Fine tuning a 12ax7

[Colas LeGrippa]

This is not a question, but a commentary.
I just finished building a SE amp with 1X 12ax7 and 1X EL34 (my design).  In order to get the max of what could yield the circuit, I decided to fine tune the preamp valve by controling it's bias. So , instead of the two usual cathode resistors that I have ever chosen quite arbitrary, I have put two 5K pots (in this case with locking nuts). So, it gives me the opportunity to fine tune the preamp tube (whatever 12AX....I put in) in order to extract all the juice out of it. I can hear clearly that when turning the cathodes pot knobs, the tube reaches it's magic spot. This can't be achieved by putting simply a 2K7 or 1K5 choosen out of nowhere.  Also, I tweaked the bias of the power tube to get alll the greasy meat out of it. I have started with a 470R bias resistor, lowering it's value until obtaining 24.88W of dissipation. I left the preamp tube cathodes unbypassed (I prefer the tone), while I have bypassed the power tube with a 100uF  cap. The circuit is extraordinary simple. The tone is, in one word: AWESOME. Less is more, like they said !!!!
From now on, I'll always experiment with pots instead of fixed values resistors. In all the preamp stages, CF, LTP,  it is a good way to build a better sounding amp, even better than the commercial ones. Those are different in voltages up to 20%, due to small differences in resistors, caps and transformers voltages and are not fine tuned individually.


Colas LeGrippa

[tubenit]

Kevin O'Connor uses the idea of a pot instead of a resistor in his SOMA 84 amp design.   I've tried this idea and it does work.

With respect, Tubenit

[kagliostro]

I'm not sure, but seems that KOC used a bit different approach

didn't he used a pot to regulate the bypass capacitor insertion, not a pot to vary the cathode resistor value ?

Franco

[Colas LeGrippa]

i couldn't say, never read one of his books. Maybe I should......Anyway I find that it is very interesting to bias precisely a preamp tube.  all the good tone seems to come out of it when tuned precisely. For example, perhaps the sweet spot of a given 12ax7 is 1k75, according to the voltage and other conditions. If we put a 1k5 because it is written on the schematics, we'll never know the good tone we are missing. I guess it is the same with the bypass caps.


Colas

[pompeiisneaks]

I think the only problem with this is that means you have to dial in your bias for every tube every time you put in a new one, seems to be a decent amount of work, and not easily maintained by the average user.

[Colas LeGrippa]

no work at all, because I leave the pots permanently installed, in the back of my amps. So, when the preamp tubes are aging, it is very easy to reajust their bias by ear, without removing the chassis.
Colas

[tubenit]

didn't he used a pot to regulate the bypass capacitor insertion, not a pot to vary the cathode resistor value

I stand corrected on this.  I found this old schematic from the "original" Carolina Blues Special  (I think this was from 2006?).

Having said that,  I have a 5k pot mounted on a vertical aluminum strip  that I have used for yrs as a temporary way of dialing in a 12A_7 bias to my liking.

With respect, Tubenit

[pompeiisneaks]

no work at all, because I leave the pots permanently installed, in the back of my amps. So, when the preamp tubes are aging, it is very easy to reajust their bias by ear, without removing the chassis.
Colas

do you just bias by ear then?  or are there bias points on the back as well to calculate the bias current etc for each tube?

~Phil

[Raybob]

no work at all, because I leave the pots permanently installed, in the back of my amps. So, when the preamp tubes are aging, it is very easy to reajust their bias by ear, without removing the chassis.
Colas

do you just bias by ear then?  or are there bias points on the back as well to calculate the bias current etc for each tube?

~Phil

Look at the OP.  His ear is the meter he is using.
"...So, it gives me the opportunity to fine tune the preamp tube (whatever 12AX....I put in) in order to extract all the juice out of it. I can hear clearly that when turning the cathodes pot knobs, the tube reaches it's magic spot...."

[Colas LeGrippa]

Obviously, I use a meter to bias the power tubes . The pot method could apply as well to cathode bias amplifiers. But in this case, we'd replace the pot by a resistor.
Tone is a matter of ears , not meters. Though I know that if the tone I prefer is when the power tubes are redplating, I got a serious problem...


Colas