Thread: A mystery to me
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Fabio Berutti
 
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As some other RATs have already pointed out, this is the so-called
ultra-linear arrangement, which is used by the largest part of HiFi amps
using PP tetrodes, particularly these days because it is getting difficult
finding OPTs fitted with cathode windings suitable for "distributed load"
stages like McIntosh or similar. I suppose You can find the original
article written by Hafler back in 1950something describing his findings
about this arrangement.
The main points about UL are the following:
- G2 is operated at the same DC voltage of the anode (in fact 2-3V higher
due to the OPT winding resistance), therefore this should be checked when
swapping tubes
- for the very same reason, tubes allowing a high G2 voltage are best
suited to UL (ie. EL34, 6L6GT, or other types where the max V is not very
different between G2 and A)
- the grid is fed back some signal, and this (negative, local) feedback
contributes to reducing output impedance and distortion. In fact, during
operation, the applied load creates a very complicated situation which is
different from the one measured in static conditions. Look for the old M-O
data sheets for the KT88, they show the anode curves in UL mode: they're
definitely neither "pentode" nor "triode".
- the behavior in terms of distortion products is half way between triode
and pentode, depending on the primary tap position
- construction is easy, efficiency is very good and so is the sound (IMHO:
all my tetrode amps are UL, including some SE types, and I think they're
good, but... I'm not an unbiased listener)
- a damping resistor should always be connected between G2 and UL tap

As per Your understanding of electronics, with that Williamson You managed
to get one of the finest pieces of tubed gear ever designed: You don't
necessarily need to be a Nobel winner to enjoy it, as far as You have ears.

My compliments

Fabio


"Jon Yaeger" ha scritto nel messaggio
...
OK, I admit that my understanding of tube electronics is rudimentary at
best. But one thing really puzzles me.

On a Williamson UL PP amp, the screen is at about the same DC potential as
the anode (as measured with a DVM). I thought that the screen had to be

at
a lower potential for normal PP operation.

What's up with that?

TIA,

Jon