Re: Bypassing cathodes with class A triode PP amplifiers

Peter Wieck said:
To be blunt, I cannot remember seeing a cap (of any size or otherwise)
bypassing an output tube cathode resistor in an audio application.

John Byrns replied:
Peter, surely this is a joke, you must be pulling our collective legs, you
really can't "remember seeing a cap bypassing an output tube cathode
resistor in an audio application"?

It seems unlikely that such a pompous little man would recognize a joke
if he fell over it. He honestly believes his extreme ignorance is
wisdom we cannot survive without.

Hey, Witless, do you actually know what a capacitor looks like? Every
handled one? Here's a short course:

1. Open your best amp. Take a pair diagonal cutters in one hand. You do
have diagonal cutters, don't you? (You can't be in DIY electronics
without diagonal cutters. There are pictures of the tools in the Boys'
Own Electrician Kit your wife gave you last Christmas. RTFM. That's
something DIYers say to clowns and newbies. It means "read the fucking
manual".)

2. Cast around inside this amp until you find a cylindrical object
about half an inch long. It will have coloured rings printed around it
and a wire coming out of each end. Use the diagonal cutters to cut both
the wires. The component will now be loose and you can pick it up.

3. Pick it up and study it. It is a resistor. Most resistors look like
that in tube amps, though a few look like cement bricks and some are
aliminum cased, but these latter have a value and a rating printed on
them as in 50KR (the R standing for the ohm symbol RTFM) 50W (the
double you standing for Watt RTFM).

4. Now look for a larger cylinder without the parallel coloured rings.
This cylinder is often black or blue, sometimes red or gray. Cut the
two wires attaching it (with the angle cutters, dolt! RTFM!!!!).

5. Pick up and study this component. It is likely to be a capacitor. It
will have printing on it comprising a value and a rating. The value
could be in so many uF or pF or a decimal fraction. The rating will be
in Volts. If it says Fuse on the outside and the bit inside is printed
so many A for Amps, you have cut out the wrong component. Try again.

6. Now try cutting out a few more components and identifying them as
either resistors or capacitors. You'll soon get the hang of it.

7. The canisters that have two connections to the bottom and printing
that says so may uF at quite a high voltage are also capacitors, used
in the power filter. The power filter is several lessons away. Don't
worry about that for now. Just try to distinguish capacitors from
resistors.

8. Now that you have identified resistors and capacitors, try something
more difficult. The glowing glass tubes are vacuum tubes. You can pull
them out. Take care not to crush the glass.

9. The vacuum tubes leave bases behind, those round white or black or
brown thingies. Use a screwdriver (RTFM) to get to the underside of
these, where you will see 4, 8 or 9 connections (in the most common
amps).

10. One of these connections is to the cathode. You identify the
cathode by finding the spec *** for the tube (RTFM) and reading it.
You can read, can't you?

11. Another way of identifying the cathode is because it usually has a
resistor and capacitor in parallel (later lesson, sorry, too
complicated for your present state of education) attached to it in the
vast majority of audio amps.

12. When you have identified the cathode of each tube, you are in a
position to find the resistor and the bypass (lesson about filters,
about three years down the road) capacitor in parallel attached to the
cathode. They are attached between the cathode and ground (later
lesson....

..... oh, ***, this is getting too complicated. Look here, Witless,
lest you electrocute yourself and rest like suet pudding on my
conscience, take the bloody amp to a proper repair hack and give him
twenty bucks to show you what a capacitor looks like and a resistor
looks like. Give him another twenty to screw the covers on properly for
you with security screw so you can't get in there again and do more
harm to the amp or yourself.

HTH.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review

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