El 84 fixed bias amp & Paul Ruby zener mod.

[nickb]

Thanks for posting the scope and analysis nickb - very clear as you say and both the 'clamp' and 'back-back' very effective at reducing xover distortion.

I wanted to ask, is xover distortion solely responsible for the 'harshness' that the OP spoke of?

And how do the overdriven EL84's sound with either of these mods installed?

Nor sure if that was directed at me but I think Alf (OP) is best placed to answer that one.

[Joe L]

Out of curiosity, which output transformer is being used?

[Alf]

Nor sure if that was directed at me but I think Alf (OP) is best placed to answer that one.

Please comment if you have already tried it !
I have installed the diodes but I'm waiting for the right opportunity to crank the amp to the required volume, dont'want to disturb too many people right now.

I would also like to hear more feedback from othe people who did the mod to a fixed bias amp..

[Joe L]

I wanted to ask, is xover distortion solely responsible for the 'harshness' that the OP spoke of?

And how do the overdriven EL84's sound with either of these mods installed?

The first amp I built with EL84s I used a transformer from a company that promises "classic tone". It really emphasized the odd harmonics of the switching distortion. I recognized the harsh sound that I heard when comparing dimed Deluxe Reverbs that both my best friend and I built, he using the "classic tone" Deluxe version and I used the Heyboer counterpart from Mojotone. Since this was an amp I was building for the "big guy" at work, I did some research into the zener mod. Since I have an excellent scope and signal generator, and know how to use it, I was able to tune the zeners for the best outcome. It definitely improved the harshness, but I still wasn't satisfied.

The best fix was to order an 18 watt output transformer from GDS Amps which are made by Heyboer. You could hear the difference from crystal clear to wide open. I ate the cost but the "big guy" was very happy

The next EL84 amp was a 36 watt version and we (I was sitting behind a technical novice giving instructions step by step as he built the amp) again used GDS transformers. We didn't even bother with the mod, it sounded great without it.

So, with a great transformer, the mod doesn't seem necessary. When needed and implemented correctly, it reduces the harsh crackling noise when the amp power section is pushed hard but the results aren't as good as using good iron to start with.

[Alf]

A great OT is often the key to a good sound imho.
My amp OT has been from the start a really old ( '64 I think) Vox AC15 OT which I like very much.
But I am always trying to improve the sound. A couple of years ago I put a conjunctive filter ( Dr Z) in it which I like too.

I am very curious if this mod will further improve the amp.