Ideal 12ax7 bias

[Colas LeGrippa]

Hello my friends !


Last week I have posted a thread about fine tuning a 12ax7 with a 10k pot (tied from cathode to ground). After performing many tests, it seems that the magic spot of that kind of valve is found  around 470 OMH, way far from 1k5 or 2k7 that usually are used in common amplifiers. In two of my amps I have soldered a 5k pot on the cathode of the first and second gain stage and those amps never sounded so good. Can someone explain to me what is happening here? More current, less voltage on the anode, more tone ??? More, why nobody seems to have discovered that a much lower value resistor makes a 12ax7 sound better ? (I am getting deaf, maybe ).


Colas LeGrippa

[tubenit]

We'd have to see the 12AX7 in the context of the specific amp schematic and also have the voltages posted for the B+ rail, plates and cathodes to really answer that question.  Another factor, IMO, is the size of cathode caps and what frequencies are being impacted and also coupling cap values. There is alot that is interactive in these amps.

Lots of factors go into answering that question.   

Having said that, I love it that you are willing to experiment and find out what works best for you!  I think that is where some of the real innovative ideas come from.

With respect, Tubenit

[jeff]

How much current are you drawing with a 470ohm cath resistor and is that within range of a 12AX7s max.dis. rating?

[jjasilli]

+1 to tubenit.  You should to plot a loadline.  That will visually show what is happening to signal. 

[Colas LeGrippa]

Well I guess that too many things are involved to really know what's goin'on without performing a lot of tests and experiments with a scope, then examining the frequency response of a given circuit according to different resistors values.....Anyway, to make a long story short, by replacing resistors by pots, we can really dial in the perfect bias of a preamp tube in a given circuit, just by ear. And f....the scope.


I love you all.


Colas LeGrippa

[tubenit]

we can really dial in the perfect bias of a preamp tube in a given circuit, just by ear.

I've heard your most excellent guitar playing!  And so, I'd trust your ear on this one!   

With respect, Tubenit

[Colas LeGrippa]

Oh, thank you Jeff, you're a brother !

[Tone Junkie]

Is that with a cap my friend or unbypassed.
Thanks Bill

[Colas LeGrippa]

unbypassed

[sluckey]

...by replacing resistors by pots, we can really dial in the perfect bias of a preamp tube in a given circuit, just by ear.

That may work for you. But it would fail miserably in a production environment. Just imagine if Leo had 100 people "dialing in" 100 Tweed Bassman amps by ear. You would have 100 amps that all sound different. By using test equipment with visual indicators you can build 100 amps that all sound the same. That's a good thing.

Not knocking your method. It works for you and that's OK. But that method would probably fail for most others. How many people would really agree that 470Ω is the magic size for a 12AX7 cathode resistor just because your ears say so to you? It's totally subjective.

[jjasilli]

Well I guess that too many things are involved to really know what's goin'on without performing a lot of tests and experiments with a scope, then examining the frequency response of a given circuit according to different resistors values.....Anyway, to make a long story short, by replacing resistors by pots, we can really dial in the perfect bias of a preamp tube in a given circuit, just by ear. And f....the scope.

I love you all.
Colas LeGrippa

It's not hard to plot a loadline, though it may sound exotic.  No test eq needed other than a volt meter.  We do need to know values: supply voltage (top of plate resistor); plate resistor value; plate voltage.  And is your cathode R feeding one or both sections of the tube?

[Colas LeGrippa]

Sluckey, I understand that industries put in their amps the same resistors even if the B+ can drift by 20 % from one amp to another.
I invite you to experiment with one of your amps. You will notice that the sound becomes louder between approx 400 to 600 OHM (K resistor unbypassed).
It is not subjective at all. It is not a question of the colour of the sound, it is question of db that any ear can easily perceive. A scope would clearly see a raise in the amplification.


It doesn't work for me like you said, IT WORKS.


Have a good day (and enjoy the 470R trick!).


Colas




 

[PRR]

I believe Leo picked the 1,500 (bypassed), not just because it is suggested by G.E., but because in testing (possibly with a pot) it gave the sound that HE liked, for his taste, with the musicians he worked with and the music and instruments they played.

May have been the intersection of Bakersfield and Surf 1959.

You may play different music, differently, different instruments, and 58 years further along the Hit Parade. Even if/when you play vintage music, you don't play it quite the same.

Also un-bypassed is different. 1,500 no-cap is a serious gain hit. 600 gets more gain. 100 overloads too easily. I dunno what the optimum unbypassed Rk may be. Leo gives little clue; the unbypassed 820 in 5F6a tonedriver is quite different from most preamp stages.