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Audio
Basic Newsletter

Our Famous Original Dyna St-70 Rebuild
Plans
E-mail | Print |

From Audio Basics, July, 1982

Hello again, I am glad that the feedback I am getting from many of you
indicates that you are happy with the information and ideas I have been
describing. Some of you have complained that you want more "meat" - technical
information on audio equipment. So, this month we will get a bit more technical
and go through the operation of a vacuum tube amplifier, what it does, what it
cannot do, and what you can do about it. The data will pertain to vacuum tube
amplifiers in general, and the Dyna St-70 specifically, as we get so many
requests for information on how to modify them.

VACUUM TUBE AMPLIFIERS

Any audio amplifier can essentially be modeled as a two stage device; a
voltage amplifier (which amplifies the amplitude of the signal) followed by a
current amplifier (which supplies the drive current to drive the now large
amplitude signal into a low impedance load - the speaker system). Although many
tube circuits will also have voltage gain in the current amplifier section, that
is not important to the discussion that follows. The closed loop gain of the
amplifier is determined by a voltage divider which sends a portion of the output
signal back to the input of the voltage amplifier out of phase. The amplifier
then does not amplify the input signal, it amplifies a signal being the
difference between input and feedback signal.

If the voltage amplifier and current amplifier were perfectly linear, the
difference between the input and feedback signal would be only a small version
of the input signal and the results would be "perfect." In theory, any
difference between input and output is supposed to be eliminated by feedback.
The feedback signal, which is a duplicate of the output signal, when subtracted
from the input signal, should create a "difference signal" which is
"pre-distorted" by the amount of distortion in the circuits, but out of phase
with the circuit's distortion. The "pre-distortion" in the "difference" signal,
when added out of phase to the actual distortion in the circuits, cancels
exactly, giving a "perfect" output signal. That is the way it is supposed to
work.

By the way, there ain't no such thing as a no feedback amplifier! There are
many different feedback schemes. One can choose to use lots of local feedback in
each stage and little overall loop feedback, but each device in itself (whether
it is transistor or tube) has internal feedback. If one attempts to make an
amplifier with "no feedback" except for that inherent in the devices themselves
then the design becomes absolutely dependent of the characteristics of each
independent device. No two tubes or transistors, even of the same type, are
identical. Even if you painstakingly select and bias each individual device one
at a time, its characteristics will change with variations in temperature,
current, voltage, and age. It will be impossible to make any two channels the
same and to keep them the same. The main characteristics of a so called "no
feedback" amplifier are: very high cost (each unit is essentially a one off),
very hot running as the devices have to be biased on very hard, unstable
operation as the unit changes characteristics with age and temperature, no two
samples will sound the same as they are device dependent, and lots of
"blow-ups." Obviously, repairs will be expensive as a repair is essentially a
re-engineering of that unit with new devices and re-biasing of each device. I
can live without it. The "wonderful" sound of a "no feedback" amplifier is the
wonderful sound of lots of instabilities and underdamped oscillations. You may
like it, I don't, it isn't music.

Thus, those that choose to build stable, repeatable, and rationally priced
amplifiers will use some feedback. The catch is in knowing what the feedback can
and cannot do in the real world, and to use the feedback properly, so that the
unit does not only measure well, but actually performs well under transient
conditions in the real world.

Now, back to that vacuum tube amplifier. Remember we mentioned that if the
voltage amplifier and the current amplifier were perfect, everything would be
just fine. Sorry folks, the internal circuits are not perfect and that is where
the troubles start.

Inasmuch as the feedback is supposed to compensate for any non-linearities
between input and output, it is nice to know what non-linearities exist.

First of all each individual active device is nonlinear. Its transfer
characteristics are exponential, not linear, be it tube, FET, or transistor.
Refer to the sketches below. If the device was perfect, its transfer function
would be a straight line, and the slope would remain the same at all
frequencies. The actual characteristics are shown in the second and third
sketches. Note that the characteristics are actually exponential. The device is
only very linear near the center-line of its operation, and the harder it is
"worked", the less linear it becomes, finally becoming 100% nonlinear when its
absolute limitations are reached. In addition, the slope of the transfer
function becomes less at higher frequencies as the gain of the device reduces.
Thus if one attempts to get the same output from any given device at higher
frequency, one will drive it into gross non-linearity sooner, as its headroom is
less. In a similar fashion, at very low frequencies (approaching DC) the device
becomes more nonlinear and its gain drops. In addition, because the slope
changes with frequency, a kind of phase distortion is introduced which is not
measured in standard IHF distortion tests, which measure only single frequency
performance.

Obviously, to optimize internal linearity, it is very desirable to operate
each device within as narrow a bandwidth and as limited an amplitude range as
possible while still covering the audio frequency range of interest.

In a vacuum tube amplifier another major non-linearity is the output
transformer. The output transformer's primary coil is just that-a very large
coil (inductor) in series with the output tubes. Obviously the coil becomes very
resistive as the frequency goes up, and in an audio amplifier this happens well
within the audio range, rolling off the high frequency output. At very low
frequencies the core of the transformer saturates giving very nonlinear bass
performance. If one wants good high frequency performance then one must have
very small output transformers so that the coil inductance is low. If one wants
good low frequency performance one must have very large transformers so the core
does not saturate. These requirements are mutually self exclusive. These
requirements become more difficult to meet as the power rating goes up. If one
attempts to "get around" this by designing a tube amplifier without output
transformers, then one is faced with the problem that output tubes have very
high output impedance and will not drive normal loudspeaker loads (8 s nominal)
without severe non-linearities.

Since the output transformer is a very narrow band and nonlinear device, it
is obviously necessary to not feed into the transformer a signal that it cannot
handle-the bandwidth of the amplifier must be limited to within the bandwidth of
the output transformer. You cannot stuff 10 pounds in a 1 pound sack.

The power supply is another source of distortion. The power supply can be
considered to be in series with and part of the output circuit. All current that
flows through the output circuit and speaker load first must flow through the
power supply. The frequency limits of a power supply are real. It can be modeled
as an inductor in series with a capacitor. Obviously at DC the capacitor's
impedance is infinitely high, and at high frequencies the inductance is
infinitely high.

Thus, at very low and very high frequencies, the power supply is not capable
at all. Unless great care is used in the power supply design, it may have
multiple resonances, and actually be high impedance at many frequencies within
the audio range. Consider also that since the power supply is part of the output
circuit, if somebody offers a "wonderful" vacuum tube amplifier with a
"wonderful" solid state power supply, you no longer have a vacuum tube
amplifier, but a solid state amplifier, so how can it be a "wonderful" tube
amplifier?

Of course the power supply is also attached to the voltage amplifier section.
Consider that all current draw by the output section causes a signal to show up
on the power supply feeds. Any given device or circuit will work best when its
supply is absolutely stable. Circuits are designed to reject power supply
variations, but the supply rejection isn't absolute. Thus the more signal that
shows up on the power supply feed to the voltage amplifier, the more distortion
and instabilities there will be, as this is a signal injected into the circuit
at the wrong place. Since we already know that the power supply is less
effective at very high and very low frequencies, obviously the power supply
related distortions will be greater at very high and very low frequencies.
Again, a very good reason to bandwidth limit the amplifier to within the
capability of its power supply.

Understand of course that we are considering basic ground rules in general.
There are many different kinds of voltage amplifier and current amplifier
configurations that work fairly well, some simple, some complex. The important
thing to know is that unless the circuits are executed to obey the guidelines
established above the distortion will be very high under real world conditions,
no matter what the linearity of each section is and no matter how high a quality
of parts are used.

Another common problem with vacuum tube amplifiers is the value chosen for
the interstage coupling capacitors. In the case of the Dynaco St-70 for example
(see attached schematic) coupling capacitors C10 and C11 are 0.1 uF. This
introduces another large low frequency roll-off within the feedback loop. Since
the amplifier actually amplifies the difference between input and feedback, and
since the feedback is the difference between input and feedback, and since the
feedback is taken off the output of the amplifier, at low frequencies the
difference signal becomes very large partially due to the roll-off caused by the
0.1 uF coupling capacitors. Now remember that we have shown that any circuit
becomes less linear with increasing amplitude and at the frequency extremes. The
roll-off caused by the small value interstage coupler makes the front end work
very hard to generate a large low frequency correction signal. This causes the
front end to run in a very nonlinear mode at low frequencies. You hear it as
"muddy bass." The "cure" is quite simple, make the interstage capacitor large
enough in value so that the loop roll-off is minimized, thus reducing the
correction required, and letting the front end run in a more linear mode. The
low frequency correction signal is easy to see on an oscilloscope. Using a low
frequency square wave as a source (20 Hz is fine) look at the signal on the
output side of the interstage coupler. Note that it looks much like the input
signal. Now look at the signal on the input side of the coupler. You will find
the circuit is generating a signal with a large bass boost! (This is true in
most tube preamps too!) What is happening is that the "flat" input signal is
rolled off by the interstage coupling capacitor. Then the rolled off signal is
fed back to the input and a correction signal is generated with a large bass
boost to make up for the roll-off. The boosted signal is then rolled off again
by the coupling capacitor and its output looks just fine. But the "monkey
motion" has ruined the voltage amplifier's linearity at low frequencies.

Now let's look at what is wrong with the original Dyna St-70 in detail. Refer
to the audio channel schematic again, keeping in mind that the "dashed" section
is our addition, the original has the input connected to V2 directly with a
piece of wire.

What we have is a typical vacuum tube amplifier with unlimited bandwidth
input acceptance (DC coupled) but with limited bandwidth output transformers and
small interstage coupling capacitors. The power supply is also limited
bandwidth, being pretty feeble at both low and high frequencies.

A very low frequency signal is rolled off by the interstage coupling
capacitors, turned into a "lump" as the output transformer core saturates, and
is further distorted as the power supply runs out of steam. The feedback signal,
being taken off after all the disasters occur is very different from the input
signal. This generates an enormous "difference" signal which drives the front
end into 100% distortion trying to make an impossible correction. Inasmuch as
the circuits are underdamped too, the "blob" makes the amp ring for a few cycles
attempting to digest the mess. Some people call these distortions and ringing,
which extends up into the mid-range, "concert hall sound." Sorry, it isn't
concert hall sound, it is distortion. If you like it you have bad taste.

At high frequencies the compensation in the voltage amplifier rolls off the
signal, the active devices roll it off further, and the output transformers
attenuate the highs further yet. This generates another huge correction signal
at high frequencies, again more than the headroom of the front end, clipping the
correction signal once again. Of course the high impedance of the supply has
further compounded the problems. The amp is driven into hard slew limiting and
all signal entering the amp while any internal device is slewing is erased. Gobs
of high frequency distortion are added and part of the music is forever lost. It
is very strange to think that some people use the St-70 to drive tweeters when
it doesn't "tweet" at all-it does kind of "squeak."

Obviously the power supply of the St-70 must be much improved. NO! Not
necessarily! Think a minute. Consider that the power bandwidth of the power
supply must be greater than the bandwidth of the audio circuit. There are two
ways to get this ratio in proper order. The expensive (and stupid) way is to
build a huge power supply-and if the amp has DC coupled inputs you can never
make it big enough. The easy and smart way is to limit the bandwidth of the
circuit to within the capabilities of the existing power supply, especially if
it is absolutely necessary to bandwidth limit the inputs anyway to make the
input bandwidth within the capabilities of the output transformers.

As mentioned earlier, the interstage coupling capacitors, C10 and C11 are too
small. Note that as long as the input is DC coupled, it is not possible to make
C10 and C11 big enough, as even a very large capacitor will have an inside the
loop roll off when compared to DC input acceptance.

To install the input bandwidth limited filter on the St-70 you will need 8
parts: (2) 10,000 resistors, (2) 470,000 resistors (5% carbon film 1/4 watt
parts from Radio Shack are just fine, and it would be better if you could use a
meter and "pair" them, so they are matched within 1%.) You will also need (2)
1000 pF capacitors (mica, polystyrene, or mylar are O.K., of about 100 volt
rating - the capacitors used should be physically small) and (2) 0.02 uF
capacitors (film) 100 volt rating, again physically as small as possible. Again,
Radio Shack will have adequate parts and if you can match them on a precision
capacitance meter it will be helpful. The capacitor values suggested are not
absolute. Anything from about 0.02 to 0.033 uF is O.K. for the larger cap, and
800 to 1200 pF for the smaller capacitor.

The new 6 dB per octave low pass and high pass filter is installed on the
input jacks on the bottom inside of the chassis. We suggest that the mono-stereo
switch wiring be eliminated as the performance is poorer when bridged mono
because of the difference between the two channels (no two output tubes or
output transformers are identical).

If you decide to eliminate the mono-stereo switch,
then do the following:

1. Remove all the wires from the input jack and mono-stereo switch except for
the two wires going directly from the input jack ground lugs to the PC card
(these are actually extensions of the leads of two 10 resistors mounted on the
card). These remain. Also remove the two original 470,000 resistors from the
jack and switch.

2. Remove the two wires running from the hot lugs of the input jack to
eyelets 7 and 17 on the PC card.

3. Connect a 10,000 resistor in series with a 0.02 uF capacitor and connect
the capacitor end of the series set to the left channel hot input jack and the
resistor end to eyelet 7 on the PC card.

4. Connect another 10,000 resistor in series with a 0.02 uF capacitor and
connect the capacitor end to the right channel hot input jack and the resistor
end to eyelet 17 on the PC card.

5. Connect a 1000 pF capacitor in parallel with a 470,000 resistor and
install the resistor between the left channel ground lug and eyelet 7 on the PC
card.

6. Connect another 1000 pF capacitor in parallel with a 470,000 resistor and
connect the resistor from the right channel ground lug on the input jack to
eyelet 17 on the PC card.

If you must keep the mono-stereo switch option, do the
following instead of the last set of instructions:

1. Remove the two wires running from the left and right channel hot lugs on
the input jacks to eyelets 7 and 17 on the PC card.

2. Connect a 10,000 resistor in series with a 0.02 uF capacitor and connect
the capacitor end to the left channel hot input lug and the resistor end to
eyelet 7 on the PC card.

3. Connect another 10,000 resistor in series with a 0.02 uF capacitor and
connect the capacitor end to the right channel hot input lug and the resistor
end to eyelet 17 on the PC card.

4. Install a 1000 pF capacitor in parallel with each of the two existing
470,000 resistors on the input jack and mono-stereo switch.

For mono operation, the amp is switched to mono, only one input jack is used
(either left or right, but not both). Connect a jumper wire from the left output
ground to the right output ground terminal. Connect a jumper wire from the left
16 tap to the right 16 tap (for 8 speakers). Take the output from the same
channel that you have the input jack connected to, using the 16 and ground
terminals (for 8 speakers). To use 4 speakers connect the jumper from the left 8
output tap to the right 8 output jack and connect the load from ground to 8 on
the channel used. This arrangement parallels the two channels for somewhat
higher power, but lower definition performance.

Now that the input bandwidth is set to a rational, finite limit, it is
possible to make the interstage coupling capacitors "big enough." You will need
to acquire four 1 uF at 400 volt film capacitors (mylar, polypropylene, or
whatever). Again Radio Shack will have adequate parts.

Locate and remove the four large identical black tubular 0.1 uF at 400 volt
capacitors from the PC card. They are positioned parallel with the front of the
chassis, one at each corner of the PC card.

Replace each with a 1.0 uF at 400 volt capacitor. The exact value of the
replacements is not critical. They can be anything between 0.8 uF to 2.0 uF at
400 volts or higher. It is important that all four new capacitors be exactly the
same.

Further detail improvements can be made to the St-70. The bias supply
capacitors in old St-70 amps should be replaced. We suggest that the two
original 50 uF capacitors (C3 and C4, located on the 7 lug terminal strip under
the chassis) be replaced with new 100 uF at 80 volt electrolytic capacitors
(again, available at Radio Shack). Note that the positive end of each cap is
connected to ground. Do not use a larger capacitor in this application or the
supply will come up too slowly, over-biasing the output tubes at turn on.

Although the original power supply is now adequate, further reductions in hum
and noise can be made by installing an additional 100 uF at 500 volt
electrolytic capacitor (a 450 volt rated cap with a 500+ volt surge rating is
adequate unless you have high line voltage) from pin 8 of the power supply tube
(V1-5AR4) to chassis ground at the ground lug near the quad filter. The positive
end of the cap goes to the tube socket, the negative end to ground.

Inasmuch as the perceived "image" and "depth" of an audio system is dependent
upon both channels having exactly the same gain and phase response, and because
the resistors in the St-70 (and other tube amps) may have drifted out of
specification over the years, it is helpful to replace all of the resistors with
new tight tolerance parts. The gain determining resistors especially should be
matched to each other within 1%. The RN60D and RL42S metal film resistors shown
on the attached St-70 parts list are a good choice. However using 1/2 watt
carbon film resistors for the RN60s and 2 watt carbon film resistors for the
RL42S types is just fine, except you will have to sort more of them to get a
tight channel-to-channel match.

Because selenium rectifiers (the small little finned block located in the
bottom middle of the chassis) become resistive with age, you may be able to
increase the voltage to your bias supply by substituting a lN4004 silicon diode
for this part (Dl). Because the negative voltage to the bias supply will now be
higher than stock, it probably will also be necessary to change the value of Rl
(10,000 2 watt resistor) to 18,000 2 watt to allow the amp to bias adjust at
1.56 volts DC across R20 in the center of rotation of P1 and P2.

The St-70 and other tube amplifiers run very hot. This tends to make solder
joints deteriorate with time. Re-solder all solder connections in the amplifier,
including all parts, leads, and the tube sockets on the PC card. Clean the input
jacks, output terminals, the bias pots, and all the tube sockets with DeoxIT-D5
(we have 5 oz. spray cans available for $20). Usually lightly "crimping" the hot
(inner) terminals of the input jacks will eliminate patch cable dropouts.

In the St-70 the noise characteristics, gain, power, and slew rate are
dependent upon having high quality tubes in the unit. We have SovtEC EL34G+
output tubes available at $80 per set of four plus $6 shipping and a SovtEC 12X4
rectifier tube available for $15 plus $6 shipping. We do not have 7199 tubes
available.

Refer to the attached schematic and parts list for other service and
adjustment notes on the St-70.

I assume you have noticed we have not spent much time on the inner details of
the circuit topography of the St-70. There may, or may not be "better" input,
phase inverter, and output circuits available. The point is that almost all tube
amps are mistakenly DC coupled and whatever the internal circuits are, they are
driven into gross nonlinearities. The important concept is that any tube amp in
which the input is limited to within the internal capabilities of the circuit
will outperform any tube amp that can be driven into internal overload, no
matter how expensive or complex the circuits may be. And the final limitations
of a tube amplifier are the output transformers. Lots of money spent trying to
achieve a "better" drive circuit probably is of little value, because the output
transformers still are the limits of performance.

THINGS YOU SHOULD NOT DO TO YOUR VACUUM TUBE AMPLIFIER (AND
WHY).

DO NOT install a solid state diode bridge to replace the vacuum tube
rectifier. The supply is operating at 500 volts with line surges over 1000
volts! There are no reliable diodes available to operate at this voltage. You
will be in great danger of blowing the diode bridge and damaging your power
transformer and filter capacitor. In addition, the solid state supply will "turn
on" instantly, and the full B+ voltage will be fed to the tubes before the
heaters have warmed up and turned the tubes on. This will tend to over voltage
the quad filter capacitor and capacitors downstream, which may damage them. The
output tubes will run hotter than normal and have a short service life. There
are no useful redeeming advantages to a solid state diode bridge.

DO NOT install solid state regulators. The "aftermarket" circuits we have
seen use transistors with inadequate voltage ratings (operating in the "blow-up"
mode) and have severe slew rate limitations. Remember, the bandwidth of your
power supply must be greater than the audio circuits, and a series bipolar
regulator is bog slow! It will change the sound, it makes it much worse!

DO NOT add external power capacitors. The long hookup wires will have lots of
inductance and impair the high frequency performance.

DO NOT rewire the amplifier internally with "magic wire." The chances are you
will screw up the lead routings, add longer lead runs than the original and
increase stray inductances. The probabilities of internal short circuits and bad
connections increase as the wires are too large for reliable termination.

DO NOT replace your capacitors with high priced and physically large "wonder
caps." The larger the physical size of a given value capacitor, the greater its
inductance will be, and the more trash it will dump into the circuit. Magic
"wonder caps" do change the sound, they make it worse!

DO NOT use polystyrene capacitors near heat generating components. They
change value with temperature, and near an output tube they may even melt.

DO NOT ship your vacuum tube amplifier to us to fix if you screw it up unless
you CALL US FIRST at 651 330-9871. Output tubes don't survive shipping, tube
amps are heavy and expensive to ship, and their performance is limited. One of
the "joys" of owning a vacuum tube amplifier is learning how to fix it yourself.
If you don't want to do this, you shouldn't own a vacuum tube amplifier.

2005 Note:

We can get much better results now with our complete $449
Ultimate
70 Rebuild Kit. It includes a new motherboard with all
new audio circuits (using the information contained herein
and much more), a new B+ supply built of modern
PC-card-mounted high efficiency, high reliability capacitors,
and a new bias supply. It also includes new tubes to replace
the now obsolete 7199 tubes. A new Input/Output jack set kit
is available too for
$65
with gold plated input jacks and high quality Pomona 5-way
binding post output jacks. No cutting or drilling is required
and step-by-step instructions, schematics, parts lists, and
wiring diagrams are provided. The kit instruction manual and
bare PC card are available for $149
if you want to provide your own parts. The price of the
manual and PC card can be applied to the later purchase of
the rest of the kit as long as they are current. The sonic
quality is really special.

Formula to Solve 6 dB/Octave High & Low Pass Filter
Such as Recommended for the St-70

Service Notes

Examining the 1.56 volt biaset test point on the St-70 can tell you much
about the condition of the amplifier. If the output tubes are old, it may be
impossible to adjust the bias pots to bring the voltage up high enough. Replace
the output tubes. A shorted output tube may cause the bias reading to run away
high. 7199 tubes are best selected by examining the output of the amp on a
scope. A low gain or noisy 7199 will show excess output hum and/or not make full
power. Fuse blowing can be caused by two problems. A hard blow soon after
turn-on indicates a power supply short, either a defective 5AR4 tube or a
shorted quad filter cap. A fuse that blows soft or after a few minutes of
operation indicates a problem with the audio circuits - probably a bad 6CA7
tube. Note that the bias setting will vary with AC line voltage so the value
isn't an absolute. It is possible to swap tubes channel to channel (except the
5AR4) one at a time to locate a defective tube.

The new input filter circuit provides -3 dB poles at 16 Hz and 17 KHz which
keeps the audio circuit working within the limits of the output transformers.
The larger C10 and C11 effectively takes them out of circuit for AC signal
purposes after the input filters have been installed. It is much more important
to install the new input filter circuits than to replace all of the resistors
and capacitors. Match R and C values channel to channel for a good gain match
between the channels.

Dyna St-70 Amplifier Suggested Modifications Parts List

C1 0.02 uF 1000V disc

P1 10 k bias trimpot

C2 0.02 uF 1000V disc

P2 10 k bias trimpot

C3 100 uF 80V electrolytic

R1 10 k 2W

C4 100 uF 80V electrolytic

R2 10 k 2W

C5A 30 uF 525V electrolytic

R3 6.8 k 2W

C5B 20 uF 525V electrolytic

R4 22 k 2W

C5C 20 uF 525V electrolytic

R5 10 k 0.5W

C5D 20 uF 525V electrolytic

R6 475 k 0.5W

C6 0.02 uF 50V film

R7 10 ohm 0.5W

C7 1000 pF 50V film

R8 330 k 2W

C8 0.05 uF 400V film

R9 1.5 M 0.5W

C8 0.05 uF 400V film

R10 270 k 2W

C10 1 uF 400V film

R11 620 0.5W

C11 1 uF 400V film

R12 47 0.5W

C12 390 pF 500V mica

R13 18 k 0.5W

D1 1N4003 or 1N4004 silicon diode

R14 47 k 2W matched within 1% of
R15

V1 5AR4 rectifier tube

R15 47 k 2W matched with 1% of R14

V2 7199 pentode, triode tube

R16 1 k 2W

V3 6CA7/EL34 pentode tube

R17 270 k 0.5W matched within 1% of
R18

V4 6CA7/EL34 pentode tube

R18 270 k 0.5W matched within 1% of
R17

L1 choke, Dynaco C-354

R19 1 k 0.5W

S1 power switch, SPST

R20 15.6 1W

F1 fuse, 3 ampere slo-blo 3AG

R21 1 k 0.5W

T1 power transformer, Dynaco PA-060

T2 output transformer, Dynaco
A-470

Schematic

Note: Input circuit (C6, R5, C7, and R6) is new and replaces the original
direct coupled input circuit. All wiring is removed from the input jacks except
R7 leads and then the new input circuit is installed. The stereo-mono switch
should be eliminated. C3, C4, C10, and C11 are revised values. Adjust bias (P1
& P2) for 1.56 volts across R20. L1 can be replaced with a 30 10 watt
resistor to make amplifier operational if L1 is defective. This input filter
circuit is also helpful for the Dyna MKIII and MKIV amplifiers.

©
2011 Audio by Van Alstine