I'm listening to the radio with such an amp right now. My problem was this
VERY hot summer: I just couldn't run my usual 6L6GC PP poweramp the whole
day without sweating just passing in front of it, therefore I put together a
real poor-man's amp aimed at 2Wpower with less than 10W dissipation. As I
said, it's truly for ambience listening. I took the schematics from
Triode (http://www.triodeel.com/compact.html) but I replaced the EL84s with
the two halves of a JJ/Tesla ECC99 (let's say roughly a 12BH7, a 3+3W double
triode with a Ri=3000 Ohms). Instead of a 6C4 the driver is (obviously) an
ECC82 used for both channels. The OPTs are readily available, "universal"
Hammond mod. 125H, connected in order to get 10K A-A. The common cathode
resistor is a 390R, 4W, metal oxide. Anode voltage is about 260V, with the
cathodes sitting at about 10V above ground (A-K=250V). Power supply is
guaranteed by a diodes bridge and a resistive Pi filter using 2x 47uF/450V
electrolytics.
I'm pretty satisfied with it, because with just 3 tubes I get the "musical
wallpaper" I need all day long (I'm a professional engineer working mostly
at home) without resorting to the sand state of matter. Now it works
connected to a couple of self-made small loudpeakers using a single Fostex
FE83 cone (2WRMS): it doesn't even sound so bad.
I never measured any parameter because I didn't care, but if the fellow
rodents are interested I can take some 'scope pictures.

Ciao to all

Fabio

"Fred Nachbaur" ha scritto nel messaggio
news:JZT4b.128124$K44.120827@edtnps84...


Engineer wrote:
john stewart wrote:

(snip)


Hi Roger- I read your query along with John B's very good
description of some of these phase splitters. Here are two more
for you to ponder, as they were used in some 30's & 40's radios.

In this scheme there is no seperate phase inverter tube or transformer.
A resister of the proper value (about 2K) is inserted into the G2 lead
of the upper pentode of a PP pair. The resulting signal on that G2 is
capacitively coupled to the G1 of the lower member of the PP pair.
Of course there is some imbalance, but that is partially corrected by
the PP output transformer. This circuit was often seen using PP 6F6's
but could be applied to other pentodes or beam tetrodes as well.


John, the screen idea is what I came up with while trying to
figure out how to phase split without an extra tube - the
purpose of the original "kluge" - but I didn't calculate any
values. Done properly, it just might just work better than a
common cathode resistor - a good experiment for someone with
an old PP 6V6 amp to play with (I don't have one or I'd try
it!)

Another notion I've intended to try out (and indeed will, one of these
days) is to use the cathode coupling (short-tail pair) and augment the
signal to the "off" phase with a voltage divider from plate of the "on"
phase to grid. This would seem to give a less distorted signal than
taking it from the screen grid.

By making this variable, the amount of asymmetry can be varied to either
side. This might actually be useful for blues harp or acoustic guitar
players, since it appears that acoustic feedback can be reduced this way.

Cheers,
Fred
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