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12BZ7 ///12AX7

Could not finda  topic for tubes, so am using amps section.
After making the amazing discovery of the E80CC as sub for AU~~E80CC blows away every/all AU's, including the *Holy Grails/Siemens,Tele's*.
http://www.r-type.org/exhib/aaa0426.htm
at least in my Defy7, the swap was more than just significant, the Defy came to life from the grave.
I found the Siemens to bea  miniscle superior to Philips = both are equal IMHO, and the valvo Red label E80's going for $$$$$$$$$$ is a scam. Don't buy into it. 
Right now that tube  is going for $50/each. When word gets out  ~~Super Tube~~~the price will rocket, as that tube is not being produced any longer. 
I've loaded up on the Philips label.

so i figured,, maybe there is a  
~~Super Tube~~ for the AX..
After days/weeks of research,,i found this
12BZ7
https://www.radiomuseum.org/tubes/tube_12bz7.html


*Big Boys*
So  ordered a  bunch off ebay, and will arrive next week
Will post a  review.
can not find much on the INET about the 12BZ7,, anyone have experience with this tube?
mozartfan

Showing 4 responses by ndevamp

ndevamp
55 posts
 
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.
ndevamp
55 posts
 
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.
ndevamp
55 posts
 
7308 is a 6dj8 equivalent, not 12au7.
The Defy 7 has the 12au7 as input tube, the 12ax7 is the driver tube. The extra current capability of a 12bh7 is unused in this circuit. Same for a 12bz7 in place of the 12ax7 at the same operating current. The miller capacitance is higher in the 12bz7 so the high frequency response will be more restricted. Without modifying the circuit, the performance overall should be worse.

ndevamp
55 posts
 
The BH/BZ/AX data sheets all look similar in the same way that a bicycle is similar to a motorbike. They have the same number of wheels and you need to balance to ride them.
Well if nothing else you’re enthusiastic about it all! But don’t be disappointed if it doesn’t work out. Do yourself a favour and have a tech check the operation.
Gain in the tube also sets the circuit dc voltages, not just the volume control position. If you look at the data sheets you’ll notice the amplification factor and current ratings are totally different for each tube, reflecting the different control grid construction and size of each.
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