12BZ7 info?

[Mike K]

I was given this 12BZ7 tube and told that it could be used in V1 for more gain. Has anybody had any experience with this valve?

Happy Holidays

[TheAmpNerd]

Originally posted by Mike K
I was given this 12BZ7 tube and told that it could be used in V1 for more gain. Has anybody had any experience with this valve?

Happy Holidays

My buddy swears by them. Though you might try it
in the PI slot. That should gain things up quite a bit.

Try it in both spots then let us know what you found out.

Don't worry it won't fry your amp either.

[Blue Strat]

Originally posted by Mike K
I was given this 12BZ7 tube and told that it could be used in V1 for more gain. Has anybody had any experience with this valve?

Happy Holidays

Not sure where you're "gain" spec is coming from, but the specs say they have the same gain as a 12AX7....u=100.

[Wakarusa]

Same amplification factor (100) but half the plate resistance.

[Old Tele man]

...which means they're probably a "better" choice for substituting for a 12AT7 with it's lower rp-value, than using an 12AX7, ie (by gm):

12AT7: gm = 0.0055A/V; rp = 10.9K; mu = 60
12BZ7: gm = 0.0032A/V; rp = 31.8K; mu = 100
12AU7: gm = 0.0022A/V; rp = _7.7K; mu = 17
12AX7: gm = 0.0016A/V; rp = 62.5K; mu = 100
_5751: gm = 0.0012A/V; rp = 58.0K; mu = 70

[TheAmpNerd]

Guys,

I'm not talking about the specs. Stick the tube in the amp.
It doesn't behave like a 12AT7 at all.

Hmmm, so it has the same gain as 12AX7 and 1/2 the plate resistance, the result in an amp is more gain,
sooner break up.

I think it has twice the plate dissapation too.

Try it and see.

[Wakarusa]

Originally posted by TheAmpNerd
I think it has twice the plate dissapation too.


12AX7 : 1.2watts
12BZ7 : 1.7watts

[Old Tele man]

...by-the-numbers:

12AU7 : 2.75 W
12AT7 : 2.50 W
12BZ7 : 1.50 W (RCA Manual RC-26, 5/68)
12AX7 : 1.20 W
_5751 : 1.10 W

...dissipation-wise, you can always go UP in Wattage without problem, but going DOWN in Wattage isn't wise...shorter tube life and premature failure will almost certainly occur...

[TheAmpNerd]

Thanks guys.

I knew it was more, just not sure how much.
Not that much more than a 12AX7 either.

Also note this tube is in a taller package then
the other 12A?7 tube types so you can't use
a standard tube shield on it.

[Old Tele man]

Ever wonder what happens when you swap preamp tubes? Aside from the "sonic" changes, each different tube also changes the circuit "gain" or voltage amplification (AV), which, in turn, affects when (sooner or later) the tube goes into distortion.

Consider the "normal," cathode-biased, preamp circuit with 12AX7, 100K-ohm plate load resistor (Ra), and 500K-ohm "next-circuit" load (Raa):

12AX7: rp = 62.5K-ohm; mu = 100

Ra' = (Ra||Raa)
AV = (mu*Ra')/(Ra' + rp)

Ra' = (100K||500K) = 83.3K
AV = (100*83.3K)/(83.2K+62.5K) = 57.1

...thus, while the tube gain is mu =100, the "combined" load (100K||500K) reduces the total "circuit" gain to less than 60!

Now, replace the 12AX7 with a 12BZ7 tube and see what happens to the total "circuit" gain:

12BZ7: rp = 31.8K-ohm; mu = 100

AV = (100*83.3K)/(83.3K+ 31.8K) = 73.7

...the total "circuit" gain actually goes UP by 30% (+2.22dB)!

What happened? The two tubes have the same gain (mu =100), so why is there at difference? Answer: tube plate resistance!

It is PLATE resistance (rp), NOT tube gain (mu), that has the most affect on circuit gain! Or, stated differently, the tube with the LOWEST plate resistance is LEAST affected by the combination of plate and "next-circuit" loading, and thus LOOSES the LEAST gain because of those circuit loadings!

[Jon Silberman]

OTm, would it possible for you to do the same evaluation as above also for the 12AT7?

Also, when the thread finishes playing out, I plan to ask the Admin's to archive it - it's highly informative on these topics.

[Wakarusa]

Of course:

Assuming we still have 100K plate and are driving into a 500K input impedence.

12AT7: rp = 10.9K-ohm; mu = 60

AV = (60*83.3K)/(83.3K+ 10.9K) = 53.06


There is, of course, a dirty secret behind some of this. Plate resistance (Rp) is a function of transconductance (Gm) which varies with plate voltage (Ep). Generally, Rp is inversely proportional to Ep and the relationship is non-linear.

The values for Rp given so far have been for Ep of 250V. For Ep = 100V, Rp is closer to 15K

(just when you thought it would be easy :))

However, you can still use the equations for basic comparisons of these tubes as long as we keep in mind that the actual gain may be a bit different in your particular amp.

[Old Tele man]

Jon Silberman...but, of course!:

12AU7: gm = 0.0022A/V; rp = _7.7K; mu = _17; Pp = 2.75W
12AT7: gm = 0.0055A/V; rp = 10.9K; mu = _60; Pp = 2.50W
12BZ7: gm = 0.0032A/V; rp = 31.8K; mu = 100; Pp = 1.50W
_5751: gm = 0.0012A/V; rp = 58.0K; mu = _70; Pp = 1.10W
12AX7: gm = 0.0016A/V; rp = 62.5K; mu = 100; Pp = 1.20W

...in the same circuit (100K-ohm plate load; 500K "next-circuit" load) and sorted by AV-order:

12BZ7: AV = 73.7 (+2.22dB)
12AX7: AV = 57.1 <---original circuit tube
12AT7: AV = 53.0 (-0.65dB)
_5751: AV = 41.0 (-2.88dB)
12AU7: AV = 15.5 (-11.3dB)

...thus, a 12BZ7 will "increase" overall circuit gain and yield MORE and sooner crunch. Notice there's not much gain drop between a 12AX7 and 12AT7.

Similar analysis would also predict the effect of each tubes' DIFFERENT rp-value upon the circuits' mid- and high-freqency response curves.

[Old Tele man]

...actually, dynamic plate resistance (rp) is fundamentally a function of plate current and not plate voltage:

rp = ( mu / gm )

gm = (3/2)*( (G^(2/3))*(Ip^(1/3) )

rp = (2/3)*(mu) / ( (G^(2/3))*(Ip^(1/3) )

where:

Ip = Plate current, amps
rp = Tube dynamic plate resistance, ohms
mu = Tube Amplification Factor (dimensionless)
G = Tube Perveance, amps-per-volt^(3/2)...(essentially a constant)

...this is because mu is essentially (but, not exactly) a constant (it typically varys less than 10-15%) value, which leaves rp to become an inverse cubic-root function of plate current, not plate voltage.

NOTE: while these are theoretical equations, they correctly demonstate the proportionality and cause and effect relationships between the various tube paramters.

Like the ROOSTER & HEN relationship: "The rooster (voltage) may CROW, but it's the hen (amperage) that gets the JOB done!" -- ie: vacuum tubes are voltage-controlled current devices...hence, the British "value" name.

[TheAmpNerd]

Yes,

In fact I have wondered about that. Now I'm also
wondering how we go about determining the
next circuit load (Raa)?

Techus Maximus....I can read the fine print
right above the arrow pointing to an old
tele man fooling around with an amp
that Wile E. Coyote is checking out.

beep beep....

[Wakarusa]

Originally posted by Old Tele man
...actually, dynamic plate resistance (rp) is fundamentally a function of plate current and not plate voltage:



Oops. You are correct. No more egg-nog before I post.

(I was thinkin' of... umm... nevermind. I'll just take my lumps :jo)

[Jon Silberman]

Calling Old Tele man!

When you wrote on the Weber board:

"...if you sent the idle by CURRENT, then you can use this equation:

Ip.q = (70%*Ppd)/Vp.q

where:
Ip.q = Plate quiescent (idle) current, amps
Vp.q = Plate quiescent (idle) potential, volts
Pp.d = Tube PLATE dissipation rating, watts

Pp.d for JJ 6V6S is way "under" rated at 14W (the original NOS 6V6GTA spec), and probably good for 16W or more (nobody knows for sure)."

Can you please explain that last sentence in super-plain English for the technically-impaired (i.e., me)? :) IOW, does it mean the JJ is "probably good for 16W or more (nobody knows for sure)." I assume this means you can get more power out of it relative to other "true" 6V6s? And what does the ability of a plate to "dissipate" do, specifically, to cause or allow more power?

Also:

"... it's part of the research I've done for a book about Leo Fenders' medium- and high-power tube amps."

What is the name and author of that book? Is it available for sale now and where? And if not yet, when is it expected out?

Thanks! Jon S.

[Old Tele man]

first, regarding:

"...if you sent the idle by CURRENT, then you can use this equation:

Ip.q = (70%*Ppd)/Vp.q"

...obviously, it should be 'set' not 'sent'...mea culpa, a typo, sorry!

Item-#1: So far, all of the "non-scientific" power-dissipation tests done by various tube sellers indicate (but don't 100% prove) that the JJ 6V6S plate dissipation capability is far greater than the NOS spec of 14 watts -- probably closer to the old NOS 6L6 spec of 19 watts -- however, WITHOUT any formal scientifically "controlled" tests, nobody's willing to stick their neck out and commit to a number (myself included).

If you want to "read the numbers" for yourself, check Lord Valves' website for the "torture" tests that he performed on some of the initial JJ 6V6S tubes...

Item-#2: Yes, I am writing a book about vacuum tube amps, the tentative title is: Leo Fenders' Medium- and High-Power Guitar Amps. No, it hasn't been published yet.

P.S.--that 70%-of-Pd may NOT be valid... the ONLY number that I've been able to substantiate is the AVERAGE-value (2/PI = 0.6366...ie: 64%) which BIAS-RITE lists as "HIGH-bias." This number represents the VOLTAGE or CURRENT point where "...maximum plate dissipation..." occurs with a sinusoidal waveform.

[TheAmpNerd]

Old Tele man,

How did you go about calculating the resistance
of the next circuilt load of 500K?

The suspense is killing me.

[Old Tele man]

...the double vertical parallel lines (ie: "||") is an electronic notation meaning "...in parallel with..."

...thus, A||B means "A-in-parallel-with-B," which equation-wise is "PRODUCT-over-the-SUM," ie:

A||B = (A*B)/(A+B)

...or, using the numbers from above:

Ra' = (100K||500K) = (100K*500K)/(100K+500K) = 83.3K

...see, it's not THAT hard...once you've memorized the magic chant, are holding your tongue just right, and have mastered Calculus...not really--just some algebra!


PS--just an approximation, ie: 1Meg VOL in parallel with 1Meg of Tone-Stack = 500K

[TheAmpNerd]

Originally posted by Old Tele man
...the double vertical parallel lines (ie: "||") is an electronic notation meaning "...in parallel with..."

...thus, A||B means "A-in-parallel-with-B," which equation-wise is "PRODUCT-over-the-SUM," ie:

A||B = (A*B)/(A+B)

...or, using the numbers from above:

Ra' = (100K||500K) = (100K*500K)/(100K+500K) = 83.3K

...see, it's not THAT hard...once you've memorized the magic chant, are holding your tongue just right, and have mastered Calculus...not really--just some algebra!


PS--just an approximation, ie: 1Meg VOL in parallel with 1Meg of Tone-Stack = 500K


Suspence indeed! Sticking the answer down there
in the post script...very funny.

Okay, that is what I've been trying to figure out where
that majic 500K number came from.

Now when I need it, I'll just click my spit shined USMC
boots together three times and utter

"There is no plce like tone...."

You're not too bad for a squid, thanks Tele.

[Blue Strat]

Originally posted by TheAmpNerd
Guys,

I'm not talking about the specs. Stick the tube in the amp.
It doesn't behave like a 12AT7 at all.

Hmmm, so it has the same gain as 12AX7 and 1/2 the plate resistance, the result in an amp is more gain,
sooner break up.

I think it has twice the plate dissapation too.

Try it and see.

Gain does not equal breakup. This is a common misconception among guitar players.

This tube breaks up do to the plate voltage lowering effect of different internal resistance.

[Wakarusa]

Originally posted by Blue Strat
Gain does not equal breakup. This is a common misconception among guitar players.

This tube breaks up do to the plate voltage lowering effect of different internal resistance.

But in this application the extra gain in the first stage will clip the following stages earlier (all else being equal). I'm betting the original post was a reference to overall behavior of the amp, not necessarily a reference to when the first stage would clip.

[Old Tele man]

...the assumption being that subsequent stages will be "hit" with signals +2.2dB above normal even though the input signal is exactly the same level...and cascading through the other circuits until eventually one becomes overdriven.

...assuming the 12BZ7 is plugged into the first preamp position, everything following is gonna see a "larger" signal than their circuits were originally designed for...the 12BZ7 circuit itself may or maynot distort, but the following circuits probably will for sure...unless the VOL is reduced proportionately to attenuate and cancel that +2.2dB gain.

[jimmisixstrings]

I read all the posts here looking for the answer to the sound/tone question, I got a lesson. Don't get me wrong, I'm happy for all the information but did anyone actually try it in a Fender amp and can they tell me how it worked out? I'm thinking about sticking one or more in my Super Reverb, any suggestions/warnings?

Thanks in advance,
jimmi

[TheAmpNerd]

I read all the posts here looking for the answer to the sound/tone question, I got a lesson. Don't get me wrong, I'm happy for all the information but did anyone actually try it in a Fender amp and can they tell me how it worked out? I'm thinking about sticking one or more in my Super Reverb, any suggestions/warnings?

Thanks in advance,
jimmi Yes,

A friend of mine tried this in his vintage deluxe amp.
He likes it.

Of course, he is also the same guy who drilled holes in the
same vintage chassis for derating.

[Timbre Wolf]

I read all the posts here looking for the answer to the sound/tone question, I got a lesson. Don't get me wrong, I'm happy for all the information but did anyone actually try it in a Fender amp and can they tell me how it worked out? I'm thinking about sticking one or more in my Super Reverb, any suggestions/warnings?

Thanks in advance,
jimmi Make sure the circuit can support the extra current draw, and take note that the glass envelope is 3/8" taller than a 12AX7. Also note that 12BZ7 can be vibration sensitive in a combo, and may be more susceptible to microphonics. If you're cool with all that, plug one in and check it out.

I use 'em in my rack setup, one for the main output of my Triaxis, and one for the main input of my Mesa 20/20 power amp. The benefit in those positions is that you get that extra couple of dB volume gain, per tube, which makes for a louder output, with earlier power-tube breakup. Or if you adjust for the same volume, you drop the noise floor several dB, yielding a more clear, clean and dynamic setup.

I've investigated them in the V1 position of my Triaxis, and found them to be tonally fairly neutral (or perhaps a tad bright), with a very clean quality, and do not recall them causing earlier breakup in Lead modes. Articulate, dynamic, tight, and snappy. By the way, I've only tried black-plate 12BZ7s (black-plate tubes seem more dynamic to me, in general), and there seemed to be no significant tonal difference between 4-5 different-construction variants.

Hope this helps.

- T

[Jack Briggs]

If you want to "read the numbers" for yourself, check Lord Valves' website for the "torture" tests that he performed on some of the initial JJ 6V6S tubes...


What site? Everytime I go to http://www.nebsnow.com/LordValve there is nothing but a pic of a dog, some description of services, some links buttons to the left, some of which contain nothing???????? Only an email link to the Lord himself is what I see. Show me what I'm missing!!!

[Shea]

A few years ago, I got some of those from Antique Electronic Supply to experiment with. They were supposed to be NOS, but they had problems with microphonic rattling right out of the box. 12BZ7s have bigger internals than 12A_7 tubes, so they're going to be more prone to microphonics.

For a while, I used one for the pi (and preceding gain stage) in a circuit that's based on the 5E3 deluxe. I didn't hear any difference from a 12AX7.

I've thought about using one of those as the PI in a Fender, since the lower plate resistance might work OK with the Fender's PI circuit, but perhaps it would have more gain and therefore overdrive the power tubes more. But I don't think I've ever gotten around to it.

Shea

[jimmisixstrings]

Thanks for the information, so the PI is where it belongs not V1 or V2? Seems as if people have very specific places they like them or percieve them to have the most impact.

[Timbre Wolf]

Seems as if people have very specific places they like them or percieve them to have the most impact. Yes - precisely! But I have not tried them as a PI ;)

- T

[FastRedPonyCar]

Gonna dig this one up from the grave and inquire what one of these might do to the sound of my Splawn? It's a heavy gain amp but really doesn't have a lot of saturation like a recto or diezel and I'm wanting a bit more saturation without having to use an overdrive.

Also, does no one offer any current production BZ7's?? Is the only option NOS? I don't mind really since they're not that expensive.

[Timbre Wolf]

I got brave and tried one of my 12BZ7 in V1 of my Victoria Regal II. Of course, the first one was microphonic. But the second one was not, and it got me where I wanted to go: louder, bright, and clear.

Drew - I'm sorry I did not answer your questions about the Splawn. I don't have that amp, but I think I can tell you that a 12BZ7 is not likely to get you "more saturation." Strangely, it kind of does the opposite: less compression, and less overdrive (in my case, at least). I think that you might want a 12AX7 that breaks up early, such as the NOS RFT ECC83.

Back to the 12BZ7: I have enjoyed these, in the past, in my rack gear, where I have brazenly used them, knowing microphonics are less likely than in a combo. Now I've got one in V1 of my Vicky combo (with one 15" speaker), and I'm surprised it is working at all. Anyone else use a 12BZ7 in a combo? Kinda livin' on the edge, here, but it is a magnificent thing to hear. :YinYang

- Thom

[phsyconoodler]

I've used large thrink tubing on microphonic preamp tubes and it helps quite a bit unless it's really bad.I just get a sze that's close and shrink it on half of the tube seems to work well.

[rockon1]

I've used large thrink tubing on microphonic preamp tubes and it helps quite a bit unless it's really bad.I just get a sze that's close and shrink it on half of the tube seems to work well.

Interesting. Ive been meaning to try that . I seem to recall seeing a tube come like that in my equipment-somewhere a while back. Bob

[Timbre Wolf]

I've used large thrink tubing on microphonic preamp tubes and it helps quite a bit unless it's really bad.I just get a sze that's close and shrink it on half of the tube seems to work well.

Thanks for the tip. I've been using Brent Jesse's brass ring tube dampers (http://www.audiotubes.com/damper.htm), and they do work fine, for marginally microphonic tubes. Your method seems more economical.

Does anyone else use 12BZ7? I know a lot of them sell on EBay, but I'm not sure where they're going.

- T

[njslim]

OK, anyone put one of these in a Vibro King? That amp could stand to be a little more open and clear, yes?

Don

[JlMMY]

I have a few of the 12BZ7s so I tried this tonight in my amp. My Vibrolux layout is:

GZ34-6L6-6L6-12AT7-12AX7-12AX7-12AT7-12AX7-12AX7 (red tube is channel two pre amp).

I put it place of 12AX7 and the amp got a little louder. I can't really crank it but I believe it would breakup sooner. The difference in the real world application is minimal. My question is, can it go in place of a 12AT7 and, if so, would this have the same effect i.e., louder, more breakup?

I would be putting it in the phase inverter slot, shown in red below.

GZ34-6L6-6L6-12AT7-12AX7-12AX7-12AT7-12AX7-12AX7