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Re: Ella mods: screen diodes

Posted by Goneill (A) on July 15, 2006 at 21:55:13

In Reply to: Re: Ella mods: diodes, resistors and bias pots posted by RHone on July 3, 2006 at 18:00:41:

OK, I’ve tried the screen diodes change and I have to agree with Al (aka tubegarden). It does seem to improve the sound, making it smoother, more musical and having more presence. Of course I might be imagining it as I’ve done quite a lot of changes recently, but I feel it’s worthwhile. Besides, it’s easy to do and if you kept your old power supply diodes when you changed them to Schottky’s then it’s free.

I found this information about what the change achieves and how it works:

Dennis Grimwood

There’s a lot to read in there, but my interpretation of it is that the screen (grid 2) is positively charged to help draw the electrons off the cathode, past the negatively charged controlling Grid (grid 1) towards the plate. Without this not so many electrons move, which is why in triode mode you lose nearly half the power of the amp. The problem is that the screen not only helps the electrons on their way to the plate, but being positive it actually attracts some of them itself. This sets up AC currents on the screen circuit as well as reducing plate current. By adding a diode to the screen the circuit is broken for AC which means the electrons continue to the plate, resulting in a cleaner sound (no AC messing up the OPT) and about 10-20% more power. As Dennis says, it makes the pentode behave more like a triode but without the resulting loss of power.

Apologies to Dennis if I’ve got this all wrong, and I recommend you read the original pages as there is a lot of interesting stuff there. This change is described in item "4. Silicon diode feed" as well as the introductory pages on Screen Grids and Ultra-Linear.

Dennis did give me one warning though. By putting the extra current through the plate you can drive the tube beyond its normal operating range. The best way to avoid problems is to make sure the grid has enough negative charge to control it. In the Ella this is easy as each tube has its own bias – just make sure you haven’t set the bias reading close to the maximum mV reading recommended for the tubes. On my KT88’s I use about 480mV which is well below the maximum of about 600mV and I have had no problem.

I’ve updated the photos and diagrams on my web page:

• My web page   (Open in New Window)

This post is made possible by the generous support of people like you and our sponsors:

Schiit Audio

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