12BZ7's

Early B.

I rolled the dice and stuck some NOS CBS 12BZ7 tubes to replace 12AX7's (GT's) in my Jolida 502 int. amp. Luckily, nothing blew up. I got the idea from Cary's Dennis Had who designed the AES SixPac monoblocks. You don't hear much about these tubes because, from what little info is on the 'net about them, they tend to be highly microphonic, unreliable, and don't last very long. They are taller than 12AX7's; some sellers market these as "super 12AX7's." To deal with the microphonic issue, I put a couple of those rubber band things around each tube. Don't know if it makes any difference at all, but it looks George Jetsonish. Anyway, here's what I found --

When I first turned on my system, I noticed that the 12BZ7's produce a slightly louder consistent hum. They also appear to run as hot as the darn power tubes. They sound brighter (in a good way) and a tad more airy than my reference Groove Tubes. I believe the soundstage increased slightly, but I'd have to do an A/B test to confirm that. And on good recordings they are certainly more detailed. The 12BZ7's provide more information, so at first, the sound appeared a bit muffled. But the most significant difference is the 12BZ7's appear to add more body to the music, especially in the bass. Now keep in mind that what I am describing are subtle differences.

Is it "better" than my favorite tubes? Well, let's just say that I'm gonna keep those 12BZ7's in the Jolida for a while. I bought them from Tube Depot for $8.95 ea. For that price you can't choose which NOS brand you're gonna get, but if you're into tube rolling and willing to take the chance on a tube that the manufacturer probably would not recommend for your amp, go for it. You may end up with a new set of reference preamp tubes for cheap.

dorokusai

You never really know until you roll, glad you had a positive experience. I roll whatever is listed as a alternate and like you, have found great deals on "uncommon" tubes....not everything is a winner, but it's fun.

Early B.

Here are some pics...

faster100

so these can sub for any amp that has 12ax7 drivers? I think i'll try them out

dorokusai

I don't like the rubber band dampers as they make me feel weird when I put them on.

organ

Very cool. Does the X7 and Z7 have identical specs?

Awesome looking Jolida btw!

Early B.

organ wrote:

Very cool. Does the X7 and Z7 have identical specs?

Almost. Here are a few quotes I pulled off the audioasylum:

“The parts of the tube are much larger so the same shield won't fit over it and it sure as heck needs to be well shielded. Each section is double the transconductance and half the plate resistance which nets the exact current draw from the power supply decoupling resistor as a 12ax7 BUT and this is a big BUT in most 12ax7 circuits dropping in a 12bz7 will elicit HUM out the wazoo. It's really a totally different tube and was designed as a humble sync splitter.”

“Each section of a 12BZ7 is pretty much 2 secions of a 12AX7 in parallel. Therefore gm is twice a 12AX7, rp is 1/2, u is = at ~100, filament current is twice and interelectrode capacitances are roughly twice.”

“You can probably get away sticking these things into circuits designed for 12AX7s w/o circuit change ASSUMING that your equipment can support the doubled filament current. Obviously, most circuit parameters will be non-optimum but damage probably won't occur. It's up to you to determine the damage potential. Also, FWIW, my experience with these tubes is that they're usually quite microphonic.”

“The 12BZ7 seems to work best with fixed bias. Just willy nilly subing a 12BX7 for a 12AX7 is liable to cause hum due to its cathode bias. Even though the anode pulls the exact same current and voltage it's grid behavior is different. Ideally anode resistor loads should be cut in half or more. Where a 12AX7 used a 100K anode resistor 56K would be nore in keeping with the 12BZ7. The 12BZ7 is more microphonic. The 12AX7 has more mica insulators stacked to reduce microphonics. The BZ7 is a beefier tube.”

“12BZ7 is in a taller bottle, the size of a 6CG7. It is similar to two 12AX7s in parallel, right down to the doubled filament current. It will work in some 12AX7 circuits, but compare the curves at the operating point in question.”

“Even though both the 12AX7 and the 12BZ7 have a mu of 100, the 2 types are not interchangeable. The difference in Rp leads to very different behavior in many circuits. The relationship between the small signal parameters is: Mu = (Rp) * (gm).”

“A 12BZ7 draws more heater current than a 12AX7. 150 ma more with a 12 volt supply, 300 ma more if you run them on 6 volts.”

“Other than that the tubes will interchange nicely. Some people prefer the 12BZ7, but do be aware that the larger plates means the 12BZ7s are much more prone to microphonics than the typical 12AX7. This additional microphonic tendency may be what people like about the tubes, they sound more "alive" is one comment I hear about them. But 12BZ7s BEG for tube damping of some sort, either my little o-ring dampers or something more sophisticated like Herbie's.”

“Yes I have tried it. It is a high mu twin triode similar to the 12AX7. While it can be substituted without rewiring, it has the same pin out (9A), the 12BZ7 draws double the heater current (600ma as compared to 300ma). Be sure that your p/s can handle the increased current draw before using it (especially in applications where you are using multiple tubes).”

“I'm a bit of a risk taker, so I tried a 12BZ7 in place of the 12AX7 in my Conrad Johnson mv55. I find the difference in sound to be wonderful: more transparent, way more, and more dynamic. A slightly greater current drain, up to 300ma instead of 150. c-j won't bless it, but my results are great and I don't expect to go back to 12AX7s.”

organ

Interesting. Thanks for the info.

Did you find your Z7's to be microphonic?

The double heater current draw is what would prevent me from trying it out in my ASL. I can see this working out in an int. amp or amplifier because the PS can handle a bigger capacity, but I'd be worried about installing one in a pre amp.

faster100

Id like to try a few in my 6v6 amp.. has 2 12ax7 drivers..

organ

faster100 wrote:

Id like to try a few in my 6v6 amp.. has 2 12ax7 drivers..

Keep a close eye(or should I say hand?) on your power transformer, if it heats up too much, the tube may be too much for the circuit. You could also sub a 5751 in there.

faster100

I had a nice set of ratheon 5751's for my audio experience pre but sold them with the pre i think..

organ

Oh yeah, I remember that pre. The GE 5751 never really worked out for me.

Early B.

organ wrote:

Did you find your Z7's to be microphonic?

Not yet, but I tried a different pair several weeks ago and one of the tubes was causing lots of havoc (i.e. static) as soon as I plugged it in, so I pulled it immediately.

Yeah, I've tried 5751's, too, and they sounded weak in my system.

I'll keep a close eye on the heat situation. If the amp gets too hot, what is the result -- blown tubes? blown fuse? blown amp?

madmax

You are a very brave man EarlyB...
madmax

RuSsMaN

Agreed Chuck.

Early B.

madmax wrote:

You are a very brave man EarlyB...

Can you and Russman elaborate? I trying to determine the level of risk associated with using these tubes. I couldn't find the answer to this question on audioasylum or anywhere else on the 'net. If it's too risky, I'll just yank them out.

Thanks.

schwarcw

Man that Jolida looks cool! Like something right out of Dr. Frankenstein's lab:eek: Seriously, a nice piece of equipment.

organ

Early,
I think they meant you're brave because a lot of us wouldn't take a risk like that:). As long as the circuit can handle it, it should be fine. Best to check with the manufacturer first. Shoot them an e-mail.

Early B.

OK, I punked out and yanked the 12BZ7's and returned the GT's to their former glory (these are the Mullard remakes).

I gotta tell ya, though, I really miss the greater impact and body the 12BZ7's provided, but it was at the expensive of a slight muddiness. When the GT's returned, they sounded tame and passive compared to the BZ7's. Id' love to find a preamp tube with both features -- the clarity and balance of the GT's with the body, detail and power of the BZ7's. I wonder if changing out my EH KT88's would make a difference. Hmmmm.

Once again, you **** have forced me to consider some more tube rolling.

madmax

It's just with the extra current for the heaters and such and no knowledge of how much for how long the jolida will put up with it. It's risky. Now if the parameters of the jolida were known or if there was a history of jolida owners doing it you could feel a little more comfortable.