bassman 12at7 phase inverter to 12ax7

[kingswayguitar]

I just lucked into a lot of tubes from a reissue Bassman (5U4GB rectifier... what's that?) for $19.95US. If they turn out to be any good, they only have the 12ax7 phase inverter and not the 12at7 I need for my '77 silverface. I've been reading about how they are very different tubes, but there also seems to be a fair bit of interchanging between the two. Should I be overly concerned? Doesn't seem to be any big hazards if I experiment a bit.

thanks
Ben

[rudy kenyon]

Ben, no worries. More gain, No pain.
The original 6G6 version of the Bassman had been fitted with a 7025/12AX7. Whether your '77 SF has a presence
control or not shouldn't matter. My brother's AB165 has a 12AT7 and no presence. Your amp should have a tag
inside the enclosure near the chassis (if a head) or in the speaker area (if a combo) -- this will allow you to
find the right chassis schematic, thus showing you which components you could change.
The 12AX7 will just deliver more gain to the pentodes, similar to a typical Marshall or HiWatt.
See Ya.

[kingswayguitar]

thanks man! love the attitude
i posted a link to the schematic here:
http://music-electronics-forum.com/t30999/
this is one of the best threads i found. sounds like i just reset the bias and fly at 'er! lock up ya' women & chillin'.
http://music-electronics-forum.com/t27308/

[kingswayguitar]

except i'm goin' the other way
12at7 to 12ax7 in PI

[Jazz P Bass]

The PI in itself does not add any real gain to the signal.

Installing a 12AX7 will 'boost' the signal a bit more than the 12AT7 will.

Is this a good thing?

The 12AX7 will not 'hurt' the amplifier.

The best route would be to compare the two tubes, one at a time.
Seeing as you only have the one, that won't work.

So run the amp with the 12AX7 & maybe purchase a 12AT7 for future comparisons.

Note: with the current production of tubes being what they are, without actually scoping the PI to see what is what, you may be flying blind.

I have seen 12AT7 tubes that put out a voltage as high as a 12AX7.

So if you get a hot AT7. it will be an exercise in futility without properly monitoring the circuit.

[kingswayguitar]

thanks
i still have the original 12at7 (36 years old) but i may take your advice and get a new 12at7 for head to head comparison.
as you can tell, i'm a complete amateur, but i'm researching whether i need to re-bias (that is, different resistor values) the PI for the 12ax7.
thanks again for everyone helping out. i love this amp to death because i can play it loud with relatively little PT distortion (unlike my traynor which is great at times when i want the 6v6 distortion)

[Jazz P Bass]

There is not a need to rebias the PI.

I would try the old tube.

[kingswayguitar]

i think im gonna have fun trying them all!!

[stokes]

The "reissue" Bassman is much different than the Bassman 10,the 5U4 is a rectifier that was used in some Tweed Bassmans and the 6g6 Blond.FWIW I prefer the At7 in the PI,the AX7 will boost the gain a bit,but the plate impedance is much different between them,I find the AX7 to be a bit too harsh,but as has been said,either will be safe,let your ears be the judge.You can get a good NOS AT7 for a good price on flea-bay,there are plenty out there.

[frus]

The PI in itself does not add any real gain to the signal.

Installing a 12AX7 will 'boost' the signal a bit more than the 12AT7 will.

not true

the following 220k resistor is kinda hard load on a 12ax7

if you put the numbers for silverface bassman into this calculator:

Balancing Long-Tailed-Pair Phase Inverter Gains

47k plate resistors, 22k tail, 220k 6L6 grid resistors, you get a gain of around 23 for 12at7, and around 19 for 12ax7
if the plate resistors were the usual 82k/100k, the gain would be approximately the same for 12ax7 and 12at7

I'll add (from experiece) that 12ax7 with 47k plate load usually sounds way thinner than with 100k

but since you won't break anything, try both and see what sounds better to you

[kingswayguitar]

this is the kind of feedback i was hoping for best case.
a bit of experience, a bit of opinion, a bit of theory.
my bassman ten has the solid state rectifier, ultralinear output transformer, no presence control, minimal nfb.
love that website by the way <frus>. if i play with the numbers/resistor values i can get close to the same gain for both. with a marshall 1959 type setup I get 28. next i'm gonna check headroom/clipping.