EL34 bias. Where do you have yours?

[eyeball987]

I remember discussing this around here about 5 years ago and was wondering where the current crop is setting their 34's in a RM100.

I used to be in upper end of the range in manual, just below 38, but now keep them in the lower range. Closer to just under 32.

There was all kinds of tech talk in the old thread about where and why and how that manual was actually off a little bit and blah blah blah. I just want to play around with the upper range again a little to see how it sounds without blowing up my Winged C's.

 

[Jaded Faith]

I'm generally a fan of biasing tubes a touch cool, between 55-60% dissipation. It will provide a bit more articulation and definition as opposed to saturated compression. Most MTS users get their gain strictly via the preamp, so this helps clean things up along with a lower gain 5751 in the PI.

 

[eyeball987]

I'm generally a fan of biasing tubes a touch cool, between 55-60% dissipation. It will provide a bit more articulation and definition as opposed to saturated compression. Most MTS users get their gain strictly via the preamp, so this helps clean things up along with a lower gain 5751 in the PI.

Hello Rob! It's good to hear from you. As the maker of my favorite mods it's always good to hear your advice. I have been using the 5751 in the PI for years now.

Being an idiot on the dissipation thing, would you say around 32 is in that range for a quad of Winged C 34's?

I wrote down a chart years ago from a former poster named Julia who put 34's in the 70% dissipation range at about 33. Would that be accurate?

 

[Jaded Faith]

A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube. Assume 500V plate voltage on the average MTS amp, some are a little lower due to component variance and your wall voltage. As such, here is the math:

Dissipation Percentage * (Tube Wattage / Plate Voltage) = Bias in mA

For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 500V class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA. Another way to say this is at 500V class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA.

To calculate for any other tube, simply sub the wattage into the formula above. Feel free to also experiment with bias values between 50-70%. I tend to prefer a cooler bias between 55-60% in most amps for less compression and more articulation. Longer tube life is another bonus of going a little lower.

When using the bias test points you are taking a mV reading across a 1 ohm resistor between the cathode and ground instead of a mA reading. This is just a simple derivative of ohms law and just be sure you use mV on your meter.

 

[eyeball987]

A general rule of thumb is that class AB amplifiers are usually operated at no more than 70% of the maximum plate dissipation of the tube. Assume 500V plate voltage on the average MTS amp, some are a little lower due to component variance and your wall voltage. As such, here is the math:

Dissipation Percentage * (Tube Wattage / Plate Voltage) = Bias in mA

For example, the aforementioned EL34 tube has a plate dissipation of 25W, so at 500V class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA. Another way to say this is at 500V class AB operation, it should be biased no higher than (0.7 * 25/500) = 35mA.

To calculate for any other tube, simply sub the wattage into the formula above. Feel free to also experiment with bias values between 50-70%. I tend to prefer a cooler bias between 55-60% in most amps for less compression and more articulation. Longer tube life is another bonus of going a little lower.

When using the bias test points you are taking a mV reading across a 1 ohm resistor between the cathode and ground instead of a mA reading. This is just a simple derivative of ohms law and just be sure you use mV on your meter.

As usual, above and beyond!

Thanks again for all of your help over the years.

 

[Jaded Faith]

My pleasure and glad it helped. I think understanding what you are doing is far more valuable than asking the general public their preferences. A few volts swing in what you get from your wall AC outlet can make your bias settings completely different from another's experience. To say "just set it to XYZ" is selling yourself short.