Replacing 12AX7s with 12BZ7s?

[DrNewcenstein]

Saw it mentioned on the Duncan forum that the 12bz7 was hotter/had more gain than the 12ax7, so I looked into it and found info to back up that claim. I've got some 12bz7s here with me now, but was wondering if anyone else had experience with this?

[DrNewcenstein]

Nevermind. Just spent that last hour testing them, and it's not worth the effort. If you've got a vintage amp using vintage 12AX7s and want more gain while keeping it period correct, then 12BZ7s might help. If it was made after 1970, throw some Sovteks in it.

[rnolan]

Oh well, at least you know now  ;)

[DrNewcenstein]

I am liking the Penta Tube 12AX7, however. Even in V1 (with a Sovtek in V2) it's a noticeable increase in gain. With Gain 1 on 10, Gain 2 on 0, MV on 5, and the EQ on 0, it's a very nice base for an AC/DC type of tone. With the Sovtek in V1 and V2 and the same setting, it's more of a Bad Moon Rising tone.

[DrNewcenstein]

I must retract my previous statement against the BZs: I'm listening again to the test tracks I recorded, and while the volume difference is definitely there, the BZs do actually appear to have twice the saturation as the Sovteks.

I'll work more with it this weekend, and try to record something worth posting so others can judge the differences.

[MarshallJMP]

The problem with these tubes is that they draw twice the heater current of a 12AX7 (300mA on 12V).Problem is that the mp-1 transformer can barely handle the 12ax7 heater current (150mA at 12V).Now the mp-1 transformer only puts out around 8 to 9V depending on the wall voltage.This tube could lower this voltage even more so you could get some brown outs and this will result in rebooting.Maybe even destroy the transformer if you use 2 of these.

Gainwise these are the same as the 12ax7 but the plate resistance is lower and the transconductance is higher.Actually one triode of a 12BZ7 is the same if you would put the 2 triodes of a 12AX7 in parallel.

As for the gain,well Sovteks aren't usually high gain and also not the best choice to use in a mp-1 IMHO.

[DrNewcenstein]

That makes sense about the BZs. Not looking to fry my MP-1, that's for sure.

My main MP-1 has GTs in it, IIRC (haven't opened it up in years, and bought it used on top of that. I'm pretty sure the guy I bought it from said he put GrooveTubes in it). Never had any trouble getting what I consider "high gain" tones from it. External tone-shaping EQ works really well for it.

The MP-1 I used for these tests was an Ebay score from a couple of years ago, and has been sitting in an unused rack with other unused rack gear pretty much the entire time, so I never even knew what tubes were in it until I started this testing.

[MarshallJMP]

If i remember correctly these BZ's are also bigger then a 12ax7?Almost as big as a EL84?

[rnolan]

Tube choice is one of those very subjective areas and you can end up with quite a few until you get the tone and gain you like.  When I had a MP1 I put some Boogie STR 12AX7As in it and they were great (for me), plenty of gain, nice and tight, low noise and beautiful feedback (best I've ever heard).  I Tend to agree with MJMP re Sovteks, there are plenty of better (IMHO) options for a MP1.  GTs are selected/tested and re-badged (as are the Boogie STRs) so they can be a variety of different makes (but often JJs).  I think Ruby do the same.  New Sensor bought the US rights to a variety of famous tubes (eg Mullard, TUNG-SOL etc) and get them made in Russia, nice tubes, I particularly love the Mullards, very 3D tube.  I've heard good things about the TUNG-SOLs but not tried them myself. (http://www.dougstubes.com/preamp-tubes/12ax7-ecc83-7025.html).

[DrNewcenstein]

If i remember correctly these BZ's are also bigger then a 12ax7?Almost as big as a EL84?

Yeah, they're like 1/2 inch taller, so there's a concern about it crashing into some of the nearby capacitors. They'd work much better in a unit where the tubes were upright IMO.

I didn't get a chance to record anything over the weekend, but I'll try again this week. Been real busy all of a sudden with random B.S.  :poop:

[Damn-Danny]

Indeed Mjmp,
the 12bz are as big El84