Quote from Digital Larry on November 15, 2012, 07:11
Well, I don't have to worry about current production tubes as I don't think anyone makes 12AB5 any more. They were spec'ed for automotive applications. I've found a fair number of NOS, pretty cheap too, compared to 6V6's.
So far, here's my take on how the screen grid operates. Please feel free to correct it.
#1 In a triode, the force pulling the electrons from the cathode to the plate (anode) is proportional to the difference between plate voltage and the cathode voltage (for a given control grid voltage).
#2 When there is a resistor in the plate circuit, the point of which is to create voltage gain from the control grid, this force varies with the amplified signal, because the plate voltage is swinging up and down with the signal. The plate also has a non-trivial internal resistance of some tens of kilohms. This fluctuating plate voltage is the main source of triode non linearity.
#3 The screen is inserted, and kept at a voltage "near" or "sightly below" the plate voltage (with no signal).
#4 As the signal comes into the control grid, controlling the current from the cathode to the plate, the force that the electrons see is now dictated mostly by the screen voltage rather than the plate voltage.
#5 As the screen is not a solid sheet, most of the electrons fly right past it and are pulled into the plate. However some of the electrons smash into the grid [y'know, the BIG ones - wink] and are collected there, accounting for some % of the total cathode current. So, the tube just became a little less efficient because the screen current does not contribute to the plate load current and output signal voltage.
#6 Due to the construction of the screen, it can't dissipate as much power as the plate without melting, and so current limiting is in order.
#7 A fluctuating screen voltage would contribute to either further linearization of the plate current (as in the ultralinear transformer arrangement), or possible further non linearity, depending on its relationship to the input signal. If we're not doing anything like trying to make an ultralinear output stage, then the best bet is to filter the screen voltage using a large cap to ground - otherwise the resistor in the screen circuit will generate a signal voltage and we'll be no better off than before, linearity wise.
#8 The practice of tapping the preamp load supply voltage from the downstream tube's screen supply is probably a cost-saving measure that as a side effect may contribute to the tube amp's "mojo" by either causing overall supply sag (compression) or some signal-frequency variation in the power tube's screen voltage. Otherwise I would tend to take a separate filtered line to power the preamp stage(s).
How am I doing so far?
You definitely want to filter the screen grid supply. In fact most Fender and Marshall push/pull amps have an inductor in series with the screen supply to help regulate it. I prefer well regulated screen voltages personally. As for the triode connection of the screens it is easy to put a switch in to get this mode so why not? It is a sort of an old school sound to it and it's useful in overdubs when recording, it also reduces power output. Stacking a triode track on top of the normal guitar track makes a nice full sound sometimes. I like to keep the preamp supply pretty clean and well regulated for the most part. I even put 1uf @ 450 volt caps on the preamp B+ connection close to the tubes if there is much distance between the preamp and the power supply in the chassis. The extra cap can help increase the punch and generally improves the sound (I figured this out back when I was modding Dynaco PAS3 preamps for the hifi crowd 30 years ago or so).
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Re: Webcor TP-4905-1A conversion to Fender Champ?
