Cathode/Fixed bias switch for half power

[amplifiednation]

So I've been playing around with half power options. I just installed a DPDT switch that takes a 50w tradional fixed bias 6L6 amp down to Cathode bias to 250R/220uf.

Everywhere I read it said the switch would cut the power by about 30%, since it's less efficient.

The switch works perfect, but I'm actually getting the opposite effect, with the cathode bias actually sounding louder! I have the fixed bias set up properly, at about 65%. The plates are some 470. When I switch to cathode bias, plate voltage drops 450v.

Any ideas?

[Phil_S]

WAG: maybe when you go to cathode bias, the amp moves from class AB to class A. So your 30W class A is about as loud as 50W AB?

What I have seen for "half power" option is for the switch to go from Pentode to Triode mode. I guess that changes the character of the amp, too. I don't know. No experience with it.

[rock_mumbles]

To reduce the power remove the 220uf cathode bypass cap ... although 1/2 power isn't much of a decrease in volume

[randalp3000]

Did that on my Princeton reverb a long time ago with the same results. I liked how the cathode bias sounded more but I did lose some headroom.

cathode bias should draw a lot more current and drop more voltage.

[Reeltarded]

Cut your power cable in half.

[amplifiednation]

So less power, but more volume and earlier breakup? I didn't think cathode bias automatically meant class A though?

[cbass]

cathode bias should draw a lot more current and drop more voltage.

Yup I,m surprised the cathode bias sounds louder.I wouldn't,t think you'd' notice much volume difference between the two

[randalp3000]

I always worry about the PT having enough current with cathode bias, hence the voltage drop. Ever notice that a good AC30 PT weighs 10lbs and is about as big or bigger than a 100 watt marshall/fender?

[pdf64]

Cathode bias on p-p 6L6 at >450V is a recipe for significant crossover distortion, resulting in a thin reedy tone at high signal levels.
Have you put it on a scope?

My guess is that it sounds louder because there's more gain due to the tubes running so hot.

As above, best to confirm that the PT is able to cope with such a high static current draw.
Pete

[jazbo8]

Cathode bias on p-p 6L6 at >450V is a recipe for significant crossover distortion, resulting in a thin reedy tone at high signal levels.
Pete

+1, cathode bias at 450V plate/screen is not recommended. Instead of switching the bias from fixed to cathode, try a pentode to triode switch.

[John_P_WI]

Taylor,

There is some mis-information / thoughts here and on the net. Cathode bias is NOT any louder than fixed bias. Cathode bias is NOT half power, cathode bias does NOT automatically indicate class A operation.

Responding to the OP question, what really needs to be done is to place 1 ohm resistors off of the cathode of the tube. The "bottom" of the resistor will then go to the fixed or cathode bias switch. This will allow you to measure the current through the tube. In your example, I would expect the cathode biased tubes to be pulling more current, or be biased hotter than the fixed bias. This is evident by the plate voltage being pulled lower.

Cathode bias can not be louder, here is why: It is less efficient, why?
Let's use a plate voltage of 400v for example and a cathode v of 20v (measured at the "top" of the cathode resistor to ground).

In Cathode bias we have an effective voltage of 400-20 or 380 v. Fixed bias does not have the cathode voltage difference of 20v. When biasing cathode tubes, and knowing the current through the 1 ohm r, calculate the power using Plate - cathode voltage for V. (400-20 = 380 as above).

Cathode bias also looses power in the cathode resistor. The resistor gets warm with use, right? Power lost.

Cathode bias can suffer from degenerative feedback, using a larger cap will make it feel more "fixed bias like" though. 30 watt amps are damn loud with good cabinets and speakers, using no cap can give a smoother feel. Yeah, not a lot of clean headroom, but great for blues.

I have played with cathode biased 6l6 power sections @ 425 v, I can assure you that there is no thinning of tone. What is noticeable is a darkening of tone at lower voltages, I attribute this to mostly to a change in preamp voltages. Anyone who has played with power scaling will agree that statement.

I for one love 6v6's screaming, cathode biased.

Pentode / Triode swithing... Softens and darkens the tone. To me I feel like I lose too much pick attack, I have used it on several amps - not much of a reduction in volume and not true triode. Kind of luke warm feeling towards it.

Power scaling would be good approach for volume reduction, either fixed or cathode biased - BUT in the D style amps where the preamp voltage on V1 is king, I would add a voltage clamp to ensure that the preamp and PI always stay at a reasonably constant voltage, and the output tubes screens, plates and bias are varied.

As the others have mentioned, verify the current through the tubes to ensure the stafety of your power transformer and OT. The PT can run warm, but not too hot to touch.

Just a few thoughts from the last 20 years...

[John_P_WI]

dbl

[Milkmansound]

Agreed - it's not quite class A

I've done it many times with good results, but you need to be conservative on the cathode bias end in order to avoid damage.

It drops headroom a little but gives much more touch sensitivity. It's a cool mod for a high power amp.

[jazbo8]

Taylor,

There is some mis-information / thoughts here and on the net. Cathode bias is NOT any louder than fixed bias. Cathode bias is NOT half power, cathode bias is NOT class A.

Agree

Pentode / Triode swithing... Softens and darkens the tone. To me I feel like I lose too much pick attack, I have used it on several amps - not much of a reduction in volume and not true triode. Kind of luke warm feeling towards it.

Perhaps some clarifications are in order, if the bias voltage remains the same, then the volume reduction would be minimum. However, if the bias is adjusted for Class A operation per the datasheet (-20V), the volume reduction should be quite noticeable as the output power drops from 50W down to 5W. Pentode connected as a triode operates just like a triode, even if it is not a "true triode". Some may prefer the soften tone.

Power scaling would be good approach for volume reduction, either fixed or cathode biased - BUT in the D style amps where the preamp voltage on V1 is king, I would add a voltage clamp to ensure that the preamp and PI always stay at a reasonably constant voltage, and the output tubes screens, plates and bias are varied.

Agree, I got very bad sound when VVR the whole amp, it lost all the sparkles created by the preamp distortion - not recommended.

Just a few thoughts from the last 20 years...

Better to get valuable information from actual players, instead of desk jockeys like myself...

[Phil_S]

...I didn't think cathode bias automatically meant class A though?

I'm no expert, and I certainly don't want so start a whole discussion on class of operation, but I think that's right. Plenty of cathode bias amps are AB. I was simply offering a WAG (wild ass guess) not something of real substance, my guess.