HAMMOND ORGAN
Power Amplifiers
The last
item that we will look at regarding a typical Hammond Organ is the power amp-lifier.
Here, the signal from the console gets increased in power enough to drive
the loud-speakers. Hammond power amplifiers are fairly standard, very conservatively
rated, and of the typical excellent, rugged Hammond construction. The earliest
power amplifiers used standard power triode tubes of the 1930s, type 2A3.
Later models used more modern tubes 6V6 or 6L6 with additional grids and called
called power pentodes, and the more recent power amplifiers used transistors
instead of tubes.
One of the on-going arguments as regards power
amplifiers for musical instruments con-cerns the relative merits of tubes
vs. transistors. Some folks state that there is a certain "warmth"
[whatever exactly such a subjective description means] regarding the sound
of a musical instrument whose speakers are driven by a tube type amplifier
as opposed to the sound of an instrument that is heard through a solid-state
or transistorized amplifier.
Since I occasionally insert my "two-cents'
worth" here and there, in my opinion, if there is any difference
at all, it is extremely subtle. I could not, if I didn't see ahead of
time what type of amplifier was in use, be able to tell the difference.
The claim for the merit of tube type power amplifiers is that if there
is any distortion, it artificially generates the second (and other even
numbered] harmonics, whereas distortion in solid state amplification circuitry
generates odd numbered harmonics. In either case, distortion is distortion
and one of the objects of all good amplifier designs is to eliminate all
distortion as much as possible. So the moral of this dissertation, if
indeed it has any moral at all, is that good amplifier design is more
important than the means of operation. There is no question that solid
state power amplifiers run cooler and consume less power than tube type
amplifiers. Regardless of amplifier type, all amplifiers for Hammond organs
do the following: accept the signal from the console, increase its level
in an input stage, and finally increase the power level to that required
to power the speakers.
One more consideration, before we look at a
few specific Hammond power amplifiers is that of the power rating. Hammond
speaker cabinets usually had a model designation that consisted of one or
more letters followed by a number such as B40, HR40, A20, etc. The first letter
was a model or style designation. The second letter, if present was an R which
indicated that there was an artificial reverberation device in the speaker
cabinet, and then the number was the amplifier power output in RMS watts.
Speaker and amplifier wattage ratings that you
find on some audio equipment can be very misleading. Often some manufacturers
will state a number which is relatively high and/or relatively meaningless.
There is only one power rating which an audio engineer or a purchaser of audio
equipment really needs to be concerned with and that is the RMS rating. The
letters stand for Root Mean Square. Without repeating the calculus definition
of this and how it is figured, which you can find by doing a Google search,
let's just say that RMS power is the actual, true amount of electrical power
that is being delivered. So if an amplifier is rated at 40 watts RMS, then
that means that at its maximum level, it will send 40 watts of electrical
power into the speakers that it is powering. Any other ratings, such as "peak"
power, "music" power, or other terms are for all practical purposes
meaningless. If I were to say that a Hammond amplifier can deliver 113.12
watts peak-to-peak, I'd be telling you the truth but it could be very misleading.
Peak power, or peak-to-peak power or any other such ratings that some manufacturers
say about their audio equipment imply more power than what is really there.
When Hammond says that the two power amplifiers in a B40 tone cabinet
each deliver 20 watts of power, they actually do deliver 20 watts to each
of the two pairs of 12" speakers in the cabinet, and if you've ever
heard a B40 tone cabinet up close, you'll quickly realize that 40 true
RMS watts of power into speakers will produce some serious volume, a lot
more than you'd want to have in your living room.
Hammond power amplifiers take a so-called balanced
input signal from the console, and develop a suitable output to drive the
speakers. A balanced output is one where there are two signal wires coming
from an output transformer whose secondary winding has a grounded center tap.
Schematically it looks like that of figure twenty-four on page thirteen.
When set up this way, any outside interference,
such as stray 60 Hz power hum gets induced into both of the two signal
wires. Since the signal phase in each is 180 degrees out of phase, it
is neutralized and grounded out at the matching transformer, and as such
has no influence. Here are two looks at an early Hammond power amplifier.
 |
Figure 31, the input stage of an early Hammond power amplifier. The ba-lanced signal comes in on the left and each side goes to one grid in the tube V1. There is 160 volts of DC on the plates of the tube and the amplified signal on the plates then passes through capacitors C1 and C2 to the next section of the power amplifier. The cathodes of the input tube are grounded through the 3900 ohm resistors, and the current flowing through to the plates causes the 6.5 volts to appear on the cathodes. Roll mouse over picture for more details.
