=20
Home =20
| |
|
Our =
Famous Original=20
Dyna St-70 Rebuild Plans
From=20
Audio Basics, July, 1982
Article By Frank Van=20
Alstine
http://www.avahifi.com/
Hello=20 again, am glad that the feedback I am getting from many of you=20 indicates that you are happy with the information and ideas I = have been=20 describing. Some of you have complained that you want more = "meat" -=20 technical information on audio equipment. So, this month we = will get a=20 bit more technical and go through the operation of a vacuum = tube=20 amplifier, what it does, what it cannot do, and what you can do = about=20 it. The data will pertain to vacuum tube amplifiers in general, = and the=20 Dyna St-70 specifically, as we get so many requests for = information on=20 how to modify them.
Vacuum Tube=20
Amplifiers
Why audio amplifier can=20
essentially be modeled as a two stage device; a voltage =
amplifier=20
(which amplifies the amplitude of the signal) followed by a =
current=20
amplifier (which supplies the drive current to drive the now large =
=20
amplitude signal into a low impedance load - the speaker system). =
Although=20
many tube circuits will also have voltage gain in the current =
amplifier=20
section, that is not important to the discussion that follows. =
The=20
closed loop gain of the amplifier is determined by a voltage =
divider=20
which sends a portion of the output signal back to the input of =
the=20
voltage amplifier out of phase. The amplifier then does not =
amplify the=20
input signal, it amplifies a signal being the difference =
between input=20
and feedback signal.
If the voltage amplifier and current amplifier = were=20 perfectly linear, the difference between the input and feedback = signal=20 would be only a small version of the input signal and the = results would=20 be "perfect." In theory, any difference between input and = output is=20 supposed to be eliminated by feedback. The feedback signal, = which is a=20 duplicate of the output signal, when subtracted from the input = signal,=20 should create a "difference signal" which is "pre-distorted" by = the=20 amount of distortion in the circuits, but out of phase with the = circuit's distortion. The "pre-distortion" in the "difference" = signal,=20 when added out of phase to the actual distortion in the = circuits,=20 cancels exactly, giving a "perfect" output signal. That is the = way it=20 is supposed to work.
By the way, there ain't no such thing as a no = feedback=20 amplifier! There are many different feedback schemes. One can = choose to=20 use lots of local feedback in each stage and little overall = loop=20 feedback, but each device in itself (whether it is transistor = or tube)=20 has internal feedback. If one attempts to make an amplifier = with "no=20 feedback" except for that inherent in the devices themselves = then the=20 design becomes absolutely dependent of the characteristics of = each=20 independent device. No two tubes or transistors, even of the = same type,=20 are identical. Even if you painstakingly select and bias each=20 individual device one at a time, its characteristics will = change with=20 variations in temperature, current, voltage, and age. It will = be=20 impossible to make any two channels the same and to keep them the = same.=20 The main characteristics of a so called "no feedback" amplifier = are:=20 very high cost (each unit is essentially a one off), very hot = running=20 as the devices have to be biased on very hard, unstable operation = as =20 the unit changes characteristics with age and temperature, no two = samples =20 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=20 of each device. I can live without it. The "wonderful" sound of = a =20 "no feedback" amplifier is the wonderful sound of lots of = instabilities=20 and underdamped oscillations. You may like it, I don't, it = isn't=20 music.
Thus, those that choose to build stable, = repeatable, and=20 rationally priced amplifiers will use some feedback. The catch = is in=20 knowing what the feedback can and cannot do in the real world, = and to=20 use the feedback properly, so that the unit does not only = measure well,=20 but actually performs well under transient conditions in the = real=20 world.
Now, back to that vacuum tube amplifier. = Remember we=20 mentioned that if the voltage amplifier and the current = amplifier were=20 perfect, everything would be just fine. Sorry folks, the = internal=20 circuits are not perfect and that is where the troubles=20 start.
Inasmuch as the feedback is supposed to = compensate for=20 any non-linearities between input and output, it is nice to = know what=20 non-linearities exist.
First of all each individual active device is = nonlinear.=20 Its transfer characteristics are exponential, not linear, be it = tube,=20 FET, or transistor. Refer to the sketches below. If the device = was=20 perfect, its transfer function would be a straight line, and = the slope=20 would remain the same at all frequencies. The actual = characteristics=20 are shown in the second and third sketches. Note that the=20 characteristics are actually exponential. The device is only = very=20 linear near the center-line of its operation, and the harder it = is=20 "worked", the less linear it becomes, finally becoming 100% = nonlinear=20 when its absolute limitations are reached. In addition, the = slope of=20 the transfer function becomes less at higher frequencies as the = gain of=20 the device reduces. Thus if one attempts to get the same output = from=20 any given device at higher frequency, one will drive it into = gross=20 non-linearity sooner, as its headroom is less. In a similar = fashion, at=20 very low frequencies (approaching DC) the device becomes more = nonlinear=20 and its gain drops. In addition, because the slope changes with = frequency, a kind of phase distortion is introduced which is not = measured=20 in standard IHF distortion tests, which measure only single = frequency =20 performance.
Obviously, to optimize internal linearity, it = is very=20 desirable to operate each device within as narrow a bandwidth = and as=20 limited an amplitude range as possible while still covering the = audio=20 frequency range of interest.
In a vacuum tube amplifier another major = non-linearity=20 is the output transformer. The output transformer's primary = coil is=20 just that-a very large coil (inductor) in series with the = output tubes.=20 Obviously the coil becomes very resistive as the frequency goes = up, and=20 in an audio amplifier this happens well within the audio range, = rolling=20 off the high frequency output. At very low frequencies the core = of the=20 transformer saturates giving very nonlinear bass performance. = If one=20 wants good high frequency performance then one must have very = small=20 output transformers so that the coil inductance is low. If one = wants=20 good low frequency performance one must have very large = transformers so=20 the core does not saturate. These requirements are mutually = self=20 exclusive. These requirements become more difficult to meet as = the=20 power rating goes up. If one attempts to "get around" this by = designing=20 a tube amplifier without output transformers, then one is faced = with=20 the problem that output tubes have very high output impedance and = will=20 not drive normal loudspeaker loads (8 s nominal) without severe=20 non-linearities.
Since the output transformer is a very narrow = band and=20 nonlinear device, it is obviously necessary to not feed into = the=20 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=20 power supply can be considered to be in series with and part of = the=20 output circuit. All current that flows through the output = circuit and=20 speaker load first must flow through the power supply. The = frequency=20 limits of a power supply are real. It can be modeled as an = inductor in=20 series with a capacitor. Obviously at DC the capacitor's = impedance is=20 infinitely high, and at high frequencies the inductance is = infinitely=20 high.
Thus, at very low and very high frequencies, = the power=20 supply is not capable at all. Unless great care is used in the = power=20 supply design, it may have multiple resonances, and actually be = high=20 impedance at many frequencies within the audio range. Consider = also=20 that since the power supply is part of the output circuit, if = somebody=20 offers a "wonderful" vacuum tube amplifier with a "wonderful" = solid=20 state power supply, you no longer have a vacuum tube amplifier, = but a=20 solid state amplifier, so how can it be a "wonderful" tube=20 amplifier?
Of course the power supply is also attached to = the=20 voltage amplifier section. Consider that all current draw by = the output=20 section causes a signal to show up on the power supply feeds. = Any given=20 device or circuit will work best when its supply is absolutely = stable.=20 Circuits are designed to reject power supply variations, but = the supply=20 rejection isn't absolute. Thus the more signal that shows up on = the=20 power supply feed to the voltage amplifier, the more distortion = and=20 instabilities there will be, as this is a signal injected into = the=20 circuit at the wrong place. Since we already know that the = power supply=20 is less effective at very high and very low frequencies, = obviously the=20 power supply related distortions will be greater at very high = and very=20 low frequencies. Again, a very good reason to bandwidth limit = the=20 amplifier to within the capability of its power supply.
Understand of course that we are considering = basic=20 ground rules in general. There are many different kinds of = voltage=20 amplifier and current amplifier configurations that work fairly = well,=20 some simple, some complex. The important thing to know is that = unless=20 the circuits are executed to obey the guidelines established = above the=20 distortion will be very high under real world conditions, no = matter=20 what the linearity of each section is and no matter how high a = quality=20 of parts are used.
Another common problem with vacuum tube = amplifiers is=20 the value chosen for the interstage coupling capacitors. In the = case of=20 the Dynaco St-70 for example (see attached schematic) coupling=20 capacitors C10 and C11 are 0.1 uF. This introduces another = large low=20 frequency roll-off within the feedback loop. Since the = amplifier=20 actually amplifies the difference between input and feedback, = and since=20 the feedback is the difference between input and feedback, and = since=20 the feedback is taken off the output of the amplifier, at low=20 frequencies the difference signal becomes very large partially due = to =20 the roll-off caused by the 0.1 uF coupling capacitors. Now = remember that=20 we have shown that any circuit becomes less linear with = increasing=20 amplitude and at the frequency extremes. The roll-off caused by = the=20 small value interstage coupler makes the front end work very = hard to=20 generate a large low frequency correction signal. This causes = the front=20 end to run in a very nonlinear mode at low frequencies. You = hear it as=20 "muddy bass." The "cure" is quite simple, make the interstage = capacitor=20 large enough in value so that the loop roll-off is minimized, = thus=20 reducing the correction required, and letting the front end run = in a=20 more linear mode. The low frequency correction signal is easy = to see on=20 an oscilloscope. Using a low frequency square wave as a source = (20 Hz=20 is fine) look at the signal on the output side of the = interstage=20 coupler. Note that it looks much like the input signal. Now = look at the=20 signal on the input side of the coupler. You will find the = circuit is=20 generating a signal with a large bass boost! (This is true in = most tube=20 preamps too!) What is happening is that the "flat" input signal = is=20 rolled off by the interstage coupling capacitor. Then the = rolled off=20 signal is fed back to the input and a correction signal is = generated=20 with a large bass boost to make up for the roll-off. The = boosted signal=20 is then rolled off again by the coupling capacitor and its = output looks=20 just fine. But the "monkey motion" has ruined the voltage = amplifier's=20 linearity at low frequencies.
Now let's look at what is wrong with the = original Dyna=20 St-70 in detail. Refer to the audio channel schematic again, = keeping in=20 mind that the "dashed" section is our addition, the original = has the=20 input connected to V2 directly with a piece of wire.
What we have is a typical vacuum tube = amplifier with=20 unlimited bandwidth input acceptance (DC coupled) but with = limited=20 bandwidth output transformers and small interstage coupling = capacitors.=20 The power supply is also limited bandwidth, being pretty feeble = at both=20 low and high frequencies.
A very low frequency signal is rolled off by = the=20 interstage coupling capacitors, turned into a "lump" as the = output=20 transformer core saturates, and is further distorted as the = power=20 supply runs out of steam. The feedback signal, being taken off = after=20 all the disasters occur is very different from the input = signal. This=20 generates an enormous "difference" signal which drives the = front end=20 into 100% distortion trying to make an impossible correction. = Inasmuch=20 as the circuits are underdamped too, the "blob" makes the amp = ring for=20 a few cycles attempting to digest the mess. Some people call = these=20 distortions and ringing, which extends up into the mid-range, = "concert=20 hall sound." Sorry, it isn't concert hall sound, it is = distortion. If=20 you like it you have bad taste.
At high frequencies the compensation in the = voltage=20 amplifier rolls off the signal, the active devices roll it off = further,=20 and the output transformers attenuate the highs further yet. = This=20 generates another huge correction signal at high frequencies, = again=20 more than the headroom of the front end, clipping the = correction signal=20 once again. Of course the high impedance of the supply has = further=20 compounded the problems. The amp is driven into hard slew = limiting and=20 all signal entering the amp while any internal device is = slewing is=20 erased. Gobs of high frequency distortion are added and part of = the=20 music is forever lost. It is very strange to think that some = people use=20 the St-70 to drive tweeters when it doesn't "tweet" at all-it = does kind=20 of "squeak."
Obviously the power supply of the St-70 must = be much=20 improved. NO! Not necessarily! Think a minute. Consider that = the power=20 bandwidth of the power supply must be greater than the = bandwidth of the=20 audio circuit. There are two ways to get this ratio in proper = order.=20 The expensive (and stupid) way is to build a huge power = supply-and if=20 the amp has DC coupled inputs you can never make it big enough. = The=20 easy and smart way is to limit the bandwidth of the circuit to = within=20 the capabilities of the existing power supply, especially if it = is=20 absolutely necessary to bandwidth limit the inputs anyway to = make the=20 input bandwidth within the capabilities of the output =20 transformers.
As mentioned earlier, the interstage coupling=20 capacitors, C10 and C11 are too small. Note that as long as the = input=20 is DC coupled, it is not possible to make C10 and C11 big = enough, as=20 even a very large capacitor will have an inside the loop roll = off when=20 compared to DC input acceptance.
To install the input bandwidth limited filter = on the=20 St-70 you will need 8 parts: (2) 10,000 resistors, (2) 470,000=20 resistors (5% carbon film 1/4 watt parts from Radio Shack are = just=20 fine, and it would be better if you could use a meter and = "pair" them,=20 so they are matched within 1%.) You will also need (2) 1000 pF=20 capacitors (mica, polystyrene, or mylar are O.K., of about 100 = volt=20 rating - the capacitors used should be physically small) and = (2) 0.02=20 uF capacitors (film) 100 volt rating, again physically as small = as=20 possible. Again, Radio Shack will have adequate parts and if you = can=20 match them on a precision capacitance meter it will be helpful. = The =20 capacitor values suggested are not absolute. Anything from about = 0.02 to =20 0.033 uF is O.K. for the larger cap, and 800 to 1200 pF for the = smaller =20 capacitor.
The new 6 dB per octave low pass and high pass = filter is=20 installed on the input jacks on the bottom inside of the = chassis. We=20 suggest that the mono-stereo switch wiring be eliminated as the = performance is poorer when bridged mono because of the = difference=20 between the two channels (no two output tubes or output = transformers=20 are identical).
If you =
decide to=20
eliminate the mono-stereo switch, then do the=20
following:
1. Remove all the wires =
from the=20
input jack and mono-stereo switch except for the two wires =
going=20
directly from the input jack ground lugs to the PC card (these =
are=20
actually extensions of the leads of two 10 resistors mounted on =
the=20
card). These remain. Also remove the two original 470,000 =
resistors =20
from the jack and switch.
2. Remove the two wires running from the hot = lugs of the=20 input jack to eyelets 7 and 17 on the PC card.
3. Connect a 10,000 resistor in series with a = 0.02 uF=20 capacitor and connect the capacitor end of the series set to = the left=20 channel hot input jack and the resistor end to eyelet 7 on the = PC=20 card.
4. Connect another 10,000 resistor in series = with a 0.02=20 uF capacitor and connect the capacitor end to the right channel = hot=20 input jack and the resistor end to eyelet 17 on the PC = card.
5. Connect a 1000 pF capacitor in parallel = with a=20 470,000 resistor and install the resistor between the left = channel=20 ground lug and eyelet 7 on the PC card.
6. Connect another 1000 pF capacitor in = parallel with a=20 470,000 resistor and connect the resistor from the right = channel ground=20 lug on the input jack to eyelet 17 on the PC card.
If you =
must keep the =20
mono-stereo switch option, do the following instead of the last =
set of =20
instructions:
1. Remove the two =
wires running=20
from the left and right channel hot lugs on the input jacks to =
eyelets=20
7 and 17 on the PC card.
2. Connect a 10,000 resistor in series with a = 0.02 uF=20 capacitor and connect the capacitor end to the left channel hot = input=20 lug and the resistor end to eyelet 7 on the PC card.
3. Connect another 10,000 resistor in series = with a 0.02=20 uF capacitor and connect the capacitor end to the right channel = hot=20 input lug and the resistor end to eyelet 17 on the PC = card.
4. Install a 1000 pF capacitor in parallel = with each of=20 the two existing 470,000 resistors on the input jack and = mono-stereo=20 switch.
For mono operation, the amp is switched to = mono, only=20 one input jack is used (either left or right, but not both). = Connect a=20 jumper wire from the left output ground to the right output = ground=20 terminal. Connect a jumper wire from the left 16 tap to the = right 16=20 tap (for 8 speakers). Take the output from the same channel = that you=20 have the input jack connected to, using the 16 and ground = terminals=20 (for 8 speakers). To use 4 speakers connect the jumper from the = left 8=20 output tap to the right 8 output jack and connect the load from = ground=20 to 8 on the channel used. This arrangement parallels the two = channels=20 for somewhat higher power, but lower definition = performance.
Now that the input bandwidth is set to a = rational,=20 finite limit, it is possible to make the interstage coupling = capacitors=20 "big enough." You will need to acquire four 1 uF at 400 volt = film=20 capacitors (mylar, polypropylene, or whatever). Again Radio = Shack will=20 have adequate parts.
Locate and remove the four large identical = black tubular=20 0.1 uF at 400 volt capacitors from the PC card. They are = positioned=20 parallel with the front of the chassis, one at each corner of = the PC=20 card.
Replace each with a 1.0 uF at 400 volt = capacitor. The=20 exact value of the replacements is not critical. They can be = anything=20 between 0.8 uF to 2.0 uF at 400 volts or higher. It is = important that=20 all four new capacitors be exactly the same.
Further detail improvements can be made to the = St-70.=20 The bias supply capacitors in old St-70 amps should be = replaced. We=20 suggest that the two original 50 uF capacitors (C3 and C4, = located on=20 the 7 lug terminal strip under the chassis) be replaced with = new 100 uF=20 at 80 volt electrolytic capacitors (again, available at Radio = Shack).=20 Note that the positive end of each cap is connected to ground. = Do not=20 use a larger capacitor in this application or the supply will = come up=20 too slowly, over-biasing the output tubes at turn = on.
Although the original power supply is now = adequate,=20 further reductions in hum and noise can be made by installing = an=20 additional 100 uF at 500 volt electrolytic capacitor (a 450 = volt rated=20 cap with a 500+ volt surge rating is adequate unless you have = high line=20 voltage) from pin 8 of the power supply tube (V1-5AR4) to = chassis=20 ground at the ground lug near the quad filter. The positive end = of the=20 cap goes to the tube socket, the negative end to = ground.
Inasmuch as the perceived "image" and "depth" = of an =20 audio system is dependent upon both channels having exactly the = same gain =20 and phase response, and because the resistors in the St-70 (and = other=20 tube amps) may have drifted out of specification over the = years, it is=20 helpful to replace all of the resistors with new tight = tolerance parts.=20 The gain determining resistors especially should be matched to = each=20 other within 1%. The RN60D and RL42S metal film resistors shown = on the=20 attached St-70 parts list are a good choice. However using 1/2 = watt=20 carbon film resistors for the RN60s and 2 watt carbon film = resistors=20 for the RL42S types is just fine, except you will have to sort = more of=20 them to get a tight channel-to-channel match.
Because selenium rectifiers (the small little = finned=20 block located in the bottom middle of the chassis) become = resistive=20 with age, you may be able to increase the voltage to your bias = supply=20 by substituting a lN4004 silicon diode for this part (Dl). = Because the=20 negative voltage to the bias supply will now be higher than = stock, it=20 probably will also be necessary to change the value of Rl = (10,000 2=20 watt resistor) to 18,000 2 watt to allow the amp to bias adjust = at 1.56=20 volts DC across R20 in the center of rotation of P1 and = P2.
The St-70 and other tube amplifiers run very = hot. This=20 tends to make solder joints deteriorate with time. Re-solder = all solder=20 connections in the amplifier, including all parts, leads, and = the tube=20 sockets on the PC card. Clean the input jacks, output = terminals, the=20 bias pots, and all the tube sockets with DeoxIT-D5 (we have 5 = oz. spray=20 cans available for $20). Usually lightly "crimping" the hot = (inner)=20 terminals of the input jacks will eliminate patch cable=20 dropouts.
In the St-70 the noise characteristics, gain, = power, and=20 slew rate are dependent upon having high quality tubes in the = unit. We=20 have Sovtec EL34G+ output tubes available at $80 per set of = four plus=20 $6 shipping and a Sovtec 5AR4 rectifier tube available for $15 = plus $6=20 shipping. We do not have 7199 tubes available.
Refer to the attached schematic and parts list = for other=20 service and adjustment notes on the St-70.
I assume you have noticed we have not spent = much time on=20 the inner details of the circuit topography of the St-70. There = may, or=20 may not be "better" input, phase inverter, and output circuits=20 available. The point is that almost all tube amps are = mistakenly DC=20 coupled and whatever the internal circuits are, they are driven = into=20 gross nonlinearities. The important concept is that any tube = amp in=20 which the input is limited to within the internal capabilities = of the=20 circuit will outperform any tube amp that can be driven into = internal=20 overload, no matter how expensive or complex the circuits may = be. And=20 the final limitations of a tube amplifier are the output = transformers.=20 Lots of money spent trying to achieve a "better" drive circuit = probably=20 is of little value, because the output transformers still are = the=20 limits of performance.
Things You Should Not Do =
To Your=20
Vacuum Tube Amplifier (And Why).
DO NOT=20
install a solid state diode bridge to replace the vacuum tube =20
rectifier. The supply is operating at 500 volts with line surges =
over 1000=20
volts! There are no reliable diodes available to operate at =
this=20
voltage. You will be in great danger of blowing the diode =
bridge and=20
damaging your power transformer and filter capacitor. In =
addition, the=20
solid state supply will "turn on" instantly, and the full B+ =
voltage=20
will be fed to the tubes before the heaters have warmed up and =
turned=20
the tubes on. This will tend to over voltage the quad filter =
capacitor=20
and capacitors downstream, which may damage them. The output =
tubes will=20
run hotter than normal and have a short service life. There are =
no=20
useful redeeming advantages to a solid state diode =
bridge.
DO NOT install solid state regulators. The = "aftermarket"=20 circuits we have seen use transistors with inadequate voltage = ratings =20 (operating in the "blow-up" mode) and have severe slew rate =20 limitations. Remember, the bandwidth of your power supply must be = greater =20 than the audio circuits, and a series bipolar regulator is bog = slow! It=20 will change the sound, it makes it much worse!
DO NOT add external power capacitors. The long = hookup=20 wires will have lots of inductance and impair the high = frequency=20 performance.
DO NOT rewire the amplifier internally with = "magic=20 wire." The chances are you will screw up the lead routings, add = longer=20 lead runs than the original and increase stray inductances. The = probabilities of internal short circuits and bad connections = increase=20 as the wires are too large for reliable termination.
DO NOT replace your capacitors with high = priced and=20 physically large "wonder caps." The larger the physical size of = a given=20 value capacitor, the greater its inductance will be, and the = more trash=20 it will dump into the circuit. Magic "wonder caps" do change = the sound,=20 they make it worse!
DO NOT use polystyrene capacitors near heat = generating=20 components. They change value with temperature, and near an = output tube=20 they may even melt.
DO NOT ship your vacuum tube amplifier to us = to fix if=20 you screw it up unless you CALL US FIRST at 952 890-3517. = Output tubes=20 don't survive shipping, tube amps are heavy and expensive to = ship, and=20 their performance is limited. One of the "joys" of owning a = vacuum tube=20 amplifier is learning how to fix it yourself. If you don't want = to do=20 this, you shouldn't own a vacuum tube amplifier.
2002 =
Note:
=20
We can get much better results now with our complete =
$249=20
Super 70i Rebuild Kit. It includes a new mother board with all new =
audio=20
circuits (using the information contained herein and much more), a =
new B+=20
supply built of modern PC-card-mounted high efficiency, high =
reliability=20
capacitors, and a new bias supply (replacing the parts on the =
7-lug=20
terminal strip as part of the new mother board layout). It also =
includes=20
new 6GH8A tubes to replace the now obsolete 7199 tubes. A new =
Input/Output=20
jack set kit is available too for $65.00 with gold plated input =
jacks and=20
high quality Pomona 5-way binding post output jacks. No cutting or =
drilling is required and step-by-step instructions, schematics, =
parts=20
lists, and wiring diagrams are provided. The kit instruction =
manual is=20
available for $25.00 separately, or the manual and bare mother =
board PC=20
card for $55.00 if you want to provide your own parts. The price =
of the=20
manual and PC card can be applied to the later purchase of the =
rest of the=20
kit as long as they are current. The sonic quality is really=20
special.
Formula to = Solve 6=20 dB/Octave High & Low Pass Filter Such as Recommended for = the=20 St-70
Service =
Notes
The new input filter circuit provides -3 dB = poles at 16=20 Hz and 17 KHz which keeps the audio circuit working within the = limits=20 of the output transformers. The larger C10 and C11 effectively = takes=20 them out of circuit for AC signal purposes after the input = filters have=20 been installed. It is much more important to install the new = input=20 filter circuits than to replace all of the resistors and = capacitors.=20 Match R and C values channel to channel for a good gain match = between=20 the channels.
Dyna St-70 Amplifier = Suggested=20 Modifications Parts List
Schematic
Note: Input circuit (C6, R5, C7, and R6) is = new and=20 replaces the original direct coupled input circuit. All wiring = is=20 removed from the input jacks except R7 leads and then the new = input=20 circuit is installed. The stereo-mono switch should be = eliminated. C3,=20 C4, C10, and C11 are revised values. Adjust bias (P1 & P2) = for 1.56=20 volts across R20. L1 can be replaced with a 30 10 watt resistor = to make=20 amplifier operational if L1 is defective. This input filter = circuit is=20 also helpful for the Dyna MKIII and MKIV = amplifiers.
=20 =20
Audio by Van Alstine, Inc.
=
2202=20
River Hills Drive
Burnsville, MN 55337-1100 USA
Voice: (952) 890-3517
=
Fax: (952)=20
894-3675
Website:
|
= =20 |