Mesa Boogie DC 3 Power tubes

[na3d]

Hi, I'd like to know if I can pull away two EL 84 power tubes (like in the rectifier) to get the amp quieter? And if yes, do I need to pull away the central two?

[Gibsonman63]

By quieter, I am assuming that you mean less volume. If there are four power tubes, you can generally pull two and cut the output power to half...

But half the power is only about -3dB, which is not much.

An attenuator would get you closer to where you want to go.

[Gibsonman63]

Sorry to highjack the thread, but my curiosity is getting the better of me.

A quick look at the schematics shows that master volume pots are linear pots and not audio taper. I have seen several MB amps where the volume comes on very fast and it is hard to dial in a reasonable volume. Would changing these to audio taper pots give the desired effect?

[Phil_S]

+1 on what Gibsonman63 says. Let's say 4x EL84 = 30W and 2x EL84 =15W. The perceived difference between 30W and 15W is that 15W is about 90% of 30W. You need to go down to 3W to cut the perceived volume in half.

Pulliing two tubes changes the primary load on the output transformer. The may sound counter-intuitive, but it doubles the primary impedance. This means the secondary (speaker) load should be doubled to properly compensate. For example, if you are using a 4 ohm speaker load with 4 tubes, go up to an 8 ohm load for two tubes. If there are multiple taps on the output transformer, the "value" of each is doubled when you pull two tubes. Some amps will be OK with a mismatch and others won't. Be careful, this can get expensive very quickly.

So, the question I'd ask is, what are you trying to achieve?