Overdriving 6L6's

[EETStudent]

Actually, this question is universal for all tubes. Ive heard two things about driving preamp and power tubes. Ive heard that more voltage on the plates results in more gain. I have also heard that raising the plate resistors, resulting in a lower voltage, will result in more gain. Which of these is correct? Im thinking about driving my 6L6's for more gain and/or output. Are the two related in this case?

[black_labb]

more voltage will result in more gain (with everything else equal) but gain doesnt mean more distortion. its used to describe high gain amps, as the premp tubes are set up to have a huge amount of gain (enough that the next stage doesnt stay linear, and starts to break up). higher voltages will actually increase the headroom to an extent along with the fidelity.

[hasserl]

I have also heard that raising the plate resistors, resulting in a lower voltage, will result in more gain.

Increasing the value of the plate resistors may result in a lower voltage to the plates, but that is not (or should not be) the goal of increasing the plate load resistors. To adjust the voltage to the plates of the tubes you should be working with design of the power supply rail, altering resistance values of dropping resistors.

But back to the plate load resistors, these resistors are there to convert current into voltage, that is their purpose. A change in the voltage at the grid results in an increase in current thru the tube. But since you need an increase in voltage, not current, in the preamp, a resistor is placed in the circuit. Ohms law says current x resistance = voltage. Increase the resistance and you also increase the voltage. Increased voltage IS gain. So, increasing the value of the plate load resistors will raise the gain of that stage.

Hope that helps
H

[stokes]

Good explanations guys,but we are missing something here.He is talking about driving the power tubes,they dont have a plate load resistor.I assume you are looking for "power tube saturation" here.A simple way to get more break up in your power tubes would be to bias them a little hotter.You could also hit them harder with your signal from the PI,which brings us back to increasing the gain of that stage,so back to black labb and hasserl's explanations.In this regard you want a bigger signal coming out of your PI but not more distortion,a bigger clean signal from the PI.

[hasserl]

Good point Stokes. I was assuming he was referring to increasing gain the preamp in order to drive the 6L6's harder. But you know what happens when you assume things.

[EETStudent]

Thanks guys. Im just confused as to how some amps experience a lot more breakup than others, but with similar configurations. I have a Gibson Mercury, 12AX7 preamp, 12AX7 PI, 2 6L6's and its very clean. The plate and grid voltages for the 6L6's are both around 460dc, with -50dc bias voltage(I know, I know thats an inaccurate judgement of the bias setting). I read somewhere that this guys amp breaks up a lot more because he lowered his plate voltage to about 400dc. Is that how it works? One of you guys mentioned increasing the signal from the PI into the 6L6's, how do I go about doing that?

[Rick Erickson]

Im just confused as to how some amps experience a lot more breakup than others, but with similar configurations. I have a Gibson Mercury, 12AX7 preamp, 12AX7 PI, 2 6L6's and its very clean.

Many Gibson amps were designed with rather low gain. This has more to do with the choice or lack of Cathode capacitors and voltage dividers, tone shaping networks etc., than it does with power supply voltage levels. Look at the schematics of various amps to see how they were designed. Gibson guitars as a general rule had pickups that produced much higher voltages than a standard Fender or Rickenbacker of the era. Since techs/engineers wanted the amps to play clean, the Gibson amps were often neutered in the design process.

RE

[Alex R]

...but yes, running amps at lower voltage can give you earlier breakup. 400volts on the plates will give a sightly softer feel to the amp and earlier breakup in volume terms. It's a simple matter of lower wattage and thus lower headroom, so you get into power tube breakup at a rather lower volume. However with 2x6L6 this is still going to be pretty loud!

[stokes]

"One of you guys mentioned increasing the signal from the PI into the 6L6's, how do I go about doing that?"
Looking at the numbers you posted,I dont think that is the route you want to go.You would be better off trying to raise the idle current on the power tubes,which will lower your plate volts some,but first measure the idle current in your power tubes to see if this is possible.If you were to try to lower the volts to the power tube plates alone,you would lower the volts all the way down the line and that may not be what you want to do.Check that current and post the results.