Ian Iveson wrote:

"Patrick Turner" wrote

...The original poster says he has a guitar amp where
12AX7 is the specified tube for phase splitter.
We could assume its configured as an LTP,
and there is probably some NFB applied to one of the two inputs to
the
LTP, since this is a common practice with LTP guitar amp
drivers...

It was the chap with the cathodyne that I had in mind. Yes most use
a long-tailed pair. It's not just about constant tone, but also
about the envelope, which at the extreme is shaped not just by the
front end, but also by the approach and onset of grid current in the
output stage, saturation of the OPT, sagging of the PS, the time it
takes and the manner in which it recovers from these things.

So you might expect a guitar amp cathodyne to include some building
out of the cathode load resistance, for example, so it deals with
grid current more gracefully and symmetrically. A bare concertina
can sound pretty bad if the cathode supplies current at the expense
of the anode.

Swap the AU7 or AT7 in such a critical circuit for an AX7 and I
would expect it to break up more suddenly, and less musically.

More generally, I suggest that blueprinting is more appropriate than
experimentation with any amplifier. Else you should at least decide
what change in sound you want *before* you modify.

...But at least I can say a Vox AC30 with 4 x EL84 in pure class A
will sound warmer than a pair of 6L6 biased for near class B...

Ah, the myth of the class A Vox. Check the load and the HT voltage.
Hot AB1, approaching AB2 I believe.

The couple of Vox AC30 I have repaired had over 12 watts dissipation in
each tube, and with the right load would clip gradually, like a
compressor.
EL34 and 6L6 tend not to do this.

The AC30 has cap coupling from whatever drive amp it has, and so cannot
sustain any grid current to give much more po than it already does,
which is about 30 watts, with about 20 being class A, which is a higher
% of class A than in most other guitar amps which are designed for
maximum loudness,
since musicians *really* want to be able to dominate once they are on
stage,
and who is going to hear them if they have a ****ant on an amp?
When most guitar amps are in gross over drive, the wave is a virtual
square wave with about 49% thd all mainly odd order, whether there
is any 2H present is utterly irrelevant.
But what guitar amps sound before they clip is of interest to those who
never over drive,
jazz musos etc.
The main harmonics in most guitar amps at low level is 3H, then 2H then
all the rest.

Its pot luck when what ever tubes are chosen that they sound musical.


We're both pretty raw amateurs when it comes to guitar amps. Could
end up looking foolish on alt.guitar.amps. Eek!

I repair a few guitar amps each year.
I dunno about you, but I am a professional



cheers, Ian

PS opera can be ok live as long as it is not in English, which
invariably sounds daft. Recorded opera sounds daft anyway.

I used to enjoy italian opera the best while painting a house.
Italian is a language begging to be sung, and since I don't understand a
word of it,
the story about ppl falling love, the jealousies, the murders, the
dramas that are all
largely implausible don't clutter my mind.

Even if I could speak italian, I'd not understand a word of opera,
as it is with operas in english, which may as well be german or french,
since opera mauls the diction and without subtitles I am stuffed.

So often when I see an opera performed and screened to the public via
the ABC TV network,
I can say I'd prefer a CD, since watching the pretentious acting and
mis-casting
with old folks in roles meant for the younger set is painful.
And folks pay $200 per ticket to go to the opera house.
There is an enormous pile of dross compared to the small
cup full of performances which are straight out gems that bring tears to
the eyes
when heard, even when you have zero idea of the drama plot.
Joan Sutherland in Acis and Galatea sung about 1962
is such a priceless item
I later saw her when she'd aged, and put a lotta weight on, and sang a
part in a role as a
pretty young thing and the object of desire amoungst the army officers
in their smart prussian uniforms.
I switched it off.

Nobody is going to poke garbage down my eyes, or ears.

Patrick Turner.