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Source: http://topnet.com.au/~hairbear/ea061967.htm
June 1967 Electronics Australia
Here is an amplifier that
should meet many a need, particularly for readers with an interest in amplified
musical instruments. With a power output of 40 watts RMS and full vibrato and
tone control facilities, it can be used with bass, rhythm or lead guitars,
electric bass or electronic organs. Alternatively, with the controls set for
level response, it can double as a high-powered public address
unit.
While, as indicated, the new amplifier has been designed with a
number of possible uses in mind, it will undoubtedly find its greatest
application in connection with electric guitars, and this is the basis on which
it is being presented.
Our last venture in this field was in the issues
from October, 1962 to January 1963. In this series, we presented a basic 12-watt
guitar amplifier (the Paymaster 102) which was expanded to a two channel 12 - 12
watt unit (the Paymaster 103).
Reflecting the whims of the market at the
time, the 103 was a compact unit, of necessarily limited power output but with
vibrato facilities (tremolo would be the more correct word) plus a second
channel which could be used for extra power output, reverberation, straight
guitar or voice public address.
The 102/103 amplifiers met a particular
need and are still being built in significant numbers. In fact, interest in them
has persisted to the point that we will almost certainly have to up-date the
designs and feature them again.
However, they have also prompted another
and persistent demand for a higher-powered single-channel amplifier suitable for
use, if necessary, with a bass guitar. The incentive to build rather than buy
has been strengthened by the fancy mark-ups which seem to apply to higher
powered amplifiers bought through “music” sources.
It is apparent also
that many would-be constructors have put aside their one time objections to the
large loudspeakers and large enclosures which are necessary to handle power of
30 watts or more. In fact, an enclosure which needs to be trundled in on castors
would seem almost to have reached the stature of a status symbol!
Perhaps
it is only fair to observe that, along with the call for higher power the basic
instruments and group playing techniques have developed greatly in recent years
and today's electric guitars are a far cry from the early units with a single
pickup coil pushed under the strings.
The question of valves v.
solid-state is also being debated by guitarists. Some prefer transistor
equipment on the basis of it's compactness, cool running and reliability. Others
stick to the “good old valves” which might get hot but don't blow up when the
Ioudspeaker leads are accidentally broken or shorted. Price enters into it also,
along with argument about “transistor tone” and “valve tone” which probably has
more to do with different response contours selected by individual
designers.
We used valves for this present amplifier, mainly because
components were conveniently available and we had built up a background of
suitable circuitry. Inevitably, at some future date, we will have to produce a
solid-state equivalent with appropriate effort to keep the cost down and to make
it reasonably proof against the type of accident that blows up costly power
transistors.
In terms of power output, a more or less accepted “norm”
appears to be about 40 watts RMS~a figure stemming partly from the economics of
ordinary amplifier design and partly from what can be accommodated by practical
“portable” loudspeaker systems.
In quoting this figure, we refer to
actual constant-tone output over the fundamental musical range, measured across
the load ~ a very practical figure for guitar applications.
How it
relates to the published figures for commercial guitar amplifiers is another
point. Actual measurements would suggest that these figures are sometimes
“optimistic” as evidenced by one very large, very imposing 60-watt amplifier
which, under test, yielded exactly 45 watts at the onset of clipping.
Our
new amplifier, as shown, delivers a measured 40 watts RMS into a 15-ohm load.
The output transformer secondary is tapped to provide a match to other load
impedances as, for example, two 15-ohm loudspeakers in parallel. As such, it
should meet most practical requirements.
For those who may want still
higher power, and are prepared to provide loudspeakers to cope, we have in mind
the possibility of substituting more expensive grain-oriented transformers and
modifying the operating conditions to provide about 60 watts RMS. We do not
expect the circuit or layout to be otherwise affected.
With powers of
this order, it is usual to have the amplifier in its own carrying case, which
sits piggy-back fashion on top of the loudspeaker enclosure. The chassis has
been designed with this in view and is relatively compact, being little longer
than is necessary to accommodate the front panel controls.
The amplifier
itself may be suited for bass, rhythm or lead guitars by simply setting the tone
controls for the desired bass/treble response contour. The ultimate result will
be dependent, however on the choice of loudspeaker system. Bass guitars need
big, husky loudspeakers in big husky enclosures, with treble response of no
great significance. At the other extreme, lead guitars can get by with less
ponderous loudspeaker systems but treble response is a “must”.
We may be
able to say more about this, in a general way, later on.
Looking now at
the circuit, it will be noted that the output valves, arranged in a “push-pull”
configuration, are a type not normally found in an audio power application. The
6DQ6A is a power valve used primarily as a power amplifier for horizontal
deflection in television receivers. Because of volume of production, it is
comparatively inexpensive.
As an output valve, the plate characteristics
are not greatly different from those of the more familiar audio types such as
the EL34 and the 6CA7. Because of its television heritage, the valve has a high
peak plate voltage rating, but this is of little consequence when the valve is
used in a strictly audio application.
If anything, however, its “top cap”
plate connection offers some advantage, in that it allows better isolation of
the plate leads from the high sensitivity input stages of the
amplifier.
A definite advantage of the 6DQ6A is that is is a good deal
shorter than either the EL34 or 6CA7. As may be seen from the photograph, the
overall height of the valve, including the insulated plate connecting caps, is
about the same as that of the transformers, making for a clean
profile.
The output valves operate under push pull class ABI, fixed-bias
conditions, a mode which avoids grid-drive problems, ensures good power supply
economy, and which minimises cross-over distortion.The total harmonic distortion
from the amplifier, incidentally, at 40 watts output, is less than 1 per
cent.
Class AB2 or class B operation would have posed additional
grid-drive, power supply and distortion problems and, fortunately, are not
necessary for the orders of power output required and achieved.
As it is,
the valves operate with a total standing current of about 100 milliamps, rising
to more than double this figure with sustained signal. 'Stopper' resistors are
included in series with each grid to inhibit oscillation during any part of the
signal cycle.
Three voltages have to be supplied to the power stage-a
nominal 370 volts for the plates, 185 volts for the screens and -34 volts for
the grids. This latter is probably the most critical of the three, for on it
depends the quiescent current of the output valves and therefore their quiescent
plate and screendissipation.
While we have suggested a figure of -34
volts, and while the bias network should give something very close to this
figure, the quiescent current in the common cathode lead to the output valves
should be checked to see that it does not exceed 100 milliamps, representing
full rated dissipation. The bias can be varied, if necessary, by varying the 18K
shunt resistor.
A single electrolytic capacitor serves to filter the bias
voltage, partly because of the high impedance of the circuit and partly because
residual hum tends to be canceled, anyway, by the push-pull connection of the
output valves.
The main HT power supply is of rather unusual
configuration, supplying separate and appropriate voltages to the output valve
plates and screens.
The main secondary winding of the power transformer
is wound for 135 volts on either side of a centre-tap, with a nominal current
rating of 150mA.
The full secondary voltage is applied across a
conventional bridge rectifier configuration, with one side of the bridge taken
to earth through the “Standby” switch. From the other side of the bridge comes
the main HT supply for the plates, just a trifle less than the peak value of the
AC input.
The screen supply is the less obvious part of the arrangement,
the positive screen supply potential being derived from the secondary centre
tap. However, examination of the circuit will show that the two diodes
connecting to earth through the “Standby” switch, and which form half of the
main bridge, represent a back-to-front full-wave rectifier system, with the
centre tap at a positive potential, rather than negative HT in a more
conventional system.
Filtering for the main HT supply could hardly be
more straightforward, since it comprises a single effective 100uF capacitor,
rated at 450VDCW. In fact, we used an available unit containing two 50uF
capacitors, and connected them in parallel. This single large capacitor not only
provides the requisite hum filtering but also serves as an effective reservoir
for peak signal current demand.
>From it also is derived supplementary
supplies for the earlier stages through a cascaded decoupling network, using
ordinary small resistors and pigtail electrolytics.
The phase splitter is
the triode section of a 6BL8, another valve widely used in television receivers.
Equal loads in the plate and cathode circuits ensure balanced signal to the
output valves each signal being about 0.9 times the amplitude of signal fed to
the phase splitter grid. Due to cathode circuit degeneration, the input
impedance to the stage is many times the value of the .47 meg grid resistor and
this has a bearing on the gain which can be expected from the preceding
stage.
We gave some thought to the use of a “long-tailed pair” type of
phase inverter but to be tied to a twin triode would have dictated much lower
gain in this portion of the circuit than we were prepared to accept.
In
designing an amplifier such as this, it is important to envisage, not only the
overall gain, but also the distribution of gain between the stages and relative
to the control functions.
Too little gain after the controls would
necessitate multiple high-gain input stages and a “front-end” which could too
easily be over-loaded by unexpectedly large input signals.
Too much gain
after the controls could magnify the unwelcome sound of “noisy” potentiometers
and dictate the use of a premium quality audio valve in the voltage amplifier
stage to minimise risk of hum and microphony.
Microphony is an important
consideration in connection with guitar amplifiers. Close proximity to the
speaker system may introduce acoustic feedback, particularly in the case of a
bass-guitar amplifier, since the frequencies are such that the vibration is
easily transmitted through solid objects.
With the phase-splitter and
output valves involved in this amplifier, we were glad to take advantage of the
6BL8 pentode section for the main voltage amplifier without, however, getting
down to a level where microphony in the stage was likely to be a
problem.
The 6BL8 pentode stage has an overall gain of about 150 times,
without external feedback, allowing for a small amount of degeneration from the
220-ohm resistor, across which the external feedback voltage is applied. The
total feedback, using the constants specified, is about 16dB, a figure which we
consider to be an advisable maximum, to minimise the risk of instability due to
phase change within the output transformer.
The circuit, as drawn, shows
the “Common” end of the output transformer secondary as being earthed and
feedback taken from the 15-ohm connection back to the cathode circuit of the
6BL8 pentode, through a 10K resistor.
The colour coding on the circuit
and the identification of the output valves should allow the feedback to be
wired in correct polarity for the A&R output transformer type 2843, as used
in our prototype.
With other types of output transformer, it may be
necessary to establish the polarity of the feedback by trial and error. In this
case, it would be logical to complete the wiring of the basic amplifier section
but to leave the feedback initially unconnected.
After switch-on and with
this portion of the amplifier operating normally, the feedback connection can be
made. If the amplifier remains stable and/or there is a drop in the level of any
test signal which is being fed through it, the feedback is negative and all is
well. If the gain increases, however, and/or the amplifier howls, it is a sure
sign that the feedback is positive.
Since it is logical to leave the
“Common” end of the secondary earthed and not to cross over the flying leads to
the output valve plates, the simplest modification is to swap over the leads
from the component board to the grids of the two output valves.
It will
be noted that no phasing capacitor is shown across the feedback resistor. Such a
capacitor is frequently used in valve amplifiers to offset the phase rotation
which commonly occurs in output transformers at supersonic frequencies and which
can cause supersonic oscillation in isolated cases.
While phasing
capacitors can be very effective for this purpose, they should really be
selected for the particular circuit and output transformer type with the aid of
an oscilloscope and square-wave generator. Any value which we might specify
would not necessarily be optimum for an amplifier built up with a different
brand or type of output transformer.
As an alternative measure, we have
specified a “step” circuit comprising a 6.8K resistor and a 220pF capacitor in
series, from the phase splitter grid to chassis. The effect of this circuit is
to produce a sharp step or reduction in the gain of the amplifier above a
certain frequency, normally selected to be just outside the audible range.
Because of reduced response in the supersonic region, an amplifier with such a
step circuit is most unlikely to become actively unstable, even with
considerable phase rotation in the output transformer and feedback
system.
The tone control network which precedes the 6BL8 pentode is a
passive system very similar in configuration to the controls used in our various
valve type Playmasters intended for use with pickups and radio tuners. The
curves have been manipulated, however, to suit them better to the present
purpose.
With amplifiers intended for reproduction from tuners and
records, there is a tendency to make the bass control most effective in the
general region of 50Hz and to look for full treble control in the region of
l0KHz. The treble control therefore has its greatest effect on musical
“overtones” rather than fundamentals, tending to make the sound more or less
“bright” according to the preference of the listener.
Guitarists,
however, seem to want to operate on the fundamentals and low order overtones and
therefore prefer a treble control which functions much lower down into the range
than is common with ordinary hi-fi amplifiers. More appropriate adjectives would
be “strident” and “piercing”.
At the bass end, they look for copious
control over frequencies in the 70-100Hz region.
The chase after
sensational effects has, in fact, produced commercial amplifiers with huge
orders of boost and cut over various parts of the spectrum and controls which
interact so much, or so lack a balance position, that it is difficult to achieve
anything like a level response.
To talk to guitarists is to realise how
confused is the whole control situation and how subjective the preference for
different kinds of sound.
In our case, we have tailored the constants so
that the amplifier can be set up for a substantially level response, allowing it
to be used for other electronic instruments, for public address or for two
dissimilar guitars, each using their own inbuilt tone facilities.
On the
other hand, maximum bass and minimum treble will give steep slope eminently
suitable for a bass guitar, while rhythm and lead guitars can boost the treble
to maximum and cut the bass back as necessary.
For the rest, as we said
earlier, it is a matter of choosing the appropriate type of loudspeaker and
enclosure.
The tone control network is preceded by three triode stages of
amplification with the vibrato “modulating” circuit and volume control
intermediate between the 12AU7 and the 12AX7. This vibrato circuit, based on a
circuit which we published in August 1964, gives full speed and depth control by
employing a light dependent resistor (LDR) in a balanced resistance
network
Electrically, the system has the same effect as if one were to
turn the volume control rapidly up and down varying the signal level without
changing any DC potentials in the amplifier. This being so, here is no tendency
to “pump” the loudspeaker cones in and out and no sound to be heard other than
the modulation of the signal itself. Guitarists who checked the amplifier during
its development voted it as about the best vibrato (or tremolo) that they had
ever used.
Another feature of the circuit is that switching the vibrato
in and out, or changing the depth, does not materially alter the average
loudness of the signal.
A small neon tube is used as the light source and
is wired in series with the plate circuit of the oscillator. A 3.3M resistor
shunts the plate of the triode to prevent the neon “going out” on the on the
positive excursion of the plate voltage, The inclusion of the resistor thus
prevents irregularities appearing in the “modulating” waveform which would
produce unwanted clicks from the amplifier. Further filtering of the
“modulating” signal is afforded by virtue of the natural time constant of the
light dependent resistor.
The vibrato or “modulating” signal is derived
from a phase shift oscillator, which consists of the second half of a 12AX7. A
high mu triode is required in this application because a phase shift oscillator
is rather critical as to gain. The oscillator frequency is varied by means of a
1M potentiometer included in the phase shift network.
A facility for
remote control of the vibrato has been provided, by way of a “shorting” type
telephone-jack on the back panel. The remote control usually consists of a
footswitch which can be either of two types, a push-on and push-off type or
push-on type with a self return spring action. Either way, they must be of such
construction as to stand up to the pressure of a somewhat large, heavily clad
foot!
In certain instances it may be inconvenient to use a footswitch,
particularly where a number of guitars are involved. In such case a player may
desire the vibrato switch to be on the amplifier within easy reach and with a
convenient switch action. An optional vibrato “hand” switch has been shown on
the circuit as being part of the depth control potentiometer, and is wired in
series with the “remote” telephone jack. Potentiometers incorporating pull -
on switches are readily available, being commonly used in television receivers.
The preamplifier valve, shown in the left foreground of the photograph,
is a 12AU7A medium mu twin triode. The valve was selected for its rugged
construction, which makes it useful in situations critical to microphonics. The
input has a low impedance network with “shorting” type jacks which tend to
present a constant load to the guitars regardless of whether there are two
guitars or only one. Having the jacks shorted when there are no input
connections also prevents “stray” signal pickup
Construction might well
begin with the mounting of the “hardware” with power and output transformers
first, followed by the can-type electrolytic and valve sockets, using the
various mounting screws to secure the tag strips where required. Orientation of
the valve sockets is indicated on the under-chassis photograph.
As will
be apparent from the photograph, the potentiometers were set back inside the
front panel with 3/16in spacers, which we managed to obtain from a parts
supplier. By using these spacers, or an equivalent thickness of washers, only
enough thread need protrude to accommodate the Iocking nut, thus allowing the
control knobs to locate fairly close to the panel.
Our own prototype
chassis was hand made, sprayed and the panel lettered with adhesive transfers.
We imagine, however, that suppliers will organise to make available pre-punched
chassis to our specifications and lettered panels to suit.
As a next
step, it is probably logical to lay in the twisted leads to the heaters and to
the 6.3V pilot lamp.
Wiring of the power supply could begin with the
“pigtail” electrolytics, the decoupling electrolytics mounting obliquely as a
matter of convenience. The two-lug tagstrip which secures the earth lead of
these electrolytics and the associated decoupling resistors is also used to
mount the 10K feedback resistor from the loudspeaker socket.
The power
diodes are mounted on a tag strip adjacent to the can type electrolytic, with
the positive end of the bridge supporting the 10K decoupling resistor. The other
end of the bridge is connected to the “Standby” switch which is mounted on the
front panel. The mains off-on switch is mounted on the back panel, adjacent to
the fuse holder. Note that the power cord should be passed through a chassis
grommet and secured by a suitable clip to obviate stress on the internal
connections.
The next logical step is to wire the power output stage,
having in mind earlier remarks about polarity of the feedback.
As can be
seen from the photograph, the screen and grid resistors of the output valves
have been wired across the sockets using the vacant lugs as anchor points.
Connection to the grid and circuit of one of the output valves is made by the
coupling capacitor from the main component panel, while the other grid circuit
is connected via a lead.
Because the top caps of the 6DQ6A's are exposed
and at full plate potential, the connectors should be of the protected type as
used in TV receivers.
The main component panel may be wired as a unit and
secured in position, using long screws with nuts as spacers. With the panel in
position the various connections can be made to the adjacent valve
sockets.
The earthed lugs at one end of the panel have been used to
secure the earthed ends of the cathode resistors and their bypass capacitors,
belonging to the first two valves. Similarly, the lugs carrying the HT have been
used to terminate one end of the load resistors to the same
valves.
Mounting the light dependent resistor and the neon posed a small
problem. In some commercial guitar amplifiers, the light dependent resistor and
neon are simply mounted adjacent to but separate from one another, on an open
tagstrip. Without shielding, however, any spurious light which may fall on the
light dependent resistor will affect the modulation characteristic of the
vibrato circuit.
We mounted our modulating assembly in a small can which
was obtained by “butchering” a discarded electrolytic capacitor. A scrap of
tinplate would do just as well. A rubber grommet retains the neon lamp while the
light dependent resistor is a neat fit in the tube.
As will be evident
from the underchassis picture, the components in the LDR circuit are mounted on
a tag strip adjacent to the input jacks, with a twin core shielded cable to the
outside lugs of the vibrato “Depth” potentiometer. A separate shielded lead is
used to connect the centre lug of the same potentiometer, to the volume
control.
Having completed the input circuitry, the amplifier should be
ready for operation.
POWER: 40 watts RMS output.
DISTORTION:
Total harmonic distortion at 40 watts output is 0.8 per cent.
INPUT
SENSITIVITY: 15mV for 40 watts output at 500Hz.
LOAD IMPEDANCE: 3.75, 8
or 15 ohms.
Chassis 16in x 7in x 11in with outward sloping front
panel.
Power transformer 240V to 270V at 150mA with centre tap, 30V bias
winding, and 6.3V at 4A with centre tap. A&R Transformer type PT5892, or
similar.
Output transformer 3.3Kohms plate to plate with 3.75, 8 and 15
ohm secondary taps. A&R transformer type OT2843, or similar.
2 Octal
valve sockets.
9-pin shielded valve socket.
2 9-pin valve
sockets.
2 6DQ6A valves, 1 6BL8 valve, 1 12AX7 valve, 1 12AU7A
valve.
4 Power diodes. types EM405, 1N3195, OA650 or similar.
1
Bias supply diode, types, BA100, 1N3193, or similar.
1 LDR, type ORP12,
B8-731-03 etc.
1 neon lamp, type NE2.
RESlSTORS
1-watt, 10
percent, unless specified.
1 x 3.3M, 1 x 2.2M, 1 x 1 M, 3 x 470K, 1 x
330K, 3 x 220K, 7 x 100K, 1 x 68K, 2 x 47K, 1 x 27K, 1 x 18K, 1 x 18K 1x 15K 1
watt, 3x 10K, 1x 10K 1 watt, 1 x 6.8K, 1 x 3.3K, 2 x 2.7K. 2 x 2.2K, 1 x 1 K, 1
x 470 ohms, 1 x 220 ohms, 2 x 47 ohms.
POTENTIOMETERS
4 x 1M log.
(C-taper).
1x 1M linear (A-taper).
CAPACI TORS
1 x 100uF 450VW
electrolytic.
1 x 100uF 350VW electrolytic
2 x 50uF 350VW
electrolytic
1 x 100uF 50VW electrolytic
5 x 25uF 6VW electrolytic
2 x
0.1uF 400V plastic
2 x .047uF 400V plastic
3 x .022uF 400V plastic
3 x
.01uF 400V plastic
1 x .0056uF L.V. plastic
1 x .001uF L.V. plastic
1
x 680pF L.V. plastic
1 x 220pF L.V. ceramic
1 x 39pF L.V.
ceramic
MISCELLANEOUS
2 x 6-way tag strips
3 x 4-way tag
strips
3 x 2-way tag strips
21 lug length of miniature resistor panel
2
single pole toggle switches.
1 pilot lamp assembly
1 fuse holder
3
“shorting” type jack sockets and plugs.
1 4-pin speaker socket and
plug.
Power flex and plug, clamp and rubber grommet, knobs, shielded cable,
hookup wire, nuts, bolts, washers, solder, etc. Remote foot switch h and
mounting if desired.
(schematic caption) Apart front the rather unusual
configuration of the power supply, the circuit follows well proven techniques.
Of particular interest is the vibrato system, which we developed some years ago,
and which gives a wide range of speed and depth without any tendency to cone
“pumping”. The gain is ample for typical commercial guitars, most of which
deliver signal levels of at least 30mV RMS.
(caption) The underside of
the new guitar amplifier has space to spare, apart from the “busy” area around
the low-level stages. Along the rear lip of the chassis is a jack for vibrato
foot control, the loudspeaker socket, mains switch and mains fuse. At the time
the photograph was taken we had not installed the clip to secure the mains
lead.
(caption) Typical contours, with the tone controls set
approximately level, as per the centre curve, and for maximum and minimum bass
and treble. Note that the maximum bass/minimum treble curves, and their
converse, tend to merge into continuous slopes, suiting the amplifier for either
bass or lead guitars, as required.
(picture caption) The preamplifier is
in the left foreground, followed logically by other valves providing vibrato,
voltage amplification and phase splitting to the two 6DQ6A output
valves.
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http://www.ozvalveamps.org/playmaster116.html | Last update: 21:32 24/09/05 |