Pulling power tubes in cathode bias amp

[reubencox]

Can you pull 2 of the EL84s in a Matchless DC30 or Vox AC30 type amp??? If so isn't it correct that you need to change your speaker load as well?

I can't find much info on pulling power tubes cept for on Marshalls.

[John Phillips]

Definitely not on an AC30. It has a single cathode resistor so you will overload the remaining two tubes.

Not certain about the Matchless but from memory they have a half-power switch, which does the same thing - if so, they must use separate cathode resistors for each pair of tubes.

With any four-tube amp, if you pull or turn off two tubes, the effective output impedance doubles, so to keep the same match you should reset the impedance selector to half - but whether it makes an important difference depends on whether the load was correctly matched with four tubes... which it isn't always. Even if it is, the worst that happens if you don't reset the impedance is that it's harder on the remaining two tubes and reduces the power further, because the amp is now running into half the matching load.

[reubencox]

Thanks for the info! :)

[Rob Sharer]

Found this thread while pondering asking a similar question (what a concept!). Short of a comprehensive list, or a deep understanding of how to evaluate each amp circuit, how is the ordinary Joe Soap musician to know which amps will stand being treated this way, i.e., having power tubes pulled?

As a Matchless user myself, I've had the case made to me by someone who would probably qualify as an expert on these amps claim that using the half-power switch significantly changes the tone, specifically resulting in a loss of low and and a spike in the highs (pretty much reflects my own experience with the switch). In a perfect world, the amp would just politely turn into the half-power version of its normal self when the switch is engaged. Is there something else going on here?

My own question was prompted by my quest to set up a stereo EL-84 rig. Whilst looking at the Mesa/Boogie 20-20 quad EL-84 stereo power amp, I began to wonder whether one could pull two of the tubes for those quieter moments. Any guesses, Sachems? Cheers,

Rob

[John Phillips]

As a Matchless user myself, I've had the case made to me by someone who would probably qualify as an expert on these amps claim that using the half-power switch significantly changes the tone, specifically resulting in a loss of low and and a spike in the highs (pretty much reflects my own experience with the switch). In a perfect world, the amp would just politely turn into the half-power version of its normal self when the switch is engaged. Is there something else going on here?Yes, the impedance mismatch when you're only running two tubes. The power supply is also under a lot less current demand, so it will get tighter and harder-sounding.

My own question was prompted by my quest to set up a stereo EL-84 rig. Whilst looking at the Mesa/Boogie 20-20 quad EL-84 stereo power amp, I began to wonder whether one could pull two of the tubes for those quieter moments.No, because it's not actually a 4-EL84 amp, it's two 2-EL84 amps :). The two sides are independent and - although they're actually fixed-bias, so you could safely pull one tube from each - it will then not be operating correctly in push-pull, so it will sound badly distorted.

You could certainly run it at 20W in mono, though... just use one side.