From: “Scott Frankland”
Subject: Re: [Fwd: [vintage_tube_hifi] How many prefer listening to Vinyl?]
Date: Fri, 24 Mar 2000 14:35:42 -0800
> > I would note that using the screen regulator seemed
> > to improve the bass response a good deal.I don’t understand
> > why doing this in place of the UL hookup would improve the
> > perceived bass response as much as it does, the actual
> > difference in output power is inconsequential.
> Opinion here… Screen regulators are NECESSARY on pentode
> connections. The linearity difference alone is amazing. I’ve noted
> the same thing you’re reporting too.
Some further speculations…
If, in a pentode amp, the screen is tied to the (unregulated) plate voltage source, then the DC “floor” of the screen will be modulated by the plate current (to one degree or another). This will no doubt have a detrimental effect on the bass quality in particular.
Since, in a pentode, the plate operates as little more than an electron collector, there is little reason to regulate it. OTOH, regulating the screen is akin to regulating the plate in a triode. A neat trick, since the pentode screen draws much less signal current than does the triode plate. By regulating the “surrogate plate” of the pentode (i.e., the screen) we prevent the screen grid from swinging with signal and thereby eliminate the conflict with the control grid for control over plate current.
Mathematically, we get a single-valued function instead of two opposing functions; i.e., the control grid swings while the screen grid remains fixed. Physically, the function of the screen then becomes simple: the screen helix becomes the central axis of a fixed accelerating field within the tube. Electrons are then relayed from space cloud to plate as a single-valued function of the control grid voltage.
One can of course regulate the screen in UL if a separate screen winding is used, but this will not prevent the screen from swinging AC across any such UL winding, again creating a double-valued “control function” over plate current. In this case, however, there will be no drop at the B+ source (the CT of the screen primary), and thus no DC modulation of the voltage source. Therefore the bass ought to be improved with this arrangement over simple UL.