to me is the system is working with the E80’s, and no shorting/crackles, whistles, pops, then just let it play.
The Defy with AU’s does not offer the fq resolution, forward sound stage, that i require ina amp,.
= I am willing to risk possible issues.
EU E80CC = 12bh7, at least from looking at the 2 designs.
I just boughta bunch of bh’s along witha bunch of bz’s.
The bh’s i will try in the AU section and the bz’s in the Defy’s AX section, will post later this week how these 2 tube rolls went off.
maybe evena YT vid with the new bz’s and bh’s.

OK so the
12bh7 = EU E99CC.

~~Ck with your amp manufacturer~~~
(a bit late for the roll to the E80CC)
https://www.tubesforamps.com/products/12bh7

Just looked at the Defy 7 schematic, audio section. It looks like the 12bz7 would work ok for the audio circuit voltage settings. BUT there is a danger it will draw too much heater current from the filament winding, along with the E80CC, which also draws double the heater current of the original 12au7. I don’t have the power supply section schematic here.
The gain of a 12bh7 is 16.5, the E80CC gain is 27, the ECC99 has a gain of 22. The 12au7 has a gain of 20. Since the 12au7 gain and transconductance sets the voltage for the rest of the input stage on the Defy 7 I’d have a tech familiar with tube circuits to check the voltages and currents over with these alternative tubes you’re trying. There might be a risk of frying the heater winding with too much heater current from these bigger tubes.