Randall Aiken   (reaiken@aikenamps.com)
    Date:
         8/14/2000 5:04 AM
 Subject:
         Re: November vs. Invader



Yes, I am using custom transformers for my amps (in fact, I just got
in four big, heavy boxes full of them the other day!). The reason is
because there is no standard transformer on the market that would
do the job, so I had to design my own, since my requirements were
very special.

As for a "standard" for 2-EL84, I don't think there is one, unless it
would be the Hammond 1650F I spec'd for the November, which
is rated at 25W, and has a primary impedance of 7.6K. It is a
good match, but a bit expensive. I don't know of any commercial
guitar amps that use it, though. I think many people consider the
Vox AC30 OT to be the defacto standard for 4-EL84, and I
suspect a lot of the boutique AC-30 cloners use aftermarket AC30
replacements as their output iron. 

As for the "best" choice for a plexi thing with EL-84, I don't know.
The Hammond certainly is different than a true plexi OT, but then,
the EL84 is quite different than the EL34, too! A great deal of the
tone of the amp comes from the way the preamp and output stage
distorts, and the point at which each stage starts to clip. The output
transformer is just part of the equation, but it is an important part. I
don't think anyone can nail the *exact* plexi tone with a 15W
EL84 amp, anyway, because a major part of that huge tone is the
high output power. However, you can create an amplifier that has
that characteristic Marshall tone at lower volumes.

My philosphy regarding transformers is a bit different than the
commonly held beliefs put forth by the "gurus". For instance, I like
large primary inductances, so the low end response will be far
enough below the passband of the amplifier to where the phase
shifts at cutoff are negligible, but not too large a primary inductance
to where the number of turns required increases the leakage
inductance and capacitance too much, which will kill the high end. I
also use quite a high interleave factor, so my upper frequency
response point is far higher than most. This is not done so the amp
will have piercing high end, it is done so the amp will not have
ringing and other transient artifacts at crossover points. A good
transformer such as this allows you to tailor the low and high
frequency response with the circuit design, instead of being at the
mercy of the output transformer, which is harder to control, and
limits you to "one sound".

Also, I don't believe that an output transformer should have an
undersized core for "saturation". True output transformer saturation,
in my opinion, is not a nice thing. When a transformer core
saturates, the primary inductance drops to zero, the low end goes
to hell in a handbasket, and the transformer will overheat, and, if
run that way long enough, will die. 

To me, it is all about reliability (and great tone, of course!). I
believe that an output transformer for a guitar amplifier should be
rated for around twice the nominal sine wave output power. This is
because the RMS power of a square wave is exactly twice the
RMS power of a sine wave. 

What this means is that if you want to design an amplifier that can
run full-out in clipping all day long, you must have iron capable of
twice the nominal output rating. In practice, amplifiers aren't usually
run continuously, because the duty cycle of operation is
considerably smaller (unless you play hardcore speed metal!). Even
so, the output transformer should be rated for around 1.5 times the
nominal power rating for best reliability. I prefer twice the rating,
for the ultimate reliability, especially since I have a built-in power
attenuator.

The other factor is the power transformer. I also rate my power
transformer secondary currents for over twice the expected
full-power output current for the same reasons. If you expect the
output to be able to produce twice the RMS power at clipping, the
power supply had best be able to provide the necessary current
without overheating. I'm not comfortable with amps whose power
transformers are too hot to touch after a night of playing -
something is eventually going to fail. If you want sag, you can easily
get it with a tube rectifier or series resistance. I don't believe an
underrated power transformer is the proper way to get sag.

Is this overkill? Perhaps, but I put my name on these amps, and I
want them to be something I am proud of, and something that will
hold up night after night on the road. I don't believe you will get that
with the dinky little iron you see in most of the amps on the market
today.

(Sorry for the long rant!)

Randall Aiken