12bz7 as V1 of the normal channel of a Twin Reverb

[chopstuck]

I picked up a trio of CBS 12bz7's and was looking to juice up some low gain
pre's. I can't sub them in a normal V1 12ax7 position as they just squeal .
I know the bz7 's have different requirements from a AX7. Does anyone have any suggestions for bias, dropping string resistors or interstage cap values or voltage dividers ? I love my silverface twin but the normal channel is useless as is and I don't want to hack it up add another triode. I'm converting the trem to brownface vibrolux. I can't steal a stage there.
Another possibility is to convert the AT7 reverb driver to one half of a BZ7 and use the other half as a stage in the normal channel. Any helpful comments ?

[David Root]

Nominal plate resistance is half of a 12AX7. This suggests 51K instead of 100K Rp.

I put the 12BZ7 into TubeCad and set up a "Fender normal" V1 stage using 51K Rp, 820 Rk, B+ 300V, .022 cplg cap, 1M vol pot, 68K input grid resistor.

That gives 60 gain, plate volts 196, Vk 1.6V, Ik=1.96 mA, z in 50K, z out 19K (nice and low), frequency response -3dB at 7Hz, (35uF bypass cap)
-3 dB at 15K5Hz, DC swing -83 to +95. I would reduce the bypass cap to 22uF.

All very Fendery except the current is of course higher, but it shouldn't squeal in a Fender V1 spot because 60 is what the 12AX7 gives with the same B+ and 100K/1k5. Also the DC swing is a bit wider than the 12AX7 IIRC, so it should stay clean.

Would be interested to know how that works.

[chopstuck]

Dave, I can't thank you enough for the info.

With the numbers you provide would that yield an equivalent gain level / structure to stock ? If I wanted to go mildly hotter, keeping the TS where it is, where would I begin to increase the gain? First stage, second ? Would I start with the cathode bypass cap or resistor or somwhere else ?


I was hoping too that the the use of the BZ7 with it's lower output impedance would make the tone controls more responsive.
Is that a fair assumption ?

I'm going to do it and report back.

I love everything Ive done to the amp so far.

DC the master vol. changed out a few tubes and reverted to BF spec per the Trainwreck pages of my old AP book.

The best change was from the monstrous orange Gauss speakers to some vintage C12n s.

[David Root]

Stock gain on a SF twin V1A w/12AX7 should be about 60, maybe a tad higher as the B+ is 315V vs my example of 300V.

I checked it in TubeCad, it is 60 as stock, assuming an AA270 circuit. If you run the 12BZ7 with the stock values gain is 74 in V1A. This combined with the lower Zout should give even higher gain into the tonestack. If you do that, then back off the gain in the TS recovery stage on the other half of the 12BZ7 by changing Rp and Rk to even lower than 51K/820, and/or try a 220K or 470K gridstopper on that stage (V1B).

Changing the vol pot to 500K would cut gain a little but watch out for tonal change.

The gridstopper also reduces possibility of blocking distortion in V1B, allowing you to keep a gainier sound, if that's where you are headed. In that case watch out for parasitics as SF Fenders generally have relatively sloppy lead dress.

There's all kinds of ways to do it, reducing coupling caps would reduce bass and possibility of blocking distortion. Change V1A coupling cap to 10 uF for example.

If you are doing a lot of this kind of unorthodox tube subbing you would find TubeCad a real help. It costs $40 but I highly recommend it. It saves 90% of the time compared with plate curves, pencil, calculator & ruler!

[chopstuck]

Thanks again Dave.
I plan on doing it this weekend if I can get the x-mas shopping done.

Chopping and sticking....

[David Root]

I'm all done with Xmas shopping! Let us know how it turns out and enjoy those vintage Jensens, those original C12Ns are really nice for a big Fender. I have a reissue, noooo comparison, although it sounded OK in my 62 Deluxe until I replaced it with a ceramic Tone Tubby.

[Sonny ReVerb]

Head over to AX84 and search for the 12BZ7 and author=zaphod. Phil Rowley did a lot of work on using the 12BZ7 in the P1eXtreme. You should be able to use his preamp values for a starting point.

[Richie]

I have tried many.many of the 12BZ7, which probably alot of others have too, and an old ham guy told me they were many that were microphonic,or hummed. And sure enough,all the ones i tried were just not useable. At least for me they weren't. And i tried alot of different brands.
I'm sure many have wanted to use these,or a cheaper way to a 12ax7,but seems they just aren't and may never be. Not saying they can't or won't work, but for me, just too noisey of hum or ringing.[just like the old ham guy told me they would]

Did Phil post any clips or speak of having any of these problems?

[chopstuck]

Yea, mine are acting microphonic. I was wondering if I could get around that with lower gain settings but one sings without an input source.

[Richie]

also doesn't make sense to me to make a tube work, rape its gain.
I can understand useing dampners and things like that.

Changing values to "make it work" or lower the gain so it may not have problems.
Then you have a tube that is running half or less than its design gain.then what is the point of trying to use it?

Sounds like a restricter plate on the stock cars..
If their was that much problem, i'd dump it and use something else. There are many other tubes besides the 12ax7 or 12bz7 in this case that could be used in the design.

It costs $40 but I highly recommend it. It saves 90% of the time compared with plate curves, pencil, calculator & ruler!

Sometimes the books data and charts don't tell everything. Or how it may act in audio use.
So, you might ask yourself, if this is such a good sub for a 12ax7, then why aren't people buying these up in hordes and useing them..? Or you'd see talk of how great they are etc..