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From: Doug Wellington USGS
Subject: TAME digest issue 1
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The Tube Amp and Music Electronics Newsletter
This newsletter is devoted to discussion of electronics in music, including,
but not limited to, tube amps, effects and instrument wiring. This newsletter
is distributed as a moderated digest. To join the list or submit questions or
articles, send email to dou–(at)–rizona.edu.
Well, folks, this is it, the first issue of the TAME digest. We currently
have 91 subscribers to the list. Keep those cards and letters coming!
I have a source list of parts distributers in the works. Let me know where
you have purchased parts, and what kind of service you got with them, and I’ll
add it to the list. I should have the first list posted by March 26th.
Beginner’s corner will include introductory articles written in an overview
format with a minimum of technical jargon.
How Unregulated Power Supplies Work
All tube amps that I have worked on have unregulated power supplies. This
type of power supply is relatively simple, and very inexpensive to build.
In this article, I’ll discuss the major parts of an unregulated power supply
and how they fit together to provide enough power for the typical tube amp.
Vacuum tubes (valves) require quite high direct current (DC) voltages to
operate optimally. The power that comes out of the typical wall outlet is
110/120 or 220/240 alternating current (AC). Tube amp power supplies must
convert this AC to DC and step the voltage up to somewhere between 400 and
600 volts and occasionally even higher.
Transformers are used to step the voltage up or down. Transformers consist
of a core with two different strands of wire wrapped around it. One strand
is the input and the other is the output. The number of windings of each
strand determines the ratio of input to output voltage. For example, if
there were equal windings on each side, the voltage would be the same going
in as coming out. On the typical tube amp in the USA, the winding ratio is
around 4:1 resulting in a voltage step up from 120 to ~480 volts. In other
countries with 220/240, the windings ratio is closer to 2:1. Many amps have
transformers with a tap into the middle of the input strand, so that they
can be switched between 110/120 and 220/240.
| | 220 | |
V in | ———| transformer | V out
110 | |
It is very important to correctly set the input voltage switch. If you have
220 volts coming out of your wall outlet, and you have the transformer set for
110, then the transformer is providing a 4:1 step up, which will result in
880 volts instead of 440. Alternatively, if you have 110 volts at the wall
and have the amp set for 220, then you will only have half the voltage that
you want. Neither of these situations is very good.
Once the voltage has been stepped up, it needs to be converted from AC
to DC. The simplest way to convert is to put a rectifier in the circuit.
Rectifiers (and diodes) limit the flow of current to one direction only.
(See the article by Tom Balon on rectifiers elsewhere in this digest.)
| | |
| transformer | |
| | |
So, looking at a somewhat crude representation of an AC current sine wave:
/ / /
/ / /
The addition of the rectifier in the circuit results in:
/ / /
/ / /
As you can see, this isn’t very good, as the result is pulses of DC at about
50 or 60 Hz, depending upon the country you are in. To get a smoother flow,
many transformers are designed with a center tap on the output winding. This
results in a reference point at the 0 voltage point in the AC sine wave.
| transformer |—— <-center tap | | ---------| |------ -------------
Relative to the center tap, there is a positive voltage that alternates on the
outer taps at all points in the sine wave (except at the exact 0 point). Now,
if we add rectifiers on both outer taps and connect them back to the center
tap, we have the following result:
| transformer |————————
—- —- —- —- —-
/ / / / /
/ / / / /
Much smoother! But still not perfect. We are still getting a rising and
falling voltage at twice the rate as before. What we need now is a way to
store up some current and then release it to fill in those “valleys” between
the pulses. There are two methods used in tube amps.
The first method is with an inductor or “choke”. Inductors look like small
transformers, but only have two connecting wires. Inductors store energy
in the form of a magnetic field. When current flows through the windings
in the inductor, energy is stored in the magnetic field. When the amount of
current coming in through the windings drops, the magnetic field collapses
and produces current in the wires, hopefully long enough to maintain a
constant level of current. In reality, the current produced by the collapse
isn’t enough to completely smooth out the pulsing.
| | =====
| transformer |—– to ground ——OOOOO–> to ground
| | /
The second method used to smooth out the pulses of current is with large
electrolytic capacitors, usually called “filter caps”. Capacitors store
electric potential, much the same way as a rechargeable battery does. (There
are several different types of capacitors. You can find more details in
many books.) Electrolytic capacitors have very high voltage capacity and
high storage capacity.
Adding filter caps to the power circuit is easier and cheaper than adding
more inductors. Filter caps usually come in two flavors. Fender uses
large axial lead (one lead on each end) electrolytics. Marshalls use large
radial lead (leads on one end) capacitors. The caps in Marshalls actually
contain two or more capacitors in one container. Filter caps can be added
before or after the inductor in the circuit.
| | =====
| transformer |—-> ground ——-OOOOO——> ground
| | / | |
———| |———>|——- – –
————- | | | | <- filter caps | | | | - - | | ground ground
This combination of using the center tapped transformer, the rectifiers, the
inductor, and the filter caps results in a relatively constant power flow
through the amplifier.
Electrolytic capacitors have a finite lifetime, usually about ten years. When
they get old, filter caps can lose some storage capacity, which will result in
a rougher power flow, possibly leading to hum in the amp. The filter caps may
also short out internally, allowing the power to flow directly to ground which
results in blown fuses.
Music Supply and Repair Service
PO Box 3328
Tucson, AZ 85722 (602) 760-0425
— Rectifiers —
The rectifier tube in your amp plays an important part in its
sonic character. I’ve experimented with several rectifiers and
have been convinced of this. While some amps will not exhibit
much of a difference in sound between rectifiers, others can be
quite dramatic. Voltage sag under peak conditions can produce
that vintage sound a lot of players like.
For a while, it was a common thing to replace the tube rectifier
with a solid state device. I’ve seen dozens of amps with these
solid state rectifiers, which consist of no more than two 1N4007
diodes, plugged into the rectifier socket. ( 2 – 1 A 1000V PIV )
A solid state device is smaller, more reliable and more efficient
than its tube counterpart. However, when replacing your tube rectifier
there are some things you should take into consideration.
With a tube rectifier, there is a “warm-up” period before B+ is
applied to the plates of the tubes. This gives the filaments in
the tubes a chance to warm up and emission to begin before applying
the full B+ voltage. Applying B+ too early causes what is known as
plate-stripping, increasing tube wear.
[Editor’s note: For beginners, there are two distinct circuits to
each tube. One circuit is connected to the heaters in each tube.
(This is what makes them glow.) Heaters affect the cathode of the
tube, causing it to start emitting electrons, creating a “cloud” of
electrons around the cathode. The second circuit provides voltage
to the plate, causing a higher potential for the electrons from the
cathode. This is the B+ voltage. If this B+ voltage is applied
before the heater makes the cathode emit, the higher potential will
actually pull the electrons out of the cathode itself, instead of
from the electron cloud around the cathode.]
For this reason, I always recommend using your standby switch and
allow 20-30 seconds or so before applying B+. Another more hi-tech
solution is a soft-start circuit that acts as an electronic standby
switch and delays the B+ on power up. I have installed many soft-start
circuits in older amps to get around this problem. You can always add
a simple standby switch.
A tube rectifier has a series resistance greater than that of a
silicon rectifier. In most cases, direct replacement of a tube
with silicon raises the raw B+ voltage. This may result in a slight
*boost* in power. You must check that the filter-caps can handle
the increase. You must also be sure the operating point of your tubes
will not be adversely affected. In the case of many Fender amps that
used 6V6 power tubes, the operating point of the output tubes was
already way over acceptable maximum. Adding solid-state rectification
without proper biasing could potentially damage the tubes and the amp.
When converting from one tube rectifier to another it is important to
take into consideration the voltage drop across the rectifier and
the filament current requirements. Replacing a 5AR4/GZ34 with a 5U4GB
is fine if your filament supply has enough margin to handle the
extra current requirements of the 5U4 style filament.
Another different example of this is converting amps from using the
6L6 power tube to an EL34. The 6L6 draws about 900 ma of filament
current. The EL34 draws 1.5 amps or ~60% more current. An inadequate
power transformer will run hotter and could burn out under these
When using a tube rectifier, it’s important the power supply
filter be properly designed. The first capacitor after the
rectifier and before the choke (if any) should not be increased
without special attention. This is usually a small cap in the
15uf to 30uf range. While a detailed discussion is rather complex,
the basic reason for this is to make sure the max current rating
of the rectifier is not exceeded.
Tube Amp Service “On the trailing edge of technology.”
3455 Homestead Rd.
Santa Clara, Ca. 95051 (408) 243-7745 PST
Coming next issue:
Converting older tube amps for use with guitars.
Beginner’s corner – How tubes work, part 1 – rectifiers
Experiences building the Guitar Player mag “Bluesmaker”
Miscellaneous modifications collected from network news
End of TAME issue 1