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....