Power Entry module (Twin fused?)

[makinrose]

Hi Guys

The old 20W amps were fixed-biased. Definitely use two bias pots for your 2x EL84 amp, then you can set how hot they run and what tone they produce (within the limits of the tube for both). Apart from being able to get more life out of the tubes and potentially eliminate the mud that comes with cathode-bias heat, the output stage can be hum-balanced, which is different than DC balance.

In fixed-bias push-pull there should always be separate bias pots for the push and pull sides. This is generally better and easier to configure than using a bias + balance pot, as TUTs show. I have to admit that early on I was enamoured with the latter but found later that the separate bias pots is a lot easier to deal with in every way.

Have fun

I'm don't think any of the Marshall El-84 based amps made during the Plexi period were fixed bias.  The 18W ones most kit builders are familiar with like the 1974X, 1953, and 1973 and the 20W Lead and Bass 2061X were cathode biased and like Vox's offering they were biased really hot with tiny or non-existent screen resistors.  Definitely amps that need some updating.  All of Marshall's bigger amp were fixed bias during the Plexi era and I think that later on the El-84 offerings became fixed bias.

I've had some success with moding them with ideas from TUT 3's AC30 chapter and cooling down the bias a bit---I think that fixed bias would be good thing to try to make them sound more Marshall and less muddy     .